This paper presents experimental investigations of cryogenic nitrogen two-phase flow in horizontal transparent tubes (diameters of 11.1mm) under terrestrial and micro-gravity (10−4g) conditions during the chilldown process, and the focus is on the film boiling region. Constant mass flow rate is achieved by a motor driven bellows, and three different mass fluxes from 9.2 to 27.6kg/m2 · s are tested in the experiments. A drop tower is applied to simulate the micro-gravity environment. During the chilldown process, we measure the time-dependent temperatures at three circumferential locations at different downstream locations. Video images are recorded for identifying the flow patterns. The experiments show that under normal gravity, the flow pattern change from dispersed flow to inverted annular flow and then to unsteady stratified flow according to different wall temperatures, the temperature differences between the lower and upper part of the test section increase with increasing flow rate. Under microgravity, when the temperature is high, the liquid chunks trend to be lifted up and confined mainly in the central core of the tube; when the temperature is low, the liquid chunks are more evenly dispersed inside the whole tube, and some touch the upper wall. It is also found that the measured wall temperatures drop more quickly under microgravity condition compared with that under normal gravity. Moreover, under microgravity condition, the measured temperatures drop more quickly with lower wall temperature. The gravity effect on the quenching curves is alleviated with increasing mass flow rate. Thus gravity effect is more important in low mass flow rate two-phase flow.

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