Boiling Heat Transfer Properties Of HFE-7100/Water Immiscible Mixture On Multiscale Hierarchically Patterned Surfaces

Boiling heat transfer of immiscible liquid mixtures can be used as a cooling method for electronic devices.The approach takes advantages of the properties of a mixture of two liquids covering an interface: one liquid has a higher density and a lower boiling point,while the other has a lower density but a higher boiling point,such as the HFE-7100/water mixture.As the heat flux increases,HFE-7100 starts to boil first,followed by boiling of water,leading to a lower-temperature onset of nucleate boiling and a higher critical heat flux.However,boiling refrigerant transition(BRT)may occur when the higher-density liquid burns out.In that case,a small increment of heat flux can cause the temperature of the heating surface to jump abruptly,as the heat transfer to HFE-7100 is suddenly replaced by the heat transfer to water.The heat transfer efficiency is thus reduced and the electronic devices may be damaged with overheating.To address the BRT problem,I investigated if and how micro/nano-structured surfaces affect the boiling heat transfer of immiscible liquid mixtures.I used the electrodeposition method and a H2 bubble pore-forming template to produce a copper surface with micro/nanostructures.I then tested the boiling heat transfer ability of HFE-7100/water mixture over the surface and compared it with smooth copper surface.Kawanami’s model was used to predict the critical height for HFE-7100 to undergo BRT.The results showed that BRT occurred on the smooth and micro/nanostructured surfaces corresponding to the predictions of the Kawanami’s model.However,the boiling heat transfer capacity of the micro/nanostructured surface was measured to be lower than that of smooth copper surface.Using a high-speed camera,I observed that the diameters and frequency of the departure bubbles were lower for the micro/nanostructured surface,in comparison with the smooth surface.To understand this surprising finding,I tested the wetting ability of the two surfaces in the immiscible liquid mixture environment.HFE-7100 showed a superior wetting ability over the micro/nanostructured surface and water could not wet and spread on the surface,which may explain the reduced boiling heat transfer property of the micro/nanostructured surface.To minimize the harmful temperature jump of the heating surface during the BRT period and enhance the boiling heat transfer ability,an array of mm-sized rods(fins)are constructed on the heating surface.These fins penetrate the HFE-7100/water interface,and reach the water space to enable nucleation and bubble formation.The BRT occurred under the conditions in agreement with the Kawanami’s model.Compared to the smooth copper surface,the fin-array surface,with a larger heat transfer area and higher nucleate site density,showed a higher boiling heat transfer ability using HFE-7100/water mixture: its critical heat flux of boiling heat transfer was significantly elevated and the onset temperature of nucleate boiling was reduced.However,the heat transfer ability of the fin-array surface still needs to be improved in the conditions when the heating surface temperature is low.To further improve the heat transfer ability of the fin-array surface at low heating-surface temperatures,surface modification was performed using the electrodeposition and H2 bubble pore-forming method as described above.The surface micro/nanostructure was found to enhance the interaction with HFE-7100 and inhibit the burnout of HFE-7100 at lower heating surface temperatures,leading to improved boiling heat transfer ability.As the heating-surface temperature increased,the modified surface,however,increased the heat conduction resistance and impeded the departure of crowded bubbles from the surface,so that the capacity of boiling heat transfer of the fin-array surface with modified porous structure decreased relative to that with smooth fins.In summary,the three projects in this thesis demonstrated the complex effects of the surface patterns on the regulation of the boiling heat transfer properties of immiscible HFE-7100/water mixture.Careful design and optimization of the surface features are needed for cooling applications over a large range of operational conditions.