HEAT EXCHANGER

Abstract: A heat exchanger comprising fins (1) and at least one heat transfer tube (31). An even number, not smaller than four, of louvers (5a, 5b, 5c, 5d) are severed and raised in an alternate and staggered manner with respect to a fin base line (3) between the remaining adjacent fin bases (1a,1b). Furthermore, the louvers (5a, 5b, 5c, 5d) raised on the same side with respect to the fin base line (3) have heights defined by a line (10a,10b) slanted at a constant angle ϑ in a stepped manner.

Description: The invention relates to a heat exchanger having, in combination, a number of fins and at least one heat transfer tube held in contact with said fins, said fins being severed to form slits therein and alternately raised to form bridge-like louvers in a staggered manner with respect to a fin base line. Such heat exchangers are used in air-conditioners such as automotive air-conditioners, package air-conditioners and room air-conditioners. In general, a heat exchanger for an air-conditioner is composed, in combination, of a number of fins and a plurality of heat transfer tubes held in contact with the fins. A severed, raised louver structure is formed on a surface of each fin in order to effectively carry out heat exchanger between coolant that flows within the heat transfer tubes and air that flows between the fins in contact with the fin surfaces. U.S. patent 3,438,433 shows a heat exchanger of this type. However, if the louvers would be arranged as proposed in that U.S. patent, a temperature boundary layer formed on the louvers would be grown without any separation, so that the heat transfer performance of the louvers on the downstream side would be deteriorated. In particular, in the case where the width of the louvers is small, the performance of the heat exchanger will considerably degrade. Thus, the heat exchanger involves a problem such that it is difficult to enhance the heat transfer efficiency by decreasing the width of the louvers. A heat exchanger that improves the above-­ noted problem is disclosed in U.S. patent 2,789,797. Namely, the latter patent shows a structure such that louvers are severed and raised in an alternate manner in a direction of air flow to form louver units, and heights of the louvers are changed between the adjacent louvers spaced in the direction of the air flow by a distance corresponding to a length of each louver. However, in the heat exchanger disclosed in U.S. patent 2,789,979, some adjacent louvers are spaced only by approximately one fourth of the fin pitch, and hence, it would be difficult to separate the temperature boundary layers along such louvers. At the same time, water droplets or dusts would be adhered to such louvers, to prevent the air from flowing smoothly and to reduce the heat transfer performance. Also, because of the prevention of the air flow,the flow resistance would be increased. Thus, the prior art heat exchangers suffer from such problems. It is the object of the present invention to provide a heat exchanger having a high heat transfer performance, and which is capable of eliminating a fear that louvers would be clogged or plugged by water droplets adhered to fin surfaces or dusts entrained in the air. According to the invention this object is obtained in that the number of said louvers grouped in a louver group defined between a first fin base and a second fin base, adjacent to said first fin base, arranged along said fin base line is an even number not smaller than four; a louver, closest to said first fin base, of said louver group has a maximum raised height; and the other louvers, of said louver group, located on the same side with respect to said fin base are arranged along a line connecting said louver having said maximum raised height and said second adjacent fin base to each other. So in the present invention an even number of louvers (more than four) are severed and raised between a remaining first fin base and a remaining second fin base located just downstream of the first fin base, in an alternate and staggered manner symmetrically with a midpoint between the adjacent first and second fin bases. Heights of every two louvers and the second fin base are changed along a line slanted a constant angle ϑ with respect to a fin base line. Advantageously, an angle defined between said fin base line and said line connecting said louver having said maximum raised height and said second fin base is in a range of 5 to 15 degrees. Preferably, said maximum raised height Hmax of the louver satisfies the following formulae: where ϑ is said angle, Pf is the fin pitch and b is the width of the louver and Re is the Reynolds number. Conveniently, said angle ϑ defined between said fin base line and a line connecting said louver having said maximum raised height and said second fin base is alter­ nately changed in every one or more louver groups. Preferably, an even number, not smaller than four, of louvers are severed and raised, alternately with respect to a fin base line, in bridge-like shapes between remaining fin bases, and the louvers located on the same side with respect to said fin base line are arranged along a line having a constant angle ϑ (>0°) with respect to said fin base line. These and other objects, features and advantages of the present invention will become more apparent by the following descriptions taken in conjunction with the accompanying drawings. In the accompanying drawings: Figs. 1 to 4 show a heat exchanger in accordance with an embodiment of the invention, Fig. 1 is a perspective view showing a primary part of the heat exchanger, Fig. 2 is a perspective view showing the overall appearance of the heat exchanger, Fig. 3 is a cross-sectional view, taken along the line III-III of Fig. 1, showing the primary part of the heat exchanger illustrating flows of fluid therealong, and Fig. 4 is a front elevational view showing a fin structure; Figs. 5 to 7 are cross-sectional views of louver portions of fins in accordance with other embodiments of the invention; Figs. 8 and 9 show comparisons in performance between the present invention and the prior art; Fig. l0 is a graph showing a relationship between louver arrangement slant angles ϑ and Reynolds numbers according to the present invention; Fig. ll is a view illustrating a maximum raised height Hmax of a heat exchanger fin structure according to an embodiment of the invention; Figs. l2 and l3 are an enlarged view of primaray parts of heat exchangers in accordance with the invention, showing water droplet adhering states; Fig. l4 is a perspective view showing a heat exchanger in accordance with still another emboodiment of the invention; Fig. l5 is a cross-sectional view of a part of the heat exchanger shown in Fig. l4; and Fig. l6 is a cross-sectional view taken along the line XVI-XVI of Fig. l5. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is a perspective view showing a heat exchanger for an automotive air-conditioner in accordance with one embodiment of the present invention and Fig. 2 is a perspective view showing the overall appearance of the heat exchanger shown in Fig. l. As shown in Fig. 2, in accordance with the embodiment, corrugated fins l each of which is bent in serpentine manner are disposed between adjacent parts of a flat fluid tube 3l which is bent also in a serpentine manner through a cold working. The corrugated fins l and the flat fluid tube 3l are brazed or soldered in a high tempera­ ture furnace to form a heat exchanger structure.Then, the heat exchanger structure is provided with an inner fluid inlet tube 33 and an inner fluid outlet tube 34. With such a structure, a heat exchange is performed between a coolant flowing within the flat fluid tube 3l and an air flowing outside the tube 3l through the corrugated fins l. In Fig. l, reference characters 5a, 5b, 5c and 5d denote severed and raised louvers (which will herein­ after be simply referred to as "louvers") formed in the fins l.Reference characters la, lb and lc denote fin bases remaining after severing and raising the louvers 5. In the embodiment shown, four louvers are formed between the remaining fin bases la, lb and lc. Fig. 3 is a cross-sectional view taken along the line III-III of Fig. l. In Fig. 3, a line 3 represents a fin base line, and lines l0a and l0b represent a direction of the louver arrangement. The louvers 5a, 5b, 5c, 5d, ... are punched in an alternate or staggered manner in the opposite directions with respect to the fin base line 3. The louvers 5a, 5b, 5c and 5d are formed to have different raised heights substantially in a symmetrical relationship with respect to a midpoint between the adjacent pair of the remaining fin bases la and lb. In other words, the respective louvers and the remaining fin bases on the same side with respect to the fin base line 3 are arranged in a stepped manner along lines l0a and l0b which slant at a predetermined constant angle ϑ with respect to the fin base line 3 that is in parallel with the flow of fluid. According to the foregoing embodiment, with such an arrangement, a distance from each adjacent louvers in the direction of air is kept substantially constant. Also, a dimension of a minimum louver gap δmin defined between the remaining fin base la and the louver 5a and between the remaining fin base lb and the louver 5d may be kept large since that minimum dimension is not restricted by the louver width in the air flow direction. When the air is caused to flow in the direction indicated by the arrow A and to enter the heat exchanger, the air flow l0l are uniformly branched between the respective louvers and the air as a whole flows linearly, since the louvers are arranged in the stepped manner along the lines l0a and l0b as shown in Fig. 3. Therefore, a possible pressure loss may be suppressed. Each thermal boundary layer l00 formed on a louver 5 is cut by every louver, without any adverse effect to downstream louvers. Thus, all the louvers may be used to fullfil their heat transfer function. As described above, the louvers and the remaining fin bases located on the same side of the fin base line 3 along the lines slanted at the constant angle ϑ with respect to the fin base one 3 are arranged in the stepped manner. Therefore, even if the width of the louvers is decreased, the louver gap may be kept sufficiently, and the air flow may well follow the respective louver substantially uniformly. The thermal boundary layers formed on the louvers will not be grown but be cut. For this reason, the "edge effect" of the respective louver may be exhibited to a maximum possible extent. Therefore, it is possible to decrease the louver width up to approximately 0.5 mm. A heat transfer efficiency of the fin structure according to the present invention is considerably superior to that of a conven­ tional fin structure. As best shown in Fig. 4, the fin structure is such that the louvers 5a, 5c (5b, 5d) embraces the remaining fin base (la, lb, lc, ...) to support the fin l on both sides in a symmetrical manner. Therefore, a mechanical resistance against a buckling deformation caused by brazing is increased. This makes it possible to thin the fin base plate much more for practical use and to reduce a cost for material of the heat exchange to provide an inexpensive heat exchanger. In the foregoing embodiment, four louvers are severed and raised between the adjacent remaining fin bases, by way of example. It is apparent that the even number, not smaller than six, of louvers may be formed. Figs. 5 and 6 show embodiments in which the even number, not smaller than six, of the louvers are formed between the adjacent remaining fin bases. More specifically, Fig. 5 shows an embodiment in which six louvers are severed and raised between the adjacent remaining fin bases, and Fig. 8 shows an embodiment in which eight louvers are severed and raised between the adjacent remaining fin bases. Also, in these embodi­ ments, the louvers are severed and raised alternately on the opposite sides of the fin base line like bridges, and heights of the louvers on each side are defined along the line inclined or slanted at a constant angle ϑ with respect to the fin base line 3 that is in parallel with the flow of the air. In the foregoing embodiments, the louver arrangement direction expressed by a slant angle ϑ defined between a fin base line and the line connecting the most raised louver and the remaining fin base is kept constant. However, according to the present invention the louver arrangement direction slanted by a constant angle ϑ with respect to the fin base line may be changed in every louver group between the remaining fin bases or in every plural louver groups. In an embodiment shown in Fig. 7, six louvers are severed and raised between the adjacent remaining fin bases in a staggered manner, with heights of the louvers located on the same side with respect to the fin base line being varied along a line slanted at a constant angle ϑ. Further, in Fig. 7, the directions of the slant defined by the angle ϑ are changed in an alternate manner in every louver group of the alternately severed and raised louvers between the remaining fin bases. In other words, in Fig. 7, a group of louvers 5a to 5f between the remaining fin bases are arranged downwardly at an angle ϑ with respect to the fin base line, whereas an adjacent group of louvers 5a to 5f are arranged upwardly at an angle ϑ with respect to the fin base line, so that the directions defined by the angle ϑ are changed in an alternate manner in every louver group. In the foregoing embodiment, the fin base portion in which the louvers are to be formed is made ductile by a cutting and raising work for the purpose of forming the louvers. After completion of the work, the louvers tend to be restored to the original shape due to springback or resiliency. As a result, compression stresses are exerted to the remaining fin bases la, lb, lc, ... to which the work is not applied. The relative positions of the remaining fin bases with respect to the louvers will not be stabilized. In order to prevent this phenomenon, buckled portions are formed in the remaining fin base plate to absorb the compression stresses with the buckled portions. The buckled portions may be formed by bending parts of the remaining fin bases in V-shapes or U-shapes in a direction perpendicular to the flow of the air, for example. Also, instead of the formation of the buckled portions in the remaining fin bases, it is possible to fold back parts of the remaining fin bases in the direction in parallel with the air flow, to thereby increase mechanical strength of the remaining fin bases to prevent the generation of the formation of the remaining fin bases. Comparative results in heat transfer performance between the fins of the heat exchanger in accordance with the embodiments of the invention and the conventional fins in which the louvers are alternately severed and raised without any remaining fin bases will be explained with reference to Figs. 8 and 9. The performance comparative experiments were conducted in accordance with a method of measuring heat transfer coefficiencies by using thermistor heaters. Each of the thermistor heaters that were used in the experiments had a thickness of l mm, a louver length b of l0 mm and an entire width of l50 mm. Eleven raws of these thermistor heaters are arranged in the air flow direction, to form a louver group corresponding to an actual fin arrangement having a fin pitch Pf of 2 mm, a louver width of l.0 mm. The thermistor heater corre­ sponding to the single louver severed and raised from the fin bases plate where heated with electric supply. The heat transfer coefficient was obtained by the following formulae: α = .....(1) Q = Q H - Q ℓ ..... (2) ΔT = Tai - Tw ..... (3) where Q : the heat quantity (W) transferred to the air Q H : the generated heat quantity (W) of the heater, Q ℓ : the heat loss (W), A : the heat transfer are (m²) over which the heater and the air were contacted, ΔT : the temperature difference (°C) between the surface of the thermistor heater and the air at the inlet, Tw : the surface temperature (°C) of the thermistor heater, and Tai : the temperature (°C) of the air at the inlet. In order to obtain the performance of the actual fin, the well known Reynolds number Re and Nusselt number Nu were used. Nu = ..... (5) where v f is the flow velocity (m/s) of the main flow, ν is the kinematic viscosity coefficient, and λ is the thermal conductivity (W/mK) of the air. Fig. 8 shows the comparison of the experimental results of the heat transfer coefficients in case of changing a relative positional shift S between the louvers on one which is diposed on the downstream side by a distance corresponding to a width of the single louver, It is appreciated that the fin according to the embodi­ ments is much superior in heat transfer performance to the conventional fin. In particular, it is appreciated that, in the conventional fin, the performance is considerably degraded at the relative positional shift S in the range of 0.4 to 0.2 mm, whereas, in the fin according to the embodiments, the performance is not changed remarkably. Fig. 9 shows this distinction more clearly. In Fig. 9, the same date are used but the heat transfer coefficients are plotted in accordance the minimum louver gaps δmin. It is preferable that, in an automotive air cooler, since moisture in the air is condensed on the fin surfaces to form water droplets, the minimum louver gap be large as much as possible. However, in this conventional fin, the minimum louver gap δmin would be increased, the relative louver positional shift S would be small so that the considerable performance reduction would be noticed as shown in Figs. 8 and 9. In contrast, in the fin according to the present invention, the heat transfer coefficients are considerably improved in the region (0.7 to 0.8 mm) in which the minimum gap is larger than that of the conventional fin. According to the fin of the invention, the fin clogging due to the water droplets formed on the fin surfaces or dusts may be prevented, to thereby provide a heat exchanger having a high heat transfer performance. Incidentally, the data shown in Figs. 8 and 9 are concerned with the louver arrangement of the louvers having a fin pitch Pf of 2 mm, a louver width b of l mm and a thickness t of 0.l mm, but these dimensions may of course be changed in accordance with the desired design. A suitable range of the slant angle of the louver arrangement in accordance with the embodiments of the invention will be explained in view of actual factors such as adhesion of water droplet or the like. A boundary layer thickness δ of a trailing flow generated downstream of the flat plate disposed in a uniform air flow corresponds to a thermal boundary layer that is generated on the louver and entailed on the downstream side to affect the performance of the downstream louvers. Therefore, it is preferable that the relative positional shift S between the upstream louver and the downstream louver be larger than the thickness δ of the boundary layer. Then, the lower limits of the arrangement slant angle ϑ may be obtained assuming that a relationship S = δ, as shown in Fig. l0. In Fig. l0, an abscissa of the graph of Fig. l0 represent a Reynolds number given by the formula (4). The parameters availably used in the air-conditioner heat exchanger are specified as follows: b = l to 2 mm, v f = l to 5 W/s, and Re = l00 to 600. A small angle ϑ in the range of 5 to l5 degrees may suffice. For a raised height of the louver severed from the fin base, a maximum raised height Hmax is restricted in view of shaping work with a limit of elongation or ductility of the fin material for the raised louver. Normally, the arrangement pitch (fin pitch) Pf of the fin base plate of the air-conditioner heat exchanger is about l.5 to 3 mm, and it is preferable to substantially establish the relationship, Hmax ≦ Pf/2. However, when the height Hmax is small as shown in Fig. ll, the louver minimum gap δmin is smaller than that given by the following formula: δmin = Hmax - t ..... (6) where t is a louver thickness (m). In this case the fin structure has a small resistance against the clogging of the louver due to the water formed on the fin surface, dusts or the like. Figs. l2 and l3 show observation results of the water adhesion states in an evaporator for an automotive air-conditioner. The observations were carried out under the condition that the air temperature was 25°C, the relative humidity was 60% and the front air flow velocity v f = 2 m/s, with cold water at a temperature of 5°C flowing through the tubes. As a result of the observations, the following phenomena were noted: (l) The water droplets 50 formed on the fin surfaces were sucked to Y-shaped, severed portions at louver proximal ends in opening portions 40 and 40' formed by cutting and raising the louvers. (2) The droplets were collected at wedge-shaped spaces formed at connecting portions between fins l and the flat tube 3l and flowed down along the connecting portions to be discharged in the direction indicated by the arrows D in Figs. l2 and l3. (3) The water drain velocity was restricted so that the water droplets 50 were always remaining at the Y-shaped, severed portions at the louver proximal ends during the operation. In particular, in the case where the minimum louver gap δmin was small, the large amount of water was left thereat. From the above observation results, it was appreciated that the minimum louver gap δmin had to be enlarged in order to prevent the clogging by the water droplets; that is, the maximum louver raised height Hmax in Fig. ll had to be increased. In the case where the maximum raised height Hmax is given in view of the work limits as follows: Hmax = Pf ..... (8) as is apparent from Fig. ll, the louver 5d and the louver 5d' are aligned with each other on the same line, so that the entailed flows of the upstream louver would affects the downstream louver to thereby reduce the heat transfer efficiency. Accordingly, based upon the foregoing discus­ sions, it is preferable that the maximum raised height Hmax be defined by the following formulae (9) and (l0) in view of the condition that the relative positional shift S of the louvers separated by the distance corre­ sponding to the width of the single louver be greater than the thickness δ of the boundary layer as illustrated in Fig. l0. Hmax ≦ -B x tan ϑ ..... (9) tan ϑ = ..... (10) Still another embodiment of the present invention will now be explained with reference to Figs. l4 to l6. Fig. l4 is a perspective view of a cross fin tube type heat exchanger constructed so that a plurality of circular tubes 47 are adapted to pass through fins l. Fig. l5 is a partial cross-sectional view taken along a line that is in parallel with the fins l in Fig. l4. Fig. l6 is a cross-sectional view of a louver group taken along the line XVI-XVI.Also, in such a heat exchanger construction, the louver cross-­ section is the same as illustrated before. Therefore, the same effects and advantages are insured in the heat exchanger shown in Figs. l4 to l6. In other words, the structure shown in Figs. l4 to l6 has a high resistance against the clogging due to the water droplets formed on the fin surfaces or the dusts entrained in the air flow, thus providing a cross-fin tube type heat exchanger having a high heat transfer performance. As described above, according to the present invention, four or more louvers having an even number are severed and raised, in series, in a staggered manner with respect to the fin base line, and every two louvers (including fin bases) are arranged in a stepped manner in a direction slanted at a constant angle ϑ with respect to the fin base line. Accordingly, a minimum louver gap may be large. The heat exchanger according to the present invention has a high clog-proof property against the water droplets, dusts or any other foreign matters with a high heat transfer perform­ ance. Also, the louvers are symmetrical with respect to the fin base plate, so that the buckling resistance strength is increased during the brazing or soldering works, which leads to a high productivity.

Claims: 1. A heat exchanger having, in combination, a number of fins (1) and at least one heat transfer tube (31) held in contact with said fins (1), said fins (1) being severed to form slits therein and alternately raised to form bridge-like louvers (5a, 5b, 5c, 5d) in a staggered manner with respect to a fin base line (3), characterized in that the number of said louvers (5a, 5b, 5c, 5d) grouped in a louver group defined between a first fin base (1a) and a second fin base (1b), adjacent to said first fin base (1a), arranged along said fin base line (3) is an even number not smaller than four; a louver(5a), closest to said first fin base (1a) of said louver group has a maximum raised height; and the other louvers (5c, 5e) of said louver group, located on the same side with respect to said fin base (3) are arranged along a line (10a, 10b) connecting said louver (5a) having said maximum raised height and said second adjacent fin base (1b) to each other. 2. A heat exchanger according to claim 1, characterized in that an angle defined between said fin base line (3) and said line (10a, 10b) connecting said louver (5a) having said maximum raised height and said second fin base (1b) is in a range of 5 to 15 degrees. 3. A heat exchanger according to claim 2, characterized in that said maximum raised height Hmax of the louver (5a) satisfies the following formulae: Hmax ≦ -b tan ϑ, and tan ϑ = where ϑ is said angle, Pf is the fin pitch and b is the width of the louver and Re is the Reynolds number. 4. A heat exchanger according to claim 1, characterized in that said angle ϑ defined between said fin base line (3) and a line (10a, 10b) connecting said louver (5a) having said maximum raised height and said second fin base (1b) is alternately changed in every one or more louver groups. 5. A heat exchanger according to any one of the preceding claims, characterized in that an even number, not smaller than four, of louvers are severed and raised, alternately with respect to a fin base line (3), in bridge­ like shapes between remaining fin bases, and the louvers located on the same side with respect to said fin base line are arranged along a line having a constant angle ϑ (>0°).with respect to said fin base line.

Inventors: KUDOH, MITUO, KUNUGI, YOSHIFUMI HITACHI TSUKUBA HOUSE 15-206, MIYAMOTO, SEIGO, SAWAHATA, YOSHITOMO, TAKENOUCHI, MASARU, TORII, TAKUJI, YOKOYAMA, MIZUHO
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: F28F1
IPC codes: F28F1, F28D1