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Spray cooling with phase change offers the means to achieve the highest rates of heat transfer from microelectronic components and other high energy density devices. The extreme complexity of the flow created by the impact of millions of droplets per second per cm 2 creates a need for a heat transfer design model which incorporates enough physical detail to yield accurate predictions while being sufficiently simplified to allow its use in routine design computations. The spray cooling group at West Virginia University is pursuing a coordinated program of laboratory experiments and computational simulations to develop a Monte Carlo-based spray cooling model that will satisfy this requirement. This paper reports progress in both the laboratory and computational fluid dynamics (CFD) phases of this study, including comparisons between single droplet impact results at We values of 161 and 633 to validate the CFD simulations and experimental measurements. A key goal of the present work is to determine the thickness of the thin liquid layer remaining in the crater formed when a liquid droplet impacts a surface covered by a preexisting liquid film. The volume of liquid in this thin liquid film in the droplet impact crater is one factor that will influence the onset of critical heat flux (CHF). Based on the present data the preexisting static film thickness is observed to have more of an effect than the initial drop parameters on the minimum liquid volume under the crater, but the crater lifetime depends on the initial drop conditions. The comparisons between experiments and simulations for these two cases show promise, but more refinement in experimental and computational technique are needed to achieve more consistent determinations of the volume of liquid under a crater. NomenclatureD = Drop diameter D U = upper crown diameter D V = Initial drop volume Fr = Froude number = H = Preexisting static liquid pool thickness H crown = Height of droplet impact crown h o = Centerline liquid film thickness h avg = Average liquid film thickness Oh = Ohnesorge number = Re = Reynolds number = 2 r = radius, measured from center of droplet impact R B = Radius of outside bottom of crown t = Time We = Weber number = V = Impact velocity Vol = Volume liquid under the impact crater Y = splashing-deposition criteria ρ = Density μ = Viscosity σ = Surface tension

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Spray cooling is a topic of current interest for its ability to uniformly remove high levels of waste heat for densely packed microelectronics. A Monte-Carlo (MC) spray cooling simulation model is under development that is based on empirical data to be a cost effective design tool that will predict accurate heat fluxes based on nozzle conditions and heater geometry. This work reports spray and single drop experiments with the goal of computing the volume beneath a drop impact cavity (sub-cavity volume) created by a single impinging droplet on an initial liquid layer. A relevant test plan for the single drop experiments in terms of We and Re numbers was created through utilization of Phase Doppler Anemometry to characterize a water spray generated by a nozzle of interest for varying flow conditions. Liquid thickness profiles of the sub-cavity formed by a single impinging drop onto a range of initial liquid film thicknesses were measured versus time via a non-contact optical thickness sensor. Time dependent sub-cavity volumes were computed by integrating these sub-cavity liquid film thickness profiles measured radially outward from the impact centerline. It is found that higher We and lower h 0 * result in a more radially uniform sub-cavity surface contour versus time, except for regions near the outer bottom cavity radius, where the liquid film was thinner. The sub-cavity volume was found to be nearly constant for a majority of the cavity lifetime and increased with We and h 0 * . These results will be incorporated in future work into the MC model to improve its predictive capability. Nomenclatured = Drop diameter Greek Letters D = Arithmetic mean droplet diameter ρ = Density D 32 = Sauter mean droplet diameter η = Index of refraction Fr = Froude number = μ = Dynamic viscosity h = Liquid film thickness σ = Surface tension R = Radial location τ = Dimensionless time (t•V axial /d) Re = Reynolds number = Superscripts T = Temperature * = Dimensionless parameter t = Time Subscripts V = Arithmetic mean velocity 0 = Initial condition Vol = Sub-cavity volume axial = Axial velocity component We = Weber number = c = Cavity z = Axial standoff distance from the nozzle tip r = Radial velocity component s = Spray

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A two-component Point Doppler Velocimeter (PDV) which has recently been developed is described, and a series of velocity measurements which have been obtained to quantify the accuracy of the PDV system are summarized. This PDV system uses molecular iodine vapor cells as frequency discriminating filters to determine the Doppler shift of laser light which is scattered off of seed particles in a flow. The majority of results which have been obtained to date are for the mean velocity of a rotating wheel, although preliminary, data are described for fully-developed turbulent pipe flow.Accuracyof thepresent wheel velocity datais approximately + i % offullscale, whileImcarity of a single channel ison theorder of +-0.5% (ie, + 0.6 m/see and ± 0.3 m/see, out of 57 m/see, respec_ely).The observed linearity of these results is on the order of the accuracy to which the speed of the rotating wheel has been set for individual data readings.The absolute accuracy of the rotating wheel data is shown to be consistent with the level of repeatability of the cell calibrations.The preliminar/turbtdem pipe flow data chow consistent turbtdence intensity values, and mean axial velocity profiles generally agree with pitot probe data. However, them is at present an offset error in the radial velocity which is on the order of 5-i0 % of the mean axial velocity.