Modeling a Freezable Water-Based Heat Exchanger for Use in Spacecraft Thermal Control

A spacecraft thermal control system typically uses water coolant loops to keep the cabin comfortable. However, the relatively high freeze point of water limits its use, because freezing water within the loop may damage the thermal control system. This paper describes a novel freeze-tolerant heat exchanger and the thermal model developed to correlate data from experiments. The radial transport of heat within the heat exchanger was modeled as a thermal circuit. The heat flows from the water into the thermally conductive shell and fins. One flow channel within the heat exchanger was lined with insulation to keep it free of ice. When the heat sink temperature is below the freeze point of water, a layer of ice builds up in the other flow channels to greatly increase the thermal resistance to heat transport, since the thermal conductivity of ice is only about th that of Al-6061. In addition, the wetted surface area is reduced by a factor of six when only the insulated channel can still flow water (all of the other channels are frozen). Together, these two effects turn down the rate of heat rejection in proportion to the net thermal load and the external heat sink environment encountered.