Vaporization subcooled boiling

When the boiling crisis occurs, the surface temperature rises. Because of the fairly good transfer coefficient of a fast-moving vapor core in an annular flow, the wall temperature rise after a dryout in the high-quality region is usually smaller than that in a subcooled boiling crisis. It is even possible to establish steady-state conditions at moderate wall temperatures, so that physical burnout may not occur immediately. Thus dryout is also described as slow burnout. 

Ivey, H. J., and D. J. Morris, 1962, On the Relevance of the Vapor Liquid Exchange Mechanism for Subcooled Boiling Heat Transfer at Higher Pressure, Rep. AEEW-R-137, UK Atomic Energy Authority, Winfrith, England. (2)... 

Levy, S., 1966, Forced Convection Subcooled Boiling—Prediction of Vapor Volumetric Fraction, Rep. 

Staub, F. W., 1967, The Void Fraction in Subcooled Boiling—Prediction of the Initial Point of Net Vapor Generation, ASME Paper 67-HT-36, National Heat Transfer Conf., ASME, New York. (3) Staub, F. W., 1969, Two Phase Fluid Modeling, The Critical Heat Flux, Nuclear Sci. Eng. 3J.T 90-199. (5)... 

Vandervort, C. L., A. E. Bergles, and M. K. Jensen, 1994, An Experimental Study of CHF in Very High Heat Flux Subcooled Boiling, Int. J. Heat Mass Transfer 37(suppl.) 161-173. (5) Vanderwater, R. G., 1956, An Analysis of Burnout in Two Phase, Liquid Vapor Flow, Ph.D. thesis, University of Minnesota, Minneapolis, MN. (5)... 

The rate of bubble collapse Rcl is primarily important in the first transition zone where the bulk liquid is subcooled. A number of studies have been published on subcooled boiling as well as the prediction of the point of net vapor generation, characteristics defining Transition Zone I, and the onset of nucleation. These studies all result in empirical correlations, and have not led to quantitative conclusions which can be generalized. The radial velocity... 

Typical flow pattern for flow boiling are shown in Fig. 5. For subcooled boiling and high liquid flow rate, the observed bubble detachment size is smaller than the gap size. The treatment of the data showed that correlation [26] obtained for a large tube flow boiling can be applied to predict subcooled flow boiling heat transfer in a confined space. For saturated flow, the vapor bubbles have a tendency to merge and produce the... 

Saha, P. and N. Zuber. 1974. Point of net vapor generation and vapor void fraction in subcooled boiling. Proceedings of the 5th International Heat Transfer Conference, Tokyo, Japan, pp. 175-179. 

It is important to note that arising naturally from the analysis, the instability region is bounded by the two roots of Equation (8), where the pressure drop versus flow rate is a minimum, maximum or point of inflection. The region of instability for a parallel channel system as bounded by these roots, which correspond to low (subcooled boiling) and high (saturated boiling) vapor qualities. This result has been rederived and confirmed by (Babelli et al, 1995), who retained the vapor to liquid density ratio, and the convective acceleration term, to derive... 

McCabe-Thie/e Example. Assume a binary system E—H that has ideal vapor—Hquid equiHbria and a relative volatiHty of 2.0. The feed is 100 mol of = 0.6 the required distillate is x = 0.95, and the bottoms x = 0.05, with the compositions identified and the lighter component E. The feed is at the boiling point. To calculate the minimum reflux ratio, the minimum number of theoretical stages, the operating reflux ratio, and the number of theoretical stages, assume the operating reflux ratio is 1.5 times the minimum reflux ratio and there is no subcooling of the reflux stream, then ... 

Regarding two pliase flow, pressurized liquid above its noniial boiling point will start to flash when released to aUiiospheric pressure, and two pliase flow will result. Two-pliase flow is also likely to occur from depressurization of tlie vapor space above a volatile liquid, especially if the liquid is viscous (e.g., greater tlian 500 cP) or has a tendency to foam. Fauske and Epstein liave provided tlie following practical calculation guidelines for two-phase flashing flows. The discharge of subcooled or saturated liquids is described by... 

Figure 8 shows that increasing the heat flux at constant mass velocity causes the peak in wall temperature to increase and to move towards lower enthalpy or steam quality values. The increase in peak temperature is thus due not only to a higher heat flux, which demands a higher temperature difference across the vapor film at the wall, but to a lower flow velocity in the tube as the peaks move into regions of reduced quality. The latter effect of lower flow velocity is probably the dominant factor in giving fast burn-out its characteristically rapid and often destructive temperature rise, for, as stated earlier, fast burn-out is usually observed at conditions of subcooled or low quality boiling. 

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