Heat Transfer Characteristics Of Oscillating Heat Pipes With Wettability Gradient Surface

Oscillating Heat Pipe(OHP)works as an effective mass and heat transfer device.Liquid evaporation in evaporator section and vapor condensation in condenser section facilitate the liquid slugs moving back and forth in the channels so as to ensure a high-rate heat transfer process.Wickless fabrication,various tube forms and outstanding thermal performance of an OHP enable itself to hold immense potential for heat dissipation equipment in the microelectronic field.Gravity plays a vital role in promoting the backflow of liquid in oscillating heat pipes.Under horizontal heating mode,oscillating heat pipes with low-bending numbers could not promise enough liquid to flow back to the evaporator without the gravity assistance.So,there would be severe dryout in the evaporator leading to rather poor thermal performance.With the aim of enhancing the heat transfer performance of oscillating heat pipes under all directions,this thesis intends to alter the wettability of wall surface based on the solid-liquid interface effect,and puts forward the introduction of wettability gradient surface,thereby intensifying the liquid slug movements and promoting the overall heat transfer performance.A superhydrophilic surface with nano-grass structure and a wettability gradient surface with a gradient of contact angles from 0 ° to 83.8 ° from the evaporation section to the condensation section were prepared by controlling chemical etching time.6-turn flat plate oscillating heat pipes were investigated visually and experimentally under different operation conditions,including surface wettability,filling ratios,heat input and orientations.The purpose is to provide preliminary solutions to severe dryout,non-startup situation and poor thermal performance for small oscillating heat pipes operated in horizontal direction.The experimental results demonstrated that the uniform superhydrophilic and wettability gradient surface could form a narrow liquid film area in the OHP channel under vertical heating mode due to the surface micro/nano structure.And thus,the heat transfer performance of the oscillating heat pipe was effectively reinforced.The superhydrophilic surface had the most significant effect on the heat transfer enhancement of the oscillating heat pipe.When the oscillating heat pipe was operated under low filling ratio and high heat input,the heat transfer performance of the wettability gradient surface and the superhydrophilic oscillating heat pipe was equivalent.Under horizontal heating mode,the wettability gradient surface showed more remarkable improvement on the thermal performance of copper oscillating heat pipe compared with the superhydrophilic surface.There were great improvements in both startup and heat transfer performance of the wettability gradient oscillating heat pipe,compared with bare copper oscillating heat pipe which scarcely started up and showed severe dry out.Due to the additional wettability-driven effect of wettability gradient oscillating heat pipe,the working fluid was easy to return to the evaporator section,promoting the circulatory flow inside the entire heat pipe.The oscillation amplitude,oscillation velocity of the vapor-liquid interface and the heat transfer capacity of the wettability gradient oscillating heat pipe were significantly improved,with the thermal resistance reduced by up to 45% at the filling ratio of 50%,while that was reduced by 26% in superhydrophilic oscillating heat pipe.Therefore,it is believed that the wettability gradient surface could function as an effective means to improve the horizontal performance of the oscillating heat pipe.Finally,a theoretical model of a single U-shape oscillating heat pipe was established,and the contact angles were introduced into the model in order to explore the effect of uniform hydrophilic surface and wettability gradient surface on the performance of oscillating heat pipes.The numerical analysis was in good agreement with the experimental results,demonstrating that the reduction of the wall contact angle increased the total heat transfer length of the liquid film.Therefore,the oscillation frequency and oscillation amplitude of the liquid slug displacement increased significantly,leading to the strengthening of the sensible heat transfer and latent heat transfer.Compared with the bare copper oscillating heat pipe,the thermal resistance of the oscillating heat pipe with uniform hydrophilic surface and wettability gradient was greatly reduced,and the heat transfer performance was effectively enhanced.