Investigation On Vapor/liquid Phase-change Heat Transfer By Nanostructured Surfaces

Boiling and condensation have been widely applied in engineered heat exchange devices such as heat pump, condensers, water-cooled nuclear reactors etc. It is beneficial to improve the heat exchange rate of those devices and working life by enhancing liquid-vapor phase change heat transfer. Surface area increment is an effective method to enhance the phase change heat transfer. The occurrence of nanostructure greatly increases the ratio of surface area to volume, therefore, it is mainly concerned that the enhanced effect of nanostructure on boiling and condensation heat transfer in this project.For flow boiling in microchannels, silicon microchannel is fabricated with integrated heater and temperature sensors by MEMS process. The nanowire array is synthesized inside the microchannel with solute etching. Experimental setup is constructed to characterize the heat transfer performance and flow pattern. The overall heat transfer coefficient in nan-owire-coated microchannels is measured at subcooling temperatures of 64℃ and 34℃, mass flux ranging from 119 kg/m2s to 571 kg/m2s with this setup. The flow pattern is recorded during the flow boiling process with high-speed camera at the meantime. The main conclu-sions are:1) The overall heat transfer coefficient on nanowire-coated microchannels is enhanced at mass flux larger than 238 kg/m2s, while it is deteriorated at mass flux of 119 kg/m2s in nanowire-coated microchannels.2) The agglomeration occurs as the increased length of nanowire array, and the cavities with different diameters are formed which is beneficial to the bubble nucleation. The onset of nucleate boiling occurs earlier at lower heat flux or superheat in nanowire-coated microchan-nels, as compared with that in plain-surface microchannels.3) With the observation of high speed camera, the two phase flow pattern can be divid-ed into three stages that is bubble nucleation, bubble filling and upstream bubble flooding. The existence of nanowire array mitigates the time ratio of the annular flow pattern to a whole period, and thus presents the different effects of nanowire array on heat transfer performance for different mass fluxes.4) The onset of two-phase flow instability is delayed on nanowire-coated microchannels as well as the fluctuation amplitude of wall temperature and pressure drop between inlet and outlet is suppressed correspondingly.For dropwise condensation, the nanowire array structure is synthesised by two-step method on copper surface. A superhydrophobic surface is obtained after the self-assembled monolayer (SAM). Experimental setup is constructed to measure the condensation heat trans-fer coefficient based on horizontally and vertically orientated condensation surface. Conden-sation heat transfer coefficient is characterized at subcooling temperature ranging from 0 to 60 K and system pressure of 60 kPa. Furthermore, the condensation flow pattern is recorded with high-speed camera. The main conclusions are:1) By gradually increasing the subcooling from 0 K, the stable dropwise condensation can be realized on nanostructured surface and condensation heat transfer coefficient is incre-aed as compared with that on flat copper surface. With continuous increase of subcooling tempereture, the heat transfer coefficient enhancement is gradually decreased and finally is steady.2) At low subcooling of △T<～10 K, the nanostructured surface shows superhydropho- bic wettability and the droplets are formed at this surface which are shown as Cassie state. As the growth of the droplets, the coalescence occurs between two or more neighboring droplets to form a bigger one. The droplet jumping behavior is shown with this coalesced droplet. At moderate subcooling of～10 K<△T<～15 K, the contact angle of most of the droplets is de-creased on nanostructured surface. Wenzel or partial wenzel state is the dominant droplet mode and the droplet sweeping becomes the main droplet removal method under gravity. At high subcooling of △T>～15 K, most of the droplets are formed in complete wenzel state and the mean diameter of the droplet is also increased, and then the droplets are removed mainly by sweeping under gravity.