Investigation On Flow And Phase Change Heat Transfer Characteristics Of Hydrophilic And Hydrophobic Surfaces

The applications of superhydrophilic?SHi?,superhydrophobic?SHS?and SLIPS?Slippery liquid-infused porous surface?surfaces in flow and phase change heat transfer fields are very important,and it has attracted more and more attentions in recent years.The applications of the surfaces with different wettability in heat transfer fields are rarely investigated.The SHS can retard the freezing time of droplet,and it also needs more investigation.The SHS can reduce the flow resistance,and simultaneously reduces the convective heat transfer,and it's comprehensive thermodynamic performance is worth investigation.The SHi can promote the liquid evaporation and enhance the evaporation heat transfer coefficient,and the role of micro-nanostructures in evaporation heat transfer is also needed to be further investigated.The condensation nucleation,coalescence,removal and sweeping of droplets on SLIPS surface are needed to be further investigated,especially for the two-phase closed thermosyphon with SLIPS condenser and SHi evaporator?TPCT-SHiSL?the roles of lubricant in condensation and evaporation need to be further investigated,and there has no research on this aspect.The main contents involved in the present thesis are as follows:The characteristics of droplet freezing retard on aluminum SHS and the relationship between temperature and surface free energy have been investigated,and the influences of micro-nanostructured morphologies,surface free energy and energy barrier of heterogeneous nucleation on droplet freezing proceses are also analyzed.The aluminum substrate has anti-icing properties after the chemical etching and fluorination modification,and the contact angle,rolling angle and contact angle hysteresis are 162.5°,1.9°and 1.1°,respectively.The surface free energy of SHS increases with the decrease of the surface temperature,and the energy barrier of heterogeneous nucleation increases with the increase of contact angle.The droplet freezing time delay of SHS is 1568 s at the temperature of-11.9?,but for the sandblasted surface the droplet freezing time delay is 159 s at the temperature of-6.6?,which indicates that SHS has good anti-icing performance.The drag reduction,heat transfer and comprehensive thermodynamic performance of SHS tubes with different diameters are investigated,and the influences of surface free energy and the work of adhesion on flow are also analyzed.The drag reductions range from 8.3%to 17.8%for the SHS tube in the present study,and the drag reduction decreases with the increase of Reynolds number,and it also increases with the decrease of diameter.The impact of SHS on drag reduction increases with the increase of the ratio of surface to volume,and the friction factor of SHS tube is smaller than that of smooth tube.The heat transfer coefficient of SHS tube is smaller than that of smooth tube because the SHS has a large amount of micro-nano scale air cavities which inhibit the heat transfer between wall and liquid.The comprehensive thermodynamic performance reduces with the increase of Reynolds number,and SHS tube with the diameter of 8.0 mm has optimal comprehensive thermodynamic performance.The SLIPS can enhance the condensation heat transfer coefficient,but the lubricant film cloaking the condensed droplets on the SLIPS easily contaminates the other surface after the condensed droplets remove from the SLIPS,which significantly inhibits the applications of SLIPS,especially for the devices such as the thermosyphon simultaneously involves boiling/evaporation and condensation phase change heat transfer.The lubricant can easily contaminate the pristine evaporator and lead the evaporator changing from hydrophilicity to hydrophobicity.The SHi is applied into the evaporator and makes it have superoleophobic properties,and alleviates the degradation of boiling/evaporation caused by lubricant pinning.The innovational gravity thermosyphon with the incorporation of SHi evaporator and SLIPS condenser is compared with various gravitational thermosypons with different configurations,and the surface free energy,energy barrier of heterogeneous nucleation and the number density of nucleation are also analyzed.The condensation heat transfer coefficient of SLIPS is2.5 fold larger than that of other surfaces due to the facilitated nucleation,coalescence and removal of droplet.The impact of lubricant on boiling/evaporation heat transfer of evaporator is significantly inhibited because the SHi evaporator alleviates the contamination of lubricant.The TPCT-SHiSL has lowest thermal resistance for the thermosyphons with differencnt configurations,and the?37?_SHiSL-PP ranges from 0.776 to 0.964,and the enhancement is up to28.9%,which indicates that the TPCT-SHiSL has optimal thermal performance.The impact of different filling ratio on the heat transfer mechanisms of gravitational thermosyphon with SHi evaporator and SLIPS condenser is investigated,and the variation trend of heat transfer coefficient of the TPCT-SHiS is similar with that of TPCT-PP.The film evaporation plays the dominant role for the TPCT-SHiSL when the filling ratios are 40%and70%at low heat fluxes and the filling ratio is 25%,and the boiling plays the dominant role for other cases.The boiling heat transfer plays the dominant role at high heat fluxes because the evaporation heat transfer is inhibited with the increase of heat fluxes.The boiling heat transfer also dominates the heat transfer of evaporator of TPCT-SHiSL at the filling ratio of 98%.For TPCT-SHiSL,the condensation heat transfer performance of SLIPS-reduces because the water is easily carried into the SLIPS condenser and inhibits the shedding of droplets with the increase of filling ratio.The heat transfer performance of TPCT-SHiSL The average?37?_25%/40%??37?_70%/40%and?37?_98%/40%are 0.823,1.010 and 1.104,respectively.Although the total thermal resistance of TPCT-SHiSL with the filling ratio of 25%is smallest for all the filling ratios,the total thermal resistance is significantly increased when the heat flux is slightly increased.The thermal resistance of evaporation is larger than that of condensation of TPCT-SHiSL for all filling ratios.Tradeoff between the total thermal resistance and the heat transfer capacity of TPCT-SHiSL shows that the optimal filling ratio is 40%.In order to reduce the evaporation thermal resistance and enhance the evaporation heat transfer,the film evaporation mechanisms of SHi copper foam have been investigated,and the flim thickness reduces with the increase of heat flux,and it becomes thinner along the height direction.The influence of velocity on convective heat transfer can be ignored because the velocity is at low-speed larminar flow.The thermal resistance along the thickness direction is much lower than that along the height direction,and the heat is mainly transferred by conduction along the thickness direction because the height of SHi copper foam is much larger than that of it's thickness.The boiling phenomenon does not occur because the boiling heat transfer is inhibited,which is caused by the SHi copper foam and low pressure environment in the chamber.The overall heat transfer coefficient of SHi copper foam is small because the overall heat transfer includes heat conduction in the liquid film and the evaporation at the liquid-vapor interface,and the heat conduction thermal resistance plays dominant role due to the heat conduction thermal resistance is much larger than that of the liquid-vapor interface.The interface evaporation heat transfer can be enhanced because the micro-nanostructures on the skeletons of the copper foam can be easily wetted by liquid and simultinously the thin cloaking film can also be formed on the structures.The evaporation heat transfer coefficient at the liquid-vapor interface is much larger than the overall heat transfer coefficient,and the evaporation heat transfer coefficients rang from 7.8 kW/?m²K?to67.9 kW/?m²K?when the heat fluxes increase from 2.3 W/cm² to 25.3 W/cm².