Drop Impingement on Wet and Dry Surfaces

Abstract

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

Droplet impact time histories for a range of Weber numbers and liquid film thicknesses for spray cooling application

“…Their results found the centerline sub-cavity film thickness to be dependent upon the initial dimensionless liquid film thickness and the impinging droplet Reynolds number, but this thickness reached a constant value at high droplet Weber and Froude numbers. 10 The purpose of the current work is to improve and expand upon the initial study by Hillen et al 7 by computing the sub-cavity volume for the range of dimensionless parameters for realistic spray conditions. Table 1 summarizes the droplet sizes and liquid film thicknesses generated by a spray impinging onto a surface measured from previous works.…”

“…Preliminary experiments by Hillen et al 7 mapped time history profiles of the liquid film thickness beneath the droplet impact cavity (referred to herein as the sub-cavity film thickness) formed by single impinging drops at different radial locations utilizing a commercially available non-contact chromatic optical measuring sensor 9 . The sub-cavity volume was then computed by integration of the profiles.…”

“…The experimental apparatus for the single drop cavity experiments was the same as in Hillen et al 7 , where a more detailed description is given. Figure 7 a) is a schematic of the single drop setup.…”

“…2 versus time to construct a more accurate film thickness traverse. The cavity traverse in Hillen et al 7 was based on the maximum bottom crown radius of the splash, resulting in a relatively low spatial resolution of the sub-cavity film thickness (sub-cavity film thickness time histories at a total of only 3 to 5 radial locations in the cavity) since the bottom crown radius was larger than the bottom cavity radius defined in Crown Splashing Layer Free Surface Impact Surface Fig. 2.…”

“…Enhanced localized heat fluxes are expected to occur in the liquid volume beneath the drop impact cavities (which will be defined as the sub-cavity volume) since the thin liquid film beneath the cavity is about 1-2 orders of magnitude thinner than the initial liquid layer and is partially comprised the cool drop liquid directly in contact with the heater surface. 7 Conversely, these small volumes also are more susceptible to boiling and subsequent dry out, contributing to the onset of critical heat flux (CHF). This is the focus of the current research, partially motivated by the numerical simulations of single drop impacts by Sarkar and Selvam 6 investigating the enhanced local heat flux of the bubble contact line.…”