Modeling and Dynamics of Two-Phase Flow Heat Exchangers using Temperature - Entropy Bond Graphs

Two-phase flow heat exchangers have many industrial and residential applications, such as in nuclear power plants, HVAC systems, steam generators, and heat pumps. Dynamic response of such heat exchangers are of interest in order to increase reliability and decrease energy consumption. This paper utilizes true bond graphs, with temperature and rate of change of entropy as power variables, to model the dynamics of two-phase flow heat exchangers. A bond graph for variable density flow is derived such that it satisfies mass, momentum, and energy equations. Due to thermofluid bond graphs requirements, a specific entropy - specific volume plane is developed and an algorithm to calculate 2-phase flow properties from this plane is discussed. For stability purposes, the resulting nonlinear system equations are nondimensionalized. Simulation results of the model for a condenser of a residential air-conditioning unit is compared with experimental data. Comparison shows good agreement in both magnitude and shape of the response between the bond graph model and the experiments.