Vibration Induced Droplet Generation from a Liquid Layer for Evaporative Cooling in a Heat Transfer Cell

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Pyrtle, Frank, III
Black, William Z.
Hartley, James G.
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During this investigation, vibration induced droplet generation from a liquid layer was examined as a means for achieving high heat flux evaporative cooling. Experiments were performed in which droplets were generated from a liquid layer using a submerged vibrating piezoelectric driver. Parameters determined during this investigation of droplet generation were droplet mass flow rate, droplet size, driver frequency, driver voltage, and liquid layer thickness. The results showed that as the liquid layer thickness was increased, the frequencies and frequency ranges at which droplet generation occurred decreased. Droplet mass flow rates were varied by adjustment of the liquid layer thickness, driver frequency, and driver voltage. The dependence of the drivers displacement, velocity, and acceleration on frequency and voltage was determined, and the drivers frequency response was related to the occurrence of droplet generation. As a result, a frequency-dependent dimensionless parameter was proposed as a method for predicting droplet generation from the surface of the liquid layer. The dimensionless parameter is a combination of the Froude number and the dimensionless driver acceleration. The measurements have shown that droplet generation occurs when the parameter is between distinct upper and lower bounds. An analytical heat transfer model of a droplet cooling heat transfer cell was developed to simulate the performance of such a cell for thermal management applications. Using droplet flow rates determined as functions of driver voltage, driver frequency, liquid layer thickness, and interception distance, the heat transfer rate of a droplet cooling heat transfer cell was predicted for varied heat source temperatures and cell conditions. The heat transfer model was formulated in such a way as to accommodate a number of parameter variations that can be used for the design of a simple heat transfer cell. The model was used to determine the effect of droplet cooling on the heat transfer rate from a heated surface, but it can also be used to determine the influence of any of the other embodied parameters that may be of interest for thermal management applications.
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