Droplet cooling can be used to extract large amounts of energy at relatively low temperatures through the latent heat of evaporation. One method of enhancing the heat transfer beyond these levels is to add dissolved gas to the liquid so that the splat increases in size as bubbles within the droplet grow, resulting in an increase in the solid/liquid and liquid vapor contact area. The bubble may also cause an increase in heat transfer within the drop, if the liquid film around the bubbles thins locally. This work is being performed in conjunction with Dr. Ken Kiger.

 

A schematic of the experimental setup is shown at right. The drops were produced by allowing liquid to drip from a glass nozzle onto the heater array. The working fluid used in this study was FC-72. The semi-transparent nature of the heater array enabled images to be made of the droplets evaporating on the surface from below using a high speed camera set at 500 fps. Recording was initiated using the same trigger signal sent to the data acquisition system, allowing heat transfer measurements to be synchronized with the high-speed images.

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A movie showing the heat transfer distribution during evaporation of a drop on the 2.7 mm heater array can be downloaded at right. Both the data and the video was obtained at 500 Hz. Each individual heater in the array is colored according to the wall heat transfer in Matlab. A gas bubble forms within within the drop by the merger of many smaller droplets, then bursts. The later this bubble bursts, the more quickly the droplet evaporates since the larger liquid-vapor contact area due the presence of the bubble allows larger mass diffusion from the drop.

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Some papers describing these results are

1). Lee J., Kim, J., and Kiger, K.T., "Time and Space Resolved Heat Transfer Characteristics of Single Droplet Cooling Using Microscale Heater Arrays", International Journal of Heat and Fluid Flow, Vol. 22, pp. 188-200, 2001.

2). Lee, J., Kiger, K.T., and Kim, J., "Enhancement of Droplet Heat Transfer Using Dissolved Gases", SAE 2002 Transactions Journal of Aerospace , pp. 736-746.

 

This work was supported by AFRL at Wright Patterson Air Force Base and the Laboratory for Physical Sciences.