Decrease of Leidenfrost temperature at quenching in subcooled liquids

Cooling of high-temperature bodies in liquids largely depends on its subcooling to the saturation temperature. An increase in subcooling leads to an increase in the surface temperature, at which the vapor film loses its stability and an intensive cooling regime begins. This temperature depends on a number of parameters, such as the properties of a liquid and a solid, the composition and topology of the surface, the value of subcooling. Within the framework of this work, it was possible to achieve a significant decrease in the temperature of the onset of an intensive cooling mode in subcooled water and ethanol by using as working sections of metal samples with a high of thermal effusivity, low roughness and a protective coating from oxidation. The obtained experimental results confirm the approximate model of the appearance of an intense cooling regime

The use of a neural network for determining the temperature of a vapor film destruction in uncooled and saturated water-ethanol mixture in a cylindrical geometry

The article presents the construction of an artificial neural network to determine the temperature of destruction of a vapor film in subcooled and saturated liquids of water-ethanol mixtures. To train the neural network, the results obtained on cylindrical samples of stainless steel, copper and nickel are used. In total, about 260 experimental points were used, which is sufficient to build a specific computational model. This article discusses a model of a neural network of the multilayer perceptron type. The trained neural network model shows a greater generalizing ability than the theoretical model for determining the temperature of vapor film destruction.

Incipience of the intensive heat transfer regime at cooling high-temperature bodies in subcooled liquid

Cooling in film boiling is usually an unwanted process in many technologies due to low intensity of heat transfer. Thus, predicting the solid wall superheat at vapor film destabilization is useful to avoid this phenomenon. In the present paper, two new semi-empirical models for evaluation of the wall superheat at destabilization of vapor film around the metallic body cooled in saturated or in subcooled liquid are proposed. Both models with fitted empirical multipliers are in good agreement with an experimental dataset. To evaluate the contribution of the natural convection in the model for temperature head at cooling in subcooled liquid, a problem about the natural convection near the vapor film, occurring during film boiling along the vertical plane, is numerically solved by means of ANES20XE CFD-code. The computational results of longitudinal velocity are in good agreement with the theoretical velocity of natural convection used in the model.

Flow Effects on the Transient Behavior of Vapor Film and Bubbles During Forced-Convective Quenching Experiments

The transient behavior of boiling phenomena during quenching of an AISI 304 stainless steel, conical-end, cylindrical probe in flowing water at 60 °C was studied. Two free-stream velocities (0.2 and 0.6 m/s) and two initial probe temperatures (850 and 950 °C) were investigated. From high-speed video recordings, undulations of the liquid vapor interface that appear periodically and propagate in the direction of the flow stream were observed during the vapor film stage. After the collapse of the vapor film, a wetting front is formed which consists of many small bubbles that coalesce rapidly in a small area while fewer and larger bubbles nucleate and grow below it. The initial temperature has a marginal effect on the size and half-life of the large bubbles. However, the water flow rate produces larger values of maximum diameter and half-life time for water flowing at 0.2 m/s than their equivalents for 0.6 m/s.

Experimental investigation on pool boiling for downward facing heating with different concentrations of Al2O3 nanofluids

An experimental study has been conducted to examine the effect of different concentrations of α-Al2O3 nanofluids on boiling heat transfer for downward facing heating. The experimental results indicated that the surface heat transfer was enhanced with the rise of the nanofluids concentration, due to the bubble generation rate and disturbance increased. For downward facing heating, bubbles were not able to escape since the bouncy force influenced, and the vapor film appeared earlier as the increase of bubble generation rate. However, the heat transfer coefficient remains at a relatively high value in the early stage of film boiling, which suppresses the deterioration of heat transfer, due to the influences of nanofluids. As the various concentrations of nanofluids increased from 0 g/L to 0.012 g/L, it was found that the enhancement of the CHF (critical heat flux) up to 20.5%. After the nanofluids boiling, the surface roughness decreased and the wettability became worse. From the experimental phenomena, under the influence of these two factors, the bubble activity was enhanced.

Heat Transfer Considerations on the Spontaneous Triggering of Vapor Explosions—A Review

Vapor explosions have been investigated both theoretically and experimentally for several decades, focusing either on the vapor film, or on mechanical aspects. Where the main interest for industry lies in the safety risks of such an event, fundamental research is focusing on all partial processes that occur during a vapor explosion. In this paper, vapor explosions are discussed from a heat transfer point of view. Generally accepted knowledge of heat transfer between hot surfaces and liquids is compared to early investigations regarding the origin of vapor explosions. Both steady state and transient models are discussed. The review of available literature suggests that vapor explosions trigger spontaneously by the collapse of the boiling film. Better understanding of the fundamental aspects of vapor explosions might give rise to future ideas on how to avoid them.

The thermo-wetting instability driving Leidenfrost film collapse

Above a critical temperature known as the Leidenfrost point (LFP), a heated surface can suspend a liquid droplet above a film of its own vapor. The insulating vapor film can be highly detrimental in metallurgical quenching and thermal control of electronic devices, but may also be harnessed to reduce drag and generate power. Manipulation of the LFP has occurred mostly through experiment, giving rise to a variety of semiempirical models that account for the Rayleigh–Taylor instability, nucleation rates, and superheat limits. However, formulating a truly comprehensive model has been difficult given that the LFP varies dramatically for different fluids and is affected by system pressure, surface roughness, and liquid wettability. Here, we investigate the vapor film instability for small length scales that ultimately sets the collapse condition at the Leidenfrost point. From a linear stability analysis, it is shown that the main film-stabilizing mechanisms are the liquid–vapor surface tension-driven transport of vapor mass and the evaporation at the liquid–vapor interface. Meanwhile, van der Waals interaction between the bulk liquid and the solid substrate across the vapor phase drives film collapse. This physical insight into vapor film dynamics allows us to derive an ab initio, mathematical expression for the Leidenfrost point of a fluid. The expression captures the experimental data on the LFP for different fluids under various surface wettabilities and ambient pressures. For fluids that wet the surface (small intrinsic contact angle), the expression can be simplified to a single, dimensionless number that encapsulates the wetting instability governing the LFP.

Non-Uniform Magnetic Field Effects on Ferrofluids Film Boiling: A Numerical Study

In this paper the effect of magnetic field on film boiling of ferrofluids on a horizontal flat plate has been investigated. The obtained results indicate that adding nanoparticles into the base fluid changes vapor film characteristics mainly due to changes in thermophysical properties of the base fluid. Also, ferrofluids enhances film boiling heat transfer on horizontal plate specially at higher volume concentration of nanoparticles. Another important result of this study is the effect of non-uniform magnetic field on horizontal film boiling characteristics. Application of magnetic field changes vapor film behavior mostly at higher values of magnetic field intensity (400) and nanoparticles volume fraction (4 percent).