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Relation of Heat flux Near CHF and Efficiency to Liquid Film Characteristics Using Small Scale Mono-Disperse Sprays
Keywords: CHF, Efficiency, Liquid film
The continuous increase on microprocessors power density is reaching the limits of conventional air cooling, therefore alternative solutions to dissipate such high power densities are considered in the electronic industry. Two phase liquid cooling has shown encouraging potential however it is difficult to implement and more studies are needed. This work is aimed at heat fluxes near the CHF in spray cooling, the efficiency of heat dissipation and its correlation with liquid film thickness. This research uses a spray generated by thermal ink-jet (TIJ) to generate a mono-disperse spray to cool small surfaces (1.3 mm x 2 mm, 3 mm x 5 mm). The drop size and position are fixed by the atomizer architecture, the frequency of drop delivery and the drop velocity can be controlled with the experimental setup. The TIJ atomizer is a good selection for drop-on-demand because it provides fast time response, high fluid delivery resolution, and excellent control by supplying a consistent stream of mono-disperse droplets from a series of nozzles that can be activated individually or in groups. These characteristics result in consistent wetted area and steady heat fluxes near the CHF. Spray cooling experiments with 33 micrometers droplet diameter impacting small copper hot substrates are performed to measure heat dissipation near the CHF at different conditions of cooling mass flow rate and number of activated nozzles. Under controlled spray conditions it is possible to achieve up to 4x107 W/m2. The liquid film characteristics of thickness and wet area are controlled by the spray, this results in the enhancement of heat transfer by conduction and convection. The liquid film thickness is measured by optical means and image processing of captured images. Higher efficiencies (eta) are achieved at smaller liquid film thickness (delta) i.e. eta≈5micrometers → delta≈0.9 . This is in agreement with results from other researchers indicating thinner film thicknesses result in better heat dissipations. Analysis of the relation between the liquid film thickness and an equivalent convective heat transfer coefficient for the film (hfilm) indicate the heat transfer by conduction becomes significant at thin liquid films while at thick liquid films there may be other mechanisms inducing heat dissipations.
Sergio Escobar-Vargas,
Hewlett Packard
Palo Alto, CA
USA


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