Experimental Study of Nucleate Pool Boiling of FC-72 on Smooth Surface under Microgravity

Abstract

Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were studied both in short-term microgravity condition utilizing the drop tower Beijing and in normal gravity conditions. The bubble behavior and heat transfer of air-dissolved FC-72 on a small scale silicon chip (10 × 10 × 0.5 mm 3 ) were obtained at the bulk liquid subcooling of 41 K and nominal pressure of 102 kPa. The boiling heat transfer performance in low heat flux region in microgravity is similar to that in normal gravity condition, while vapor bubbles increase in size but little coalescence occurs among bubbles, and then forms a large bubble remains attached to the heater surface during the whole microgravity period. Thermocapillary convection may be an important mechanism of boiling heat transfer in this case. With further increasing in heat flux to the fully developed nucleate boiling region, the vapor bubbles number as well as their size significantly increase in microgravity. Rapid coalescence occurs among adjacent bubbles and then the coalesced large bubble can depart from the heating surface during the microgravity period. The reason of the large bubble departure is mainly attributed to the momentum effects caused by the coalescence of small bubbles with the large one. Hence, the steady-state pool boiling can still be ob- tained in microgravity. In the high heat flux regime near the critical heat flux, significant deterioration of heat transfer was observed, and a large coalesced bubble forms quickly and almost covers the whole heater surface, leading to the occurrence of the critical heat flux in microgravity condition.

Bubble dynamics in nucleate pool boiling on micro-pin-finned surfaces in microgravity

“…4 shows boiling heat transfer curves chip PF30-60 and PF50-120 under different gravity conditions at atmosphere pressure. The boiling curves of chip S under different gravity conditions [21] are also shown for comparison. By the way, the liquid subcooling for smooth surface experiment is about 41 K (corresponding to 15 C of bulk liquid temperature) which is also caused by ambient temperature.…”

Influences of wake-effects on bubble dynamics by utilizing micro-pin-finned surfaces under microgravity

“…where v b is bubble rising; a l is acceleration of rising bubble at the microgravity condition; C D is the drag coefficient, when the body is a vapor bubble of negligible density, C D can be commonly defined based on an equivalent projected area, pr 2 , as given by Xue [20], …”

“…Previous studies have noted that the dynamic behavior of bubbles is strongly influenced by many factors such as the surface geometry of heater [5][6][7][8][9][10][11][12][13][14], thermocapillary convection [15][16][17][18][19][20] non-condensate gas [16][17][18][19][20][21], and the gravity level [22][23][24][25][26][27][28], and the effects of these factors have been reported in several research literatures.…”

“…It was found that micro-pin-finned structure promoted the growth, coalescence and detachment of bubbles, and the mean temperature of micro-pin-finned surface kept almost steady or quasisteady state under microgravity conditions. Some researchers [15][16][17][18][19][20] postulated that thermocapillary or Marangoni convection, typically masked by dominant buoyancydriven convection in terrestrial conditions, plays a significant role in bubble nucleation and growth in microgravity, and therefore influences heat transfer effectiveness. These convection effects are the result of fluid motion along the vapor-liquid interface and away from the heater surface, induced by surface tension gradients.…”

“…Straub [16] suggested that non-condensable gasses caused Marangoni convection, leading to the departure of bubbles, while Aktinol's experiments [17] showed that the variation of capillary flow induced from non-condensable gas was very small and the resulting capillary flow had only a second order effect on the flow field. Moreover, Kannengieser [18] and Zhao et al [19,20] further proposed that these gasses were not the only cause of Marangoni convection, but Henry et al [21] suggested that non-condensable gases actually decreased the strength of Marangoni convection.…”