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

Review of spray cooling – Part 1: Single-phase and nucleate boiling regimes, and critical heat flux

This article presents a theoretical study of the deformation of a spherical liquid droplet impinging upon a flat surface. The study accounts for the presence of surface tension during the spreading process. The theoretical model is solved numerically utilizing deforming finite elements and grid generation to simulate accurately the large deformations, as well as the domain nonuniformities characteristic of the spreading process. The results document the effects of impact velocity, droplet diameter, surface tension, and material properties on the fluid dynamics of the deforming droplet. Two liquids with markedly different thermophysical properties, water and liquid tin, are utilized in the numerical simulations because of their relevance in the industrial processes of spray cooling and spray deposition, respectively. The occurrence of droplet recoiling and mass accumulation around the splat periphery are standout features of the numerical simulations and yield a nonmonotonic dependence of the maximum splat radius on time.

Modeling of the deformation of a liquid droplet impinging upon a flat surface

This paper presents a systematic study of liquid droplet impact on three polymer surfaces:  poly(methyl methacrylate), poly(methyl methacrylate/n-butyl methacrylate), and poly(n-butyl methacrylate). Changing from one surface to the next represents an incremental variation in solid surface tensions of 5−6 mJ/m2. These surfaces were prepared through careful experimental procedures that were used for the determination of solid surface tensions from contact angles. Our data for the maximum spreading diameter of water and formamide impacting on these surfaces were compared with those predicted from literature models. Of the models selected, we modified the model of Pasandideh-Fard et al. [Phys. Fluids 1996, 8, 650] and the results yielded a least error of only 5.09 ± 5.05% in the determination of the maximum spreading diameter. The improved model was also compared with literature data, and good agreement was found. Of course, any such comparisons would rely on accurate experimental impact dynamics data on care...

On the maximum spreading diameter of impacting droplets on well-prepared solid surfaces.

With the increasing power density of electronic chips, large radar, laser diode array and other equipments, the conventional heat dissipation methods are difficult to achieve the desired thermal control requirements increasingly. Spray cooling has attracted widespread attention due to its advantages in high heat flux removal such as less flow rate demand, high heat dissipation capacity, low superheat degree, no temperature overshoot and no contact thermal resistance with the heating surface. As of today, lots of researchers engage in this field and numerous achievements of spray cooling are obtained theoretically and experimentally. In this paper, an overview with spray cooling was completed. The current research progresses of heat transfer mechanisms of spray cooling in the three stages (single-phase regime, two-phase regime and critical heat flux regime) were summarized, and the influence factors, spray characteristics, heating surface characteristics, fluid characteristics and external environment characteristics, were analyzed in detail. The flash evaporation cooling, a special form of spray cooling, was also explored by a number of studies due to its irreplaceable advantage in low pressure environment or in space. Film flash evaporation and droplet flash evaporation significantly improve the cooling capacity of system and utilization of working fluid. In fact, the application of flash evaporation cooling is profound for development and expansion of spray cooling. Additionally, spray cooling system and nozzle were also elaborated in the paper.

Spray cooling and flash evaporation cooling: The current development and application

Liquid spray cooling of a heated surface

This paper investigates the utility and accuracy of the use of fine-wire thermocouples for time resolved thermometry in turbulent flames. The problem, if course, is that there is no unique time constant in a turbulent flame with widely fluctuating temperature. An analytical formulation is presented which shows that while little error may be expected in determination of r.m.s. values of temperature, there may be a substantial error made in deduction of the mean temperature, under certain circumstances. Experimentally, in a premixed methane-air flame, a comparison is made of a time resolved compensated thermocouple measurement with a simultaneous measurement by molecular Rayleigh scattering, and the results compared with the theoretical predictions. The thermocouple, compensated through a single time constant, behaved admirably, under these conditions.

William M. Grissom

Papers

Liquid spray cooling of a heated surface

Time Resolved Thermometry by Simultaneous Thermocouple and Rayleigh Scattering Measurements in a Turbulent Flame