We investigated experimentally the start-up characteristics of a mechanically pumped two-phase loop (MPTCL), with CO2 as working fluid, and a single evaporator that consists of a bent inner ring and an outer ring constructed by stainless tubes with hydraulic diameter of 2.6 mm and length of 9 m, along which totally 54 pieces of heating element are distributed. Experiments were performed in the following conditions: mass flow rates of 1.1, 2.1, and 3.3g/s; heat loads ranged from 50 to 300W, with the heat-load ratios of the inner ring to the outer ring 2.2:1, 1:1, and 1:2.2 at the operational temperature of −15°C, respectively. During the start-up cases, we detected a reverse flow accompanying with pressure spike, which can be understood as explosive boiling, and a subsequent temporal dry-out phenomenon at the outlet of the evaporator, as a result of explosive boiling. The back flow together with the pressure spike is helpful to set up a two-phase flow all along the evaporator, though it may have negative effect on the loop, especially, when coincident explosive boiling happens. However, such a pressure spike that depends on initial superheating should be controlled to avoid possible harm to the loop.

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