Film dryout

The CHF in high-quality annular flow uses a liquid film dryout model, i.e., the concept of liquid film dryout (zero film flow rate) in annular flow. Sugawara (1990) reported an analytical prediction of CHF using FIDAS computer code based on a three-fluid and film dryout model. 

Sudo, Y., T. Usui, and M. Kaminaga, 1991, Experimental Study of Falling Water Limitation under a Counter-current Flow in a Vertical Rectangular Channel, JSMEInt. J. Ser. H 34 169-174. (5) Sugawara, S., 1990, Analytical Prediction of CHF by FIDAS Code Based on Three-Fluid and Film-Dryout Model, J. Nuclear Sci. Technol. 27. 12 29. (5)... 

Revellin, R., Haberschill, P., Bonjour, J. and Thome, J.R., Conditions of Liquid Film Dryout during Saturated Flow Boiling in Microchannels, Chem. Engng. 5d., 63, 5795-5801,(2009). 

Jensen and Tang suggest that the annular mist region (where the critical heat flux is governed by film dryout) occurs for qualities greater than xa given by... 

CHF is also referred as boiling crisis and burnout. DNB is generally used to describe the CHF condition in pool boiling and also in flow boiling where rapid bubble formation causes vapor blanket on the heater surface. Thus, CFIF at low quality is referred as DNB such as in PWR core. Dryout is used to describe liquid film dryout in annular flow. At high flow quality, dryout is expected with increase in heat flux such as in BWR core. 

A physical model of ONB for the explosive boiling and dryout, was suggested. In order to understand why dryout occurred even at a low value of vapor quality x, it is important to keep in mind that the liquid film does not cover the entire heated surface of the micro-channel, and two-phase flow is characterized by an unsteady cyclic behavior. The following assumptions are made in the development of the model ... 

The basic mechanism of dryout almost invariably involves the rupture of a residual thin liquid film, either as a microlayer underneath the bubbles or as a thin annular layer in a high-quality burnout scenario. Bankoff (1994), in his brief review of significant progress in understanding the behavior of such thin films, discussed some significant questions that still remain to be answered. 

The primary mode of heat transfer at the wall is forced convection of the vapor phase. As the liquid does not wet the heating surface during film boiling, heat transfer due to drop-wall collisions is relatively small, resulting in low wall-drop heat transfer (only a few percent of the total heat input). Most of the droplet evaporation occurs because of vapor-drop heat transfer. Just after dryout, the... 

Drop deposition onto the liquid film downstream reduces the number of liquid drops formed at each axial position that remain entrained until dryout. 

In saturated annular flow boiling, on the other hand, a liquid film annulus normally covers the heating surface and acts as a cooling medium. Thinning of the liquid film annulus, therefore, indicates approaching dryout. The behavior of bubble layers and liquid annuli are of interest to visual observers. 

Figure 5.10 Dryout of liquid-film thickness. (From Hewitt, 1970. Copyright 1970 by Hemisphere Publishing Corp., New York. Reprinted with permission.)...

Since the liquid film thickness is zero at dryout, Bennett et al. (1967a) suggested that the mass balance on the wall for a small length increment, AZ, upstream of the dryout, be in the form... 

Levy, Healzer, and Abdollahian (1980) predicted the dryout flux in vertical pipes by a semiempirical adiabatic model (Levy and Healzer, 1980) for liquid film flow and entrainment. It starts with a heat balance along the flow direction and a mass balance perpendicular to the flow direction ... 

For a very thin liquid film, the value of 3 cannot be evaluated, and it should be replaced by a new parameter 3, using the generalized turbulent boundary-layer profile in an adiabatic flow as in Reference (Levy and Healzer, 1980). GF can be solved stepwise along the pipe until the G value goes to zero, where dryout occurs. This analysis was performed to compare the calculated g"rit with Wurtz data (Wurtz, 1978) and also to compare with the predictions by the well-known Biasi et al. correlation (1968), as shown in Figure 5.90. For the limited data points compared, the agreement was good. 

Hill, W. S., and W. M. Rohsenow, 1982, Dryout Drop Distribution and Dispersed Flow Film Boiling, MIT Heat Transfer Lab. Rep. 85694-105, Massachusetts Institute of Technology, Cambridge, MA. (4)... 

Fig. 4.54 Types of boiling crisis, a film boiling b dryout...

FIGURE 15.138 Limiting cases for postdryout heat transfer in the dispersed flow film-boiling region (from Hewitt et al. [13], with permission. Copyright CRC Press, Boca Raton, FL). (ze0 is the point at which dryout occurs and Zcq is the point at which the equilibrium quality reaches unity.)... 

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