Numerical analysis of the thermal and hydraulic characteristics of CO2/propane mixtures in printed circuit heat exchangers

Abstract

The supercritical CO2 Brayton cycle has great prospects for application in the field of solar thermal power generation,where the thermoelectric conversion efficiency is significantly affected by the ambient temperature. Meanwhile, the real gas effect and thermal efficiency advantage of CO2 can only be fully used near the critical temperature. In this case, a scheme of using CO2-based mixtures as heat transfer fluids is proposed. As an important component of the supercritical Brayton cycle, a printed circuit heat exchanger (PCHE) is used as the object with CO2/propane mixtures as the heat transfer fluid. The thermal and hydraulic performance of the PCHE is investigated by numerical analysis. The general regulation of mixture components on heat transfer and friction features is obtained for various pressure ratios, inlet temperatures and mass flow rates. The results indicate that CO2/propane mixtures exhibit lower pressure loss and higher heat transfer coefficients as heat transfer fluids. The fanning friction factor decreases by 8.3% and the Nusselt number increases by 16.9% when the molar fraction of propane varies from 0 to 0.5. The heat transfer capacity of the mixture does not change significantly with pressure since its specific heat decreases slightly with increasing pressure. Compared with mixtures, the pressure drop of pure CO2 is more obviously affected by the mass flow rate. New thermal–hydraulic correlations explaining the thermophysical property variations and the mixing ratio effects are proposed which predict all data with errors of less than ±2%.