Models of heat transfer to fluids at supercritical pressure with influences of buoyancy and acceleration

Abstract

Physically-based semi-empirical models of buoyancy-influenced and inertia-influenced heat transfer to fluids at supercritical pressure taking careful account of the temperature dependence of fluid properties are presented in this paper. The first model is concerned with buoyancy-influenced convective heat transfer in a vertical heated tube to fluid flowing either in the upwards direction (the buoyancy aided case) or the downwards direction (the buoyancy-opposed case). It provides a detailed picture of how the mode of heat transfer changes with increase of buoyancy influence from one of forced convection, when any such influence is negligibly small, to combined forced and free convection and then eventually to a mode of heat transfer which exhibits characteristics very similar to those of free convection. The second model is concerned with inertia-influenced heat transfer due to thermally-induced bulk flow acceleration. It shows how acceleration of a strongly heated flow can cause reduced turbulence production, deterioration of heat transfer effectiveness and even laminarisation of a turbulent flow. These two models yield criteria for screening experimental data in terms of the magnitude of buoyancy and inertia influences. A third model is presented which describes heat transfer under conditions where influences of buoyancy and inertia due to thermally-induced bulk flow acceleration are encountered together. It is shown how buoyancy-influenced flow and heat transfer can be modified as a result of thermally-induced bulk flow acceleration. The paper ends with a brief outline of how the models might be used for correlating data on buoyancy-influenced and inertia-influenced heat transfer taking account of the strong temperature dependence of fluid properties which can be encountered in the case of fluids at supercritical pressure.