Leidenfrost temperature

As was shown before, the Leidenfrost temperature is the second transformation of heat transfer mechanisms. Empirical correlations have been established by film boiling data obtained from water at high pressure levels. For a wide range of steam-water mixture velocities, the correlation for hFB reported by Bishop et al. (1965), as shown in Eq. (4-37), is recommended for use in design. 

Top spray systems During top-spray cooling of an overheated core, the wall temperature is usually higher than the Leidenfrost temperature, which causes water to be sputtered away from the wall by violent vapor formation and then pushed upward by the chimney effect of the steam flow generated at lower elevations (as shown in Fig. 4.25). A spray-cooling heat transfer test with BWR bundles was reported by Riedle et al. (1976). They found the dryout heat flux to be a function of spray rate and system pressure. The collapsed level required to keep the bundle at saturation for various pressures compared reasonably well with that in the literature (Duncan and Leonard, 1971 Ogasawara et al., 1973). 

Considering a surface temperature which is higher than the Leidenfrost temperature of the liquid in this study, it is assumed that there exists a microscale vapor layer which prevents a direct contact of the droplet and the surface. Similar to Fujimoto and Hatta (1996), the no-slip boundary condition is adopted at the solid surface during the droplet-spreading process and the free-slip... 

Fig. 15 shows the detailed structure of the droplet from a viewing angle of 60°. Experimental images show that a hole is formed in the center of the droplet for a short time period (3.4 4.8 ms) and the center of the liquid droplet is a dry circular area. The simulation also shows this hole structure although a minor variation exists over the experimental images. As the temperature of the surface is above the Leidenfrost temperature of the liquid, the vapor layer between the droplet and the surface diminishes the liquid-solid contact and thus yields a low surface-friction effect on the outwardly spreading liquid flow. When the droplet periphery starts to retreat due to the surface-tension effect, the liquid in the droplet center still flows outward driven by the inertia, which leads to the formation of the hole structure. 

In a recent review, Klimenko and Snytin [122] have shown that, for a stationary surface, the Leidenfrost temperature depends strongly on the material and coolant property combinations however, experiments involving extensive combinations of solids and coolants have not been performed, and minimum heat fluxes and Leidenfrost temperatures have not been measured when the surface is in motion. Experimental data [123,124] show that the Leidenfrost temperature depends not only on the water subcooling but also on the fluid motion and the thermophysical properties through the thermal admittance, a = Vfcpc. The results demonstrate conclusively the importance of the material properties and fluid motion on the Leiden-... 

The Leidenfrost effect describes the hovering of a droplet above a hot surface. In the case of release agents for aluminum demolding, the emulsion droplet may not touch the metallic mold surface because of the rapid evaporation of the water contained in the droplet. This phenomenon occurs above a certain temperature, called the Leidenfrost temperature. The formulations are optimized for a maximum Leidenfrost temperature, so that the product efficiency is maximized even at elevated temperatures. 

Most concern the use of specific surfactants or surfactant mixtures that permit the lowering of the Leidenfrost temperature [2]. Others concern the increase of the oil viscosity contained in the emulsion [3 - 5] and/or the use of specific organic waxes [6]. Another approach concerns the thickening of the emulsion or the increase of its elongationai viscosity [7]. 

It should be realized that the Leidenfrost superheat, A7 LDF = (TLDF - Tsat), is a function not only of pressure but also of droplet size, flow conditions, and force fields. Furthermore, experimental results obtained by Berger (Drew Mueller, 1937) for stagnant ether droplets falling on a horizontal, heated surface indicated a possible effect of surface material and roughness, as the minimum surface temperature necessary for the spheroidal state changes from 226°F (108°C) on a smooth surface of zinc to 240°F (116°C) on that of a rough surface, and from 260°F (127°C) on a smooth surface of iron to 284°F (140°C) on that of a rough surface. 

Leidenfrost, W. 1959. Measurement of heat conductivity of milk of different water content in a temperature range of 20-100°C. Fette Seifen. Anstrichmitt 61, 1005-1010. 

They have to evaluate their formulations at elevated temperatures, as the Leidenfrost effect hinders the initial wetting. 

Experiments show that a drop of liquid evaporates rapidly when placed on a horizontal surface heated to a few degrees above the saturation temperature the evaporation is slow, however, if the surface is well above saturation. In the first case the liquid can wet the surface, creating nucleate boiling with bubble formation, liquid film evaporation, and high rates of heat transfer. In the second case the drop remains suspended on a poorly conducting vapor film, which prevents direct contact between the liquid and the hot surface. The latter is the Leidenfrost problem of film boiling and is our concern here. 

The temperature drop corresponding to point C is called the critical temperature drop, and the flux at point C is the peak flux. In the third segment, line CD in Fig. 13.4, the flux decreases as the temperature drop rises and reaches a minimum at point D. Point D is called the Leidenfrost point. In the last segment, line DE, the flux again increases with AT and, at large temperature drops, surpasses the previous maximum reached at point C. 

Because, by definition, h = q[A)IAT, the plot of Fig. 13.4 is readily convertible into a plot of h vs. AT This curve is shown in Fig. 13.5. A maximum and a minimum coefficient are evident in Fig. 13.5. They do not, however, occur at the same values of the temperature drop as the maximum and minimum fluxes indicated in Fig. 13.4. The coefficient is normally a maximum at a temperature drop slightly lower than that at the peak flux the minimum coefficient occurs at a much higher temperature drop than that at the Leidenfrost point. The coefficient is proportional to AT in the first segment of the line in Fig. 13.4 and to between AT and AT in the second segment. 

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