Effect of axial wall conduction and ambient heat-in-leak on the performance of a two-fluid counter-flow cryogenic heat exchanger, using finite element method

Abstract

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.

A two-fluid, single-pass, counter-flow, cryogenic heat exchanger is investigated for the effect of ambient heat-in-leak to both the fluids and axial conduction in the wall separating the two fluids. In most cryogenics applications, the performance of heat exchangers deteriorates significantly due to heat-in-leak from the ambient and axial wall conduction. Here a small, counter-flow heat exchanger, of the type used in Joule-Thomson refrigerators, experiencing heat-in-leak to both hot & cold fluids, and axial wall conduction, is analyzed adopting the finite element method. Five non-dimensional parameters, including those to account for ambient heat-in-leak and axial wall conduction are defined to present the result. The effect of these parameters on the heat exchanger performance has been analyzed. The set of non-dimensional governing equations for hot fluid, cold fluid and the wall are obtained by energy balance. These governing equations are solved by FEM using Galerkin’s method.Validation is carried out by comparing the results obtained using the present methodology with those published in the literature for limited parameters. The excellent match between the two validates the solution methodology used. The effect of ambient heat-in-leak and axial wall conduction are studied for their effect on the fluid temperature profiles. Ambient heat-in-leak and axial wall conduction are found to increase the hot fluid exit temperature. The effect of axial wall conduction is found to be more concentrated towards the heat exchanger ends. Further, the effect of ambient heat-in-leak and axial wall conduction and varying non-dimensional parameters are studied for their effect on the hot fluid effectiveness and performance degradation.