Recently, the demand for cryogenic cooling technology has increased, driven by the need to improve the sensitivity, response time, and noise reduction of space telescopes. Cryogenic Loop Heat Pipes (CLHP) are effective heat transport devices for such demand. To estimate the performance of a CLHP, it is necessary to predict the flow patterns in the condenser. Flow patterns are generally estimated from flow regime maps, but they are constructed based on visualization results of room-temperature fluids. In cryogenic fluids such as nitrogen and hydrogen, there is no visualization example of condensate flow because of the difficulty of experimentation. Therefore, there are challenges in applying previous flow regime maps on pattern estimation in CLHP steady-state models. To investigate transition conditions of flow patterns and modify previous flow regime maps for cryogenic condense flow, backlight measurement of the condenser was conducted in the nitrogen-charged CLHP. The experiment was conducted in an 80K cryogenic vacuum chamber. The evaporator has a porous wick made of SUS316L with a pore radius of 1.0 ?m. The heat transport distance was 2000 mm. 300 mm length visualization condenser was installed upstream of the main condenser. The CLHP started operation at a low heat load of 2 W on the evaporator and operated steadily until 8 W. At all heat load conditions, dark films considered to be vapor-liquid interfaces were seen on top and bottom surfaces of the channel. At 2–6 W heat load, the bottom dark film was thicker than the top. Although the flow patterns were estimated to be stratified at 2 W and wavy at 4 – 8 W from the steady-state model calculation, wavy flow was not observed. For the next experiment, it is planned to add another condenser upstream of the visualization condenser to understand the overall picture of flow pattern transitions.
Gomi et al. (Sun,) studied this question.