Evaluation of equilibrium and non-equilibrium evaporation models for many-droplet gas-liquid flow simulations

Advances and challenges in explaining fuel spray impingement: How much of single droplet impact research is useful?

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

At the impact of a liquid droplet on a smooth surface heated above the liquid's boiling point, the droplet either immediately boils when it contacts the surface ("contact boiling"), or without any surface contact forms a Leidenfrost vapor layer towards the hot surface and bounces back ("gentle film boiling"), or both forms the Leidenfrost layer and ejects tiny droplets upward ("spraying film boiling"). We experimentally determine conditions under which impact behaviors in each regime can be realized. We show that the dimensionless maximum spreading γ of impacting droplets on the heated surfaces in both gentle and spraying film boiling regimes shows a universal scaling with the Weber number We (γ~We(2/5)), which is much steeper than for the impact on nonheated (hydrophilic or hydrophobic) surfaces (γ~We(1/4)). We also interferometrically measure the vapor thickness under the droplet.

Drop impact on superheated surfaces.

Review of drop impact on heated walls

Heat-transfer measurements of evaporating liquid droplets

Evaporation of a liquid droplet on a hot plate

Countercurrent flow of air/water and steam/water through a horizontal perforated plate

Countercurrent flow limiting phenomenon above a perforated plate has been studied in a steam/water environment. Water was injected as a vertical jet and the injection height above the perforated plate was changed from 0 cm to 35.6 cm. The 15 hole perforated plate has a rectangular cross section with a perforation ratio of 0.423. The weep-points and total dumping points have been determined for low and high water injection heights above the perforated plate and the results have been compared to those of the horizontal water spray experiments. The data corresponding to high water injection heights were similar to those of the horizontal water spray experiments. However, a different behavior was observed for the weep-point data with low water inlet heights. The dumping point was little affected by the water inlet position above the perforated plate. The dimensionless effective steam flow rate defined for the experiments with horizontal water spray was used to correlate the data corresponding to both the onset of weeping and the total dumping points. The correlation was successful for the weep-point data with high water injection heights. However the dimensionless parameter was redefined for the weeping-point data with low water injection heights.

Countercurrent steam/water flow above a perforated plate-vertical injection of water. Topical report, August 1980-July 1981

The study of steam condensation in countercurrent stratified flow of steam and subcooled water has been carried out in a rectangular channel, with an inclination angle 33/sup 0/ from the horizontal. The variables in this experiment were the inlet water and steam flow rates and the inlet water temperature. Condensation heat transfer coefficients were determined as functions of local steam and water flow rates and the degree of subcooling. Correlations are given for the local Nusselt number for the smooth and for the rough surface regimes, and also for the dimensionless wave amplitude. A turbulence-centered model is also considered. It is shown that better agreement with the data can be obtained if the characteristic lengths in the turbulent Nusselt number and turbulent Reynolds number are taken to be wave amplitude and the friction velocity, rather than the water layer thickness and 0.3 times the mean water velocity. A new correlation is presented based on the wave parameters.

M.C. Yuen

Papers

Heat-transfer measurements of evaporating liquid droplets

Evaporation of a liquid droplet on a hot plate

Countercurrent flow of air/water and steam/water through a horizontal perforated plate

Countercurrent steam/water flow above a perforated plate-vertical injection of water. Topical report, August 1980-July 1981

Countercurrent steam-water flow in a flat-plate geometry