Experimental investigations on heat transfer to H2O/CO2 mixtures in supercritical region

Supercritical H2O/CO2 mixtures are the working fluids in a novel power generation system with coal gasified in supercritical water, but their heat transfer behaviors have received less attention compared with those of the supercritical water. Here, we experimentally investigated the heat transfer to these important mixtures. First, a stable and accurate apparatus was designed and established to measure the forced convection heat transfer coefficients (HTCs) of supercritical H2O/CO2 mixtures. The measurement uncertainty of this apparatus is lower than those reported in literature attributed to the well controls of the pressure and the component mass fraction throughout the apparatus. Then HTCs were measured for the H2O/CO2 mixtures that flow in a vertically upward smooth tube with the fluid pressure of 24 MPa, temperatures from 400 to 460 °C, CO2 mass fractions up to 20.1% and mass velocities up to 1270 kg m−2 s−1. Results show that the supercritical mixtures have similar convection heat transfer behavior to supercritical pure fluids, and that the forced convection heat transfer coefficients of these mixtures can be well predicted using the correlation developed for supercritical pure fluids. These results lay the foundation for designing heat transfer devices with supercritical mixtures as heat transfer fluids.