Boiling two-phase flow under microgravity

Abstract

To study the effects of reduced gravity on the flow regime and the heat transfer characteristics of a boiling two-phase horizontal flow, parabolic flight experiments were performed by using an aircraft. The gravity level during the parabolic flight and the duration time were about −0.01 g e ∼ +0.01 g e and 20 s, respectively. Under earth gravity, many small bubbles are detached very frequently from the heater rod surface, flowing into the upper stream due to the buoyancy 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 become much larger along the heater rod, surrounding the heater rod in the downstream. This tendency was more noticeable in the cases of lower inlet fluid velocity, higher heat flux and lower inlet fluid subcooling. The local heat transfer coefficients at the bottom of the heater rod tend to decrease slightly under microgravity compared with those under earth gravity because of the reduction of the heat removal due to natural convection. On the other hand, the local heat transfer coefficients at the top of the heater rod tend to increase slightly under microgravity. However, the differences of the local heat transfer coefficients are very small in spite of large differences of the flow regimes under earth gravity and microgravity.