Ph.D. Thesis Defense by Sally M. Sellers
Tuesday, August 22, 2000

(Dr. William Black, advisor)

"Heat Transfer Resulting from the Evaporation of Liquid Droplets on a Horizontal Heated Surface"


 Cooling a heated surface with thin-film evaporation of water has been shown to provide high heat transfer rates while maintaining the surface temperature at levels below approximately 120?C.  One method to produce and control a thin-film on a heated surface is to utilize a droplet generator mechanism and control the position and flow rate of the droplets as they strike the surface.  The resulting process is generally referred to as droplet or spray cooling.  Specifically, the term droplet cooling is used to describe the heat transfer phenomenon that occurs when liquid droplets impact a hot surface and cool it by evaporation.  Because the phase change process is very efficient, droplet cooling has the potential to remove a large amount of energy from a small heated surface. This high heat transfer capability is a possible solution to many thermal management problems, especially some of the cooling problems that exist in the microelectronic industry.

 In this experimental investigation, the heat flux resulting from droplet cooling on a horizontal surface was examined.  All tests were conducted with water at atmospheric pressure.  A continuous jet droplet generator and a deflection mechanism were used to control the diameter, velocity, impact frequency, and spacing of the drops.  Two orifices with diameters of 50 and 144 ?m were utilized to produce droplet diameters from 97 to 392 ?m, at droplet generation frequencies between 2.5 and 38 kHz.  These droplet parameters generated surface averaged mass flow rates from 1.22 ? 10-2 to 17.5 ? 10-2 g/(cm2s).  Two different heater designs were used including a thin-film resistive heater to measure moderate heat fluxes (less that approximately 100 W/cm2) and a traditional, thermally-controlled, calibrated heater assembly to measure higher heat fluxes.

 Regardless of the test heater used, the critical heat flux results were comparable.  Trends in the heat transfer data suggest a slight dependence on droplet size and velocity, while the droplet spacing can affect the heat flux by over 25 percent in some cases.  The measurements of heat flux have shown that droplet cooling can achieve cooling rates as high as 297 W/cm2 at a surface temperature of 126?C for a surface averaged mass flow rate of 0.175 g/(cm2s).  This value is more than twice the heat flux of traditional pool boiling of 126.5 W/cm2 for water at one atmosphere.  In addition, the value for the critical heat flux for droplet cooling is about 1.5 times the critical heat flux value predicted for jet impingement under equivalent conditions.  A generalized correlation for a nondimensional critical heat flux as a function of the Weber number, Strouhal number, and a dimensionless spacing is presented for the data obtained in the investigation.