A thermal-hydraulic analysis model of shell-and-tube heat exchangers with supercritical working fluids near the pseudo-critical temperature

Increasing the operating pressure and temperature of working fluids to supercritical state can significantly enhance the efficiency and power density of thermodynamic cycles, including steam Rankine cycles and CO₂ Brayton cycles. Such working fluids could undergo the pseudo-critical state in heat exchangers, wherein the pressure and temperature locate near the extension of the saturation curve in the supercritical region, and exhibit apparently more drastic property variations than normal states. In this study, a thermal-hydraulic model is specifically developed for these heat exchangers to solve critical performance metrics including the heat transfer capacity and pressure drops. The preliminary validation of the model has been obtained by comparing the model result with the published experimental data for a supercritical CO₂–Air heat exchanger. The average deviation between model predictions and experimental data are <10 %, and the computational time for running the current model is only about one-tenth of that for a previously validated model. We further discuss various discretization strategies of the model, including the finite volume method and moving boundary method, for a supercritical CO₂ and a supercritical steam heat exchanger, respectively. The finite volume method brings a uniform discretization of the computation domain and could achieve a high accuracy when the discretization is properly refined. On the contrary, the moving boundary method refines the discretization for only part of the domain, where the temperature-dependent gradient of the specific heat of the used supercritical fluid achieves a threshold value. This work determines the optimal threshold value that enables the developed model using this method to provide decent accuracy but takes 50 % less computation time compared to the finite volume method. Relevant findings provide an important basis for design and optimization of multi-scale heat exchangers with supercritical working fluids for future energy and aviation fields.