A computer model for heat exchange processes in mobile air-conditioning systems
The last few years have seen a rapid growth in the number of cars equipped with air-conditioning systems. The space available to fit the system is limited and the under bonnet environment is hostile. Moreover, the depletion of the stratospheric ozone has led to legislation on the phasing out of the chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs ). These substances are used as refrigerants in most refrigeration, heat pump and air-conditioning systems in service today. The aim of this research project was to study existing air-conditioning systems used in automotive applications to develop a model that simulates the components of these systems. This provides a better understanding of the effect of using different refrigerants in the system and its performance. Experimental studies of the performance of the different heat exchanger geometries used provided inputs to the model developed. Automotive air-conditioning condensers and evaporators simulation models were developed and used to compare the performance of these heat exchangers using CFC and HCFC refrigerants and the non-ozone depleting replacements. Thermodynamic properties of the new refrigerants were derived from the equation of state. The evaporator was simulated taking into consideration the mass transfe r associated with the heat transfer in humid conditions. Two types of compact heat exchangers were modelled, round tube with plane fin and plate tube with corrugated fin. These cover most automotive, domestic and industrial applications. The basic performance data of various geometries were determined experimentally. An existing thermal wind tunnel was re-instrumented and modified to improve accuracy at the low air velocities was used in this study. A new data logger linked to a personal computer was used with newly written software to collect and analyse the test data. The results for all geometries tested were correlated and presented in non-dimensional form. The test data were used to determine the effect of various geometrical parameters on the performance for an optimisation of condenser and evaporator designs. The model developed is being used by industrial collaborators for the design of heat exchangers in automotive air-conditioning systems.