| Spray cooling can be used to transfer large amounts of energy at low temperatures through the latent heat of evaporation. Heat transfer rates much higher than can be attained in pool boiling are possible with sprays since the vapor can be removed from the surface more easily. We are investigating the fundamental mechanisms by which sprays transfer energy using FC-72 and FC-87 using a 7.0 mm microheater array. This work is being performed in conjunction with Dr. Ken Kiger.
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| A schematic of test rig is shown at right. A spray nozzle from ISR is used to cool a microheater array using FC-72. A high speed video camera is used to simultaneously obtain images of the liquid on the surface through the heater array. |
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| By varying the amount of gas in the test liquid, we can determine the effect of gas on spray cooling. It has been found that dissolved gas shifts the spray cooling curves to the right by increasing the saturation temperature of the liquid–an example is shown on right. The primary effect of dissolved gas on sprays is to shift the heat transfer curves to higher temperatures by increasing the saturation temperature of the liquid. Although heat transfer is degraded at lower wall temperatures when gas is present, both the CHF and spray efficiency increase. |
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Images of the droplets at wall temperatures before CHF, at CHF, and after CHF for cases with and without gas for similar superheats are shown below:
It is seen that many single droplets form near CHF for the degassed fluid. A mixture of individual droplets and larger distorted pools of moving liquid form when gas is present. The reason for higher heat transfer when gas is present may be because more liquid is in contact with the wall. |
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We have also used a Total Internal Reflection technique to visualize and make measurements of the liquid–solid contact area and the three-phase contact line length. It was found that the wall heat flux did not depend on the wetted area fraction of liquid on the surface, but correlated very well with the contact line length. The implications of this conclusion are that it may be possible to improve the control and magnitude of the heat flux if one can similarly enhance and/or control the contact line length on the heated surface. |
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A paper describing dissolved gas effects on spray cooling heat transfer will be presented at the 2003 ASME IMECE in Washington DC this November. The submitted paper can be downloaded here.
This work was previously supported by AFRL at Wright Patterson Air Force Base and the Laboratory for Physical Sciences. It is currently being supported by ONR and Parker Hannefin.
