Mechanism for nucleation jet enhancement of nucleate pool boiling

The very high heat transfer rates in nucleate pool boiling are due to the rapid vapor bubble formation and departure from the heated surface. This paper presents observations and analyses of high-frequency bubble departures from very thin wires. The analysis shows that the bubbles entrained in experimentally observed nucleation jets were already at the stage of heat transfer-controlled growth when they were rapidly ejected from the surface. The momentum in the expanding liquid around the bubble, perhaps augmented by natural convection from the heated surface, most likely pulls the bubbles from the surface into the jet flow. The jet flows themselves are shown to be not due to the rising of the bubbles in the liquid because the bubbles are very small so their buoyancy was much less than the drag at the observed velocities of 120 mm/s. The jet streams were postulated to be due to the bubble growth in a relatively large cavity (similar in size to the observed bubble diameter of about 6 gm first observed about 0.2 mm above the surface) that channeled the liquid flow driven by the bubble growth in the vertical direction. The pulsed jet flows caused by the rapid bubble growth and frequent departures increase the convection heat transfer inside the cavities in addition to increasing the convection along the surface outside the cavities due to entrainment of additional liquid into the jets that increases the heat transfer along the surface.