Patent Publication Number: US-5289874-A

Title: Heat exchanger with laterally displaced louvered fin sections

Description:
TECHNICAL FIELD 
     The field of this invention relates to tube and fin heat exchangers and more particularly to louvered fin arrangements therefor. 
     BACKGROUND OF THE DISCLOSURE 
     Heat exchangers of the type used for radiators in vehicle engine cooling systems or condensers in vehicle air conditioning systems utilize tubes carrying a coolant or refrigerant requesting that need to be cooled. The heat exchanger also commonly has fins, also called air centers, interposed between the tubes to effectively increase the contact with air for heat transfer to the air. The impetus for increasing the efficiency of heat exchangers is dictated by the need for more fuel efficient and aerodynamic motor vehicles. 
     The aerodynamic shape of many motor vehicles dictate that the hood line of the motor vehicle be lowered resulting in less space available in the engine compartment particularly in the vertical direction. Two of the largest components in the engine compartment are the radiator and condenser. The lower hood lines dictate for radiators or condensers with less core face area. Any decrease in core face area, overall size and weight of the radiator or condenser must therefor be accompanied by an increase in efficiency for heat transfer for a given air flow. 
     Often, louvers are incorporated in the fins to improve the heat transfer efficiency. Various fin and louver arrangements have been utilized to increase the heat transfer by increasing the turbulence of air about the fins and tubes. This increase in heat transfer is often accompanied by an undesirable increase in air pressure drop across the heat exchanger. It has also been found that various louver arrangements may be more advantageous than others. U.S. Pat. No. 4,693,307 issued to Scarselletta on Sep. 15, 1987, which is incorporated herein by reference, discloses a heat exchanger with fins having both louvered and non-louvered sections to have both low air pressure drop and high heat transfer performance at a constant air mass flow. U.S. Pat. No. 4,958,681 issued to co-inventor Durgaprasad S. Kadle, which is also incorporated herein by reference, discloses bypass channels positioned near the tube outer surface. 
     What is needed is an improved louver arrangement which improves efficiency over the teachings of the known prior art. 
     SUMMARY OF THE DISCLOSURE 
     In accordance with one aspect of the invention, a heat exchanger includes a plurality of layers of tubes with each layer having a plurality of tubes spaced from each other and with each layer spaced from each other to form a gap therebetween. A folded strip forming a plurality fins is arranged between adjacent pairs of tubes within each layer of tubes. The fins have a first series of louvered sections axially aligned with the tubes of each layer. The fins have a second series of louvered sections interposed between adjacent sections of the first series of louvered sections. Each louvered section of the second series is laterally aligned with the gap between each layer, i.e. is laterally misaligned or displaced with respect to the tubes. Each louvered section of the second series is spaced from adjacent sections of said first series to form plain non-louvered sections interposed between the first and second series of louvers. 
     Desirably, the heat exchanger further includes a front edge section of the fins extending in front of a front layer of said tubes. The front edge section has a louvered section spaced from the louvered section of the first series aligned with the front layer. A plain non-louvered section is interposed between the louvered section of the front edge section and the louvered section aligned with the front layer. 
     Desirably, the heat exchanger furthermore has a rear edge section of said fins extending behind a rear layer of said tubes. The rear edge section has a louvered section spaced from the louvered section of the first series aligned with the rear layer. A plain non-louvered section is interposed between the louvered section of the rear edge section and the louvered section aligned with said rear layer. Preferably, the plain non-louvered section laterally spans a side of the tubes. 
     The combination of the louvered sections of the fin in specific arrangements in combination with other known parameters gives rise to enhanced performance. One of these other parameters are total louvered area not more than 60% and not less than 40% of the total fin area. Another parameter is the thickness and stacked density of the fins is not more than 12% nor less than 2.5% of the space between the adjacent flat tubes. The fin pitch and span is not more than 1.2/mm nor less than 0.68/mm. Pitch and span is defined in detail in the Scarselletta reference. 
     According to a broader aspect of the invention, a folded strip forming a plurality fins is arranged between adjacent pairs of tubes within at least one layer of tubes. The fins have at least one louvered section laterally aligned with the tubes of at least one layer. The fins have at least one louvered section being laterally misaligned with at least one layer of tubes and being spaced from the laterally aligned louvered section to form a plain non-louvered section interposed between the aligned and misaligned louvered sections. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference now is made to the accompanying drawings in which: 
     FIG. 1 is a front elevational view of a radiator for a motor vehicle engine cooling system in accordance with the invention; 
     FIG. 2 is an enlarged isometric view of a section of the radiator shown in FIG. 1; 
     FIG. 3 is an enlarged fragmentary front elevational view of the radiator shown in FIG. 1; 
     FIG. 4 is cross-sectional view taken along lines 4-4 shown in FIG. 3; 
     FIG. 5 is an isometric view of a second embodiment of a radiator; 
     FIG. 6 is a cross sectional view taken along lines 6-6 shown in FIG. 5; 
     FIG. 7 is a view similar to FIG. 6 illustrating a third embodiment of a radiator; 
     FIG. 8 is an isometric view of a fourth embodiment of a radiator; 
     FIG. 9 is a cross-sectional view taken along lines 8--8 shown in FIG. 8; 
     FIG. 10 is a graph illustrating comparative test results for on-car performance of radiators having various louver arrangements on its fins for a three layer radiator as illustrated in FIGS. 8 and 9; and 
     FIG. 11 is a graph illustrating comparative test results for various louver arrangements on the fins of a single layer radiator as illustrated in FIGS. 1 and 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, a radiator 10 used in an engine cooling system of a motor vehicle has a pair of vertically oriented tanks 11 and 12 interconnected by a core 13. The tanks 11 and 12 have an inlet pipe 17 and outlet pipe 19 respectively by which the radiator 10 is connected to the rest of the cooling system. In addition, a fill pipe 31 and overflow pipe 33 are openly connected to let the radiator be filled and allowed to overflow respectively. 
     The radiator core 13 has a series of coolant carrying tubes 14 that fluidly interconnect tank 11 with tank 12. The tubes 14 are flattened tubes with top and bottom relatively flat surfaces 16 and two arcuate front and rear edges 18 and 21 as shown in FIG. 2. The tubes 14 are stacked in a vertical layer 20 with the front and rear edges 18 and 21 vertically aligned. 
     The tubes 14 are vertically spaced apart to form gaps 22 therebetween with fins 15 positioned therein. The fins 15 are also referred to as air centers. As more clearly shown in FIGS. 2 and 3, each fin 15 is part of a corrugated strip 24. Each strip 24 has its crests 26 bonded to an adjacent tube 14. The fin 15 extends from the front 27 of the radiator to the rear 29 thereof at right angles from the longitudinal axis 28 of the tubes. The fin 15 has a plurality of louvers generally indicated as 25 arranged in different sections and at different angles as further described in detail. 
     The fin 15 has a louvered section 30 that is laterally aligned with the tubes 14 within layer 20. As more clearly shown in FIG. 4 the louvered section 30 includes louvers 32 that are inclined at an acute angle and are positioned toward the front edge 18 of tube 14 and louvers 34 that have a negative angle and are positioned toward the rear edge 21 of tube 14. 
     The fin 15 has front edge section 36 that extends in front of tube edge 18 and has a rear edge section 38 that extends behind tube edge 21. The front edge section 36 has a series of louvers 40 that are laterally misaligned from the tube 14, i.e. the louvers 40 are laterally positioned in front of the front edge 18 of tube 14. Similarly, the rear edge section 38 has a series of louvers 42 that are laterally misaligned from the tube 14, i.e. the louvers 42 are positioned behind the rear edge 21 of tube 14. 
     The series of louvers 40 and 42 are spaced from the louvered section 30 to form plain non-louvered sections 44 interposed therebetween. The plain non-louvered fin sections 44 laterally span the front edge 18 and rear edge 21 of the tube 14. The front edge 18 and rear edge 21 are substantially aligned with the midpoint 45 of a respective plain section 44. 
     Referring now to FIGS. 5 and 6, a second embodiment of a radiator is illustrated. In the radiator 10a, two layers 20a and 23a of tubes 14a are spaced apart with a gap 47 therebetween. The fins 15a have a first series of louvered sections 30a and 48a that are laterally aligned with the respective layers 20a and 23a. A second series of louvers 46 are laterally misaligned with the layers 20a and 23a, i.e. the louvers 46 are aligned with the gap 47 between the two layers. The louvers 46 are angled similarly to the louvers in section 48. Louvers in section 30 are oppositely angled. The louver sections 30a and 48a are spaced apart from louvers 46 to form plain non-louvered sections 44a interposed therebetween. The plain non-louvered fin sections 44a laterally span either the front edge 18a and rear edge 21a of a tube 14a. The front edge 18a and rear edge 21a are proximate with the midpoint 45a of a respective plain section 44a. 
     Reference is now made to FIG. 7 which discloses a variation of the fin arrangement for a radiator having two layers 20b and 23b of tubes 14b. In the variation shown in FIG. 7, the fin 15b has louvered section 30b and 48b wherein each includes louvers 32b that are inclined at an acute angle and are positioned toward the front edge 18b of respective tubes 14 and louvers 34b that have a negative angle and are positioned toward the rear edge 21b of tube 14b. Similarly, louvered section 46b include louvers 51b angled similarly to louvers 32b and louvers 53 angled similarly to louvers 34b. Louvers 53 are positioned forward of louvers 51 within each louver section 46b. 
     FIGS. 8 and 9 are now referred to in describing a radiator having three layers 20c, 23c and 55c of tubes 14c being spaced apart with gaps 47c and 57c therebetween. The fins 15c have a first series of louver sections 30c, 48c and 58c that are laterally aligned with the respective layers 20c, 23c and 55c. A second series of louvered sections 46c and 56c are laterally misaligned with the layers 20c, 23c, and 55c, i.e. the louvers 46c and 56c are aligned with the gaps 47c and 57c between the two layers. Louvered sections 30c, and 55c each includes louvers 32c that are inclined at an acute angle and are positioned toward the front edge 18c of respective tubes 14c and louvers 34c that have a negative angle and are positioned toward the rear edge 21c of tube 14 c. Similarly, louvered section 48c include louvers 62c angled similarly to louvers 32c and louvers 64c angled similarly to louvers 34c. Louvers 64c are positioned forward of louvers 62c. The louvers in section 46c are angled similarly to louvers 32c. Louvers in section 56c are oppositely angled similar to louvers 34c. The series of louvers 30c, 48c and 55c are spaced apart from louvered sections 46c and 56c to form plain non-louvered sections 44c interposed therebetween. The plain non-louvered fin sections 44c laterally span either the front edge 18c or rear edge 21c of tubes 14c. The front edge 18c and rear edge 21c are substantially aligned with the midpoint 45c of a respective plain section 44c. 
     The louvered section 46c has its louvers angled oppositely from the adjacent sets of louvers 34c and 62c in adjacent sections 30c and 48c respectively. Similarly, louvered section 56c has its louvers angled oppositely from the adjacent louvers 64c and 32c of sections 48c and 55c. 
     The fin 15c has front edge section 36c that extends in front of tube edge 18c and has a rear edge section 38c that extends behind tube edge 21c. The front edge section 36c has a series of louvers 40c that are laterally misaligned from the tube 14c, i.e. the louvers 40c are laterally positioned in front of the front edge 18c of tube 14. Similarly, the rear edge section 38c has a series of louvers 42c that are laterally misaligned from the tube 14c, i.e. the louvers 42c are positioned behind the rear edge 21c of tube 14c. 
     The series of louvers 40c and 42c are spaced from the louvered section 30c to form plain non-louvered sections 44c interposed therebetween. The plain non-louvered fin sections 44c laterally span the front edge 18c and rear edge 21c of the tube 14c. The front edge 18c and rear edge 21c are substantially aligned with the midpoint 45c of a respective plain section 44c. 
     The performance of a radiator constructed in accordance with the above description compares favorably with various radiator constructions of the prior art. As appreciated in the U.S. Pat. No. 4,693,307 issued to Scarselletta, the addition of louvers on the fins increase the heat exchange capacity at the expense of restrictivity of the radiator. Restrictivity is the mechanical energy required to produce an air flow through the radiator. The increase of louver concentration improves heat transfer significantly only till a threshold value. Further increase of louver concentration only improves heat transfer slowly and increases restrictivity very rapidly. A performance versus restrictivity graph plotted for different louver concentrations is shown in FIG. 10 for the radiator construction as shown in FIGS. 8 and 9. The curve plotted for different louver concentrations has a decided &#34;knee&#34; 70 which corresponds to a concentration of louvers set at 44%. As described in the Scarselletta patent, performance increases have been shown for louver concentrations at not less than 40% and not more than 60%. 
     In accordance with the present invention, while maintaining the louver concentrations as set forth in the Scarselletta patent, heat transfer efficiency can be further enhanced if the louvers are properly positioned as hereinbefore described. The heat transfer capacity for a given radiator of the multi-layer type as illustrated in FIGS. 8 and 9 can be increased to substantially the same level as a heavily louvered fin of the prior art with significantly less restrictivity. Compared to the hybrid louver radiator as taught by Scarselletta, the present invention provides increased heat transfer capability with only an incremental increase in pressure drop that places the plotted point above the previous &#34;knee&#34; that was previously thought to be the maximum curve achievable. 
     The graph illustrated in FIG. 11 for a single layered radiator as illustrated in FIGS. 1 and 2 again pictorially represents the improvement achieved by the louvered sections 40 and 42 as compared to previous single layered radiators. The heat transfer capacity peaks out with the enhanced extended center before dropping down lower for a fully louvered center. The peak occurs because of the existence of a heat sink in the fins that is provided by the plain non-louvered sections 44 aligned with the front edge 18 and rear edge 21 of the tubes 14. The plain non-louvered section 44 provides for increased heat conduction away from the tubes 14 to the edges 36 and 38 of the fins 15. In contrast fully louvered fins loses the conduction path to the edges of the fin and thus, the heat transfer capacity decreases. 
     The combination of the louvered sections of the fin in the above described specific arrangements in combination with other known parameters gives rise to enhanced performance. One of these other parameters are total louvered area not more than 60% and not less than 40% of the total fin area. Another parameter is the thickness and stacked density of the fins is not more than 12% nor less than 2.5% of the space between the adjacent flat tubes. The fin pitch and span is not more than 1.2/mm nor less than 0.68/mm. 
     Variations and modifications are possible without departing from the scope and spirit of the present invention as defined by the appended claims.