Heat Transfer Coefficient Calculation for Developed Ammonia Boiling in the Evaporator Channel of a Thermal Sink

The subject of this article is the heat transfer of ammonia in the channels of thermal sink evaporators. The objective was to determine a sufficiently simple correlation acceptable for engineering practice, which could be used to calculate the heat transfer in cylindrical channels of a thermal sink, designed for two-phase systems of thermal control systems of uncrewed spacecraft. For this purpose, experiments were performed on two thermal sink having an evaporator channel of ~7 mm diameter made of aluminum alloy and stainless steel, with channel surface roughness Ra 3.33 µm and 0.12 µm. The experiments were carried out with a subcooled liquid or two-phase flow at the channel inlet. The results of the experiments were compared with Kupriyanova's formula obtained under markedly different conditions: with ammonia boiling in a large volume on the external surface of 5...6 mm diameter tubes at –40 °C...+20 °C. It is shown that Kupriyanova's formula can be used on the ground and in microgravity conditions to calculate heat transfer coefficients during the developed boiling of ammonia in the range of flow parameters: saturation temperature +35 °C...+75 °C; mass velocity 27...200 kg/(sec-m²); liquid subcooling to saturation temperature at thermal sink inlet 0 °C...30 °С; mass vapor quality at the inlet 0...0.7. Difference of calculated and experimental values of heat transfer coefficients did not exceed 30 %.