Challenges in Improving the Operational Efficiency of Two-Phase Heat Transfer Systems in Space

For high-power spacecraft with extensive heat transfer distances, thermal control systems based on mechanically pumped two-phase loops (2ph-MPL) offer a highly effective solution. The development of these systems began in the 1980s, yet the first high-power satellites equipped with 2ph-MPLs by Thales Alenia Space-France were only launched in 2021-2022. Despite this progress, there is substantial potential to enhance the efficiency and reliability of 2ph-MPL technology. This report addresses several key challenges and presents preliminary research findings that could further advance the capabilities of 2ph-MPLs: 1) Pressure regulation and cavitation prevention: A heat-controlled accumulator (HCA) is used to regulate pressure within the loop, maintaining the working fluid's boiling point or ensuring a sufficient cavitation margin at the pump inlet. This report discusses a method for determining the required HCA volume and examines 2ph-MPL performance under critical operating conditions, where pressure control may become compromised. 2) Impact of non-condensable gases (NCGs): During the operation of an ammonia-based loop, non-condensable gases can accumulate due to radiolysis. Experimental and theoretical approaches were employed to assess the impact of NCGs on loop performance. 3) Hysteresis-induced overheating: When the heat load is activated, abnormal and prolonged overheating of payloads may occur due to a hysteresis effect, which requires significant wall overheating of the cold plate to initiate boiling. This report provides recommendations to mitigate this issue. 4) Component design and optimization: The design of 2ph-MPL components requires balancing numerous technical factors and parameters to achieve optimal performance. A methodology for multi-criteria, multi-parameter optimization is presented, including an example of its application in designing a satellite heat rejection subsystem with minimal mass per unit of dissipated power.