Non-local entropy evolution in heat exchangers with elliptical and circular tube geometries

Numerical simulations of the entropy transportation process inside a plate and tube heat exchanger are presented. Entropy levels around tubes with circular and elliptical cross sections are analyzed and compared. The purpose of the comparison is to determine how the entropy flux contributes to increase, or decrease, the value of the entropy in certain regions of the flow field. This is accomplished by numerically integrating the Reynolds averaged transport equations for the mass, momentum, energy and entropy, in a three dimensional domain. A k-ε-2L model is used to account for the fluctuations appearing in the laminar-to-turbulent transitional regime of the flow. The importance of entropy convection, relative to the generation of entropy, is underlined by means of a direct comparison through a series of visualization maps. This approach allows some useful insights about the evolution of the entropy field in complex geometries. For example, in this case certain troublesome regions where the entropy field reaches high values can be identified and, therefore, design recommendations can be stated in order to increase the thermal efficiencies of the devices under consideration.