Heater Size and Orientation Effect on Pool Boiling of FC-72

A recent study on pool boiling for upward facing square heaters reported two pool boiling regimes depending on the relative importance of buoyancy and surface tension forces. At higher gravity levels and/or with larger heaters when the ratio of heater size L_h (length of a side for a square heater) to capillary length L_c was greater than 2.1, boiling was buoyancy dominated and the heat transfer results were heater size independent. Boiling was surface tension dominated and heat transfer results were heater size dependent when L_h/L_c<2.1 (small heaters and/or low gravity conditions). This paper studies the effects of orientation on the balance between buoyancy and surface tension forces. The threshold value of L_h/L_c for transition between pool boiling regimes was found to be 1.8 for heaters oriented at 45°, 90°, and 135°.