The Impact of Modeling Complexity and Two-Phase Flow Parameters on the Accuracy of System Modeling for Unitary Air Conditioners

Abstract

Current air-conditioning system models seek to balance the level of code complexity with the speed of execution. Ideally, the simplest code possible that can accurately predict system performance is desired. To further understand this issue, two methods of modeling unitary air-conditioning systems are compared: a detailed, local approach and a simplified, global approach. These methods have been applied to two commercial unitary air conditioners for which extensive experimental data were obtained. The global approach predicted absolute system performance as well as or better than the local approach. In a separate analysis, the effects of two-phase modeling parameters on system model predictions were considered. The two-phase friction factors, heat transfer coefficients, and void fraction models were independently varied. The resulting system performance predictions were sensitive to variations in the air-side heat transfer coefficients and the void fraction model and not as sensitive to variations in the friction factor. Both the void fraction model and the condenser parameters mainly affect the compressor discharge pressure and the compressor power consumption. The evaporator parameters mainly affect the refrigerant mass flow rate and the heat transfer rates in both heat exchangers.