Modeling and optimization of triple tube heat exchangers. Theoretical formulation, CFD model and experimental contrast

Abstract

This paper presents a theoretical model that calculates the thermal parameters of a triple tube heat exchanger (TTHE) analogously to the formulation used for double tube heat exchangers. The model calculates the outlet temperatures, heat exchange and thermal efficiency of the TTHE from the inlet conditions and flow properties in a simple and efficient formulation that allows for the easy calculation of multiple cases to analyze the TTHE behavior under different conditions. The model also proposes a theoretical optimum distribution of the flows to optimize heat transfer. The theoretical model is compared with detailed CFD simulations and experimental data of straight 1-meter-long and U-shaped 6-meter-long TTHE systems. The theoretical model shows average errors of 7% and 12%, and the CFD simulation shows 5% and 10% errors, both for straight and U-shaped TTHEs, respectively. The model error is reasonably low since the overall heat transfer coefficients used in the model are based on heat transfer theoretical correlations, the accuracy of which is usually low. In addition, a parametric study on the distribution of the inner tube and the outer annulus mass flow rate ratio was performed to determine the optimum distribution of a particular case