Patent Publication Number: US-2023146747-A1

Title: Refrigerant distributor, heat exchanger, and air-conditioning apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to a double-channel refrigerant distributor including an inner pipe and an outer pipe, a heat exchanger, and an air-conditioning apparatus. 
     BACKGROUND ART 
     There has been known a refrigerant distributor configured to distribute refrigerant through the use of a double-channel pipe having an inner pipe and an outer pipe. Such a refrigerant distributor including a double-channel pipe has a refrigerant outflow hole (also called “orifice”) provided in the lowermost part of the inner pipe. Refrigerant having flowed out through the refrigerant outflow hole is ejected into a space between the inner pipe and the outer pipe, flows into a heat transfer pipe through the outer pipe, and exchanges heat with air through the heat transfer pipe (see, for example, Patent Literature 1). 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2012-2475 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the related-art refrigerant distributor, for various reasons, the refrigerant hardly undergoes transition in flow condition to an annular flow, and regardless of annular drainage in a typical flow pattern map, there are imbalances in the distribution of a liquid phase across a vertical cross-section of the refrigerant distributor. Examples include a case in which a refrigerant inflow pipe is short, a case in which one heat exchanger is constituted by connecting a heat exchanger to a heat exchanger via a connecting pipe having a bend, or other cases. The related-art refrigerant distributor has suffered from imbalances in the distribution of refrigerant due to such imbalances in the distribution of a liquid phase. 
     The present disclosure was made under such circumferences, and has as an object to provide a refrigerant distributor configured to reduce imbalances in the distribution of a liquid phase across the refrigerant distributor and appropriately distribute refrigerant, a heat exchanger, and an air-conditioning apparatus. 
     Solution to Problem 
     A refrigerant distributor according to an embodiment of the present disclosure includes an outer pipe through which refrigerant flows and to which a plurality of heat transfer pipes are connected at predetermined spacing from each other, an inner pipe, housed in the outer pipe, through which the refrigerant flows and that has a refrigerant outflow hole through which the refrigerant flows out of the inner pipe into the outer pipe, and a structural part with which the inner pipe or the outer pipe is provided, in which the refrigerant enters an undeveloped state of two-phase gas-liquid flow, and through which the refrigerant flows into the inner pipe. The refrigerant outflow hole is provided such that an angle θ between a lower end of the inner pipe on a vertical line passing through a center of the inner pipe and a position of presence of the refrigerant outflow hole as seen from the center of the inner pipe falls within a range of 10 degrees≤θ≤80 degrees. The refrigerant outflow hole comprises a sole refrigerant outflow hole provided in a vertical cross-section of the inner pipe at a position where the refrigerant outflow hole is provided. 
     Advantageous Effects of Invention 
     The refrigerant distributor according to the embodiment of the present disclosure has an inner or outer pipe provided with a structural part in which refrigerant enters an undeveloped state of two-phase gas-liquid flow. The refrigerant having passed through the structural part flows into the inner pipe in an undeveloped state of two-phase gas-liquid flow. Only one refrigerant outflow hole is provided in a vertical cross-section of the inner pipe at a position where the refrigerant outflow hole is provided. The refrigerant outflow hole is provided such that an angle θ between a lower end of the inner pipe on a vertical line passing through the center of the inner pipe and the position of presence of the refrigerant outflow hole falls within a range of 10 degrees≤θ≤80 degrees. Therefore, the refrigerant outflow hole is provided only near the liquid surface of the refrigerant. This allows the refrigerant distributor to, even when the refrigerant flows into the inner pipe in an undeveloped state of two-phase gas-liquid flow, evenly distribute the refrigerant into a space formed between the inner pipe and the outer pipe, making it possible to appropriately distribute the refrigerant. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a refrigerant circuit diagram of an air-conditioning apparatus according to Embodiment 1. 
         FIG.  2    is a side schematic view of an outdoor heat exchanger of the air-conditioning apparatus according to Embodiment 1. 
         FIG.  3    is a top schematic view of the outdoor heat exchanger of the air-conditioning apparatus according to Embodiment 1. 
         FIG.  4    is a diagram showing states of refrigerant in an inner pipe of the air-conditioning apparatus according to Embodiment 1. 
         FIG.  5    is a vertical cross-sectional view of a refrigerant distributor of the air-conditioning apparatus according to Embodiment 1 as taken along line A-A in  FIG.  3   . 
         FIG.  6    is a vertical cross-sectional view, intended to explain the effects of the air-conditioning apparatus according to Embodiment 1 that shows a relationship between the liquid surface of refrigerant in the inner pipe and a refrigerant outflow hole. 
         FIG.  7    is a diagram, intended to explain the effects of the air-conditioning apparatus according to Embodiment 1 that shows a range of influence of refrigerant outflow holes on the refrigerant and a flow condition of the refrigerant. 
         FIG.  8    is a diagram, intended to explain the effects of the air-conditioning apparatus according to Embodiment 1, that shows the characteristics of the amounts of refrigerant that are distributed in a case in which the refrigerant outflow holes are provided in a lower part of the inner pipe. 
         FIG.  9    is a vertical cross-sectional view, intended to explain the effects of the air-conditioning apparatus according to Embodiment 1 that shows a relationship between the liquid surface of refrigerant in the inner pipe and a refrigerant outflow hole. 
         FIG.  10    is a diagram, intended to explain the effects of the air-conditioning apparatus according to Embodiment 1 that shows a range of influence of refrigerant outflow holes on the refrigerant and a flow condition of the refrigerant. 
         FIG.  11    is a diagram, intended to explain the effects of the air-conditioning apparatus according to Embodiment 1, that shows the characteristics of the amounts of refrigerant that are distributed in a case in which the refrigerant outflow holes are provided in an upper part of the inner pipe. 
         FIG.  12    is a vertical cross-sectional view showing a relationship between the liquid surface of refrigerant in the inner pipe and a refrigerant outflow hole in the air-conditioning apparatus according to Embodiment 1. 
         FIG.  13    is a diagram showing a range of influence of refrigerant outflow holes on the refrigerant and a flow condition of the refrigerant in the air-conditioning apparatus according to Embodiment 1. 
         FIG.  14    is a diagram showing the characteristics of the amounts of refrigerant that are distributed in a case in which the refrigerant outflow holes are provided in the liquid surface in the inner pipe in the air-conditioning apparatus according to Embodiment 1. 
         FIG.  15    is a top schematic view of an outdoor heat exchanger of an air-conditioning apparatus according to Embodiment 2. 
         FIG.  16    is a vertical cross-sectional view of a refrigerant distributor of the air-conditioning apparatus according to Embodiment 2 as taken along line A-A in  FIG.  15   . 
         FIG.  17    is a vertical cross-sectional view of a refrigerant distributor of the air-conditioning apparatus according to Embodiment 2 as taken along line B-B in  FIG.  15   . 
         FIG.  18    is a side schematic view of a second outdoor heat exchanger of an air-conditioning apparatus according to Embodiment 3. 
         FIG.  19    is a side schematic view of an outdoor heat exchanger according to a first example of an air-conditioning apparatus according to Embodiment 4. 
         FIG.  20    is a side schematic view of an outdoor heat exchanger according to a second example of the air-conditioning apparatus according to Embodiment 4. 
         FIG.  21    is a cross-sectional schematic view of upper outer and inner pipes of the outdoor heat exchanger according to the second example of the air-conditioning apparatus according to Embodiment 4 as taken along line A-A in  FIG.  20   . 
         FIG.  22    is a side schematic view of an outdoor heat exchanger according to a third example of the air-conditioning apparatus according to Embodiment 4. 
         FIG.  23    is a side schematic view of an outdoor heat exchanger according to a fourth example of the air-conditioning apparatus according to Embodiment 4. 
         FIG.  24    is a diagram showing the angle of a refrigerant outflow hole in an inner pipe in an air-conditioning apparatus according to Embodiment 5. 
         FIG.  25    is a diagram showing a flow pattern map (Bakers map) drawn by plotting flow conditions of the refrigerant inside the inner pipes under conditions of experimentation conducted by the inventors on the refrigerant in the distributors according to Embodiments 1 to 5. 
         FIG.  26    is a diagram showing a modified Bakers flow pattern map drawn in Embodiment 6 under refrigerant inflow conditions that are identical to those of  FIG.  25   . 
         FIG.  27    is a diagram showing a relationship between the flow passage cross-sectional area of an inner pipe and the rate of improvement in refrigerant distribution brought about by a refrigerant outflow hole in Embodiment 6. 
         FIG.  28    is a vertical cross-sectional view of a refrigerant distributor of an air-conditioning apparatus according to Embodiment 7. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes, with reference to the drawings, an air-conditioning apparatus having a refrigerant distributor according to an embodiment. In the drawings, identical components are described with reference to identical signs, and a redundant description is given only when necessary. The present disclosure may encompass all combinations of components, described in any of the following embodiments that can be combined with each other. 
     Embodiment 1 
     &lt;Air-Conditioning Apparatus  100 &gt; 
       FIG.  1    is a refrigerant circuit diagram of an air-conditioning apparatus  100  according to Embodiment 1. As shown in  FIG.  1   , the air-conditioning apparatus  100  includes an outdoor unit  10  and a plurality of indoor units  11 ,  12 , and  13 . The indoor units  11 ,  12 , and  13  are connected in parallel to one another. Refrigerant circulates through the outdoor unit  10  and the plurality of indoor units  11 ,  12 , and  13 . The air-conditioning apparatus  100  is a variable refrigerant flow air-conditioning apparatus. It should be noted that Embodiment 1 is not intended to limit the number of indoor units  11 ,  12 , and  13  that are connected to the outdoor unit  10 . 
     The air-conditioning apparatus  100  has a refrigerant circuit in which a compressor  1 , a four-way valve  2 , an outdoor heat exchanger  3 , expansion valves  5 , indoor heat exchangers  6 , and an accumulator  8  are connected to one another by a refrigerant pipe  26  and a refrigerant pipe  27 . The outdoor heat exchanger  3  and each of the indoor heat exchangers  6  exchange heat between refrigerant and air flowing inside on the wind generated by a fan  4  and fans  7 . 
     During cooling operation, high-temperature and high-pressure gas refrigerant compressed by the compressor  1  flows via the four-way valve  2  into the outdoor heat exchanger  3  through the refrigerant pipe  26 , which connects the four-way valve  2  to the outdoor heat exchanger  3 . After having flowed into the outdoor heat exchanger  3 , the refrigerant exchanges heat with the wind generated by the fan  4  and then flows out through the refrigerant pipe  27 , which connects the outdoor heat exchanger  3  to the expansion valves  5 . In the case of heating operation, that is, in a case in which the outdoor heat exchanger  3  functions as an evaporator, the refrigerant flows in a direction opposite to that in which the refrigerant flows in a case in which the outdoor heat exchanger  3  functions as a condenser. 
     &lt;Outdoor Heat Exchanger  3 &gt; 
       FIG.  2    is a side schematic view of the outdoor heat exchanger  3  of the air-conditioning apparatus  100  according to Embodiment 1.  FIG.  3    is a top schematic view of the outdoor heat exchanger  3  of the air-conditioning apparatus  100  according to Embodiment 1. The black arrows in  FIG.  2    represent the flow of refrigerant in a case in which the outdoor heat exchanger  3  functions as an evaporator. 
     The outdoor heat exchanger  3 , which is mounted in the outdoor unit  10  of the air-conditioning apparatus  100 , causes heat exchange to be performed between the refrigerant and outside air sucked through an air inlet by the fan  4 . The outdoor heat exchanger  3  is disposed below the fan  4 . 
     As shown in  FIG.  2   , the outdoor heat exchanger  3  has a refrigerant distributor  30 , a plurality of heat transfer pipes  31 , and a plurality of fins  32 . The refrigerant distributor  30  is disposed in a horizontal direction. The plurality of heat transfer pipes  31  are provided at spacings from each other, and each have one end inserted in the refrigerant distributor  30 . The fins  32  are attached to the heat transfer pipes  31 , and are provided between the heat transfer pipes  31 . The fins  32  transfer heat to the heat transfer pipes  31 . 
     &lt;Refrigerant Distributor  30 &gt; 
     As shown in  FIG.  2   , the refrigerant distributor  30  is a double-pipe structure including an inner pipe  33  and an outer pipe  34 . To the outer pipe  34 , the plurality of heat transfer pipes  31  are connected in a direction of extension of the outer pipe  34 . Refrigerant having flowed into a space between the inner pipe  33  and the outer pipe  34  is distributed to the plurality of heat transfer pipes  31 . 
     The inner pipe  33  is kept horizontal in a direction of pipe extension. Refrigerant containing liquid refrigerant flows in through one end of the inner pipe  33 . A cap  36  is provided at the furthest downstream end of the inner pipe  33  in the flow of refrigerant in a case in which the outdoor heat exchanger  3  functions as an evaporator. The refrigerant pipe  27  of the refrigeration cycle circuit is connected to the furthest upstream end of the inner pipe  33  in the flow of refrigerant in a case in which the outdoor heat exchanger  3  functions as an evaporator. 
     As shown in  FIGS.  2  and  3   , the inner pipe  33  has refrigerant outflow holes  35  (also called “orifices”) formed therein at a spacing from each other in the direction of pipe extension of the inner pipe  33  and between the heat transfer pipes  31 . Providing the refrigerant outflow holes  35  between the heat transfer pipes  31  makes it possible to bring about further improvement in refrigerant distribution performance of the refrigerant distributor  30  than in a case in which the refrigerant outflow holes  35  are provided in the inner pipe  33  directly below the heat transfer pipes  31 . It should be noted that the refrigerant outflow holes  35  may be formed in the inner pipe  33  directly below the heat transfer pipes  31 . Further, the inner pipe  33  is provided with a flow inlet  41 . The flow inlet  41  has a length L as an entrance length. Assuming that D is the inside diameter of the inner pipe  33 , L&lt;5D holds. 
       FIG.  4    is a diagram showing states of refrigerant in the inner pipe  33  of the air-conditioning apparatus  100  according to Embodiment 1. As shown in  FIG.  4   , the refrigerant is present in two states, namely gas-phase refrigerant and liquid-phase refrigerant, in the inner pipe  33 , which is a shower pipe. In Embodiment 1, the refrigerant outflow holes  35  are provided at around the angle θ′ of the liquid surface AL of the liquid-phase refrigerant. 
       FIG.  5    is a vertical cross-sectional view of the refrigerant distributor  30  of the air-conditioning apparatus  100  according to Embodiment 1 as taken along line A-A in  FIG.  3   .  FIG.  5    is a diagram showing a state where refrigerant is flowing in a state of semi-annular flow through the inner pipe  33 .  FIG.  5    shows an example in which a refrigerant outflow hole  35  is provided at the angle θ′ of the liquid surface AL of the liquid-phase refrigerant. 
     The angle θ at which the refrigerant outflow hole  35  is provided, that is, the angle θ between a lower end of the inner pipe  33  on a vertical line passing through the center of the inner pipe  33  and the position of presence of the refrigerant outflow hole  35  as seen from the center of the inner pipe  33 , needs only fall within the range of 10 degrees≤θ≤80 degrees. 
     More specifically, the angle at which the refrigerant outflow hole  35  is provided is determined by Formula (1). Formula (1) is a prediction formula, based on the Nusselt&#39;s liquid membrane estimation formula, in which results of experimentation conducted by the inventors are reflected. 
     
       
         
           
             
               
                 
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     where x is the distance of projection of the refrigerant outflow hole  35  onto a horizontal line orthogonal to a direction of pipe extension passing through the center of the inner pipe  33 , Ja is the Jacob number, Ga is the Galileo number, Pr L  is the liquid Prandtl number, v L  is a coefficient of liquid kinematic viscosity, L is the entrance length of the inner pipe, D is the inside diameter of the inner pipe, Ga=gD 3 /v L   2 , Ja=CpL/Δiv, CpL is the specific heat at constant pressure, Δiv is the latent heat, and L&lt;5D. 
     The quantities of state and the values of physical properties are estimated by the pressure of inflow into the refrigerant distributor  30 . 
       FIG.  6    is a vertical cross-sectional view, intended to explain the effects of the air-conditioning apparatus  100  according to Embodiment 1, that shows a relationship between the liquid surface AL of refrigerant in the inner pipe  33  and a refrigerant outflow hole  35 .  FIG.  6    shows a case in which the liquid phase of refrigerant flowing through the inner pipe  33  is a semi-annular flow, and also shows a case in which the refrigerant outflow hole  35  is provided in the lowermost part of the inner pipe  33 .  FIG.  7    is a diagram, intended to explain the effects of the air-conditioning apparatus  100  according to Embodiment 1 that shows a range of influence of refrigerant outflow holes  35  on the refrigerant and a flow condition of the refrigerant.  FIG.  8    is a diagram, intended to explain the effects of the air-conditioning apparatus  100  according to Embodiment 1, that shows the characteristics of the amounts of refrigerant that are distributed in a case in which the refrigerant outflow holes  35  are provided in a lower part of the inner pipe  33 . 
     In the case shown in  FIGS.  7  and  8   , as shown in  FIG.  6    the refrigerant outflow holes  35  are provided in the lowermost part of the inner pipe  33 . In  FIGS.  7  and  8   , the refrigerant outflow holes  35  are assigned sings A to G in alphabetical order by proximity to the flow inlet  41 . In  FIGS.  7  and  8   , the dashed lines represent the range of influence of each separate refrigerant outflow hole  35 , and at some point in time, refrigerant within the dashed lines passes through the refrigerant outflow holes  35  to be distributed. In a case in which the flow pattern of the refrigerant is a semi-annular flow, as shown in  FIG.  8   , the amounts of liquid refrigerant that are distributed to the upstream refrigerant outflow holes A to D are larger than the amounts of liquid refrigerant that are distributed to the downstream refrigerant outflow holes E to G. 
       FIG.  9    is a vertical cross-sectional view, intended to explain the effects of the air-conditioning apparatus  100  according to Embodiment 1, that shows a relationship between the liquid surface AL of refrigerant in the inner pipe  33  and a refrigerant outflow hole  35 .  FIG.  9    shows a case in which the liquid phase of refrigerant flowing through the inner pipe  33  is a semi-annular flow, and also shows a case in which the refrigerant outflow hole  35  is provided at position θ=90 degrees in the inner pipe  33 . That is, the refrigerant outflow hole  35  is located above the liquid surface AL.  FIG.  10    is a diagram, intended to explain the effects of the air-conditioning apparatus  100  according to Embodiment 1 that shows a range of influence of refrigerant outflow holes  35  on the refrigerant and a flow condition of the refrigerant.  FIG.  11    is a diagram, intended to explain the effects of the air-conditioning apparatus  100  according to Embodiment 1, that shows the characteristics of the amounts of refrigerant that are distributed in a case in which the refrigerant outflow holes  35  are provided in an upper part of the inner pipe  33 . In the case shown in  FIGS.  10  and  11   , as shown in  FIG.  9   , the refrigerant outflow holes  35  are provided at position θ=90 degrees in the inner pipe  33 . In a case in which the flow pattern of the refrigerant is a semi-annular flow, as shown in  FIG.  11   , the amounts of liquid refrigerant that are distributed to the upstream refrigerant outflow holes A to C are larger than the amounts of liquid refrigerant that are distributed to the downstream refrigerant outflow holes D to G. 
       FIG.  12    is a vertical cross-sectional view showing a relationship between the liquid surface AL of refrigerant in the inner pipe  33  and a refrigerant outflow hole  35  in the air-conditioning apparatus  100  according to Embodiment 1.  FIG.  12    shows a case in which the liquid phase of refrigerant flowing through the inner pipe  33  is a semi-annular flow. In Embodiment 1, the refrigerant outflow hole  35  is provided near the liquid surface AL in the inner pipe  33 . Only one refrigerant outflow hole  35  is provided in a vertical cross-section of the inner pipe  33 .  FIG.  13    is a diagram showing a range of influence of refrigerant outflow holes  35  on the refrigerant and a flow condition of the refrigerant in the air-conditioning apparatus  100  according to Embodiment 1.  FIG.  14    is a diagram showing the characteristics of the amounts of refrigerant that are distributed in a case in which the refrigerant outflow holes  35  are provided in the liquid surface AL in the inner pipe  33  in the air-conditioning apparatus  100  according to Embodiment 1. In the case shown in  FIGS.  13  and  14   , as shown in  FIG.  12   , the refrigerant outflow holes  35  are provided at position of the liquid surface AL in the inner pipe  33 . Even in a case in which the flow pattern of the refrigerant is a semi-annular flow, as shown in  FIG.  14   , the amounts of liquid refrigerant that are distributed to the refrigerant outflow holes A to G are evener than in  FIGS.  8  and  11   . 
     Therefore, in the air-conditioning apparatus  100  according to Embodiment 1, the refrigerant outflow holes  35  are provided near the liquid surface AL even in a case in which a sufficient entrance length cannot be ensured (L&lt;5D). Thus, the air-conditioning apparatus  100  according to Embodiment 1 makes it possible to distribute gas and liquid relatively evenly to the space formed between the outer pipe  34  and the inner pipe  33 . Therefore, the refrigerant distributor  30  can appropriately distribute refrigerant. 
     Embodiment 2 
     Embodiment 1 has illustrated the case of one outdoor heat exchanger  3 . Embodiment 2 illustrates a case in which a first outdoor heat exchanger  3   a  and a second outdoor heat exchanger  3   b  are connected to each other by a bent inner pipe  33   r.    
       FIG.  15    is a top schematic view of an outdoor heat exchanger  3  of an air-conditioning apparatus  100  according to Embodiment 2. As shown in  FIG.  15   , the outdoor heat exchanger  3  includes a first outdoor heat exchanger  3   a  and a second outdoor heat exchanger  3   b . A first refrigerant distributor  30   a  of the first outdoor heat exchanger  3   a  and a second refrigerant distributor  30   b  of the second outdoor heat exchanger  3   b  are connected to each other by a bent inner pipe  33   r  having a bend having a curvature. The bent inner pipe  33   r  connects an inner pipe  33  of the first outdoor heat exchanger  3   a  to an inner pipe  33  of the second outdoor heat exchanger  3   b.    
       FIG.  16    is a vertical cross-sectional view of the first refrigerant distributor  30   a  of the air-conditioning apparatus  100  according to Embodiment 2 as taken along line A-A in  FIG.  15   . As shown in  FIG.  16   , the flow pattern of refrigerant flowing through the inner pipe  33  of the first refrigerant distributor  30   a  of the first outdoor heat exchanger  3   a  is a semi-annular flow. The angle θ 1  of a refrigerant outflow hole  35  is for example θ1=0 degrees, which indicates the lowermost part of the inner pipe  33 . 
       FIG.  17    is a vertical cross-sectional view of the first refrigerant distributor  30   a  of the air-conditioning apparatus  100  according to Embodiment 2 as taken along line B-B in  FIG.  15   . As shown in  FIG.  17   , the flow pattern of refrigerant flowing through the inner pipe  33  of the second refrigerant distributor  30   b  of the second outdoor heat exchanger  3   b  is a separated flow. The angle θ 2  of a refrigerant outflow hole  35  is for example θ2=|45 degrees|, which indicates a horizontal direction orthogonal to a direction of pipe extension passing through the center of the inner pipe  33 . 
     The angle θ 2  of a refrigerant outflow hole  35  of the second refrigerant distributor  30   b  is larger within the range of −180 degrees to 180 degrees than the angle θ 1  of a refrigerant outflow hole  35  of the first refrigerant distributor  30   a  (θ2&gt;θ1). 
     In the air-conditioning apparatus  100  according to Embodiment 2, the flow pattern of refrigerant flowing through the inner pipe  33  of the first refrigerant distributor  30   a  before passing through the bent inner pipe  33   r  is a semi-annular flow. The flow pattern of refrigerant flowing through the inner pipe  33  of the second refrigerant distributor  30   b  after having passed through the bent inner pipe  33   r  is a separated flow. Therefore, as shown in  FIG.  17   , the liquid surface AL of the refrigerant rises, with the result that there is deterioration in refrigerant distribution performance. In Embodiment 2, the angle θ 2  of a refrigerant outflow hole  35  of the second refrigerant distributor  30   b  is larger than the angle θ 1  of a refrigerant outflow hole  35  of the first refrigerant distributor  30   a . This makes it possible to bring about improvement in refrigerant distribution performance of the first and second refrigerant distributors  30   a  and  30   b.    
     The bent inner pipe  33   r  may be an L-shaped pipe fitting (elbow), or may be one formed by bending an outer pipe  34  of the first refrigerant distributor  30   a.    
     Embodiment 3 
     As with Embodiment 2 shown in  FIG.  15   , Embodiment 3 is configured such that an outdoor heat exchanger  3  includes a first outdoor heat exchanger  3   a  and a second outdoor heat exchanger  3   b . In such a configuration of Embodiment 3, the second outdoor heat exchanger  3   b  has an inner pipe  33  whose diameter becomes smaller toward one terminal end. 
       FIG.  18    is a side schematic view of a second outdoor heat exchanger  3   b  of an air-conditioning apparatus  100  according to Embodiment 3. As shown in  FIG.  18   , the second outdoor heat exchanger  3   b  has an inner pipe  33   a  and an inner pipe  33   b . As shown in  FIG.  15   , the inner pipe  33  of the first outdoor heat exchanger  3   a  is connected to the inner pipe  33   a  (see  FIG.  15   ) of the second outdoor heat exchanger  3   b  via the bent inner pipe  33   r  (see  FIG.  15   ). The inside diameter of the inner pipe  33   a  of the second outdoor heat exchanger  3   b  is equal to the inside diameter of the inner pipe  33  of the first outdoor heat exchanger  3   a . The inner pipe  33   a  is connected to the inner pipe  33   b . The inside diameter of the inner pipe  33   b  is smaller than the inside diameter of the inner pipe  33   a . A cap  36  is provided at a terminal end of the inner pipe  33   b . That is, the inside diameter of the terminal end of the inner pipe  33   b  of the second outdoor heat exchanger  3   b , at which the cap  36  is provided, is smaller than the inside diameter of a starting end of the inner pipe  33   a  of the second heat exchanger to which the bent inner pipe  33   r  is connected. 
     The air-conditioning apparatus  100  according to Embodiment 3 makes it possible to prevent the flow pattern from changing from a semi-annular flow to a separated flow due to a decrease in flow rate of refrigerant at a terminal end of the second refrigerant distributor  30   b  of the second outdoor heat exchanger  3   b . This makes it possible to bring about improvement in flow robustness of refrigerant distribution characteristics. 
     Although Embodiment 3 has illustrated a case in which the second outdoor heat exchanger  3   b  has the inner pipe  33   a  and the inner pipe  33   b , the inner pipe  33  of the second outdoor heat exchanger  3   b  may be a pipe whose inside diameter becomes gradually smaller from the starting end toward the terminal end. 
     Embodiment 4 
     Embodiment 4 is configured such that a structural part C in which refrigerant flowing through an inner pipe  33  enters an undeveloped state of two-phase gas-liquid flow is provided upstream of the inner pipe  33 . Note here that the “undeveloped state of two-phase gas-liquid flow” refers to a state where the refrigerant flowing through the inner pipe  33  is in a state of not being a two-phase gas-liquid flow and in a state of being a stratified flow. 
     First Example of Structural Part 
       FIG.  19    is a side schematic view of an outdoor heat exchanger  3  according to a first example of an air-conditioning apparatus  100  according to Embodiment 4.  FIG.  19    is a diagram showing a structural part C 1  of a first example of a refrigerant distributor  30  according to the air-conditioning apparatus  100  according to Embodiment 4. 
     In  FIG.  19   , a lower inner pipe  33 _ 1  is provided with a refrigerant outflow hole  35  (not illustrated) at position described in Embodiment 1. Further, a relation of connection between a plurality of heat transfer pipes  31  and a lower outer pipe  34 _ 1  is similar to that of Embodiment 1. Furthermore, an upper outer pipe  34  is provided on top of the plurality of heat transfer pipes  31  and fins  32  (not illustrated). A relation of connection between the upper outer pipe  34  and the plurality of heat transfer pipes  31  is similar to the relation of connection between the lower outer pipe  34 _ 1  and the plurality of heat transfer pipes  31 . 
     At an end of the upper outer pipe  34  through which refrigerant flows out, an outflow pipe  42  whose diameter is smaller than that of the upper outer pipe  34  is provided. 
     As shown in  FIG.  19   , the lower inner pipe  33 _ 1  is housed in the lower outer pipe  34 _ 1  and has an upstream side further extended than the lower outer pipe  34 _ 1 . The extended portion of the lower inner pipe  33 _ 1  is a linear flow inlet  41  serving as an entrance through which the refrigerant flows into the lower outer pipe  34 _ 1 . The flow inlet  41 , which is the extended portion of the lower inner pipe  33 _ 1 , is also referred to as “structural part C 1 ”. 
     Assuming that D is the inside diameter of the flow inlet  41  and L is the length of the flow inlet  41 , L&lt;10×D holds. It is more desirable that L&lt;5×D hold. 
     Refrigerant having passed through such a structural part C 1  enters an undeveloped state of two-phase gas-liquid flow, and then flows into the lower inner pipe  33 _ 1 . Then, the refrigerant, which is in an undeveloped state of two-phase gas-liquid flow, passes through a refrigerant outflow hole  35  (not illustrated) from the lower inner pipe  33 _ 1 , and then flows out to the lower outer pipe  34 _ 1 . After having flowed out to the lower outer pipe  34 _ 1 , the refrigerant flows into the upper outer pipe  34  through the plurality of heat transfer pipes  31 . After having flowed into the upper outer pipe  34 , the refrigerant flows into the outflow pipe  42  and flows out of the outdoor heat exchanger  3  through the outflow pipe  42 . 
     Examples of methods for estimating a flow pattern of refrigerant include flow pattern maps such as Baker&#39;s maps. Many of these flow pattern maps represent a sufficiently developed state of gas-liquid flow, that is, a pattern of flow in a case in which a sufficient entrance length is provided. 
     Based on the results of the latest refrigerant visualization experiment conducted by the inventors, it was newly found that flow patterns calculated by Baker&#39;s maps or other diagrams obtained by mounting in actual units are not developed in flow and are therefore different from actual flow patterns. Specifically, in many of the cases of annular flow patterns on flow pattern maps, laminar flows and wavy flows were observed. Based on the results of the experimentation conducted by the inventors, this trend was found predominantly when the entrance length of the lower inner pipe  33 _ 1  fell within the range of L&lt;10×D, and was particularly evident in a case in which L&lt;5D. Therefore, in a case in which there is no sufficient entrance length upstream of the lower inner pipe  33 _ 1 , the refrigerant outflow hole  35  of the lower inner pipe  33 _ 1  is positioned near the interface of a laminar flow or a wavy flow (θ=10 degrees to 80 degrees). 
     (Effects) 
     Therefore, the refrigerant distributor  30 , which has the structural part C 1 , of the air-conditioning apparatus  100  according to Embodiment 4 makes it possible to evenly distribute a two-phase gas-liquid flow by providing the lower inner pipe  33 _ 1  with the structural part C 1 , bringing about improvement in distribution performance. 
     Second Example of Structural Part 
       FIG.  20    is a side schematic view of an outdoor heat exchanger  3  according to a second example of the air-conditioning apparatus  100  according to Embodiment 4.  FIG.  20    is a diagram showing a structural part C 2  of a second example of the refrigerant distributor  30  according to the air-conditioning apparatus  100  according to Embodiment 4. 
     In  FIG.  20   , the outdoor heat exchanger  3  has a divider  51 _ 1  provided inside a lower outer pipe  34 _ 1  and a divider  51 _ 2  provided inside an upper outer pipe  34 _ 2  to bring about improvement in velocity of flow of refrigerant and improvement in performance. 
     As shown in  FIG.  20   , the divider  51 _ 1  is provided inside the lower outer pipe  34 _ 1 . The divider  51 _ 1  divides the interior of the lower outer pipe  34 _ 1  into a lower outer pipe  34 _ 1 _ 1  and a lower outer pipe  34 _ 12  in a direction parallel with an axis of the outer pipe  34 _ 1 . At an end of the lower outer pipe  34 _ 1 _ 1  through which refrigerant flows in, a flow inlet  41  whose diameter is smaller than that of the lower outer pipe  34 _ 1 _ 1  is provided. To an outflow side of the lower outer pipe  34 _ 1 _ 2 , an outflow pipe  42  whose diameter is smaller than that of the lower outer pipe  34 _ 1 _ 2  is connected. 
     In  FIG.  20   , a relation of connection between a plurality of heat transfer pipes  31  and the lower outer pipe  34 _ 1  is similar to that of Embodiment 1. The upper outer pipe  34 _ 2  and an upper inner pipe  33 _ 2  are provided on top of the plurality of heat transfer pipes  31  and fins  32  (not illustrated). A relation of connection between the upper outer pipe  34 _ 2  and the plurality of heat transfer pipes  31  is similar to the relation of connection between the lower outer pipe  34 _ 1  and the plurality of heat transfer pipes  31 . 
     The upper outer pipe  34 _ 2  houses the upper inner pipe  33 _ 2 . As in the case of Embodiment 1, the upper inner pipe  33 _ 2  is provided with refrigerant outflow holes  35 . The divider  51 _ 2  is provided inside the upper outer pipe  34 _ 2 . The divider  51 _ 2  is provided above the divider  51 _ 1 , and divides the interior of the upper outer pipe  34 _ 2  into an upper outer pipe  34 _ 2 _ 1  and an upper outer pipe  34 _ 2 _ 2  in a direction parallel with an axis of the outer pipe  24 _ 2 . Specifically, the divider  51 _ 2  divides the inner periphery of the upper outer pipe  34 _ 2  and the upper inner pipe  33 _ 2  from each other in a direction parallel with the axis of the outer pipe  24 _ 2 . 
     The upper outer pipe  34 _ 2  is further extended than the upper inner pipe  33 _ 2 . The interior of the upper outer pipe  34 _ 2 _ 1  forms a confluence space S_ 1 . To the confluence space S_ 1 , the plurality of heat transfer pipes  31  are connected, and in the confluence space S_ 1 , flows of refrigerant having passed through the flow inlet  41 , the lower outer pipe  34 _ 1 _ 1 , and the plurality of heat transfer pipes  31  merge with one another. 
     The confluence space S_ 1  is also referred to as “structural part C 2 ”. The flows of refrigerant having merged with one another in the confluence space S_ 1  flow into the upper inner pipe  33 _ 2 . Further, the flows of refrigerant having merged with one another in the confluence space S_ 1  partly flow into the upper inner pipe  33 _ 2  after having been turned back by the divider  51 _ 2 . 
     The confluence space S_ 1  is structured such that assuming that A 1  is the flow passage cross-sectional area of the confluence space S_ 1  and AS is the flow passage cross-sectional area of the upper inner pipe  33 _ 2 , A 1 &gt;AS holds. 
     Such a structure causes the refrigerant to decrease in two-phase gas-liquid flow when flowing into the upper inner pipe  33 _ 2 , which is small in flow passage cross-sectional area, from the confluence space S_ 1 , which is large in flow passage cross-sectional area, but in the confluence space S_ 1 , the refrigerant enters an undeveloped state of two-phase gas-liquid flow. 
       FIG.  21    is a cross-sectional schematic view of the upper outer and inner pipes  34 _ 2 _ 2  and  33 _ 2  of the outdoor heat exchanger  3  according to the second example of the air-conditioning apparatus  100  according to Embodiment 4 as taken along line A-A in  FIG.  20   . 
       FIG.  21    shows an example in which in the upper inner pipe  33 _ 2 , a refrigerant outflow hole  35  is provided at the angle θ′ of the liquid surface AL of the liquid-phase refrigerant as in the case of Embodiment 1 described with reference to  FIG.  5   . 
     The angle θ′ at which the refrigerant outflow hole  35  is provided is an angle between a lower end of the inner pipe  33 _ 2  on a vertical line passing through the center of the inner pipe  33 _ 2  and the position of presence of the refrigerant outflow hole  35  as seen from the center of the inner pipe  33 _ 2 , and needs only fall within the range of 10 degrees≤θ′≤80 degrees. 
     In  FIG.  20   , refrigerant having flowed out of the refrigerant outflow hole  35  of the upper inner pipe  33 _ 2  passes through the upper outer pipe  34 _ 2 _ 2  and the plurality of heat transfer pipes  31  in sequence and flows into the lower outer pipe  34 _ 1 _ 2 . After having flowed into the lower outer pipe  34 _ 1 _ 2 , the refrigerant flows into the outflow pipe  42  and flows out of the outdoor heat exchanger  3 . 
     (Effects) 
     The refrigerant distributor  30 , which has the structural part C 2 , of the air-conditioning apparatus  100  according to Embodiment 4 provides the upper outer pipe  34 _ 2  with the structural part C 2 . This results in an undeveloped two-phase gas-liquid flow, as the flow passage cross-sectional area A 1  of the confluence space S_ 1  and the flow passage cross-sectional area AS of the upper inner pipe  33 _ 2  are different from each other. As a result, a region where a two-phase gas-liquid flow is undeveloped is formed upstream of the upper inner pipe  33 _ 2 . In this case, the refrigerant outflow hole  35  of the upper inner pipe  33 _ 2  is positioned near the interface of a laminar flow or a wavy flow (θ=10 degrees to 80 degrees). 
     Therefore, the refrigerant distributor  30 , which has the structural part C 2 , of the air-conditioning apparatus  100  according to Embodiment 4 makes it possible to evenly distribute a two-phase gas-liquid flow, bringing about improvement in distribution performance. 
     Third Example of Structural Part 
       FIG.  22    is a side schematic view of an outdoor heat exchanger  3  according to a third example of the air-conditioning apparatus  100  according to Embodiment 4.  FIG.  22    is a diagram showing a structural part C 3  of a third example of the refrigerant distributor  30  according to the air-conditioning apparatus  100  according to Embodiment 4. 
     As shown in  FIG.  22   , a divider  61  is provided inside a lower outer pipe  34 _ 1 . The divider  61  divides the lower outer pipe  34 _ 1  into a lower outer pipe  34 _ 1 _ 1  and a lower outer pipe  34 _ 1 _ 2 . Specifically, the divider  61  divides the inner periphery of the lower outer pipe  34 _ 1  and a lower inner pipe  33 _ 1  from each other. 
     The lower outer pipe  34 _ 1 _ 1  is further extended than the lower inner pipe  33 _ 1 . The lower outer pipe  34 _ 1 _ 1  has an opening port (not illustrated) in a lower surface thereof. To the opening port, a refrigerant inflow pipe  62  is connected. 
     The interior of the lower outer pipe  34 _ 1  constitutes an inflow space S_ 2 . Into the inflow space S_ 2 , refrigerant flows from the refrigerant inflow pipe  62 . 
     The inflow space S_ 2  is also referred to as “structural part C 3 ”. Refrigerant having flowed into the inflow space S_ 2  flows into the lower inner pipe  33 _ 1 . 
     The inflow space S_ 2  is structured such that assuming that A 2  is the flow passage cross-sectional area of the inflow space S_ 2  and AS is the flow passage cross-sectional area of the lower inner pipe  33 _ 1 , A 2 &gt;AS holds. 
     Such a structure causes the refrigerant to decrease in two-phase gas-liquid flow when flowing into the lower inner pipe  33 _ 1 , which is small in flow passage cross-sectional area, from the inflow space S_ 2 , which is large in flow passage cross-sectional area, but in the inflow space S_ 2 , the refrigerant enters an undeveloped state of two-phase gas-liquid flow. 
     In  FIG.  22   , a relation of connection between a plurality of heat transfer pipes  31  and the lower outer pipe  34 _ 1  is similar to that of Embodiment 1. An upper outer pipe  34 _ 2  is provided on top of the plurality of heat transfer pipes  31  and fins  32  (not illustrated). A relation of connection between the upper outer pipe  34 _ 2  and the plurality of heat transfer pipes  31  is similar to the relation of connection between the lower outer pipe  34 _ 1  and the plurality of heat transfer pipes  31 . 
     At an end of the upper outer pipe  34 _ 2  through which refrigerant flows out, an outflow pipe  42  whose diameter is smaller than that of the upper outer pipe  34 _ 2  is provided. 
     Refrigerant having flowed into the lower inner pipe  33 _ 1  passes through a refrigerant outflow hole  35  (not illustrated) from the lower inner pipe  33 _ 1 , and then flows out to the lower outer pipe  34 _ 1 . After having flowed out to the lower outer pipe  34 _ 1 , the refrigerant flows into the upper outer pipe  34 _ 2  through the plurality of heat transfer pipes  31 . After having flowed into the upper outer pipe  34 _ 2 , the refrigerant flows into the outflow pipe  42  and flows out of the outdoor heat exchanger  3 . 
     In this case, the refrigerant outflow hole  35  of the lower inner pipe  33 _ 1  is positioned near the interface of a laminar flow or a wavy flow (θ=10 degrees to 80 degrees). 
     Although  FIG.  22    has illustrated a case in which the refrigerant inflow pipe  62  is provided on the lower surface of the lower outer pipe  34 _ 1 _ 1 , the number of refrigerant inflow pipes  62  is not limited to 1. Further, the refrigerant inflow pipe  62  may be fitted, for example, to an upper or side surface of the lower outer pipe  34 _ 1 _ 1 . 
     (Effects) 
     The refrigerant distributor  30  of the air-conditioning apparatus  100  according to Embodiment 4 has the structural part C 3 , which is a portion of the lower outer pipe  34 _ 1 _ 1  further extended than the lower inner pipe  33 _ 1 , and the structural part C 3  has the inflow space S_ 2 . The lower inner pipe  33 _ 1  is housed in and protected by the lower outer pipe  34 _ 1 . This makes it unnecessary to increase the thickness of the lower inner pipe  33 _ 1  to ensure strength, making it possible to achieve a reduction in wall thickness of the lower inner pipe  33 _ 1  and savings in space. Further, since the lower inner pipe  33 _ 1  is not exposed to the outside, the wall thickness of the lower inner pipe  33 _ 1  can be reduced. 
     The refrigerant distributor  30 , which has the structural part C 3 , of the air-conditioning apparatus  100  according to Embodiment 4 brings about an undeveloped state of two-phase gas-liquid flow by providing the lower outer pipe  34 _ 1 _ 1  with the structural part C 3 , making it possible to evenly distribute the two-phase gas-liquid flow through the inner pipe  33 _ 1 . This results in improvement in distribution performance of the refrigerant distributor  30 . 
     Further, connecting the refrigerant inflow pipe  62  to the lower outer pipe  34 _ 1 _ 1  makes it possible to check an increase in piping space resulting from the pipe routing of the refrigerant inflow pipe  62  or other pipes, making it possible to bring about improvement in mountability of the outdoor heat exchanger  3 . 
     Fourth Example of Structural Part 
       FIG.  23    is a side schematic view of an outdoor heat exchanger  3  according to a fourth example of the air-conditioning apparatus  100  according to Embodiment 4.  FIG.  23    is a diagram showing a structural part C 4  of a fourth example of the refrigerant distributor  30  according to the air-conditioning apparatus  100  according to Embodiment 4. 
     In  FIG.  23   , a lower inner pipe  33 _ 1  is provided with a refrigerant outflow hole  35  (not illustrated) at position described in Embodiment 1. Further, a relation of connection between a plurality of heat transfer pipes  31  and a lower outer pipe  34 _ 1  is similar to that of Embodiment 1. Furthermore, an upper outer pipe  34 _ 2  is provided on top of the plurality of heat transfer pipes  31  and fins  32  (not illustrated). A relation of connection between the upper outer pipe  34 _ 2  and the plurality of heat transfer pipes  31  is similar to the relation of connection between the lower outer pipe  34 _ 1  and the plurality of heat transfer pipes  31 . 
     At an end of the upper outer pipe  34 _ 2  through which refrigerant flows out, an outflow pipe  42  whose diameter is smaller than that of the upper outer pipe  34 _ 2  is provided. 
     As shown in  FIG.  23   , the lower inner pipe  33 _ 1  is housed in the lower outer pipe  34 _ 1  and has an upstream side further extended than the lower outer pipe  34 _ 1 . An extended portion of the lower inner pipe  33 _ 1  is linear. Furthermore, a bent inflow pipe  63  is provided upstream of the extended linear portion of the lower inner pipe  33 _ 1 . The bent inflow pipe  63  is also referred to as “structural part C 4 ”. 
     Assuming that DR is the flow passage inside diameter of the bent inflow pipe  63  and L 2  is the length of the linear portion of the lower inner pipe  33 _ 1  further extended than the outer pipe  34 _ 1 _ 2 , L 2 &lt;5×DR holds. 
     Refrigerant having passed through such a structural part C 4  enters an undeveloped state of two-phase gas-liquid flow. Then, the refrigerant, which is in an undeveloped state of two-phase gas-liquid flow, flows into the lower inner pipe  33 _ 1 . After having flowed into the lower inner pipe  33 _ 1 , the refrigerant passes through the refrigerant outflow hole  35  (not illustrated) from the lower inner pipe  33 _ 1 , and then flows out to the lower outer pipe  34 _ 1 . After having flowed out to the lower outer pipe  34 _ 1 , the refrigerant flows into the upper outer pipe  34 _ 2  through the plurality of heat transfer pipes  31 . After having flowed into the upper outer pipe  34 _ 2 , the refrigerant flows into the outflow pipe  42  and flows out of the outdoor heat exchanger  3 . 
     In this case, the refrigerant outflow hole  35  of the lower inner pipe  33 _ 1  is positioned near the interface of a laminar flow or a wavy flow (θ=10 degrees to 80 degrees). 
     Although  FIG.  23    has illustrated a case in which the lower inner pipe  33 _ 1  is provided with the bent inflow pipe  63 , the bent inflow pipe  63  may be formed by bending part of the lower inner pipe  33 _ 1 . 
     (Effects) 
     The refrigerant distributor  30 , which has the structural part C 4 , of the air-conditioning apparatus  100  according to Embodiment 4 subjects gas-liquid refrigerant flowing through the bent inflow pipe  63  to centrifugal force by providing the bent inflow pipe  63 . This causes the refrigerant flowing through the bent inflow pipe  63  to enter an undeveloped state of two-phase gas-liquid flow. 
     Therefore, the refrigerant distributor  30 , which has the structural part C 4 , of the air-conditioning apparatus  100  according to Embodiment 4 makes it possible to evenly distribute a two-phase gas-liquid flow by providing the lower outer pipe  34 _ 1  with the structural part C 4 , bringing about improvement in distribution performance. 
     Embodiment 5 
     Providing the structural parts C 1  to C 4  described in Embodiment 4 causes refrigerant flowing into the inner pipe  33  to enter an undeveloped state of two-phase gas-liquid flow. As a result of the inventors&#39; analysis, they found a more appropriate angle of a refrigerant outflow hole  35  in this case. Embodiment 5 is intended to define a more appropriate angle φ of a refrigerant outflow hole  35  in the case of an undeveloped state of two-phase gas-liquid flow. The angle φ is an angle between a lower end of the inner pipe  33  on a vertical line passing through the center of the inner pipe  33  and the position of presence of the refrigerant outflow hole  35  as seen from the center of the inner pipe  33 . 
       FIG.  24    is a diagram showing the angle φ of a refrigerant outflow hole  35  in an inner pipe  33  in an air-conditioning apparatus  100  according to Embodiment 5. 
     In  FIG.  24   , φ is the optimum angle of the refrigerant outflow hole  35 , φ D0  is the liquid-surface angle in a case in which it is assumed that the gas-liquid slip ratio of the refrigerant is 1 and the gas-liquid interface of the refrigerant is flat and horizontal, φ DS  is the wetting boundary angle in a pipe circumferential direction that is used, for example, in the prediction of an evaporative transfer coefficient in consideration of the gas-liquid slip ratio and inertial force of the refrigerant, and AS is the flow passage cross-sectional area of the inner pipe  33 . 
     In a case in which φ DS  is defined as the liquid-surface angle of a flow pattern, the angle φ of the refrigerant outflow hole  35  is expressed as φ D0 &lt;φ&lt;φ DS . 
     Note here that φ D0  and φ DS  are computed according to Formulas (5) and (6), respectively, using Formulas (2) to (4) for liquid surface angle, proposed by Mori et al., that are used in the prediction of the evaporative heat transfer coefficient of a horizontal smooth pipe. 
     
       
         
           
             
               
                 
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     Note here that the variables in the formulas are as follows and the refrigerant quality, the densities, the mass velocity, the latent heat, or other variables represent values measured at the inlet of the inner pipe  33 . Further, in the inner pipe  33 , the thermal flow rate takes on a sufficiently small value of q=0.001. Further, the mass velocity is defined as G=(M×3600)/{(D/2) 2 ×π}, where M [kg/h] is the refrigerant mass flow rate and d [m] is the inside diameter of the inner pipe  33 . Further, the quantifies of state of the refrigerant such as the densities and the evaporative latent heat can be estimated, for example, by using a common table of physical property values and the physical property calculation software “Refprop”. 
     x: Refrigerant quality [-],
 
ρ G : Refrigerant gas density [kg/m 3 ],
 
ρ L : Refrigerant liquid density [kg/m 3 ],
 
G: Mass velocity [kg/(m 2 s)],
 
D: Inside diameter of inner pipe  33  [m],
 
g: Gravitational acceleration [m/s 2 ],
 
Δh G : Evaporative latent heat [kJ/kg],
 
q: Intratubular surface circumference average thermal flow rate [kW/m 2 ]
 
     The wetting boundary angle φ DS  in a pipe circumferential direction as calculated by the formulas of Mori et al. is a boundary angle with a very thin region taken into account, as the formulas are formulas obtained by an analysis based on a measurement database of heat transfer coefficients and a heat transfer coefficient is high in heat transfer coefficient contribution in a very thin liquid film region. On the other hand, the angle φ of optimum distribution of a refrigerant outflow hole  35  at which to achieve appropriate distribution in refrigerant distribution should be an angle that is smaller than a portion in which the liquid film is thick to some extent, that is, φ DS . Further, this angle φ of optimum distribution is present at an angle that is larger than the liquid-surface angle φ D0  in a case in which, as shown in  FIG.  24   , it is virtually assumed that the gas-liquid slip ratio is 1 and the gas-liquid interface is flat and horizontal. 
     According to the comparison results of the analysis conducted by the inventors using Formulas (2) to (6) and the refrigerant visualization experiment, it is found that the angle φ of optimum distribution is nearly equal to 1.5φ D0 . Further, it is found that although the angle of the liquid surface is particularly dominantly affected by the quality of refrigerant, although the angle of the liquid surface is affected by the flow rate and quality of refrigerant and the gas-liquid density ratio. Assume the maximum flow under a representative condition of heating rated operation in the range of 0.05 to 0.80, which highly frequently occurs as the evaporator inlet quality of common air-conditioning equipment. It is found that in this case, the optimum distribution angle is present in the range of 80 degrees to 10 degrees and an increase in quality leads to a decrease in optimum distribution angle. 
     Further, Formulas (6) and (7) are φ D0  and φ DS  prediction formulas obtained by the analysis conducted by the inventors using Formulas (2) to (6). Formulas (6) and (7) represent a relationship between the flow passage cross-sectional area AS [mm 2 ] of the inner pipe  33 , which is a dominant shape parameter of the inner pipe  33  in a case in which the flow condition of refrigerant during heating rated operation common to air-conditioning equipment is taken into account as a representative condition, and the angle φ of optimum distribution. When the angle φ of optimum distribution satisfies φ D0 &lt;φ&lt;φ DS , the distribution performance of the inner pipe  33  can be improved. 
       [Math. 7] 
       ϕ D0 =(−0.0408× AS+ 74.124)×0.62  (7)
 
       [Math. 8] 
       ϕ DS =(−0.0408 ×AS +74.124)×1.2  (8)
 
     Therefore, the refrigerant distributor  30  of the air-conditioning apparatus  100  according to Embodiment 5 makes it possible to place the angle φ of a refrigerant outflow hole  35  at more appropriate position, thus making it possible to more evenly distribute refrigerant. 
     Embodiment 6 
       FIG.  25    is a diagram showing a flow pattern map (Baker&#39;s map) drawn by plotting flow conditions of the refrigerant inside the inner pipes  33  under conditions of experimentation conducted by the inventors on the refrigerant in the distributors according to Embodiments 1 to 5. 
     The inventors attempted to reduce imbalances in liquid phases due to the internal gravities of the inner pipes  33  by designing the inside diameters of the inner pipes  33  to attain a flow condition for an annular flow or an annular spray flow on the Baker&#39;s map. 
     However, it was confirmed by the refrigerant visualization experiment that even under conditions of an annular flow and an annular spray flow on a flow pattern map as shown in  FIG.  25   , the refrigerant actually flows in a wavy flow or a laminar flow. 
     This is presumably due to the fact that many flow pattern maps such as Baker&#39;s maps are often constructed based on water-air experiments with sufficient entrance lengths. As a result of the refrigerant visualization experiment conducted by the inventors, it was found that under conditions for the maximum flows of refrigerant flowing through the heat exchangers, the flows often became undeveloped and laminar, provided the inside diameters D [m] of the inner pipes  33  fell within the range of D D A /6, where D A  [m] is the inside diameter of an inner pipe  33  within a range of an annular flow, an annular spray flow, and a slug flow on the Bakers map. 
     As a result, it was made clear based on the refrigerant visualization experiment that an actual flow pattern can be largely predicated by modifying a Baker&#39;s flow pattern map and causing an inner pipe  33  to have an inside diameter D of D A /6. 
       FIG.  26    is a diagram showing a modified Baker&#39;s flow pattern map drawn in Embodiment 6 under refrigerant inflow conditions that are identical to those of  FIG.  25   . In  FIG.  26   , the inside diameter of the inner pipe  33  is D A /6. As shown in  FIG.  26   , it is confirmed that the conditions of an annular flow and an annular spray flow on the Baker&#39;s flow pattern map shown in  FIG.  25    are laminar flows and the flow pattern of refrigerant as observed by the actual refrigerant visualization largely agrees with the flow pattern of refrigerant shown in  FIG.  26   . Therefore, with the inside diameter of the inner pipe  33  being D≥D A /6, a flow of refrigerant inside becomes undeveloped and laminar as in the cases of Embodiments 1 to 5. Therefore, for example, the distribution performance of a two-phase gas-liquid flow can be improved by positioning the refrigerant outflow holes  35  of the lower inner pipe  33 _ 1  near the interface (θ=10 degrees to 80 degrees) of a laminar flow or a wavy flow. 
     It should be noted that the horizontal axis of the Bakers map is (G L ×λ×φ mod )/G G  and the vertical axis is G G /λ, and that G G =W G /A m , G L =W L /A m , W G =W×x, W L =W×(1−x), and A m =(D/2) 2 ×π, 
     where G L  is the liquid-phase mass velocity [kg/m 2 s], G G  is the gas-phase mass velocity [kg/m 2 s], W L  is the liquid-phase mass flow rate [kg/s], W G  is the gas-phase mass flow rate [kg/s], A m  is the flow passage cross-sectional area of the inner pipe  33  [m 2 ], x is the quality [-], ρ is the density [kg/m 3 ], μ is the coefficient of viscosity [Pa·s], and σ is the surface tension [N/m]. 
     
       
         
           
             
               
                 
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     The values followed by the subscripts A and W are the values of the physical properties of air and water, respectively, at 20 degrees C. under atmospheric pressures, and aw is the air-water surface tension in this state. 
     Further, according to the refrigerant visualization experiment conducted by the inventors using common fluorocarbon refrigerant, it was found that the refrigerant flows in a laminar flow under most flow conditions with the flow passage cross-sectional area AS of the inner pipe  33  being equal to 31.6 mm 2  to 201.1 mm 2  and that positioning the refrigerant outflow holes  35  at an angle near the liquid surface AL (θ=10 degrees to 80 degrees) as shown in Embodiments 1 to 5 is particularly highly effective in improving imbalances in distribution. 
       FIG.  27    is a diagram showing a relationship between the flow passage cross-sectional area AS of the inner pipe  33  and the rate of improvement in refrigerant distribution brought about by the refrigerant outflow holes  35  in Embodiment 6. As shown in  FIG.  27   , in the region R_ 1 , where 0&lt;AS&lt;31.6 mm 2 , the refrigerant easily undergoes transition in flow pattern to an annular flow in many cases, so that the effect of improvement in distribution brought about by the angle of the refrigerant outflow holes  35  is low. 
     Meanwhile, in the region R_ 2 , where 31.6 mm 2 ≤AS≤201.1 mm 2 , the effect of improvement in distribution is high, as it is a region of undeveloped flow patterns of wavy and laminar flows. In the region R_ 3 , where AS&gt;201.1 mm 2 , the flow passage cross-sectional area of the inner pipe  33  is large for a heat exchanger that is used in common air-conditioning equipment, so that there are tendencies turning toward a decrease in the inertial force and deterioration in distribution. This leads to a decrease in the effect of improvement in distribution. 
     Embodiment 7 
       FIG.  28    is a vertical cross-sectional view of a refrigerant distributor  30  of an air-conditioning apparatus  100  according to Embodiment 7. 
     In each of Embodiments 1 to 6, the angle θ 1  of a refrigerant outflow hole  35  is not limited to particular orientations, and the effect of improvement in distribution can be brought about by positioning the refrigerant outflow hole  35  near the liquid surface AL. On the other hand, in Embodiment 7, the orientation of the angle θ 1  of a refrigerant outflow hole  35  at which the refrigerant distributor  30  is mounted in a heat exchanger, that is, the direction of opening of the refrigerant outflow hole  35 , is set as follows. Specifically, in a case in which the refrigerant distributor  30  is mounted in a heat exchanger, the refrigerant outflow hole  35  is provided at position on a windward side of the refrigerant distributor  30  and in a range near the liquid surface AL (θ=10 degrees to 80 degrees). Doing so makes it possible to distribute much liquid refrigerant to a region where there are great differences in temperature among flat tubes. 
     The embodiments are presented as examples, and are not intended to limit the scope of claims. The embodiments may be carried out in other various forms, and various omissions, substitutions, and changes can be made without departing from the spirit of the embodiments. These embodiments and modifications thereof are encompassed in the scope and spirit of the embodiments. 
     REFERENCE SIGNS LIST 
       1 : compressor,  2 : four-way valve,  3 : outdoor heat exchanger,  3   a : first outdoor heat exchanger,  3   b : second outdoor heat exchanger,  4 : fan,  5 : expansion valve,  6 : indoor heat exchanger,  7 : fan,  8 : accumulator,  10 : outdoor unit,  11 ,  12 ,  13 : indoor unit,  26 ,  27 : refrigerant pipe,  30 : refrigerant distributor,  30   a : first refrigerant distributor,  30   b : second refrigerant distributor,  31 : heat transfer pipe,  32 : fin,  33 ,  33   a ,  33   b ,  33 _ 2 : inner pipe,  33   r  bent inner pipe,  34 ,  34 _ 1 ,  34 _ 1 _ 1 ,  34 _ 1 _ 2 ,  34 _ 2 _ 1 ,  34 _ 2 _ 2 : outer pipe,  35 : refrigerant outflow hole,  36 : cap:  41 : flow inlet,  42 : outflow pipe,  51 _ 1 ,  51 _ 2 ,  61 : divider,  62 : refrigerant inflow pipe,  63 : bent inflow pipe,  100 : air-conditioning apparatus, AL: liquid surface, C, C 1  to C 4 : structural part, L: length of extended inner pipe, D: inside diameter of extended inner pipe, A 1 : flow passage cross-sectional area of confluence space, A 2 : flow passage cross-sectional area of inflow space, AS: flow passage cross-sectional area of inner pipe, DR: flow passage inside diameter of bent inflow pipe, L 2 : length of linear portion of inner pipe extended, poo: liquid-surface angle, φ Ds : liquid-surface angle, θ, φ, θ 1 : angle of refrigerant outflow hole, θ: angle of liquid surface, R_ 1 , R_ 2 , R_ 3 : region, S_ 1 : confluence space, S_ 2 : inflow space