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Heat Transfer & Transport Phenomena in Microscale

ISBN Druckformat: 1-56700-150-5

BOILING NUCLEATION, BUBBLE DYNAMICS AND HEAT TRANSFER ON A MICRO FILM SURFACE HEATED AT AN EXTREMELY HIGH RATE

Abstrakt

Boiling nucleation, bubble dynamics and heat transfer on a micro platinum film surface immersed in liquid and heated pulsewise are investigated at a rate of wall temperature rise up to 93×106 K/s. The heater temperature at boiling incipience saturates at a higher rate of temperature rise. The saturated value for ethyl alcohol agrees with the homogeneous nucleation temperature rather than that of the thermodynamic limit, for a system pressure range from 0.1 to 2.0 MPa, whereas it is approximately 19 К lower for water at atmospheric pressure. Concurrent generation of a large number of fine bubbles with uniform size is observed on the heater, which the authors have named 'caviarwise bubble generation'. The number of bubbles increases with the increase in heater temperature in a similar way to that calculated by the homogeneous nucleation theory. Fine bubbles generated due to spontaneous nucleation grow to coalesce into a single bubble on the heater surface and the heat transfer rate from the surface decreases markedly. The coalesced bubble contracts after growth and the heat transfer rate increases abruptly during the collapse. Evaporation by excess energy stored in the super heated liquid layer before nucleation is shown to be the dominant process rather than heat transfer from the film surface to the fluid after nucleation, for growth of the coalesced bubble.
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