Graduate Thesis Or Dissertation
 

The study of active sites and bubble formation frequency in nucleate boiling heat transfer

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  • The heat-transfer mechanism studied in this experiment was pool boiling from a flat horizontal surface facing upward with the bulk liquid at its saturation temperature. The hot-film anemometer probe was introduced to boiling heat-transfer analysis as a possible device for counting active nucleation sites and for other investigations such as measuring bubble frequency. Although the existing hot-film probe had a high sensitivity to bubble motion, the strong agitation from the bubbles at the higher heat fluxes stripped the thin sensing film from the glass probe surface, indicating that some improvement on the hot-film probe was needed for it to be useful in boiling heat transfer analysis. The largest heat flux examined was 113,300 B.t.u./(hr.)(sq. ft.) at which the measured density of active sites was 7,420/sq. ft. The correlation of active sites with the heat flux was represented by where q= 12.5 ([superscript n]/[subscript A]) where q = heat flux, B.t.u./(hr.)(sq. ft); n = number of active sites; A = heating surface area, sq. ft. The statistical analysis of the three sets of data at heat fluxes of 45,800, 73,700 and 113,300 B.t.u./(hr. )(sq. ft.) showed that, at these heat-flux levels, the distribution of active sites followed the Poisson distribution within a five percent level of significance. It was concluded that the hot-film probe was not an ideal device to measure bubble formation frequency because of the inability to detect bubble boundaries.
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