Effects Of Hydrophilic/Hydrophobic Surface On Transport Performance Of An Oscillating Heat Pipe

The oscillating heat pipe (OHP) is a promising technology for dissipating high grade heat and has a wide variety of applications. The considerable interest in such a heat pipe is due to its simple structure and high heat transfer performance. Heat is transferred by the sensible heat of liquid slugs and the phase change heat transfer of the vapor plugs, and the slug oscillations play a dominant role in the heat transfer. The surface wettability remarkably influenced the slug motion and thermal performance of OHPs. Effects of surface wetting characteristics on the heat transfer and slug motion of oscillating heat pipes were investigated experimentally and theoretically. The treated surfaces of the OHPs included the superhydrophilic, hydrophilic, copper, hydrophobic, and superhydrophobic surfaces with the average contact angles of0°,12.9°,73.4°,141.5°and155.5°, respectively. Deionized water was used as the working fluid and the heat mode was vertical bottom-heating mode.The liquid slug movements became stronger both in superhydrophilic and hydrophilic OHPs as opposed to the copper OHP, while the global heat transfer performance of the superhydrophilic and hydrophilic OHPs increased for the six-turn OHP. The thin liquid film length at the end of slug increased with the heat input and the surface hydrophilicity increasing. The liquid-vapor interface length for the superhydrophilic and hydrophilic surfaces was higher than that for the copper surface. Compared with the copper channel, the average liquid-vapor interface length for the superhydrophilic and hydrophilic surface was increased by43%-406%and0%-106%, respectively. The average amplitude of the liquid slug for the superhydrophilic and hydrophilic OHPs increased by0%-196%and0%-120%, respectively, in comparison with that of copper OHPs. The average velocity of the liquid slug in the superhydrophilic and hydrophilic OHPs increased by0%-344%and0%-300%, respectively. The thermal performance of OHPs mainly depended on slug oscillations. The heat transfer performance of superhydrophilic OHPs and hydrophilic OHPs increased by5%～15%and15%-25%, respectively, in comparison with that of copper OHPs. The hydrophilic OHP also showed a better startup performance than the copper OHP. The dryout phenomenon occurred in the hydrophobic OHP with motionless liquid slugs and the thermal resistance was significantly increased in the hydrophobic OHP.Effects of superhydrophobic surface and superhydrophobic and superhydrophilic hybrid surface on the fluid flow and heat transfer of OHPs were investigated. The inner surfaces of the OHPs were hydrophilic surface (copper), hybrid surface (superhydrophilic evaporation and superhydrophobic condensation section), and uniform superhydrophobic surface, respectively. Droplets roll easily on the superhydrophobic surface. Experimental results showed that the superhydrophobic surface influenced the slug motion and thermal performance of OHPs. Visualization results showed that the liquid-vapor interface was concave in the copper OHP. A thin liquid film existed between the vapor plug and the OHP wall. On the contrary, the liquid-vapor interface took a convex profile in the OHP with superhydrophobic surface and the liquid-vapor interface length in the hybrid surface OHP was higher than that in the uniform superhydrophobic surface OHP. The liquid slug movements became stronger in the hybrid surface OHPs as opposed to the copper OHP, while the global heat transfer performance of the hybrid surface OHPs increased by5%-20%. Comparing with the copper OHPs, the average amplitude and velocity of the liquid slug movements in the hybrid surface OHPs increased by0%-116%and0%-347%, respectively. However, the average amplitude and velocity of the liquid slug movements in the uniform superhydrophobic OHPs was decreased by5%～100%and15%～100%, respectively. The partial dryout phenomenon took place in OHPs with uniform superhydrophobic surface. The liquid slug movements became weaker and the thermal resistance was increased by10%-35%in the superhydrophobic surface OHPs.The effect of surface contact angle on the oscillation flow and heat transfer in the OHPs was investigated theoretically. The evaporation and condensation heat transfer rates were calculated through the film thickness and the effect of advancing and receding contact angles at the end of the slug as the slug movement was considered in the model. Numerical results showed that the surface contact angles have a profound effect on the OHP characteristics. The maximum displacement and velocity of OHP increased with the decrease of surface contact angle and increase of inner diameter. Compared to the copper OHP (θa=85°,θ=33°), the maximum displacement and velocity of the superhydrophilic surface OHP (θa=1°,θr=1°) was increased by47%-75%and70%-97%, respectively. The film thickness and length at the end of the slug increased with the decrease of the surface contact angle and the corresponding heat transfer performance increasing. The maximum liquid film thickness and dynamic meniscus length for the superhydrophilic surface OHP was increased by42%-57%and19%-26%, respectively, compared to the copper OHP. The heat transfer performance was significantly increased in the superhydrophilic OHPs. The thermal resistance of superhydrophilic OHP was decreased by65%-73%, compared to copper OHP.Analysis and prediction of the annular condensation length or transition from annular flow to slug-bubbly flow pattern is important for the condensation heat transfer because the pressure drop and heat transfer coefficient is dramatically correlated with the flow patterns. The condensation liquid was pulled into the corner of the noncircular microchannels due to the effect of surface tension. The thickness in the meniscus region increased along the channel with the decrease of the thin-liquid film region. The condensation annular flow completed when the short side wall of the microchannel was entirely covered by the meniscus region. At this point, the vapor liquid interface presented a circular shape, and the transition flow occurred. The location of transition flow depends on the short side length and the meniscus area ratio (MAR) at the breakup point under the same experimental conditions. The location of the transition flow moves toward the outlet with the increase of the short side length and decrease of the aspect ratio in the rectangular and trapezoidal microchannels. The transition flow occurs further downstream, and the annular flow regime is expanded in the trapezoidal and triangular microchannels compared with the rectangular microchannel with the same hydraulic diameter. The analysis was compared with the experimental data for different cross section microchannels. Most of the predicted values agreed very well with the experimental data with a MRD of30.50%and a MAD of5.15mm. Based on the numerical results and introducing a new parameter-meniscus area ratio (MAR), a dimensionless correlation with was proposed to predict the transition flow location at different cross section microchannels.