▪ Abstract This review examines recent advances made in predicting boiling heat fluxes, including some key results from the past. The topics covered are nucleate boiling, maximum heat flux, transition boiling, and film boiling. The review focuses on pool boiling of pure liquids, but flow boiling is also discussed briefly.

Boiling heat transfer

Recent demands for superior material properties and more efficient use of materials and production time are forcing manufacturers to develop intelligent processing techniques for enhanced process control in order to better dictate the end product. In the heat treatment and processing of metallic alloys, the desire to obtain parts of enhanced and uniform mechanical properties is requiring increased control over heat removal rates and enhanced temperature control. In particular, spray quenching has been shown to be an effective means to control and enhance the cooling rates of heat treatable aluminum alloys. This study presents a detailed and thorough parametric study of the Leidenfrost point (LFP), which serves as the temperature boundary between the transition and film boiling regimes. Sessile drop evaporation experiments were conducted with acetone, benzene, FC-72, and water on heated aluminum surfaces with either polished, particle blasted, or rough sanded finishes to observe the influential effects of fluid properties, surface roughness, and surface contamination on the LFP. A weak relationship between surface energies and the LFP was observed by performing droplet evaporation experiments with water on polished copper, nickel, and silver surfaces. Additional parameters which were investigated and found to have negligible influence on the LFP included liquidmore » subcooling, liquid degassing, surface roughness on the polished level, and the presence of polishing paste residues. The accumulated LFP data of this study were used to assess several existing models which attempt to identify the mechanisms which govern the LFP. The disagreement between the experimental LFP values and those predicted by the various models suggests that an accurate and robust theoretical model which effectively captures the LFP mechanisms is currently unavailable.« less

The Leidenfrost point : Experimental study and assessment of existing models

Spray water cooling heat transfer at high temperatures and liquid mass fluxes

Film boiling on a horizontal plate — new correlation

The past efforts in applying linear Taylor instability theory to the prediction of heat transfer during film boiling on a horizontal surface have suffered from the fact that empirical correlations must be used to define the shape of vapor-liquid interfaces and to determine the transport of mass and heat across these interfaces. The objective of this study is to clarify the physics of film boiling and to predict heat transfer coefficients through complete numerical simulation of the evolving interface between superposed layers of immiscible fluids. A coordinate transformation technique supplemented by a numerical grid generation method and a second-order projection method are combined to solve for the flow and temperature fields associated with an evolving interface. From the numerical simulation, the film thickness and, in turn, the heat transfer coefficient are found to vary both spatially and temporally. Increased wall superheat not only thickens the vapor film in the valley but also enlarges the vapor bulge. The effect of increased system pressure is to slow down the growth of the interface.

Numerical Simulation of Saturated Film Boiling on a Horizontal Surface

>Film boiling is studied for water and Freon-11 (CCl₃F) at atmospheric pressure on a flat horizontal aluminum heating surface 8 in. square. High speed motion pictures were taken from which interbubble distances, bubble periods, breakoff diameters, and geometric arrangement of bubbles were determined. The results show that the minimum heat flux in film boiling from a horizontal surface is definitely a type of Taylor hydrodynamic instability. A new experimental technique was developed for obtaining film boiling on a large area without the necessity of using huge heat fluxes to go over the peak of the boiling curve. Once film boiling was established, the film boiling region could be traversed by changing the heat output from the gas burner beneath the slab. (auth)

Earl Ramon Hosler

Papers

Film Boiling on a Horizontal Plate