Modelling and optimization of a forced convection heat exchanger for Mars waste heat rejection for power cycle and cryocooling applications

Abstract

A forced convection heat exchanger for waste heat rejection to the Martian atmosphere is modelled for both a power generation and cryogenic refrigeration application. The modeling is based on existing as well as recent, experimentally validated heat transfer and pressure drop correlations for low-Reynolds-number, moderate-Knudsen number finned tube arrays and represents the first detailed examination of optimal heat exchanger design and performance in this regime. These correlations were developed from heat exchanger performance data taken in Mars-like conditions and are described in a separate, companion work [1]. A heat exchanger model based on the effectiveness-NTU method is developed using these correlations and the heat exchanger geometry is optimized using an open-source solver to minimize the overall system mass for a range of candidate heat exchanger materials and heat transfer rates. The optimal heat exchangers generally consisted of shallow, staggered banks of 1–2 mm diameter tubes and 2–4 mm fin spacing, although in some conditions bare tube arrays were optimal. These geometries tended to be preferred due to their low relative pressure drop, a necessity given the power requirement of producing high pressure head in the thin atmosphere. Curve fits that capture the heat exchanger performance are generated based on the input conditions in order to develop a reduced order model that can be used to integrate the heat exchanger model with an existing recuperated Brayton cycle high-temperature gas cooled reactor (HTGR) power cycle model. This power cycle model is used to determine the impact of using convective heat rejection on optimal system mass and compare this approach to radiative heat rejection. For a 40 kWe power output, the optimal overall cycle mass is found to decrease by 78 % when using convective heat rejection compared to a radiator; this is due to the decreased mass of the heat rejection system as well as the lower optimal rejection temperature which results in a lower recuperator mass. The heat exchanger itself has a mass of only 37 kg, a frontal area of 12.7 m2, and requires 2134 W of fan power to operate. The system is shown to perform well over a wide range of Mars ambient conditions. A high-power liquid Oxygen (LOx) reverse Brayton cryocooler cycle is also modeled to determine the benefit of using convective heat rejection for in-situ resource utilization (ISRU) applications. Across a range of power source options and ambient temperatures, using a convective heat rejection system reduced total system mass by up to 43 % and frontal area by up to 95 % compared to a radiator and also reduced the mass-optimal cycle power draw by increasing the optimal cycle efficiency. However, the magnitude of the mass reduction depends strongly on the specific power of the electrical source; the forced convection system provides only a 7 % improvement over a radiator when coupled with a photovoltaic power source due to the increased power demand of the fan. Overall, this technology offers significant performance advantages over radiative heat rejection systems for waste heat rejection applications on Mars and therefore may significantly benefit future crewed Mars missions.