| The present work provides experimental data and flow visualization results for forced-convection boiling over a gold-film flat surface heater in microgravity. Specifically, the nucleate boiling curve, heater surface temperatures, heat transfer coefficient, and heat fluxes for various forced-flow conditions are presented. A flow boiling regime map based on photographic results of bubble sizes and patterns is developed to facilitate physical understanding. Second, a thin gold-film semi-transparent heater is designed and used to generate a single bubble. Bubble generation, growth, and departure in microgravity with different flow rates were observed with a CCD camera. It was observed that a high flow rate would offset the microgravity effects. As a result, the bubble generation frequency, Weber number, and bubble shape tend to be similar to that in normal gravity. Critical heat flux (CHF) and the transition from nucleate boiling to film boiling during forced flow is another important aspect in the present research. Predicting the CHF is significant in heat transfer design because film boiling often leads to catastrophic increases in the heater temperature. In particular, visualization of bubble generation on a platinum wire heater is provided by the CCD camera. A numerical study is carried out to investigate the transient conjugated heat transfer for high-specific-heat material in single phase and two-phase flow under both normal gravity and microgravity. The transient temperature distribution along the heater surface was calculated according to an energy balance. Experiments were conducted to test the validity of the numerical model. The influence of gravity on the process was studied, and it is observed that the transient forced convection boiling heat transfer process in microgravity is slightly shorter than that in normal gravity. |
