A study of vertical turbulent film boiling and interfacial effects



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Theoretical and experimental studies were made of vertical, turbulent film boiling. The physical process is characterized by vapor moving within a thin, continuous vapor film next to a heated wall under the influence of buoyancy and pressure forces. Liquid in the bulk phase is separated from the heater wall by the vapor layer, but interfacial waves strongly influence the momentum and energy transport processes. It was shown that this complex process could be modeled mathematically by a two step analysis. First, a quasi-steady state model led to predictions of the heat transfer rates at any instant based on the velocity and temperature profiles that exist at that instant. The results of the quasi-steady state solution were then applied to the unsteady state condition, which is characterized by interfacial waves and a continually changing vapor film thickness, to determine the heat transfer rates at each instant of time. These results were averaged over time and space to give local and overall heat transfer rates expressed in dimensionless form. The analysis considers the effects of both the interfacial velocity and the interfacial roughness. The theory correlated data taken from systems where large waves were encountered and from relatively wave-free systems. Experimental measurements were made of the instantaneous, local surface heat flux by means of a low mass, surface thermocouple and a heat meter. Interfacial waves were found to be so large under some conditions as to cause occasional liquid-solid contact. Spectral analysis showed that the surface heat flux variations were random and caused by interfacial activity, and that the spectrum of these variations covered a broad frequency range from about 10 to 100 cps.