Microstructure Effect On Subcooled Flow Boiling And Sessile Droplet Evaporation

Boiling and evaporation are two phase change phenomena which are commonly found in the nature and industry.They are also found in marine boiling heat transfer equipments and low temperature multi-effect distillation system.Microstructures are usually fabricated on surfaces to improve the nucleate density and to change surface wettability.With microstructured surfaces,boiling heat transfer can be greatly enhanced.Furthermore,they are also helpful to enhance the sessile droplet evaporation.In this dissertation,research on the microstructure effect on boiling heat transfer and sessile droplet evaporation was carried out.A two-step electrodeposition method was applied to fabricate a micro-porous surface.By varying the current density,the surface structure could be modified from a cauliflower-like structure to a honeycomb-like structure.In this case,the cauliflower-like structured surfaces were hydrophobic.After the structured surface was fabricated,the surfaces were used for the flow boiling experiment and it was found that the structured surface affects the boiling heat transfer.The experimental results showed the hydrophobic cauliflower-like structure could obviously enhance the boiling heat transfer.The hydrophilic honeycomb-like structure with bigger cavities worked better for lower superheat;however,the advantage did not last to the higher superheat,where,the cauliflower-like surface with smaller cavities transferred much more heat.The microstructures worked well before the heat flux reaching CHF(Critical Heat Flux).However,the surface wettability determines the CHF.With correct application,the microstructured surface can enhance the efficiency of marine boiling heat transfer equipments obviously.To understand the microstructure effect on the sessile droplet evaporation,research was conducted on the hybrid surface effect on droplet evaporation.The surface was made by electrowetting a piece of smooth hydrophilic MWCNT(Multi-walled Carbon Nanotube)surface.Half of the surface was smooth and hydrophilic,and another half was rough and hydrophobic.The results showed that the droplet shape is different and the droplet size affects electrowetting on the MWCNT surface,and it is concluded that MWCNT is electricity conductive,hence the droplet size effect should be taken into consideration during the electrowetting process.When a droplet is rested on the surface consisting of different wettabilities,the droplet shape turned out to be asymmetric,with a contact angle on the hydrophobic side less than 60°,which is smaller than that on thehydrophilic side which is about 75°.The evaporation for the asymmetric droplet appeared to be asymmetrically as well.The evaporation on the hydrophobic side was faster than on the hydrophilic side,which was proved by numerical simulation thereafter.To better understand the experimental results and to find out the main factor,a simulation was conducted on the asymmetric droplet evaporation,and it was found that the curvature was the main factor affecting the evaporation.The results from the study on the sessil droplet evaporation on structurally hybrid surface can alo be used to analyze the effect of falling film thickness on the evaporation process,and the structurally hybrid surface can be used to improve the distribution of falling film and hence the sea water evaporation.