Transient Response of Two-Phase Heat Exchanger With Varying Convection Coefficients

Transient changes of fluid and wall temperatures in a two-phase heat exchanger are investigated in this article, particularly with respect to spatial and temporal effects of varying convection coefficients. The coupled energy equations for both sides of the heat exchanger are solved directly with an integral method. Varying convection coefficients are related to changes of vapor fraction between the inlet and outlet of the heat exchanger. Unlike past numerical studies encountering difficulties with instability, stiffness, and lack of convergence, the current integral formulation provides a reliable alternative and efficient procedure for transient response within the heat exchanger. Furthermore, complex inversion from a transformed domain is not needed, in contrast to conventional methods with Laplace transforms. In this article, past integral methods are extended to cases with varying convection coefficients, arising from changes of phase fraction on the two-phase side of the heat exchanger, as well a multiple step-changes of temperature. The predicted results show close agreement with past data, including numerical simulations with a dynamic simulator.