Local heat transfer coefficients for cryogenic hydrogen flow in circular tubes with constant wall temperature

Abstract

Cryogenic storage technologies as liquid hydrogen (LH2) or as cryo-compressed hydrogen (CcH2) are developed to store large amounts of hydrogen onboard heavy-duty vehicles. In both applications, the cryogenic hydrogen needs to be heated to ambient temperatures before being converted in the fuel cell. The available heat source onboard heavy-duty vehicles is the coolant of the fuel cell or hydrogen combustion engine with temperatures up to 350 K. For CcH2 tanks, the hydrogen can enter the heat exchanger with temperatures around 40 K. Thus, the maximum occurring temperature difference inside the heat exchanger can exceed 300 K. Without thorough knowledge of the local heat transfer inside the heat exchanger, freezing can block the heat exchanger and, thus, cause a shutdown of the hydrogen supply of the fuel cell. In the field of rocket engines, several correlations are proposed for heat transfer to hydrogen with high wall-to-bulk temperature ratios $T_w/T_b$.

CFD simulations are used to thoroughly investigate the local heat transfer for a wide range of operating conditions as they occur in CcH2 storage applications. The hydrogen pressure is considered in the range from 15 bar to 400 bar and the temperature in a range of 40 K to 150 K. The existing correlations are compared to the CFD simulation data and an improved correlation is proposed which correlates the CFD data with a maximum deviation of 25%.