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Over the course of the past decade, the automotive industry has made efforts to reduce the depth of the brazed aluminum, louver fin evaporators typically used in the air conditioning system for automobiles by increasing their compactness. Increasing fin and louver densities have led to the possibility of condensate affecting the air side performance of automotive evaporators. Condensate can “bridge” the space between two adjacent fins or louvers. Condensate bridging of the fins or louvers can alter the flow of air through the evaporator, causing a change in the heat transfer and friction characteristics. This study attempts to quantify these changes and determine which parameters will have an impact on them. Wind tunnel tests measuring the air side sensible heat transfer coefficient and pressure drop were conducted to examine how the air side heat transfer and friction characteristics of automotive evaporators are changed by the presence of condensate.

Condensate that forms on the air side surface of an evaporator can have a significant impact on the air side performance of brazed aluminum, louver fin automotive evaporators. Condensate can “bridge” the space between two adjacent fins or louvers and alter the flow of air through the evaporator, causing a change in the heat transfer and friction characteristics. This study attempts to determine how condensate drains from an evaporator in the hopes of improving the air side fin geometry and obtain an evaporator that retains a minimum amount of condensate. Using a table-top wind tunnel apparatus, qualitative observations of condensate draining from a single column of louver fins brazed to a refrigerant tube were made. The amount of condensate retained in an evaporator core was determined using the experimentally-verified dip test method.

This paper assesses the effect of material thermal properties on the performance and manufacture of brazed radiators made of aluminum and of copper/brass. Analysis is presented to show that copper/brass cores can be closely weight competitive with brazed aluminum. This is because the higher thermal conductivity of copper allows use of thinner fins. The thinner fins result in approximately 38% lower air pressure drop for the same frontal area and performance. If the cu/br design is designed for the same air pressure drop as the aluminum core, one may obtain frontal area reductions as large as 16%. Analysis of the effect of the different core materials and material thickness on the core heating time is presented. It is concluded that the thick header controls the core pre-heat time. Further, it is estimated that the required pre-heat time for cu/br and aluminum cores would be approximately equal.

The louvered fin geometry is widely used for heat transfer to air in automotive heat exchangers. The only published correlation for heat transfer and friction in the corrugated louver fin geometry is that of Davenport. However, this correlation is strictly empirical, since it is based on a multiple regression correlation using dimensional parameters assumed to be important. Further, the correlation does not include the dimensions of the inlet/exit and the internal flow redirection louvers. The objective of the present work is the development of an improved, rationally based correlation. The correlation is based on dividing the total fin surface into four different regions, and applying rationally based heat transfer and friction equations to each of these regions. The resulting correlation is semi-analytical, since it is based on heat transfer mechanisms and theoretical relationships expected to apply the various regions of the fin.

This paper describes a new radiator core design concept that may be used to replace the conventional multi-row “tube and center” design. It offers several mechanical and thermal design advantages for copper-brass radiators. Conventional multi-row radiator cores must be made with a space between the tubes in the air flow direction. This is required to obtain a sufficient ligament in the header. This space between the tube rows causes a loss of the potential thermal performance. The new concept described alleviates the need for a space between the tube rows. The design allows for zero space between the tube rows. This will provide excellent heat conduction from the tubes to the fin over the full fin depth in the air flow direction. Further, it will allow louvering of the fins over the full fin depth, which will also contribute to higher thermal performance. The tube ends are reshaped to a larger minor radius.

A flow visualization study of the louvered fin geometry, commonly used in automotive heat exchangers was performed. Flow visualization was performed using a dye injection technique with 10:1 scale models. The geometrical parameters, louver pitch, louver angle, and fin pitch were varied to determine their effect on the flow structure. Tests covered louver pitch based Reynolds numbers of 400 - 4000. Data are presented in the form of a dimensionless flow efficiency (defined in terms of the mean flow angle, relative to the louver angle) and Reynolds number. Correlations are developed to predict the flow efficiency as a function of dimensionless geometrical groups and Reynolds number. A discussion of the flow structure is also presented.

This paper describes improvements in the design of conventional copper-brass radiators that should provide increased thermal performance, durability, and reduced material cost. A 1-row, and an improved 2-row design is described that has lower air and coolant pressure drop, and lower material costs than a conventional multi-row design. A parametric study describes how changes in core geometry affect thermal and pressure drop performance, and core weight. Several mechanical design improvements are identified, which reduce stress in the radiator tube-header joint. Design equations supporting the design recommendations are also given or referenced. Work in progress leading to reduced fin corrosion is also reviewed.

This paper presents quantitative information on stress in copper/brass radiators. Strain gages were used to measure the stresses in the unsupported tubes near the header, and to determine thermal stress resulting from uneven expansion of the tubes and the side support bar. Strain measurements were performed on plain and ribbed “turbulator” tubes. The compressive strength of various fin arrays was also measured. An analytical model was developed to predict the compressive strength of the fin array as a function of the fin geometry parameters. A finite element stress analysis model was used to predict the stress in the tubes and in the tube/header solder joint. The predicted values agree well with the measured stress. Mechanical design changes, intended to affect reduced stress, were evaluated using the finite element model.