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

The influences of wake on bubble dynamics under various heat fluxes have been studied in microgravity. Nucleate pool boiling experiments of FC-72 on silicon chips fabricated with micro-pin-fins were conducted in 10(-3) gravitational acceleration for 3.6 s. Experimental results indicated that wake field had little influence on bubble dynamics at low heat flux, q = 12.53 W/cm(2), but its effects on bubble dynamics became very great at moderate heat flux, q = 27.89 W/cm(2), and high heat flux, q = 39.54 W/cm(2). More importantly, wake-effects appeared even more significant in microgravity. In the wake field, the horizontal flow of the liquid on micro-pin-finned silicon chips promoted the collision, coalescence and movement of bubbles, and the vertical flow of liquid phase exerted an upward force on bubbles, which can effectively shorten the growth cycle and decrease the departure radius of bubbles. Furthermore, fresh liquid could easily be inhaled into the micro-pin-finned structure owning to the negative pressure in wake region, which can supply sufficient liquid for the growth of bubble, avoiding film boiling. The interaction between the micro-pin-finned structure and the wake effect promoted the process of bubble coalescence and departure effectively, so the process of heat transfer was significantly improved on micro-pin-finned surface. In addition, the flow field and bubble behavior in wake region were also briefly analyzed based on some reasonable simplifications and hypotheses. The theoretical analyses showed that the durations of wake-effects lasted longer than the time periods of the bubble in next generation, and the bubble diameters were also smaller than the thickness of Wake region. Therefore, the dominant bubble of next generation was influenced by the wake field during its ebullition cycle, which is consistent with the experimental phenomenon. (C) 2016 Elsevier Ltd. All rights reserved