BOILING 2-PHASE FLOW UNDER MICROGRAVITY

To study the effects of reduced gravity on the flow regime and the hea t transfer characteristics of a boiling two-phase horizontal flow, par abolic flight experiments were performed by using an aircraft. The gra vity level during the parabolic flight and the duration time were abou t - 0.01 g(e) approximately + 0.01 g(e) and 20 s, respectively. Under earth gravity, many small bubbles are detached very frequently from th e heater rod surface, flowing into the upper stream due to the buoyanc y and resulting in a stratified flow in the cases of lower inlet fluid velocity and higher heat flux. Under microgravity conditions, bubbles are hardly detached from the heater rod, growing and coalescing to be come much larger along the heater rod, surrounding the heater rod in t he downstream. This tendency was more noticeable in the cases of lower inlet fluid velocity, higher heat flux and lower inlet fluid subcooli ng. The local heat transfer coefficients at the bottom of the heater r od tend to decrease slightly under microgravity compared with those un der earth gravity because of the reduction of the heat removal due to natural convection. On the other hand, the local heat transfer coeffic ients at the top of the heater rod tend to increase slightly under mic rogravity. However, the differences of the local heat transfer coeffic ients are very small in spite of large differences of the flow regimes under earth gravity and microgravity.