Numerical investigation of ammonia boiling heat transfer in rectangular microchannel under high pressure

Ammonia boilers are commonly used as expendable radiators during the return phase of spacecraft. The aim of this study is to investigate the bubble behavior and heat transfer characteristics within a vertical rectangular microchannel of a plate-fin ammonia boiler under gravity at an absolute pressure of 354–615 kPa. In order to achieve this, a single rectangular microchannel unit was intercepted from the ammonia boiler structure to construct a simulation model, and the high-pressure boiling process of ammonia in the rectangular microchannel was numerically investigated using the VOF model. Good agreement was obtained by comparing the numerical results with the experimental data with the error within 8 %. The results show that the wake flow at the lower end of the primary bubbles has an enhanced effect on the heat transfer, which is about 50–60 %. The maximum horizontal dimension of primary bubbles increases with increasing superheat, and significant bubble coalescence occurs when 45 % of the rectangular microchannel spacing is exceeded. In addition, the effect of different saturation temperatures on the heat transfer performance on the hot high temperature wall was investigated. Significant heat transfer deterioration was found to occur at saturation temperatures below 4 °C (superheat above 15 °C). The reason found in this study was that bubble coalescence significantly increases the percentage of gas-phase contact area on the high-temperature wall (from 25 % to 54 %) and weakens the wake enhancement effect.