Effect of Void Fraction Model on the Dynamic Performance of Moving Boundary Heat Exchanger

Transient simulation of refrigeration systems is a relevant tool in the development and optimization of household refrigerators. These models should be accurate but fast. The core of transient refrigeration system simulation is the heat exchanger model as it imposes the system pressures. Dynamic pressure variation inside the heat exchanger is governed by the apparent density and internal energy of the refrigerant. For two-phase flow, the void fraction determines the apparent density. Several void fraction models are available in literature; each model provides different values of void fraction for the same refrigerant conditions. Unfortunately, a 1% error in void fraction could lead to 16% error in apparent density at a void fraction of 95%. In the current study a moving boundary heat exchanger model was developed to simulate the dynamic performance during pull-down in a typical four component vapor compression system. In this model, the void fraction can be calculated based on 12 different void fraction models. A parametric study is performed on the effect of void fraction models on the performance of small refrigeration systems using a system simulation software. System pressures and charge distribution were compared for the different void fraction model.