Modelling of heat addition to near-critical and supercritical fluids

This work presents a coupled flow model to describe heat addition to near-critical and supercritical fluids. In a first approximation only forced convection is considered, while buoyancy forces, viscous and volume expansion losses are neglected. The steady, one-dimensional balance equations are solved simultaneously through an iterative procedure, taking into account real gas effects. The analysis shows that the intrinsic high compressibility of near-critical fluids, mirrored in the high peaks of the thermodynamic response functions, strongly reduces the region of validity for the incompressible flow assumption. Indeed, depending on the initial conditions, compressible flow effects may occur at Mach numbers below 0.1. For a given mass flow rate, the occurrence of heat transfer deterioration (HTD) correlates directly with the maximum amount of heat that a compressible flow can absorb. The latter is a rapid decreasing function of the local Mach number, which increases upon heat addition due the combined effect of thermal acceleration and enhanced fluid compressibility. The coupled approach also provides a natural explanation for the empirically observed dependence of HTD upon the heat flux to mass flux ratio as well as upon the ratio between the isobaric thermal expansion coefficient and the isobaric specific heat capacity.