To clarify the reason of inconsistent trends in the existing heat transfer data in microgravity nucleate boiling, the method to acquire the local heat transfer data and to observe in detail the bubble behavior from underneath was developed andintroduced in the experiment by the use of the NASDA TR-1A ballistic rocket. The pool boiling experiment was conducted using ethanol as test liquid at an almost constant temperature of 25 to 30°C. The system pressure was varied from 0.01 to 0.48 MPa withcorresponding liquid subcooling from 3 to 95 K. The steady state heat transfer is realized in microgravity at the conditions of moderate and high subcooling where the bubble size is determined so that the rate of evaporation of microlayer balances that of condensation at the bubble surface. On the other hand, the transition to burnout is unavoidable even at low heat flux when the liquid of low subcooling is concerned. The nucleate boiling heat transfer is dominated by the isolated or primary bubbles justgenerated on the heat transfer surface even in the high heat flux or the low liquid subcooling region where these bubbles lift the large coalesced one covering the entire heating surface. Two opposite trends of the heat transfer enhancement anddeterioration are possible depending on the behavior of dry patches extending in the microlayer underneath the bubbles contacting directly with the surface.
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