Boiling natural convection

Monde, M., H. Kusuda, and H. Uehara, 1982, Critical Heat Flux during Natural Convective Boiling in Vertical Rectangular Channels Submerged in Saturated Liquid, Trans. ASME, J. Heal Transfer 704 300-303. (5)... 

Natural Convection Boiling (to Point A on the Boiling Curve)... 

Stephan, K. Bubble formation and heat transfer in natural convection boiling. In Hahne, E. Grigull, U. Heat transfer in boiling. Washington Hemisphere 1977, p. 3-20... 

Stephan, K. Abdelsalam, M. Heat transfer correlations for natural convection boiling. Int. J. Heat Mass Transfer 23 (1980) 73-87... 

K. Stephan and J. Mitrovic, Heat Transfer in Natural Convective Boiling of Refrigerants and Refrigerant-Oil-Mixtures in Bundles of T-Shaped Finned Tubes, in Advances in Enhanced Heat Transfer—1981, R. L. Webb, T. C. Carnavos, E. L. Park Jr., and K. M. Hostetler eds., HTD vol. 18, pp. 131-146, ASME, New York, 1981. 

K. Stephan and P. Preusser, Heat Transfer in Natural Convection Boiling of Polynary Mixtures, in Proc. 6th Int. Heat Transfer Conference, Toronto, Canada, vol. 1, p. 187,1978. 

Natural convective boiling Nucleate boiling Transition boiling... 

The lower Emit of applicability of the nucleate-boiling equations is from 0.1 to 0.2 of the maximum limit and depends upon the magnitude of natural-convection heat transfer for the liquid. The best method of determining the lower limit is to plot two curves one of h versus At for natural convection, the other ofh versus At for nucleate boiling. The intersection of these two cui ves may be considered the lower limit of apphcability of the equations. 

Natural convection is self-induced and is created by the density differences, which are temperature related the boiling of water in a kettle is an example of free convection. Forced convection is caused by an external force being applied by mechanical means such as a fan or pump the cooling of a warm bottle in cool flowing water is an example of forced convection. 

The conclusion to be drawn from the above examples and many others is that softness in a boiling system, preceding the boiling channel inlet, may cause flow oscillations of low frequency. It is probably the pressure perturbations arising from the explosive nature of nucleate boiling that initiates the oscillation, and the reduced burn-out flux which follows probably corresponds to the trough of the flow oscillation, as a reduction in flow rate always drops the burn-out flux in forced-convection boiling. 

Figure 2.42 shows boiling curves obtained in an annular channel with length 24 mm and different gap size (Bond numbers). The heat flux q is plotted versus the wall excess temperature AT = 7w — 7s (the natural convection data are not shown). The horizontal arrows indicate the critical heat flux. In these experiments we did not observe any signs of hysteresis. The wall excess temperature was reduced as the Bond number (gap size) decreased. One can see that the bubbles grew in the narrow channel, and the liquid layer between the wall and the base of the bubble was enlarged. It facilitates evaporation and increases latent heat transfer. 

Confined boiling of water and surfactant solutions under condition of natural convection causes a heat transfer enhancement. Additive of surfactant leads to enhancement of heat transfer compared to water boiling in the same gap size however, this effect decreased with decreasing gap size. For the same gap size, CHF decreases with an increase in the channel length. CHF in surfactant solutions is significantly lower than in water. 

The mechanism of heat transfer from a submerged surface to a pool of liquid depends on the temperature difference between the heated surface and the liquid Figure 12.54. At low-temperature differences, when the liquid is below its boiling point, heat is transferred by natural convection. As the surface temperature is raised incipient boiling occurs, vapour... 

Figure 1.1 Pool boiling regimes A-B, natural convection B-C, nucleate boiling C-D, partial film boiling D-E, stable film boiling.

Similarly, in Figure 2.21, NR is plotted versus AT for boiling -pentate, benzene, and ethyl alcohol on a flat chromium surface (Cichelli and Bonilla, 1945). The value for m in all three cases was chosen to be 3. In the cases considered, the measured q" from experimental data was interpreted as "oil, neglecting the natural-convection contributions because the flux levels in the experiments were... 

Figure 2.24 Comparison of experimental results of bubble period with predictions of a model involving different mechanisms (a) nucleate boiling only (b) nucleate boiling and natural convection (c) nucleate boiling, natural convection, and microlayer evaporation. (From Judd, 1989. Copyright 1989 by American Society of Mechanical Engineers, New York. Reprinted with permission.)...

A boiling heat transfer model incorporating nucleate boiling, natural convection, and microlayer evaporation was formulated as... 

Equation (2-113) means that any cylindrical cavity for any liquid-solid combination under a given pressure has a minimum heat flux below which boiling will not be stable, and a transition between natural convection and stable nucleate boiling (bumping) is always observed. 

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