Patent Publication Number: US-2022236015-A1

Title: Flat tube, multi-channel heat exchanger, and air conditioning and refrigeration system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Section 371 National Stage Application of International Application No. PCT/CN2020/093677, filed Jun. 1, 2020 and published as WO 2020/239120 on Dec. 3, 2020, not in English, which claims priority and rights to Chinese Patent Applications No. 201920820825.6, No. 201920820935.2, and No. 201920819017.8 filed on May 31, 2019, which are incorporated herein by reference in their entireties . 
    
    
     FIELD 
     Embodiments of this application belong to the technical field of heat exchange device manufacturing, and specifically, relates to a flat tube, a multi-channel heat exchanger with the flat tube, and an air conditioning and refrigeration system with the multi-channel heat exchanger. 
     BACKGROUND 
     As an alternative technology for copper tube fin heat exchangers, multi-channel heat exchangers have attracted growing attention in the field of air conditioning technologies, and have developed rapidly in recent years. One of difficulties in the application of multi-channel heat exchangers to the field of air-conditioning heat pumps is that during operating under a low temperature condition, a heat exchange capability decreases rapidly due to frost, thereby greatly reducing heat exchange performance of multi-channel heat exchangers. 
     SUMMARY 
     This application is made when the applicant realizes and discovers the following technical problems in a heat exchanger in the related art: 
     It is found by the applicant that when a heat exchanger in the related art is applied in a heat pump system, a heat exchange temperature difference on a windward side is large, and in an air intake direction, the heat exchange temperature difference decreases and a heat exchange volume of the heat exchanger continuously decreases. In addition, air humidity is also large on the windward side, and decreases along with the air intake direction. As a result, frosting concentrated on the windward side, a wind resistance increase, and an air volume decrease are caused, and a heat exchange capability of the heat exchanger decreases quickly. 
     Objectives of embodiments of this application are to solve at least one of technical problems existing in the prior art, so as to alleviate a heat exchange capability decrease of a heat exchanger, and improve heat exchange efficiency under a frosting condition. 
     An air conditioning and refrigeration system according to some embodiments of the present disclosure is provided and includes a multi-channel heat exchanger, wherein the multi-channel heat exchanger includes: 
     a first header, a second header, and a plurality of flat tubes, 
     wherein the flat tube has a first longitudinal side face and a second longitudinal side face opposite to and parallel to each other in a thickness direction of the flat tube, and a third longitudinal side face and a fourth longitudinal side face opposite to and parallel to each other in a width direction of the flat tube; 
     wherein a distance between the first longitudinal side face and the second longitudinal side face is less than a distance between the third longitudinal side face and the fourth longitudinal side face; the flat tube has a plurality of flow channels extending in a length direction of the flat tube, and the plurality of flow channels of the flat tube are parallel to each other, and are distributed to be spaced apart in the width direction of the flat tube; a center line of the width direction of the flat tube divides the flat tube into a first part and a second part; a flow cross-sectional area of the first part is A 1 , a flow cross-sectional area of the second part is A 2 , and A 2 &gt;A 1 ; and the first part and the second part of the flat tube are arranged in a direction from an air inlet side to an air outlet side; 
     wherein the plurality of flat tubes are arranged in parallel in a thickness direction of the flat tube, a first end of the flat tube is connected to the first header, and a second end of the flat tube is connected to the second header, so as to connect the first header and the second header; and 
     wherein air flows through the first part of the flat tube, and then flows through the second part of the flat tube. 
     An air conditioning and refrigeration system according to some embodiments of the present disclosure is provided and includes 
     a multi-channel heat exchanger includes: 
     a first header, a second header, and a plurality of the flat tubes, 
     wherein the flat tube has a first longitudinal side face and a second longitudinal side face opposite to and parallel to each other in a thickness direction of the flat tube, and a third longitudinal side face and a fourth longitudinal side face opposite to and parallel to each other in a width direction of the flat tube; a distance between the first longitudinal side face and the second longitudinal side face is less than a distance between the third longitudinal side face and the fourth longitudinal side face; the flat tube has a plurality of flow channels extending in a length direction of the flat tube, and the plurality of flow channels of the same flat tube are parallel to each other, and are distributed to be spaced apart in the width direction of the flat tube; the flat tube is divided in the width direction of the flat tube into a first part, a second part, and a third part with same widths; and a flow cross-sectional area of the first part is A 1 , a flow cross-sectional area of the second part is A 2 , a flow cross-sectional area of the third part is A 3 , A 2 &gt;A 1 , and/or A 2 &gt;A 3 ; 
     wherein the plurality of flat tubes are arranged in parallel in a thickness direction of the flat tube, a first end of the flat tube is connected to the first header, and a second end of the flat tube is connected to the second header, so as to connect the first header and the second header; and 
     a first part, a second part, and a third part of the flat tube are arranged in a direction from an air inlet side to an air outlet side; 
     wherein air flows through the first part of the flat tube, and then flows through the second part of the flat tube and then the third part of the flat tube. 
     A multi-channel heat exchanger is provided and includes: 
     a first header, a second header, and a plurality of the flat tubes 
     wherein the flat tube has a first longitudinal side face and a second longitudinal side face opposite to and parallel to each other in a thickness direction of the flat tube, and a third longitudinal side face and a fourth longitudinal side face opposite to and parallel to each other in a width direction of the flat tube; a distance between the first longitudinal side face and the second longitudinal side face is less than a distance between the third longitudinal side face and the fourth longitudinal side face; the flat tube has n groups of flow channels extending in a length direction of the flat tube, and the n groups of flow channels are distributed to be spaced apart in the width direction of the flat tube; and a flow cross-sectional area of a first group of the flow channels is A 1 , a flow cross-sectional area of k th  group of the flow channels is A k , a flow cross-sectional area of an n th  group of the flow channels is An, 1&lt;k≤n, A k ≥1.2 A k−1  and k is an integer greater than 1; 
     wherein the plurality of flat tubes are arranged in parallel in a thickness direction of the flat tube, a first end of the flat tube is connected to the first header, and a second end of the flat tube is connected to the second header, so as to connect the first header and the second header; and 
     a first group of flow channels, a k th  group of flow channels, an n th  group of flow channels of the flat tube are arranged in a direction from an air inlet side to an air outlet side; and 
     a first to n th  group of fins, wherein the first to n th  group of fins are all installed between a first longitudinal side face of the flat tube and a second longitudinal side face of an adjacent flat tube, and the first to n th  groups of fins are sequentially arranged in a width direction of the flat tube, the first group of fins corresponds to the first group of flow channels, the k th  group of fins corresponds to the k th  group of flow channels, the n th  group of fins corresponds to the n th  group of flow channels. 
     The additional aspects and advantages of this application are partially given in the following description, and some of them become obvious from the following description, or are understood through practice of this application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and/or additional aspects and advantages of this application become obvious and easy to understand from the description of the embodiments with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic structural diagram of a multi-channel heat exchanger according to an embodiment of this application; 
         FIG. 2  is a schematic structural diagram of a multi-channel heat exchanger from a side view according to an embodiment of this application (an arrow denotes an air flow direction); 
         FIG. 3  is a schematic structural diagram of fins of a multi-channel heat exchanger from an angle of view according to an embodiment of this application. 
         FIG. 4  is a schematic structural diagram of fins of a multi-channel heat exchanger from another angle of view according to an embodiment of this application; 
         FIG. 5  is a schematic structural diagram of a flat tube and fins of a multi-channel heat exchanger according to an embodiment of this application; 
         FIG. 6  is a schematic structural diagram of a flat tube and fins of a multi-channel heat exchanger from an end face view according to an embodiment of this application; 
         FIG. 7  is a cross-sectional view at A-A in  FIG. 6  (an arrow denotes an air flow direction); 
         FIG. 8  is a diagram of heat exchange volume comparison between a multi-channel heat exchanger according to an embodiment of this application and a conventional multi-channel heat exchanger; 
         FIG. 9  is a diagram of frosting amount comparison between a multi-channel heat exchanger according to an embodiment of this application and a conventional multi-channel heat exchanger; 
         FIG. 10  is a transverse cross-sectional view of a flat tube of a multi-channel heat exchanger according to a first embodiment of this application; 
         FIG. 11  is a transverse cross-sectional view of a flat tube of a multi-channel heat exchanger according to a second embodiment of this application; 
         FIG. 12  is a transverse cross-sectional view of a flat tube of a multi-channel heat exchanger according to a third embodiment of this application; 
         FIG. 13  is a transverse cross-sectional view of a flat tube of a multi-channel heat exchanger according to a fourth embodiment of this application; 
         FIG. 14  is a schematic structural diagram of a transversely inserted fin according to an embodiment of this application; 
         FIG. 15  is a schematic diagram of a heat exchange volume and water content of a heat exchanger; 
         FIG. 16  is a transverse cross-sectional view of a flat tube of a multi-channel heat exchanger according to an embodiment of this application; 
         FIG. 17  is a cross-sectional view at A-A in  FIG. 6  according to another embodiment (an arrow denotes an air flow direction); and 
         FIG. 18  is a transverse cross-sectional view of a flat tube of a multi-channel heat exchanger according to the embodiment corresponding to  FIG. 17 . 
     
    
    
     REFERENCE SIGNS 
     multi-channel heat exchanger  100 ; 
     first header  10 , second header  20 ; 
     flat tube  30 , first longitudinal side face  30   a,  second longitudinal side face  30   b,  third longitudinal side face  30   c,  fourth longitudinal side face  30   d,  flow channel  30   e,  first part/first group of flow channels  31 , second part/second group of flow channels  32 , third part  33 /third group of flow channels  33 , fourth group of flow channels  34 , fifth group of flow channels  35 , sixth group of flow channels  36 , seventh group of flow channels  37 ; and 
     fin  40 , slat  40   a,  first fin/first group of fins  41 , second fin/second group of fins  42 , notch  43 , third fin  44 . 
     DETAILED DESCRIPTION 
     Embodiments of this application are described in detail below, and examples of the embodiments are shown in the accompanying drawings. Throughout the accompanying drawings, a same or similar number denotes a same or similar component or a component with a same or similar function. The embodiments described below with reference to the accompanying drawings are examples, and are merely intended to explain this application, but shall not be understood as a limitation on this application. 
     The following describes a multi-channel heat exchanger  100  according to an embodiment of this application with reference to  FIG. 1  to  FIG. 9  and  FIG. 14  to  FIG. 16 . 
     As shown in  FIG. 1  and  FIG. 2 , the multi-channel heat exchanger  100  in this embodiment of this application includes a first header  10 , a second header  20 , a plurality of flat tubes  30 , a plurality of first fins  41 , and a plurality of second fins  42 . 
     As shown in  FIG. 1 , an axial direction of the first header  10  may be parallel to an axial direction of the second header  20 , and the first header  10  and the second header  20  may be arranged in parallel and spaced apart from each other. The first header  10  and the second header  20  are distributed in a length direction of the flat tube  30 . The first header  10  may be used as an inlet header, the second header  20  may be used as an outlet header; or the first header  10  may be used as an outlet header, and the second header  20  can be used as an inlet header. 
     The plurality of flat tubes  30  are arranged in parallel in a thickness direction of the flat tube  30 , and the thickness direction of the flat tube  30  may be parallel to the axial direction of the first header  10  and the axial direction of the second header  20 . The plurality of flat tubes  30  may be disposed to be spaced apart in the axial direction of the first header  10  and the axial direction of the second header  20 . A first end of the flat tube  30  is connected to the first header  10 , and a second end of the flat tube  30  is connected to the second header  20 , so as to connect the first header  10  and the second header  20 . In this way, a heat exchange medium can flow along a path: the first header  10 —the flat tube  30 —the second header  20  or along a path: the second header  20 —the flat tube  30 —the first header  10 . The first header  10  may be provided with a first interface, and the second header  20  may be provided with a second interface. The first interface and the second interface are configured to connect to an external pipeline, so as to connect the heat exchanger to an entire air conditioning system or another heat exchange system. 
     As shown in  FIG. 2 ,  FIG. 5 , and  FIG. 16 , the flat tube  30  has a first longitudinal side face  30   a,  a second longitudinal side face  30   b,  a third longitudinal side face  30   c,  and a fourth longitudinal side face  30   d.  The first longitudinal side face  30   a  and the second longitudinal side face  30   b  are opposite and parallel to each other in the thickness direction of the flat tube  30 , and the third longitudinal side face  30   c  and the fourth longitudinal side face  30   d  are opposite to each other in the width direction of the flat tube  30 . A distance between the first longitudinal side face  30   a  and the second longitudinal side face  30   b  is less than a distance between the third longitudinal side face  30   c  and the fourth longitudinal side face  30   d,  that is, a thickness of the flat tube  30  is less than a width of the flat tube  30 . 
     In practical application of the multi-channel heat exchanger  100 , air flows through a gap between two flat tubes  30 , that is, air passes through the first longitudinal side face  30   a  and the second longitudinal side face  30   b.  As shown in  FIG. 16 , in the flat tube  30  in this application, the first longitudinal side face  30   a  and the second longitudinal side face  30   b  are arranged in parallel, that is, the thickness of the flat tube  30  is constant in an air intake direction, so that the flat tube  30  has little impact on air flow. 
     As shown in  FIG. 16 , the flat tube  30  has a plurality of flow channels  30   e  extending in the length direction of the flat tube  30 , and the plurality of flow channels  30   e  of the same flat tube  30  are parallel to each other, and are distributed to be spaced apart in the width direction of the flat tube  30 . A center line of the width direction of the flat tube  30  divides the flat tube  30  into a first part  31  and a second part  32 . A flow cross-sectional area of the first part  31  is A 1 , a flow cross-sectional area of the second part  32  is A 2 , and A 2 &gt;A 1 . The first part  31  and the second part  32  of the flat tube  30  are arranged in a direction from an air inlet side to an air outlet side. 
     It can be understood that if only a heat exchange effect of the flat tube  30  itself is considered, because the flow cross-sectional area of the second part  32  is greater than the flow cross-sectional area of the first part  31 , more refrigerant can pass through the cross-sectional area of the second part  32 . In this case, a heat exchange effect of the second part  32  of the flat tube  30  is better than that of the first part  31  of the flat tube  30 . 
     A quantity of flow channels  30   e  in the first part  31  may be equal to or different from a quantity of flow channels  30   e  in the second part  32 . 
     In some embodiments, as shown in  FIG. 16 , the center line of the width direction of the flat tube  30  does not pass through the flow channel  30   e.  In this case, the flow channels  30   e  in the first part  31  all are complete flow channels  30   e,  and the flow channels  30   e  in the second part  32  all are complete flow channels  30   e.  In this case, a sum of flow cross-sectional areas of the flow channels  30   e  in the first part  31  is A 1 , and a sum of flow cross-sectional areas of the flow channels  30   e  in the second part  32  is A 2 . 
     In some other embodiments, the centerline of the width direction of the flat tube  30  passes through one flow channel  30   e.  In this case, a flow channel  30   e  in the middle is divided by the center line into two sections: one located in the first part  31 , and the other located in the second part  32 . A sum of flow cross-sectional areas of flow channels  30   e  located in the first part  31  and a flow cross-sectional area of a side, located in the first part  31 , of the flow channel  30   e  in the middle is A 1 . A sum of flow cross-sectional areas of flow channels  30   e  located in the second part  32  and a flow cross-sectional area of a side, located in the second part  32 , of the flow channel  30   e  in the middle is A 2 . 
     As shown in  FIG. 6 , fins  40  are provided between a first longitudinal side face  30   a  of the flat tube  30  and a second longitudinal side face  30   b  of an adjacent flat tube  30 . The fin  40  has two opposite ends in the thickness direction of the flat tube  30 . The two ends of the fin  40  are respectively connected to a first longitudinal side face  30   a  and a second longitudinal side face  30   b  of adjacent flat tubes  30 . 
     As shown in  FIG. 5  and  FIG. 7 , the fins  40  in this application are classified into first fins  41  and second fins  42 . The first fins  41  and the second fins  42  are installed between the first longitudinal side face  30   a  of the flat tube  30  and the second longitudinal side face  30   b  of the adjacent flat tube  30 , and the first fins  41  and the second fins  42  are arranged in the width direction of the flat tube  30 . The first fin  41  has two opposite ends in the thickness direction of the flat tube  30 , and the two ends of the first fin  41  are respectively connected to first parts  31  of adjacent flat tubes  30 . The second fin  42  has two opposite ends in the thickness direction of the flat tube  30 , and the two ends of the second fin  42  are respectively connected to second parts  32  of adjacent flat tubes  30 . An air-side heat transfer coefficient of the second fin  42  is greater than an air-side heat transfer coefficient of the first fin  41 . 
     In related art, to improve energy efficiency of a multi-channel heat pump heat exchanger is mainly to improve a problem of frosting. In the case of operating under a low temperature condition, especially when the temperature is about 0° C., water content in the air is large. In this case, an outdoor unit of an air conditioner operates in an evaporator mode, moisture in the air may condense or frost directly, and therefore adhere to the heat exchanger, which will easily cause wind resistance of the heat exchanger to increase and an air volume to decrease, thereby decreasing heat exchange performance of the heat exchanger quickly, and affecting heat exchange efficiency of the heat exchanger. 
     In the related art, a plurality of flow channels in a flat tube are uniformly arranged, structures of the flow channels are the same, and corresponding fins are also arranged in a same manner. As shown in  FIG. 8  and  FIG. 9 , for a flat tube of such a structure, during actual use, a heat exchange temperature difference on a windward side is relatively large, and therefore a heat exchange volume of the heat exchanger on the windward side is large. A heat exchange volume of the heat exchanger on a leeward side is relatively small, and in addition, air on the windward side has a large moisture content. There is a large amount of frost in a fin region on the windward side, and there is a relatively small amount of frost on fins on the leeward side. In this way, the windward side may be easily blocked by a large amount of frost, thereby decreasing heat exchange performance of the heat exchanger quickly, and affecting a heat exchange effect of the entire heat exchanger. 
     As shown in  FIG. 8  and  FIG. 9 , in the multi-channel heat exchanger  100  in this application, the flow cross-sectional area of the second part  32  on the leeward side is designed to be greater larger than that of the first part  31  on the windward side, and the air-side heat transfer coefficient of the second fin  42  on the leeward side is greater than the air-side heat transfer coefficient of the first fin  41  on the windward side. This can balance impact of reduction of the heat exchange temperature difference on the heat exchange volume and the amount of frost to an extent, and can improve the heat exchange volume on the leeward side, reduce the amount of frost on the windward side, and alleviate a heat exchange performance decrease. An overall heat exchange effect can be greatly improved. 
     It should be noted that the windward side mentioned above means a side through which air flows first, and the leeward side mentioned above means a side through which air flows later, that is, the air flows through the first part  31  of the flat tube  30  and then flows through the second part  32  of the flat tube  30 . 
     According to the multi-channel heat exchanger  100  in this application, cross-sectional areas of flow channels  30   e  inside the flat tube  30  are designed in combination with air-side heat transfer coefficients of fins  40  in different regions, so that an internal flow area of the flat tube  30  on the windward side is decreased, to reduce a refrigerant flow volume, and meanwhile to reduce heat exchange between fins on the windward side and the air and reduce heat exchange of refrigerant with the air. In this way, under a frosting condition, a degree of frosting on the windward side can be reduced, thereby reducing frost blockage of the heat exchanger, and further improving heat exchange performance of the heat exchanger under a frosting condition. 
     In some embodiments, A 2 ≥1.2 A 1 , for example, A 2 =1.5 A 1 . it is found that through a large quantity of experiments that when the flow cross-sectional areas of the first part  31  and the second part  32  meet the foregoing relationship, frost blockage of the heat exchanger can be effectively reduced, the amount of frost is more evenly distributed in the width direction of the flat tube, and heat exchange performance of the heat exchanger is improved under a frosting condition. 
     In some embodiments, the first part  31  has a plurality of flow channels  30   e,  the second part  32  has a plurality of flow channels  30   e,  and a flow cross-sectional area of any one of the flow channels  30   e  located in the first part  31  is less than a flow cross-sectional area of any one of the flow channels  30   e  located in the second part  32 . 
     In some embodiments, as shown in  FIG. 16 , the first part  31  has a plurality of flow channels  30   e,  the second part  32  has a plurality of flow channels  30   e,  and a flow cross-sectional area of any one of the flow channels  30   e  located in the first part  31  is less than a flow cross-sectional area of at least one flow channel  30   e  located in the second part  32 . 
     In some embodiments, as shown in  FIG. 16 , lengths of all of the flow channels  30   e  in the thickness direction of the flat tube  30  are the same. In this way, distances from different flow channels  30   e  to the first longitudinal side face  30   a  and the second longitudinal side face  30   b  of the flat tube  30  are equivalent, which helps meet a reliability requirement of the entire multi-channel heat exchanger  100 . 
     The fins  40  of the multi-channel heat exchanger  100  in this embodiment of this application may be of a wavy type or a transversely inserted type. As shown in  FIG. 3  to  FIG. 7 , the fins  40  are of a wavy type, and as shown in  FIG. 14 , the fins  40  are of a transversely inserted type. 
     In the embodiment shown in  FIG. 3  to  FIG. 7 , both ends of the plurality of first fins  41  are sequentially connected end to end in the length direction of the flat tube  30  to form a wavy shape, and the plurality of first fins  41  may be formed as a wavy overall fin. One first fin  41  is formed between a crest and a trough of the wavy overall fin that are adjacent, and the crest and the trough of the wavy overall fin are respectively connected to a first longitudinal side face  30   a  and a second longitudinal side face  30   b  of two adjacent flat tubes  30 . 
     Certainly, as shown in  FIG. 14 , the first fin  41  may be of a transversely inserted type. The plurality of first fins  41  are arranged in parallel and spaced apart in the length direction of the flat tube  30 , one side of the first fins  41  has a plurality of notches  43 , and the first part  31  of the flat tube  30  is separately inserted into the notches  43 . 
     In the embodiment shown in  FIG. 3  to  FIG. 7 , both ends of the plurality of second fins  42  are sequentially connected end to end in the length direction of the flat tube  30  to form a wavy shape, and the plurality of second fins  42  may be formed as a wavy overall fin. One second fin  42  is formed between a crest and a trough of the wavy overall fin that are adjacent, and the crest and the trough of the wavy overall fin are respectively connected to a first longitudinal side face  30   a  and a second longitudinal side face  30   b  of two adjacent flat tubes  30 . 
     As shown in  FIG. 3 , a distance between two adjacent fins  40  in the length direction of the flat tube  30  is Fp. When both ends of the plurality of fins  40  are sequentially connected end to end in the length direction of the flat tube  30  to form a wavy shape, Fp is a distance in a wavy length direction between a crest and a trough of the wavy overall fin that are adjacent. In other words, Fp is a distance in the length direction of the flat tube  30  between an end, connected to a first longitudinal side face  30   a,  of the first fin  40  and an end, connected to a second longitudinal side face  30   b,  of the second fin  40 . When the fin  40  is of a transversely inserted type, Fp is a surface-to-surface distance of two adjacent fins  40  in the length direction of the flat tube  30 . 
     In some embodiments, a distance between two adjacent first fins  41  in the length direction of the flat tube  30  is Fp 1 , a distance between two adjacent second fins  42  in the length direction of the flat tube  30  is Fp 2 , and Fp 2 &lt;Fp 1 . In other words, a density of the second fins  42  is larger, so that the second part  32  connected to the second fins  42  can better dissipate heat. 
     As shown in  FIG. 3  to  FIG. 7 , the fins  40  may be provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 . As shown in  FIG. 3 , an louver length of the slat  40   a  of the fin  40  is L, and L is a length of the slat  40   a  along both ends of the fin  40 . The louver length L of the slat  40   a  is usually less than a length of the fin  40 . 
     As shown in  FIG. 3  to  FIG. 7 , the fins  40  may be provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 . As shown in  FIG. 4 , an louver angle of the slat  40   a  of the fin  40  is R, and the louver angle R of the slat  40   a  is a surface-to-surface angle between the slat  40   a  and a body of the fin  40 . 
     As shown in  FIG. 3  to  FIG. 7 , the fins  40  may be provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 . As shown in  FIG. 4 , an louver pitch between slats  40   a  of two adjacent fins  40  is Lp, Lp is a distance in the width direction of the flat tube  30  between slats  40   a  of two adjacent fins  40 , for example, a distance from a center point of a slat  40   a  to a center point of its adjacent slat  40   a.    
     In some embodiments, the multi-channel heat exchanger  100  has at least one of the following characteristics: a. the first fins  41  and the second fins  42  each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver length of the slat  40   a  of the first fin  41  is L 1 , an louver length of the slat  40   a  of the second fin  42  is L 2 , and L 2 &gt;L 1 ; b. the first fins  41  and the second fins  42  each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver angle of the slat  40   a  of the first fin  41  is R 1 , an louver angle of the slat  40   a  of the second fin  42  is R 2 , and R 2 &gt;R 1 ; c. the first fins  41  and the second fins  42  each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver pitch between slats  40   a  of two adjacent first fins  41  is Lp 1 , an louver pitch between slats  40   a  of two adjacent second fins  42  is Lp 2 , and A 2 /Lp 2 ≥A 1 /Lp 1 ; or d. the second fin  42  is provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , and the first fin  41  is provided with no slats  40   a.    
     For example, in an embodiment, the multi-channel heat exchanger  100  meets the following: a. the first fins  41  and the second fins  42  each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver length of the slat  40   a  of the first fin  41  is L 1 , an louver length of the slat  40   a  of the second fin  42  is L 2 , and L 2 &gt;L 1 . In this way, an air-side heat transfer coefficient or heat dissipation performance of the second fin  42  is larger than that of the first fin  41 , and in combination with the second part  32  having a larger flow cross-sectional area, heat exchange between fins on the windward side and the air can be reduced, thereby reducing heat exchange of refrigerant with the air. In this way, under a frosting condition, a degree of frosting on the windward side can be reduced, thereby reducing frost blockage of the heat exchanger, and further improving heat exchange performance of the heat exchanger under a frosting condition. 
     In another embodiment, the multi-channel heat exchanger  100  meets the following: b. the first fins  41  and the second fins  42  each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver angle of the slat  40   a  of the first fin  41  is R 1 , an louver angle of the slat  40   a  of the second fin  42  is R 2 , and R 2 &gt;R 1 . In other words, the louver angle of the slat  40   a  of the second fin  42  is larger, and the air is more likely to flow into the slat  40   a  of the second fin  42  to exchange heat with the second fin  42 . In this way, an air-side heat transfer coefficient or heat dissipation performance of the second fin  42  is larger than that of the first fin  41 , and in combination with the second part  32  of the flat tube having a larger flow cross-sectional area, heat exchange between fins on the windward side and the air can be reduced, thereby reducing heat exchange of refrigerant with the air. In this way, under a frosting condition, a degree of frosting on the windward side can be reduced, thereby reducing frost blockage of the heat exchanger, and further improving heat exchange performance of the heat exchanger under a frosting condition. 
     In still another embodiment, the multi-channel heat exchanger  100  meets the following: c. the first fins  41  and the second fins  42  each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver pitch between two adjacent first fins  41  is Lp 1 , an louver pitch between two adjacent second fins  42  is Lp 2 , and A 2 /Lp 2 ≥A 1 /Lp 1 . A ratio of the flow cross-sectional area of the second part  32  corresponding to the second fins  42  to the louver pitch is larger. In this way, an air-side heat transfer coefficient or heat dissipation performance of the second fin  42  is larger than that of the first fin  41 , and in combination with the second part  32  of the flat tube having a larger flow cross-sectional area, heat exchange between fins on the windward side and the air can be further reduced. In addition, heat transfer tolerance of air entering the second fin is improved. Under a frosting condition, a degree of frosting on the windward side can be reduced, thereby reducing frost blockage of the heat exchanger, and further improving heat exchange performance of the heat exchanger under a frosting condition. 
     In yet another embodiment, the multi-channel heat exchanger  100  meets the following: d. the second fin  42  is provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , and the first fin  41  is provided with no slats  40   a.  In this way, an air-side heat transfer coefficient or heat dissipation performance of the second fin  42  provided with the slat  40   a  is larger than that of the first fin  41 , and in combination with the second part  32  having a larger flow cross-sectional area, heat exchange between fins on the windward side and the air can be further reduced, thereby accelerating a speed of the air passing through the first fin. In this way, under a frosting condition, a degree of frosting on the windward side can be reduced, thereby reducing frost blockage of the heat exchanger, and further improving heat exchange performance of the heat exchanger under a frosting condition. 
     In other embodiments, the multi-channel heat exchanger  100  meets a plurality of the foregoing conditions a, b, c, and d. Details are not described herein. 
     An air conditioning and refrigeration system is further disclosed in this application. 
     The air conditioning and refrigeration system in this application includes the multi-channel heat exchanger  100  in any one of the foregoing embodiments, and air flows through a first part  31  of a flat tube  30 , and then flows through a second part  32  of the flat tube  30 . In actual implementation, a fan of the air conditioning and refrigeration system can be disposed facing the multi-channel heat exchanger  100 , and in a direction of air passing through the multi-channel heat exchanger  100 , the first part  31  of the flat tube  30  is located upstream of the second part  32 . 
     According to the air conditioning and refrigeration system in this application, cross-sectional areas of flow channels  30   e  inside the flat tube  30  are designed in combination with air-side heat transfer coefficients of fins  40  in different regions, to balance heat exchange efficiency on a windward side and a leeward side of the multi-channel heat exchanger  100 . Frost is not easy to form, and heat exchange efficiency of air conditioning and refrigeration system is high. 
     Other components, such as a compressor and throttle valve, and other operations of the air conditioning and refrigeration system according to the embodiments of this application are known to a person of ordinary skill in the art, and details are not described herein. 
     The following describes a multi-channel heat exchanger  100  according to an embodiment of this application with reference to  FIG. 1  to  FIG. 9 ,  FIG. 14 ,  FIG. 15 ,  FIG. 17 , and  FIG. 18 . 
     As shown in  FIG. 1  and  FIG. 2 , the multi-channel heat exchanger  100  in this embodiment of this application includes a first header  10 , a second header  20 , a plurality of flat tubes  30 , a plurality of first fins  41 , a plurality of second fins  42 , and a plurality of fourth fins  44 . 
     As shown in  FIG. 1 , an axial direction of the first header  10  may be parallel to an axial direction of the second header  20 , and the first header  10  and the second header  20  may be arranged in parallel and spaced apart from each other. The first header  10  and the second header  20  are distributed in a length direction of the flat tube  30 . The first header  10  may be used as an inlet header, the second header  20  may be used as an outlet header; or the first header  10  may be used as an outlet header, and the second header  20  can be used as an inlet header. 
     The plurality of flat tubes  30  are arranged in parallel in a thickness direction of the flat tube  30 , and the thickness direction of the flat tube  30  may be parallel to the axial direction of the first header  10  and the axial direction of the second header  20 . The plurality of flat tubes  30  may be disposed to be spaced apart in the axial direction of the first header  10  and the axial direction of the second header  20 . A first end of the flat tube  30  is connected to the first header  10 , and a second end of the flat tube  30  is connected to the second header  20 , so as to connect the first header  10  and the second header  20 . In this way, a heat exchange medium can flow along a path: the first header  10 —the flat tube  30 —the second header  20  or along a path: the second header  20 —the flat tube  30 —the first header  10 . The first header  10  may be provided with a first interface, and the second header  20  may be provided with a second interface. The first interface and the second interface are configured to connect to an external pipeline, so as to connect the heat exchanger to an entire air conditioning system or another heat exchange system. 
     The flat tube  30  in this embodiment of this application is first described with reference to  FIG. 18 . 
     As shown in  FIG. 18 , the flat tube  30  has a first longitudinal side face  30   a,  a second longitudinal side face  30   b,  a third longitudinal side face  30   c,  and a fourth longitudinal side face  30   d.  The first longitudinal side face  30   a  and the second longitudinal side face  30   b  are opposite and parallel to each other in the thickness direction of the flat tube  30 , and the third longitudinal side face  30   c  and the fourth longitudinal side face  30   d  are opposite to each other in the width direction of the flat tube  30 . A distance between the first longitudinal side face  30   a  and the second longitudinal side face  30   b  is less than a distance between the third longitudinal side face  30   c  and the fourth longitudinal side face  30   d,  that is, a thickness of the flat tube  30  is less than a width of the flat tube  30 . 
     In practical application of the multi-channel heat exchanger  100 , air flows through a gap between two flat tubes  30 , that is, air passes through the first longitudinal side face  30   a  and the second longitudinal side face  30   b.  As shown in  FIG. 18 , in the flat tube  30  in this application, the first longitudinal side face  30   a  and the second longitudinal side face  30   b  are arranged in parallel, that is, the thickness of the flat tube  30  is constant in an air intake direction, so that the flat tube  30  has little impact on air flow. 
     As shown in  FIG. 18 , the flat tube  30  has a plurality of flow channels  30   e  extending in the length direction of the flat tube  30 , and the plurality of flow channels  30   e  of the same flat tube  30  are parallel to each other, and are distributed to be spaced apart in the width direction of the flat tube  30 . The flat tube  30  is evenly divided in the width direction of the flat tube  30  into a first part  31 , a second part  32 , and a third part  33 . To be specific, the flat tube  30  is evenly divided in the width direction of the flat tube  30  into a first part  31 , a second part  32 , and a third part  33  that have a same width. A flow cross-sectional area of the first part  31  is A 1 , a flow cross-sectional area of the second part  32  is A 2 , a flow cross-sectional area of the third part  33  is A 3 , A 2 &gt;A 1 , and/or A 2 &gt;A 3 . The first part  31 , the second part  32 , and the third part  33  of the flat tube  30  are arranged in a direction from an air inlet side to an air outlet side. 
     It can be understood that if only a heat exchange effect of the flat tube  30  itself is considered, because the flow cross-sectional area of the second part  32  is greater than the flow cross-sectional area of the first part  31 , more refrigerant can pass through the cross-sectional area of the second part  32 . In this case, a heat exchange effect of the second part  32  of the flat tube  30  is better than that of the first part  31  of the flat tube  30 . Because the flow cross-sectional area of the second part  32  is greater than the flow cross-sectional area of the third part  33 , more refrigerant can pass through the cross-sectional area of the second part  32 . In this case, the heat exchange effect of the second part  32  of the flat tube  30  is better than that of the third part  33  of the flat tube  30 . 
     A quantity of flow channels  30   e  in the first part  31  may be equal to or different from a quantity of flow channels  30   e  in the second part  32 , so as to adjust flow cross-sectional areas. 
     In some embodiments, trisection lines in the width direction of the flat tube  30  do not pass through the flow channel  30   e.  In this case, the flow channels  30   e  in the first part  31  all are complete flow channels  30   e,  the flow channels  30   e  in the second part  32  all are complete flow channels  30   e,  and flow channels  30   e  in the third part  33  all are complete flow channels  30   e.  In this case, a sum of flow cross-sectional areas of the flow channels  30   e  in the first part  31  is A 1 , a sum of flow cross-sectional areas of the flow channels  30   e  in the second part  32  is A 2 , and a sum of flow cross-sectional areas of the flow channels  30   e  in the third part  33  is A 3 . 
     In some other embodiments, as shown in  FIG. 18 , trisection lines in the width direction of the flat tube  30  pass through the flow channel  30   e.  In this case, one or two flow channels  30   e  are divided by the corresponding trisection lines into two parts. In the embodiment shown in  FIG. 18 , two trisection lines both pass through two flow channels  30   e.  One section of one flow channel  30   e  is located in the first part  31 , the other section is located in the second part  32 ; one section of the other flow channel  30   e  is located in the second part  32 , and the other section is located in the third part  33 . A 1  represents a sum of flow cross-sectional areas of flow channels  30   e  completely located in the first part  31  and a flow cross-sectional area of the section, located on the side of the first part  31 , of the flow channel  30   e.  A 2  represents a sum of flow cross-sectional areas of flow channels  30   e  completely located in the second part  32  and a flow cross-sectional area of the section, located on the side of the second part  32 , of the flow channel  30   e.  A 3  represents a sum of cross-sectional areas of flow channels  30   e  completely located in the third part  33  and a flow cross-sectional area of the section, located on the side of the third part  33 , of the flow channel  30   e.    
     It can be understood that the second part  32  is located in the middle of the width direction of the flat tube  30 . During actual use, heat exchange between the first part  31  and the outside and between the third part  33  and the outside air has a good effect, which facilitates installation and use of the flat tube and the heat exchanger. 
     In related art, to improve energy efficiency of a multi-channel heat pump heat exchanger is mainly to improve a problem of frosting. In the case of operating under a low temperature condition, especially when the temperature is about 0° C., water content in the air is large. In this case, an outdoor unit of an air conditioner operates in an evaporator mode, moisture in the air may condense or frost directly, and therefore adhere to the heat exchanger, which will easily cause wind resistance of the heat exchanger to increase and an air volume to decrease, thereby decreasing heat exchange performance of the heat exchanger quickly, and affecting heat exchange efficiency of the heat exchanger. 
     In the related art, a plurality of flow channels in the flat tube are uniformly arranged with a same flow channel size. For a flat tube of such a structure, during actual use, as a heat exchange temperature difference of the heat exchanger decreases along with an air intake direction, a heat exchange volume of the heat exchanger on the windward side is large, and a heat exchange volume of the heat exchanger on the leeward side is small. In this way, the windward side of the heat exchanger may be easily blocked by a large amount of frost, thereby affecting a heat exchange effect of the entire heat exchanger. 
     According to the flat tube  30  in this application, a heat exchange effect of a middle region can be improved or enhanced by designing a flow cross-sectional area in the middle region as the largest, thereby balancing impact of reduction of air intake heat exchange temperature difference on the heat exchange volume to an extent. Reducing a flow cross-sectional area of the flat tube in a windward region can increase a heat exchange volume on the leeward side, reducing frosting on the windward side, and greatly improve an overall heat exchange effect. 
     It should be noted that the windward side mentioned above means a side through which air flows first, and the leeward side mentioned above means a side through which air flows later, that is, the air flows through the first part  31  of the flat tube  30  and then flows through the second part  32  of the flat tube  30 . 
     According to the flat tube  30  in this application, cross-sectional areas of the flow channels  30   e  inside the flat tube  30  are redesigned so that a flow cross-sectional area in the middle region is the largest. The first part, the second part, and the third part of the flat tube  30  are arranged in a direction from an air inlet side from an air outlet side. In this way, under a frosting condition, a degree of frosting on the windward side can be reduced, thereby reducing frost blockage of a heat exchanger, and further improving heat exchange performance of the heat exchanger under a frosting condition. 
     In some embodiments, A 2 ≥1.2 A 1  or A 2 ≥1.2 A 3 . In actual implementation, A 2 ≥1.2 A 1  and A 2 ≥1.2 A 3 , for example, A 2 =1.8 A 1 , and A 2 =1.2 A 3 . It is found by the inventor through a large quantity of experiments that when the flow cross-sectional areas of the first part  31  and the second part  32 , and the flow cross-sectional areas of the third part  33  and the second part  32  meet the foregoing relationship, frost blockage of the heat exchanger can be greatly reduced, and refrigerant can be appropriately allocated among the flow channels. A heat exchange capability of the third part  33  can be effectively utilized, thereby further improving heat exchange performance of the heat exchanger under a frosting condition. 
     In some embodiments, A 1 =A 3 . In actual implementation, a plurality of flow channels  30   e  are arranged symmetrically along a center line of the width direction of the flat tube  30  to facilitate extrusion processing and molding of the flat tube  30 . 
     In some embodiments, the first part  31  has a plurality of flow channels  30   e,  the second part  32  has a plurality of flow channels  30   e,  and the third part  33  has a plurality of flow channels  30   e.  A flow cross-sectional area of any one of the flow channels  30   e  located in the first part  31  is less than a flow cross-sectional area of at least one flow channel  30   e  located in the second part  32 , and a flow cross-sectional area of any one of the flow channels  30   e  located in the third part  33  is less than a flow cross-sectional area of at least one flow channel  30   e  located in the second part  32 . 
     In some embodiments, as shown in  FIG. 18 , the first part  31  has a plurality of flow channels  30   e,  the second part  32  has a plurality of flow channels  30   e,  and the third part  33  has a plurality of flow channels  30   e.  A flow cross-sectional area of any one of the flow channels  30   e  located in the first part  31  is less than a flow cross-sectional area of any one of the flow channels  30   e  located in the second part  32 , and a flow cross-sectional area of any one of the flow channels  30   e  located in the third part  33  is less than a flow cross-sectional area of any one of the flow channels  30   e  located in the second part  32 . 
     In actual implementation, as shown in  FIG. 18 , a size of a flow cross-sectional area of the flow channel  30   e  is negatively related to a distance from the flow channel  30   e  to a center line of the width direction of the flat tube  30 , and a flow cross-sectional area of a flow channel  30   e  close to the center line of the width direction of the flat tube  30  is larger than a flow cross-sectional area of a flow channel  30   e  away from the center line. 
     In some embodiments, as shown in  FIG. 18 , lengths of all of the flow channels  30   e  in the thickness direction of the flat tube  30  are the same. In this way, distances from different flow channels  30   e  to the first longitudinal side face  30   a  and the second longitudinal side face  30   b  of the flat tube  30  are equivalent, which facilitates even heat exchange of the entire multi-channel heat exchanger  100  to improve reliability of the flat tube. 
     In the multi-channel heat exchanger  100  in this application, as shown in  FIG. 6 , fins  40  are provided between a first longitudinal side face  30   a  of the flat tube  30  and a second longitudinal side face  30   b  of an adjacent flat tube  30 . The fin  40  has two opposite ends in the thickness direction of the flat tube  30 . The two ends of the fin  40  are respectively connected to a first longitudinal side face  30   a  and a second longitudinal side face  30   b  of adjacent flat tubes  30 . 
     As shown in  FIG. 5  and  FIG. 17 , the fins  40  in this application are classified into first fins  41 , second fins  42 , and fourth fins  44 . The first fins  41 , the second fins  42 , and the fourth fins  44  are installed between the first longitudinal side face  30   a  of the flat tube  30  and the second longitudinal side face  30   b  of the adjacent flat tube  30 , and the first fins  41 , the second fins  42 , and the fourth fins  44  are sequentially arranged in the width direction of the flat tube  30 . The first fin  41  has two opposite ends in the thickness direction of the flat tube  30 , and the two ends of the first fin  41  are respectively connected to first parts  31  of adjacent flat tubes  30 . The second fin  42  has two opposite ends in the thickness direction of the flat tube  30 , and the two ends of the second fin  42  are respectively connected to second parts  32  of adjacent flat tubes  30 . The fourth fin  44  has two opposite ends in the thickness direction of the flat tube  30 , and the two ends of the fourth fin  44  are respectively connected to third parts  33  of adjacent flat tubes  30 . 
     The flat tube  30  is divided in the width direction into the first part  31 , the second part  32 , and the third part  33  by flow cross-sectional area, and the first fin  41 , the second fin  42 , and the fourth fin  44  are correspondingly arranged outside these parts. In this way, a heat dissipation effect of each part can keep at a high level. 
     According to the multi-channel heat exchanger  100  in this application, cross-sectional areas of the flow channels  30   e  inside the flat tube  30  are redesigned so that a flow cross-sectional area in the middle region is the largest. In this way, under a frosting condition, a heat exchange effect of the middle region, namely, the second part can be improved while reducing a degree of frosting on the windward side and reducing frost blockage of the heat exchanger, thereby further improving heat exchange performance of the heat exchanger under a frosting condition. 
     The fins  40  of the multi-channel heat exchanger  100  in this embodiment of this application may be of a wavy type or a transversely inserted type. As shown in  FIG. 3  to  FIG. 9  and  FIG. 17 , the fins  40  are of a wavy type, and as shown in  FIG. 18 , the fins  40  are of a transversely inserted type. 
     In the embodiment shown in  FIG. 3  to  FIG. 9  and  FIG. 17 , both ends of the plurality of first fins  41  are sequentially connected end to end in the length direction of the flat tube  30  to form a wavy shape, and the plurality of first fins  41  may be formed as a wavy overall fin. One first fin  41  is formed between a crest and a trough of the wavy overall fin that are adjacent, and the crest and the trough of the wavy overall fin are respectively connected to a first longitudinal side face  30   a  and a second longitudinal side face  30   b  of two adjacent flat tubes  30 . 
     Certainly, as shown in  FIG. 14 , the first fin  41  may be of a transversely inserted type. The plurality of first fins  41  are arranged in parallel and spaced apart in the length direction of the flat tube  30 , one side of the first fins  41  has a plurality of notches  43 , and the first part  31  of the flat tube  30  is separately inserted into the notches  43 . 
     In the embodiment shown in  FIG. 3  to  FIG. 9  and  FIG. 17 , both ends of the plurality of second fins  42  are sequentially connected end to end in the length direction of the flat tube  30  to form a wavy shape, and the plurality of second fins  42  may be formed as a wavy overall fin. One second fin  42  is formed between a crest and a trough of the wavy overall fin that are adjacent, and the crest and the trough of the wavy overall fin are respectively connected to a first longitudinal side face  30   a  and a second longitudinal side face  30   b  of two adjacent flat tubes  30 . 
     In the embodiment shown in  FIG. 3  to  FIG. 9  and  FIG. 17 , both ends of the plurality of fourth fins  44  are sequentially connected end to end in the length direction of the flat tube  30  to form a wavy shape, and the plurality of fourth fins  44  may be formed as a wavy overall fin. One fourth fin  44  is formed between a crest and a trough of the wavy overall fin that are adjacent, and the crest and the trough of the wavy overall fin are respectively connected to a first longitudinal side face  30   a  and a second longitudinal side face  30   b  of two adjacent flat tubes  30 . 
     Certainly, as shown in  FIG. 14 , the fourth fin  44  may be of a transversely inserted type. The plurality of fourth fins  44  are arranged in parallel and spaced apart in the length direction of the flat tube  30 , one side of the fourth fins  44  has a plurality of notches  43 , and the third part  33  of the flat tube  30  is separately inserted into the notches  43 . 
     In some embodiments, an air-side heat transfer coefficient of the second fin  42  is greater than an air-side heat transfer coefficient of first fin  41 , and the air-side heat transfer coefficient of the second fin  42  is greater than an air-side heat transfer coefficient of the fourth fin  44 . 
     In the related art, a plurality of flow channels in a flat tube are designed in a same manner, and corresponding fins are also designed in a same manner. For a flat tube of such a structure, during actual use, an air heat exchange temperature difference is decreasing, and therefore a heat exchange volume of the heat exchanger is also decreasing. A heat exchange volume of the heat exchanger on the windward side is large, and a heat exchange volume of the heat exchanger on the leeward side is small. The heat exchange volume decreases along with the air intake direction, and in addition, air on the windward side has a largest moisture content. As a result, there is a large amount of frost on fins on the windward side, and there is a small amount of frost on fins on the leeward side. In this way, the windward side may be easily blocked by a large amount of frost, thereby affecting a heat exchange effect of the entire heat exchanger. 
     According to the multi-channel heat exchanger  100  in this application, the flow cross-sectional area of the second part  32  is designed to be greater larger than that of the first part  31 , and the flow cross-sectional area of the second part  32  is designed to be greater larger than that of the third part  33 . The air-side heat transfer coefficient of the second fin  42  is greater than the air-side heat transfer coefficient of the first fin  41  on the windward side, and the air-side heat transfer coefficient of the second fin  42  is greater than the air-side heat transfer coefficient of the fourth fin  44 . This can balance impact of reduction of the heat exchange temperature difference on the heat exchange volume and the amount of frost, and can improve the heat exchange volume on the leeward side and a heat exchange volume of the flat tube and the fins located on a back side of an air flow direction, and reduce the amount of frost on the windward side. A temperature step difference of the entire heat exchanger is small, and an overall heat exchange effect can be greatly improved. 
     It should be noted that the windward side mentioned above means a side through which air flows first, and the leeward side mentioned above means a side through which air flows later, that is, the air flows through the first part  31  of the flat tube  30 , then flows through the second part  32  of the flat tube  30 , and at last, flows through the third part  33  of the flat tube. The first part  31 , the second part  32 , and the third part  33  of the flat tube  30  are arranged in a direction from an air inlet side from an air outlet side. 
     According to the multi-channel heat exchanger  100  in this application, cross-sectional areas of flow channels  30   e  inside the flat tube  30  are designed in combination with air-side heat transfer coefficients of fins in different regions, so that an internal flow area of the flat tube  30  on the windward side is decreased, to reduce a refrigerant flow volume, and meanwhile to reduce heat exchange between fins on the windward side and the air and heat exchange of refrigerant with the air, and improve the heat exchange volume of the flat tube and the fins located on the back side of the air flow direction. In this way, under a frosting condition, a degree of frosting on the windward side can be reduced, thereby reducing frost blockage of the heat exchanger, making a frosting position move backward, and further improving heat exchange performance of the heat exchanger under a frosting condition. 
     As shown in  FIG. 3 , a distance between two adjacent fins  40  in the length direction of the flat tube  30  is Fp. When both ends of the plurality of fins  40  are sequentially connected end to end in the length direction of the flat tube  30  to form a wavy shape, Fp is a distance in a wavy length direction between a crest and a trough of the wavy overall fin that are adjacent. In other words, Fp is a distance in the length direction of the flat tube  30  between an end, connected to a first longitudinal side face  30   a,  of the first fin  40  and an end, connected to a second longitudinal side face  30   b,  of the second fin  40 . When the fin  40  is of a transversely inserted type, Fp is a surface-to-surface distance of two adjacent fins  40  in the length direction of the flat tube  30 . 
     In some embodiments, a distance between two adjacent first fins  41  in the length direction of the flat tube  30  is Fp 1 , a distance between two adjacent second fins  42  in the length direction of the flat tube  30  is Fp 2 , a distance between two adjacent fourth fins  44  in the length direction of the flat tube  30  is Fp 3 , Fp 2 &lt;Fp 1 , and/or Fp 2 &lt;Fp 3 . In other words, a fin density of the second fins  42  is larger, so that the second part  32  connected to the second fins  42  can better dissipate heat. In this way, under a frosting condition, a status of frosting on the windward side can be reduced, so that more air can rapidly flow to the back side, thereby improving heat exchange performance of the heat exchanger under a frosting condition. 
     As shown in  FIG. 3  to  FIG. 9  and  FIG. 17 , the fins  40  may be provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 . As shown in  FIG. 3 , an louver length of the slat  40   a  of the fin  40  is L, and L is a length of the slat  40   a  along both ends of the fin  40 . The louver length L of the slat  40   a  is usually less than a length of the fin  40 . 
     As shown in  FIG. 3  to  FIG. 9  and  FIG. 17 , the fins  40  may be provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 . As shown in  FIG. 4 , an louver angle of the slat  40   a  of the fin  40  is R, and the louver angle R of the slat  40   a  is a surface-to-surface angle between the slat  40   a  and a body of the fin  40 . 
     As shown in  FIG. 3  to  FIG. 9  and  FIG. 17 , the fins  40  may be provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 . As shown in  FIG. 4 , an louver pitch between slats  40   a  of two adjacent fins  40  is Lp, Lp is a distance in the width direction of the flat tube  30  between slats  40   a  of two adjacent fins  40 , for example, a distance from a center point of a slat  40   a  to a center point of its adjacent slat  40   a.    
     In some embodiments, the multi-channel heat exchanger  100  has at least one of the following characteristics: a. the first fins  41 , the second fins  42 , and the fourth fins  44  each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver length of the slat  40   a  of the first fin  41  is L 1 , an louver length of the slat  40   a  of the second fin  42  is L 2 , an louver length of the slat  40   a  of the fourth fin  44  is L 3 , L 2 &gt;L 1 , and/or L 2 &gt;L 3 ; b. the first fins  41 , the second fins  42 , and the fourth fins  44  each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver angle of the slat  40   a  of the first fin  41  is R 1 , an louver angle of the slat  40   a  of the second fin  42  is R 2 , an louver angle of the slat  40   a  of the fourth fin  44  is R 3 , R 2 &gt;R 1 , and/or R 2 &gt;R 3 ; c. the first fins  41 , the second fins  42 , and the fourth fins  44  each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver pitch between two adjacent first fins  41  is Lp 1 , an louver pitch between two adjacent second fins  42  is Lp 2 , an louver pitch between two adjacent fourth fins  44  is Lp 3 , Lp 2 &gt;Lp 1 , and Lp 2 &gt;Lp 3 ; or d. the second fin  42  is provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , and the first fin  41  and the fourth fin  44  are provided with no slats  40   a.    
     For example, in an embodiment, the multi-channel heat exchanger  100  meets the following: a. the first fins  41 , the second fins  42 , and the fourth fins  44  each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver length of the slat  40   a  of the first fin  41  is L 1 , an louver length of the slat  40   a  of the second fin  42  is L 2 , an louver length of the slat  40   a  of the fourth fin  44  is L 3 , L 2 &gt;L 1 , and/or L 2 &gt;L 3 . In this way, an air-side heat transfer coefficient or heat dissipation performance of the second fin  42  is larger than that of the first fin  41 , an air-side heat transfer coefficient or heat dissipation performance of the second fin  42  is larger than that of the fourth fin  44 , and in combination with the second part  32  having a larger flow cross-sectional area, heat exchange between fins on the windward side and the air can be reduced, thereby reducing heat exchange of refrigerant with the air. In this way, under a frosting condition, a status of frosting on the windward side can be reduced, thereby improving heat exchange performance of the heat exchanger under a frosting condition. 
     In another embodiment, the multi-channel heat exchanger  100  meets the following: b. the first fins  41 , the second fins  42 , and the fourth fins  44  each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver angle of the slat  40   a  of the first fin  41  is R 1 , an louver angle of the slat  40   a  of the second fin  42  is R 2 , an louver angle of the slat  40   a  of the fourth fin  44  is R 3 , and R 2 &gt;R 1 , and/or R 2 &gt;R 3 . In other words, the louver angle of the slat  40   a  of the second fin  42  is larger, and the air is more likely to flow into the slat  40   a  of the second fin  42  to exchange heat with the second fin  42 . In this way, an air-side heat transfer coefficient or heat dissipation performance of the second fin  42  is larger than those of the first fin  41  and the fourth fin  44 , and in combination with the second part  32  of the flat tube having a larger flow cross-sectional area, heat exchange between fins on the windward side and the air can be further reduced, thereby reducing heat exchange of refrigerant with the air. In this way, under a frosting condition, a degree of frosting on the windward side can be reduced, thereby reducing frost blockage of the heat exchanger, and further improving heat exchange performance of the heat exchanger under a frosting condition. 
     In still another embodiment, the multi-channel heat exchanger  100  meets the following: c. the first fins  41 , the second fins  42 , and the fourth fins  44  each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver pitch between two adjacent first fins  41  is Lp 1 , an louver pitch between two adjacent second fins  42  is Lp 2 , an louver pitch between two adjacent fourth fins  44  is Lp 3 , Lp 2 &gt;Lp 1 , and Lp 2 &gt;Lp 3 . The louver pitch of the second fins  42  is larger. In this way, an air-side heat transfer coefficient or heat dissipation performance of the second fin  42  is larger than those of the first fin  41  and the fourth fin  44 , and in combination with the second part  32  of the flat tube having a larger flow cross-sectional area, heat exchange between fins on the windward side and the air can be reduced, thereby reducing heat exchange of refrigerant with the air. In this way, under a frosting condition, wind resistance on the windward side can be reduced, and meanwhile, a status of frosting on the windward side can be reduced, thereby reducing frost blockage of the heat exchanger, and further improving heat exchange performance of the heat exchanger under a frosting condition. 
     In yet another embodiment, the multi-channel heat exchanger  100  meets the following: d. the second fin  42  is provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , and the first fin  41  and the fourth fin  44  are provided with no slats  40   a.  An air-side heat transfer coefficient or heat dissipation performance of the second fin  42  provided with the slat  40   a  is larger than those of the first fin  41  and the fourth fin  44 , and in combination with the second part  32  having a larger flow cross-sectional area, heat exchange between fins on the windward side and the air can be reduced, thereby reducing heat exchange of refrigerant with the air, so as to facilitate installation and use of the heat exchanger. In addition, under a frosting condition, a heat exchange effect on the windward side is reduced, a heat exchange effect of the middle of the heat exchanger in the air intake direction is enhanced, and a heat exchange temperature difference distribution and a frosting association relationship are adjusted. A degree of frosting on the windward side can be reduced, thereby reducing frost blockage of the heat exchanger, and further improving heat exchange performance of the heat exchanger under a frosting condition. 
     In other embodiments, the multi-channel heat exchanger  100  meets a plurality of the foregoing conditions a, b, c, and d. Details are not described herein. 
     An air conditioning and refrigeration system is further disclosed in this application. 
     The air conditioning and refrigeration system in this application includes the multi-channel heat exchanger  100  in any one of the foregoing embodiments, and air flows through a first part  31  of a flat tube  30 , and then flows through a second part  32  of the flat tube  30  and then through a third part  33  of the flat tube  30 . In actual implementation, a fan of the air conditioning and refrigeration system can be disposed facing the multi-channel heat exchanger  100 , and in a direction of air passing through the multi-channel heat exchanger  100 , the first part  31  of the flat tube  30  is located upstream of the second part  32 , and the second part  32  of the flat tube  30  is located upstream of the third part  33 . 
     According to the air conditioning and refrigeration system in this application, cross-sectional areas of flow channels  30   e  inside the flat tube  30  are designed in combination with air-side heat transfer coefficients of fins in different regions, to balance heat exchange efficiency on a windward side and a leeward side of the multi-channel heat exchanger  100  and enhance a heat exchange effect of the middle of the heat exchanger. Frost is not easy to form, and heat exchange efficiency of air conditioning and refrigeration system is high. 
     The following describes a multi-channel heat exchanger  100  according to an embodiment of this application with reference to  FIG. 1  to  FIG. 13 . 
     As shown in  FIG. 1  and  FIG. 2 , the multi-channel heat exchanger  100  in this embodiment of this application includes a first header  10 , a second header  20 , a plurality of flat tubes  30 , and a first to n th  groups of fins. 
     As shown in  FIG. 1 , an axial direction of the first header  10  may be parallel to an axial direction of the second header  20 , and the first header  10  and the second header  20  may be arranged in parallel and spaced apart from each other. The first header  10  and the second header  20  are distributed in a length direction of the flat tube  30 . The first header  10  may be used as an inlet header, the second header  20  may be used as an outlet header; or the first header  10  may be used as an outlet header, and the second header  20  can be used as an inlet header. 
     The plurality of flat tubes  30  are arranged in parallel in a thickness direction of the flat tube  30 , and the thickness direction of the flat tube  30  may be parallel to the axial direction of the first header  10  and the axial direction of the second header  20 . The plurality of flat tubes  30  may be disposed to be spaced apart in the axial direction of the first header  10  and the axial direction of the second header  20 . A first end of the flat tube  30  is connected to the first header  10 , and a second end of the flat tube  30  is connected to the second header  20 , so as to connect the first header  10  and the second header  20 . In this way, a heat exchange medium can flow along a path: the first header  10 —the flat tube  30 —the second header  20  or along a path: the second header  20 —the flat tube  30 —the first header  10 . The first header  10  may be provided with a first interface, and the second header  20  may be provided with a second interface. The first interface and the second interface are configured to connect to an external pipeline, so as to connect the heat exchanger to an entire air conditioning system or another heat exchange system. 
     The flat tube  30  in this embodiment of this application is first described with reference to  FIG. 10  to  FIG. 13 . 
     As shown in  FIG. 10  to  FIG. 13 , the flat tube  30  has a first longitudinal side face  30   a,  a second longitudinal side face  30   b,  a third longitudinal side face  30   c,  and a fourth longitudinal side face  30   d.  The first longitudinal side face  30   a  and the second longitudinal side face  30   b  are opposite and parallel to each other in the thickness direction of the flat tube  30 , and the third longitudinal side face  30   c  and the fourth longitudinal side face  30   d  are opposite to each other in the width direction of the flat tube  30 . A distance between the first longitudinal side face  30   a  and the second longitudinal side face  30   b  is less than a distance between the third longitudinal side face  30   c  and the fourth longitudinal side face  30   d,  that is, a thickness of the flat tube  30  is less than a width of the flat tube  30 . 
     In practical application of the multi-channel heat exchanger  100 , air flows through a gap between two flat tubes  30 , that is, air passes through the first longitudinal side face  30   a  and the second longitudinal side face  30   b.  As shown in  FIG. 10  to  FIG. 13 , in the flat tube  30  in this application, the first longitudinal side face  30   a  and the second longitudinal side face  30   b  are arranged in parallel, that is, the thickness of the flat tube  30  is constant in an air intake direction, so that the flat tube  30  has little impact on air flow. 
     As shown in  FIG. 10  to  FIG. 13 , the flat tube  30  has n groups of flow channels extending in a length direction of the flat tube  30 , and the n groups of flow channels are distributed to be spaced apart in a width direction of the flat tube  30 ; and a flow cross-sectional area of a first group of the flow channels  31  is A 1 , . . . , a flow cross-sectional area of k th  group of the flow channels is A k , . . . , a flow cross-sectional area of an n th  group of the flow channels is An, 1&lt;k≤n, A k ≥1.2 A k−1 , and k is an integer greater than 1. 
     It can be understood that if only a heat exchange effect of the flat tube  30  itself is considered, because a sum of flow cross-sectional areas of a next group of flow channels in the width direction of the flat tube  30  is 1.2 times greater than a sum of flow cross-sectional areas of a previous group of flow channels, a heat exchange effect of a region of the flat tube  30  is gradually enhanced along with the width direction of the flat tube  30 . 
     In related art, to improve energy efficiency of a multi-channel heat pump heat exchanger is mainly to improve a problem of frosting. In the case of operating under a low temperature condition, especially when the temperature is about 0° C., water content in the air is large. In this case, an outdoor unit of an air conditioner operates in an evaporator mode, moisture in the air may condense or frost directly, and therefore adhere to the heat exchanger, which will easily cause wind resistance of the heat exchanger to increase and an air volume to decrease, thereby decreasing heat exchange performance of the heat exchanger quickly, and affecting heat exchange efficiency of the heat exchanger. 
     In the related art, as shown in  FIG. 8  and  FIG. 9 , a plurality of flow channels in a flat tube are designed in a same manner. For a flat tube of such a structure, during actual use, a heat exchange temperature difference is decreasing, and therefore a heat exchange volume is decreasing. A heat exchange volume of a region, on a windward side, of the flat tube is large, and a heat exchange volume of a region, on a leeward side, of the flat tube is small. In this way, a temperature step difference of the heat exchanger is large, and a heat exchange effect on the leeward side is poor, thereby affecting a heat exchange effect of the entire heat exchanger. 
     According to the flat tube  30  in this application, a heat exchange effect of a region on the leeward side can be improved by designing a large flow cross-sectional area in the region on the leeward side, thereby balancing impact of reduction of the heat exchange temperature difference on the heat exchange volume to an extent. A heat exchange volume on the leeward side can be increased, a temperature step difference of the entire heat exchanger is small, and an overall heat exchange effect can be greatly improved. 
     It should be noted that the windward side mentioned above means a side through which air flows first, and the leeward side mentioned above means a side through which air flows later, that is, the air flows through a region corresponding to the first group of flow channels of the flat tube  30 , then flows through a region corresponding to the k th  group of flow channels, and at last, flows through a region corresponding to the n th  group of flow channels. 
     According to the flat tube  30  in this application, flow cross-sectional areas of the flow channels  30   e  inside the flat tube  30  are redesigned so as to increase a cross-sectional area of the region on the leeward side. In this way, under a frosting condition, heat exchange of the flat tube on the windward side can be reduced, thereby reducing a difference of heat exchange effects among various part of the flat tube, and further improving overall heat exchange performance of the heat exchanger under a frosting condition. 
     A quantity of flow channels  30   e  in each group may be the same or different. In the embodiment shown in  FIG. 10  to  FIG. 13 , each group includes a same quantity of flow channels  30   e.    
     In some embodiments, as shown in  FIG. 10  to  FIG. 12 , each group includes a plurality of flow channels  30   e,  and flow cross-sectional areas of all flow channels  30   e  in a same group are equal. Certainly, in some other embodiments, as shown in  FIG. 13 , each group includes a single flow channel  30   e.    
     Shapes of all flow channels  30   e  in a same group are the same, to facilitate extrusion and molding of the flat tube  30 . 
     As shown in  FIG. 10 , the flat tube  30  has a first group of flow channels  31 , a second group of flow channels  32 , and a third group of flow channels  33  distributed in the length direction of the flat tube  30 . Each group includes two flow channels  30   e,  each flow channel  30   e  of the flat tube  30  is rectangular, and a dimension of each flow channel  30   e  in the thickness direction of the flat tube  30  is equal. A dimension of a flow channel in a next group in the width direction of the flat tube  30  is greater than a dimension of a flow channel in a previous group in the width direction of the flat tube  30 . 
     As shown in  FIG. 11 , the flat tube  30  has a first group of flow channels  31 , a second group of flow channels  32 , and a third group of flow channels  33  distributed in the length direction of the flat tube  30 . Each group includes three flow channels  30   e,  each flow channel  30   e  of the flat tube  30  is rectangular, and a dimension of each flow channel  30   e  in the thickness direction of the flat tube  30  is equal. A dimension of a flow channel in a next group in the width direction of the flat tube  30  is greater than a dimension of a flow channel  30   e  in a previous group in the width direction of the flat tube  30 . 
     As shown in  FIG. 12 , the flat tube  30  has a first group of flow channels  31 , a second group of flow channels  32 , a third group of flow channels  33 , and a fourth group of flow channels  34  distributed in the length direction of the flat tube  30 . Each group includes four flow channels  30   e,  each flow channel  30   e  of the flat tube  30  is rectangular, and a dimension of each flow channel  30   e  in the width direction of the flat tube  30  is equal. A dimension of a flow channel in a next group in the thickness direction of the flat tube  30  is greater than a dimension of a flow channel  30   e  in a previous group in the thickness direction of the flat tube  30 . 
     As shown in  FIG. 13 , the flat tube  30  has a first group of flow channels  31 , a second group of flow channels  32 , a third group of flow channels  33 , a fourth group of flow channels  34 , a fifth group of flow channels  35 , a sixth group of flow channels  36 , and a seventh group of flow channels  37  distributed in the length direction of the flat tube  30 . Each group includes one flow channel  30   e,  each flow channel  30   e  of the flat tube  30  is rectangular, and a dimension of each flow channel  30   e  in the thickness direction of the flat tube  30  is equal. A dimension of a flow channel in a next group in the width direction of the flat tube  30  is greater than a dimension of a flow channel  30   e  in a previous group in the width direction of the flat tube  30 . 
     In the multi-channel heat exchanger  100  in this application, as shown in  FIG. 6 , fins  40  are provided between a first longitudinal side face  30   a  of the flat tube  30  and a second longitudinal side face  30   b  of an adjacent flat tube  30 . The fin  40  has two opposite ends in the thickness direction of the flat tube  30 . The two ends of the fin  40  are respectively connected to a first longitudinal side face  30   a  and a second longitudinal side face  30   b  of adjacent flat tubes  30 . 
     As shown in  FIG. 5  to  FIG. 7 , the fins  40  in this application are classified into a first group of fins  41  to an nth group of fins. The first group of fins  41  to the n th  group of fins are all installed between a first longitudinal side face  30   a  of a flat tube  30  and a second longitudinal side face  30   b  of an adjacent flat tube  30 , and the first group of fins  41  to the n th  groups of fins are sequentially arranged in the width direction of the flat tube  30 , the first group of fins  41  corresponds to the first group of flow channels  31 , . . . , the k th  group of fins corresponds to the k th  group of flow channels, . . . , the n th  group of fins corresponds to the n th  group of flow channels. 
     The flat tube  30  is provided with n groups of flow channels in the width direction, so that the n groups of flow channels correspond to the n groups of fins, and a heat dissipation effect of each part of the multi-channel heat exchanger  100  can keep at a high level. 
     According to the multi-channel heat exchanger  100  in this application, cross-sectional areas of the flow channels  30   e  inside the flat tube  30  are redesigned so that a flow cross-sectional area of the flat tube  30  gradually increases along with an air direction. In this way, under a frosting condition, a degree of frosting on the windward side can be reduced, and heat exchange performance of a region, located on a back side of an air intake direction, of the heat exchanger can be enhanced, thereby reducing frost blockage of the heat exchanger, and further improving heat exchange efficiency of the heat exchanger under a frosting condition. 
     The fins  40  of the multi-channel heat exchanger  100  in this embodiment of this application may be of a wave type or a transversely inserted type. As shown in  FIG. 3  to  FIG. 7 , the fins  40  are of a wave type, and as shown in  FIG. 14 , the fins  40  are of a transversely inserted type. 
     In the embodiment shown in  FIG. 3  to  FIG. 7 , both ends of the plurality of fins  40  are sequentially connected end to end in the length direction of the flat tube  30  to form a wavy shape, and the plurality of fins  40  may be formed as a wavy overall fin. One fin  40  is formed between a crest and a trough of the wavy overall fin that are adjacent, and the crest and the trough of the wavy overall fin are respectively connected to a first longitudinal side face  30   a  and a second longitudinal side face  30   b  of two adjacent flat tubes  30 . 
     Certainly, the fin  40  may be of a transversely inserted type. The plurality of fins  40  are arranged in parallel and spaced apart in the length direction of the flat tube  30 , one side of the fins  40  has a plurality of notches  43 , and the flat tube  30  is separately inserted into the notches  43 . 
     In some embodiments, an air-side heat transfer coefficient of the k th  group of fins is greater than an air-side heat transfer coefficient of a (k−1) th  group of fins. 
     In the related art, as shown in  FIG. 8  and  FIG. 9 , a plurality of flow channels in a flat tube are designed in a same manner, and corresponding fins are also designed in a same manner. For a flat tube of such a structure, during actual use, a heat exchange temperature difference is decreasing. Therefore, a heat exchange volume of a region corresponding to the flat tube and the fins on the windward side is large, and a heat exchange volume of a region corresponding to the flat tube and the fins on the leeward side is small. In addition, the air has a decreasing moisture content along with the air intake direction. There is a large amount of frost on the fins on the windward side, and there is a small amount of frost on the fins on the leeward side. In this way, a temperature step difference of the heat exchanger is large, and however, relatively high heat exchange performance leads to a large amount of frost. A heat exchange effect on the leeward side is poor, and the windward side may be easily blocked by a large amount of frost, thereby affecting a heat exchange effect of the entire heat exchanger. 
     According to the multi-channel heat exchanger  100  in this application, impact of reduction of the heat exchange temperature difference on the heat exchange volume and the amount of frost can be balanced to an extent, and the heat exchange volume on the leeward side can be improved by designing A k ≥1.2 A k−1  and designing an air-side heat transfer coefficient of the k th  group of fins to be greater than an air-side heat transfer coefficient of a (k−1) th  group of fins. The amount of frost on the windward side can be reduced, a heat exchange performance decrease can be alleviated, and an overall heat exchange effect can be greatly improved. 
     It should be noted that the windward side mentioned above means a side through which air flows first, and the leeward side mentioned above means a side through which air flows later, that is, the air flows through the first group of fins corresponding to the first group of flow channels of the flat tube, then flows through the k th  groups of fins corresponding to the k th  group of flow channels of the flat tube, and at last, flows through the n th  group of fins corresponding to the n th  group of flow channels of the flat tube. 
     According to the multi-channel heat exchanger  100  in this application, cross-sectional areas of flow channels  30   e  inside the flat tube  30  are designed in combination with air-side heat transfer coefficients of fins in different regions, so that an internal flow area of the flat tube  30  on the windward side is decreased, to reduce a refrigerant flow volume, and meanwhile to reduce heat exchange between fins on the windward side and the air and reduce heat exchange of refrigerant with the air. In this way, under a frosting condition, a degree of frosting on the windward side can be reduced, thereby reducing frost blockage of the heat exchanger, and further improving heat exchange performance of the heat exchanger under a frosting condition. 
     As shown in  FIG. 3 , a distance between two adjacent fins  40  in the length direction of the flat tube  30  is Fp. When both ends of the plurality of fins  40  are sequentially connected end to end in the length direction of the flat tube  30  to form a wave shape, Fp is a distance in a wave length direction between a crest and a trough of the wavy overall fin that are adjacent. In other words, Fp is a distance in the length direction of the flat tube  30  between an end, connected to a first longitudinal side face  30   a,  of the first fin  40  and an end, connected to a second longitudinal side face  30   b,  of the second fin  40 . When the fin  40  is of a transversely inserted type, Fp is a surface-to-surface distance of two adjacent fins  40  in the length direction of the flat tube  30 . 
     In some embodiments, a distance between two adjacent fins  40  in the first group of fins  41  in the length direction of the flat tube  30  is Fp 1 , a distance between two adjacent fins  40  in the second group of fins  42  in the length direction of the flat tube  30  is Fp 2 , . . . , a distance between two adjacent fins  40  in the k th  group of fins in the length direction of the flat tube  30  is Fpk, . . . , a distance between two adjacent fins  40  in a n th  group of fins in the length direction of the flat tube  30  is Fpn, and Fpk&gt;Fp(k−1). In other words, density of a next group of fins is larger, so that a heat exchange effect with the leeward side of the heat exchanger can be effectively improved. 
     As shown in  FIG. 3  to  FIG. 7 , the fins  40  may be provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 . As shown in  FIG. 3 , an louver length of the slat  40   a  of the fin  40  is L, and L is a length of the slat  40   a  along both ends of the fin  40 . The louver length L of the slat  40   a  is usually less than a length of the fin  40 . 
     As shown in  FIG. 3  to  FIG. 7 , the fins  40  may be provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 . As shown in  FIG. 4 , an louver angle of the slat  40   a  of the fin  40  is R, and the louver angle R of the slat  40   a  is a surface-to-surface angle between the slat  40   a  and a body of the fin  40 . 
     As shown in  FIG. 3  to  FIG. 7 , the fins  40  may be provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 . As shown in  FIG. 4 , an louver pitch between slats  40   a  of two adjacent fins  40  is Lp, Lp is a distance in the width direction of the flat tube  30  between slats  40   a  of two adjacent fins  40 , for example, a distance from a center point of a slat  40   a  to a center point of its adjacent slat  40   a.    
     In some embodiments, the multi-channel heat exchanger  100  has at least one of the following characteristics: a. the first group to the n th  group of fins each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver length of the slat  40   a  of the first group of fins  41  is L 1 , . . . , an louver length of the slat  40   a  of the k th  group of fins is Lk, . . . , an louver length of the slat  40   a  of the n th  group of fins is Ln, and Lk&gt;L(k−1); b. the first group to the n th  group of fins each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver angle of the slat  40   a  of the first group of fins  41  is R 1 , . . . , an louver angle of the slat  40   a  of the k th  group of fins is Rk, . . . , an louver angle of the slat  40   a  of the n th  group of fins is Rn, and Rk&gt;R(k−1); or c. the first group to the n th  group of fins each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver pitch between two adjacent fins in the first group of fins  41  is Lp 1 , . . . , an louver pitch between two adjacent fins in the k th  group of fins is Lpk, . . . , an louver pitch between two adjacent fins in the n th  group of fins is Lpn, and Lpk&gt;Lp(k−1). 
     For example, in an embodiment, the multi-channel heat exchanger  100  meets the following: a. the first group to the n th  group of fins each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver length of the slat  40   a  of the first group of fins  41  is L 1 , . . . , an louver length of the slat  40   a  of the k th  group of fins is Lk, . . . , an louver length of the slat  40   a  of the n th  group of fins is Ln, and Lk&gt;L(k−1). In this way, an air-side heat transfer coefficient or heat dissipation performance of a next group of fins is larger than that of a previous group of fins, and in combination with a next group of flow channels having a larger flow cross-sectional area, heat exchange between fins on the windward side and the air can be further reduced, thereby reducing heat exchange of refrigerant with the air. In this way, under a frosting condition, a degree of frosting on the windward side can be reduced, thereby reducing frost blockage of the heat exchanger, and further improving heat exchange performance of the heat exchanger under a frosting condition. 
     In another embodiment, the multi-channel heat exchanger  100  meets the following: b. the first group to the n th  group of fins each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver angle of the slat  40   a  of the first group of fins  41  is R 1 , . . . , an louver angle of the slat  40   a  of the k th  group of fins is Rk, . . . , an louver angle of the slat  40   a  of the n th  group of fins is Rn, and Rk&gt;R(k−1). In other words, an louver angle of a slat  40   a  of a next group of fins is larger, and the air is more likely to flow into the slat  40   a  of the next group of fins to exchange heat with the next group of fins. In this way, an air-side heat transfer coefficient or heat dissipation performance of a next group of fins is larger than that of a previous group of fins, and in combination with a next group of flow channels having a larger flow cross-sectional area, heat exchange between fins on the windward side and the air can be further reduced, to reduce heat exchange between fins on the windward side and the air and reduce heat exchange of refrigerant with the air. In this way, under a frosting condition, a degree of frosting on the windward side can be reduced, thereby reducing frost blockage of the heat exchanger, and further improving heat exchange performance of the heat exchanger under a frosting condition. 
     In still another embodiment, the multi-channel heat exchanger  100  meets the following: c. the first group to the n th  group of fins each are provided with a plurality of slats  40   a  arranged in the width direction of the flat tube  30 , an louver pitch between two adjacent fins in the first group of fins  41  is Lp 1 , . . . , an louver pitch between two adjacent fins in the k th  group of fins is Lpk, . . . , an louver pitch between two adjacent fins in the n th  group of fins is Lpn, and Lpk&gt;Lp(k−1). An louver pitch of a next group of fins is larger. In this way, an air-side heat transfer coefficient or heat dissipation performance of a next group of fins is larger than that of a previous group of fins, and in combination with a next group of flow channels having a larger flow cross-sectional area, heat exchange between fins on the windward side and the air can be further reduced, thereby reducing heat exchange of refrigerant with the air. In this way, under a frosting condition, a degree of frosting on the windward side can be reduced, thereby reducing frost blockage of the heat exchanger, and further improving heat exchange performance of the heat exchanger under a frosting condition. 
     In other embodiments, the multi-channel heat exchanger  100  meets a plurality of the foregoing conditions a, b, and c. Details are not described herein. 
     An air conditioning and refrigeration system is further disclosed in this application. 
     The air conditioning and refrigeration system in this application includes the multi-channel heat exchanger  100  in any one of the foregoing embodiments, and air sequentially flows through a first group of fins  41 , . . . , a k th  group of fins, . . . , an n th  group of fins. In actual implementation, a fan of the air conditioning and refrigeration system can be disposed facing the multi-channel heat exchanger  100 . 
     According to the air conditioning and refrigeration system in this application, cross-sectional areas of flow channels  30   e  inside the flat tube  30  are designed in combination with air-side heat transfer coefficients of fins in different regions, to balance heat exchange efficiency on a windward side and a leeward side of the multi-channel heat exchanger  100 . Frost is not easy to form, and heat exchange efficiency of air conditioning and refrigeration system is high. 
     Other components, such as a compressor and throttle valve, and other operations of the air conditioning and refrigeration system according to the embodiments of this application are known to a person of ordinary skill in the art, and details are not described herein. 
     In the description of this specification, descriptions with reference to terms such as “an embodiment”, “some embodiments”, “illustrative embodiment”, “example”, “specific example”, or “some examples” mean that specific features, structures, materials, or characteristics described with reference to the embodiment or example are included in at least one embodiment or example of this application. In this specification, illustrative descriptions of the foregoing terms do not necessarily mean a same embodiment or example. Moreover, the described specific features, structures, materials, or characteristics can be combined in any one or more embodiments or examples in an appropriate manner. 
     Although the embodiments of this application are shown and described, a person of ordinary skill in the art can understand that various changes, modifications, substitutions, and variants can be made based on these embodiments without departing from the principle and purpose of this application. The scope of this application is defined by the claims and their equivalents.