Heat‐in‐leak and longitudinal wall heat conduction influence on three‐fluid cross‐flow heat exchanger performance

Abstract

AbstractThe combined effect of the longitudinal wall heat conduction in the partitioning wall and ambient heat‐in‐leak (HL) from the surrounding to the heat exchanger is a serious factor affecting the performance of a multifluid heat exchanger. The performance of a three‐fluid cross‐cocurrent flow heat exchanger under such circumstances is investigated in this paper. The governing equations for three‐fluids and two separating walls are formulated on the basis of energy balance equations and solved using the finite element method (FEM). Nondimensional parameters (C1,2, C3,2, R1, R3, Ht, Hb, , , , and the number of transfer units [NTU]) are generated and used for the investigation. Effectiveness‐number of transfer plots method is used to analyze the combined effect of wall heat conduction and HL from the surrounding. The combined effect reduces the hot fluid effectiveness significantly for all the parameters considered. The effect of HL alone is predominant as compared with longitudinal wall conduction alone. However, the negligence of the above two parameters in the thermal design of the heat exchanger contributes to a significant loss in hot fluid effectiveness. For a balanced mass flow rate of hot and cold fluid, the drop in the hot fluid effectiveness is found to be 37% approximately (with C3,2 = 0.5, R1 = R3 = 1, Ht = Hb = 0.05, and = = 0.025 at NTU = 10). The results present insight and important considerations involved in the design of such heat exchangers. Also with the application of the FEM, for any number of elements, the stiffness matrix can be generated by writing a program and the exit temperatures are calculated very easily.