A spacecraft thermal control system must keep the vehicle, avionics and atmosphere (if crewed) within a defined temperature range. Water coolant loops are typically used to transport heat to or from the cabin of a crewed spacecraft via heat exchangers to the heat sink systems that reject the heat to space. Water is non-toxic and good for heat transport, but it has a high freeze point. Thus, there is concern that the water loop can freeze and damage the thermal control system unless a low freeze point intermediate fluid loop is included. Incorporating a freeze-tolerant water/ice heat exchanger can eliminate this risk and offers a novel approach to spacecraft thermal control, since parts of the heat exchanger can be selectively frozen to passively increase the turndown of the heat rejection rate. In addition, it has the potential to simplify the thermal control system (for example, a secondary loop between the coolant water loop and the radiator may no longer be needed) and thereby reduce its size and mass.This paper describes the design for a Self-Regulating Freezable Heat exchanger (SRFHX) and the results of experiments conducted to characterize its performance. At low load conditions the SRFHX begins to freeze the water along the shell and fins. The buildup of ice passively turns down the rate of heat rejection in proportion to the spacecraft net thermal load and the external heat sink environment encountered. Experiments were conducted to measure the heat transfer performance and operability of a SRFHX over a broad range of test conditions and put it through 191 freeze/thaw cycles without damage.
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