An Improved Moving Boundary Heat Exchanger Model with Pressure Drop

A literature review indicates that almost all moving boundary heat exchanger models used in dynamic simulations of heat pumps rely on the common hypothesis that the refrigerant pressure drop is negligible. In fact, it is important to include the momentum balance in some applications, such as electronics cooling where microchannels are commonly used and significant pressure drop is observed and large-scale heat exchangers in solar thermal plants where tube length can be longer than several hundred meters. In addition, a comprehensive and robust switching approach is needed to handle transitions between different model states due to phase change. It is found that the current switching methods in the literature exhibit several shortcomings which may cause serious errors and stability issues when simulating cycling transients of vapor compression systems. The objective of this paper is to propose an improved moving boundary formulation that aims to fill in the above research gaps. Specifically, two different approaches are presented to account for the refrigerant pressure drop across the heat exchanger. A novel and comprehensive switching scheme is introduced to ensure smooth transition between different model representations under large disturbances. The proposed model is validated using measured data. The validation shows that the proposed heat exchanger model along with other supporting component models can reasonably capture the start-up transients of a flash tank vapor injection heat pump system.