BOILING NUCLEATION ON A VERY SMALL FILM HEATER SUBJECTED TO EXTREMELYRAPID HEATING - EFFECT OF AMBIENT-PRESSURE ON BUBBLE FORMATION BY FLUCTUATION NUCLEATION

The effect of ambient pressure on the dynamics of bubble formation in liquid heated rapidly to the limit of superheat is investigated by hea ting a fine film heater immersed in ethyl alcohol at a high rate of te mperature rise up to 20 x 10(6) K/s. The heater surface temperature at boiling incipience increases with the increase of the rate of tempera ture rise and saturates at the rate higher than about 5 x 10(6) K/s. T he saturated temperature agrees well with that of homogeneous nucleati on in an ambient pressure range from 0.1 to 2.0 MPa. The nucleated bub bles become smaller to coalesce with the increase of ambient pressure and the measured number density of them reaches 7.0 x 10(10) l/m(2) fo r the maximum. The increase of the number density versus the surface t emperature agrees qualitatively with that predicted by the fluctuation nucleation theory. As the result, the bubble formation observed at a higher rate of temperature rise is concluded to be due to fluctuation nucleation.