Development of an Advanced High Temperature Heat Exchanger Design for Hydrogen Production

This paper deals with the development of an advanced high temperature heat exchanger design for hydrogen production by the sulfur iodine thermochemical cycle from advanced nuclear reactor concepts. The offset strip-fin hybrid plate type compact heat exchanger concept is chosen, and the material of manufacture is the liquid silicon impregnated carbon composite. The offset strip-fin is chosen as a method of heat transfer enhancement due to the boundary layer restart mechanism between the fins that has a direct effect on enhancing heat transfer. The effect of the fin thickness, pitch in flow direction, and the aspect ratio of the offset fins on the flow field and heat transfer are studied in 2-D using Computational Fluid Dynamics (CFD) techniques, and the results are then compared with the analytical calculation results. The preprocessor GAMBIT is used to create a computational mesh, and the CFD software package FLUENT, that is based on the finite volume method is used to produce numerical results. Proper dimensions of the strip fins need to be chosen in order to have an optimized heat transfer enhancement coupled with a reduced pressure drop. The study is conducted with helium gas as the working fluid with varied of Reynolds number values. The flow and heat transfer is considered to become periodically fully developed after a certain entrance length hence numerical simulations were performed using periodic boundary conditions. Two-dimensional numerical simulations were also performed for the whole length of the heat exchanger which has 37 such periodic modules. Comparison study was performed between the cases of fins with rectangular and curved geometry. Attempt has also been made in order to validate the coefficient of fin thickness (C_fin) value using CFD techniques, which has been used in the existing empirical correlations to suit this type of heat exchanger geometry. The model developed in this paper is used to investigate the heat exchanger design parameters in order to find an optimal design.