Numerical investigation of the influence of heat exchanger U-bends on temperature profile and heat transfer of secondary working fluids

In this paper, numerical investigations, conducted using computational fluid dynamics, on the enhancement of internal convection heat transfer following a heat exchanger U-bend under laminar flow conditions in secondary working fluids are described. Under laminar flow conditions enhanced mixing within the heat exchanger U-bends is known to occur due to the development of secondary flows, known as Dean vortices. Numerical investigations indicated that within the U-bend, secondary flows partially invert temperature profiles resulting in a significant localised decrease in average fluid temperature at the pipe surface. As a result, downstream heat transfer enhancement is
observed, the magnitude of which can exceed that typical of a pipe combined entry condition in some circumstances by greater than 20% for up to twenty pipe diameters downstream. Heat transfer enhancement was found to increase with increasing U-bend radius, but to decrease with increasing heat exchanger pipe radius and internal Reynolds number