Heat transfer to supercritical fluids in horizontal heated flow of emerging energy systems

A critical review of heat transfer to supercritical pressure fluids in horizontal flow under heating conditions is presented with the goal of obtaining a perspective on its peculiarities and guiding the development of emerging energy systems. The involving fluids include water, carbon dioxide, refrigerants, and hydrocarbon fuels. The flow channel is a circular tube, heated mostly by electricity. Experimental observations show that heat transfer in the lower half, especially at the bottom side, is much more sensitive to the change of operating conditions. The thermal entrance effect causes a regular enhancement of heat transfer, in contrast to the fluctuant impairment in vertical flow. Further summary leads to some general rules of supercritical heat transfer in horizontal flow. First, the axial profile of wall temperatures can be classified into three types, the boundaries of which correspond to buoyancy parameter of 10 and 90. Secondly, the layout of thermocouples in existing experiments is not always proper, as buoyancy-affected cases were mishandled by only one thermocouple circumferentially. A preliminary design process is proposed to guide a proper experimental program. Finally, existing Nusselt correlations are assessed by local and average heat transfer data. An idea of quantifying thermal entrance effect is proposed on the basis of asymptotic hypothesis, and a promising strategy is recommended towards the development of a more general correlation.