Critical heat flux, mechanism

Mechanisms of CHF in Forced Convective Boiling in Channels. Detailed reviews of critical heat flux mechanisms in forced convective boiling are given by Hewitt [291], Tong and Tang [5], Collier and Thome [3], and Katto [101]. The more commonly accepted mechanisms for the occurrence of critical heat flux in forced convection are as follows ... 

FIGURE 15.122 Tentative map of regimes of operation of various forced convective critical heat flux mechanisms (from Hewitt and Semaria [300], with permission from Taylor Francis, Washington, DC. All rights reserved). 

Y. Katto, Critical Heat Flux Mechanisms, in Convective Flow Boiling, J. C. Chen ed., pp. 29-44, Taylor Francis, Washington, DC, 1996. 

C. Unal, P. Sadasivan, and R. M. Nelson, On the Hot-Spot Controlled Critical Heat Flux Mechanism in Pool Boiling of Saturated Fluids, in Pool and External Flow Boiling, V. K. Dhir and A. E. Bergles eds., pp. 193-201, ASME, New York, 1992. 

The heat transfer mechanism of a vapor-liquid mixture in which the critical heat flux has been exceeded can be classified as partial or stable film boiling. The differ-... 

Dean, R A., R. S. Dougall, and L. S. Tong, 1971, Effect of Vapor Injection on Critical Heat Flux in a Subcooled R-l 13 (Freon) Flow, Proc. Int. Symp. on Two-Phase Flow Systems, Haifa, Israel. (6) Deane, C. W., and W. M. Rohsenow, 1969, Mechanism and Behavior of Nucleate Boiling Heat Transfer to the Alkali Liquid Metals, USAEC Rep. DSR 76303-65, Massachusetts Institute of Technology, Cambridge, MA Also in 1970, Liquid Metal Heat Transfer and Fluid Dynamics J. C. Chen and A. A. Bishop, Eds., ASME Winter Annual Meeting, New York. (4)... 

Oberjohn, W. J., and R. H. Wilson, 1966, The Effect of Non-uniform Axial Flux Shape on the Critical Heat Flux, ASME Paper 66-WA/HT-60, Winter Annual Meeting, ASME, New York. (5) Ogasawara, H. et al., 1973, Cooling Mechanism ofthe Low Pressure Coolant-Injection System of BWR and Other Studies on the Loss-of-Coolant-Accident Phenomena, ANS Topical Meeting Water Reactor Safety, p. 351, Salt Lake City, UT. (4)... 

Macbeth (M5) has recently written a detailed review on the subject of burn-out. The review contains a number of correlations for predicting the maximum heat flux before burn-out occurs. These correlations include a dependence upon the tube geometry, the fluid being heated, the liquid velocity, and numerous other properties, as well as the method of heating. Sil-vestri (S6) has reviewed the fluid mechanics and heat transfer of two-phase annular dispersed flows with particular emphasis on the critical heat flux that leads to burn-out. Silvestri has stated that phenomena responsible for burn-out, due to the formation of a vapor film between the wall and the liquid, are believed to be substantially different from phenomena causing burn-out due to the formation of dry spots that produce the liquid-deficient heat transfer region. It is known that the value of the liquid holdup at which dry spots first appear is dependent on the heat flux qmi. The correlations presented by Silvestri and Macbeth (S6, M5) can be used to estimate the burn-out conditions. 

The results given in Fig. 9.20 show that for very small values of Bo (Bo < 0.01), the surface temperature also increases monotonically with the heat flux. This supports the theory of a heat removal mechanism that does not change, unlike that observed in the pool boiling in plain media. For the system shown in Fig. 9.20 and for d = 1.1 mm, the transition from the surface-wetted condition to the formation of a vapor film occurs at the critical heat flux... 

V. I. Gomelauri and T. S. Magrakvelidze, Mechanism of Influence of Two Dimensional Artificial Roughness on Critical Heat Flux in Subcooled Water Flow, Therm. Eng. (25/2) 1-3,1978. 

R. A. Pabisz Jr. and A. E. Bergles, Using Pressure Drop to Predict the Critical Heat Flux in Multiple Tube, Subcooled Boiling Systems, Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, vol. 2, pp. 851-858, Edizioni ETS, Pisa, Italy, 1997. 

Mechanisms of CHF in Fool Boiling. The mechanism of the critical heat flux phenomenon in pool boiling has been the subject of widespread interest and controversy. Recent reviews relating to mechanisms are presented by Katto [157], Dhir [87], and Bergles [158]. The postulated mechanisms can be approximately classified into four types as follows ... 

Hydrodynamic instability mechanism. Here, instabilities occur in the vapor-liquid interfaces leading to the breakdown of the vapor release mechanisms and to vapor accumulation at the surface leading to critical heat flux. 

Eventually, the velocity of vapor in the jets becomes so large that the jets themselves become unstable near the interface as a result of Helmholtz instability (of wavelength XH) as shown in Fig. 15.63). The breakup of the jets destroys the efficient vapor-removal mechanism, increases vapor accumulation at the interface, and leads to liquid starvation at the surface and to the critical heat flux phenomenon. If jet breakup occurs at a vapor velocity UH within the jets, the critical heat flux q"m is given by... 

The region in which the mechanisms of critical heat flux are similar to those for pool boiling (region 1 in Fig. 15.86) is bounded by xa and by a transition quality x, to an intermediate region (region 2 in Fig. 15.86), with xt given by... 

Boundary layer separation models. In this class of model, the critical heat flux phenomenon is considered to be analogous to the phenomenon of boundary layer separation from a permeable plate through which gas is flowed in a direction normal to the flow over the plate. This mechanism was initially suggested by Kutateladze and Leontiev [312] and was further developed by Tong [313] and others and more recently by Celata et al. [314], This method of prediction leads to an equation of the form... 

In general, the processes modeled in the prediction methods for subcooled and low-quality critical heat flux mentioned above are clearly important ones there seems scope for fundamental work on the precise mechanisms involved in the near-wall region. 

A. Miyra, A. Marroquin, and H. Auracher, Critical Heat Flux and Minimum Heat Flux of Film Boiling of Mixtures in Forced Convection Boiling, in Proc. 4th World Conference on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, Brussels, Belgium, vol. 2, pp. 873-880,1997. 

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