Liquid convective heat transfer in microchannels exhibits high efficiency.However,the large pressure drop causes much concern for practical application.Water flowing in hydrophobic tubes shows low pressure drop owing ...Liquid convective heat transfer in microchannels exhibits high efficiency.However,the large pressure drop causes much concern for practical application.Water flowing in hydrophobic tubes shows low pressure drop owing to the slippage on the tube walls.Super-hydrophobic/hydrophilic micro aluminum tubes of 0.68mm inner diameter were fabricated with a two step procedure of chemical etching and then surface chemical modification.A kind of micro-nanometric hierarchical structure was formed on the surface,which could trap air serving as the slip agent.Heat transfer and fluid flow of deioned water flowing laminary in the super-hydrophobic/hydrophilic microchannels were studied experimentally.The results showed that the air-layer existing in the micro-nano hierarchical structures of the super-hydrophobic surface decreased flow resistance evidently but decreased heat transfer coefficient only a little,which was still higher than the superficial heat transfer coefficient while considering the heat conduction of stationary nanolayer of air.It was supposed that eddy flow was generated in the micro-nano bubbles by the slip flow of water on the tube wall,which enhanced the heat transfer.展开更多
Here we report a multiyear study on the surface roughness length and bulk transfer coefficients over the degraded grassland and cropland surfaces in a semiarid area of China. Eddy covariance measurement and the meteor...Here we report a multiyear study on the surface roughness length and bulk transfer coefficients over the degraded grassland and cropland surfaces in a semiarid area of China. Eddy covariance measurement and the meteorological profile observation data were used to analyze characteristics of these parameters on the diurnal, seasonal, and annual scales. Significant seasonal and annual variations of the aerodynamic roughness length are observed over the two surfaces. A large variation of kB-1 is measured during the day. Both kB-1 and the bulk transfer coefficients exhibit significant seasonal and annual variations. During the growing season (May to October), average Cd and Ch are 3.1×10-3 and 2.5×10-3 over the degrade grassland surface, and 4.7×10-3 and 3.1×10-3 over the cropland surface respectively. During the non-growing season, average Cd and Ch are 2.3×10-3 and 2.0×10-3 over the degrade grassland surface, and 2.9×10-3 and 2.2×10-3 over the cropland surface respectively.展开更多
文摘Liquid convective heat transfer in microchannels exhibits high efficiency.However,the large pressure drop causes much concern for practical application.Water flowing in hydrophobic tubes shows low pressure drop owing to the slippage on the tube walls.Super-hydrophobic/hydrophilic micro aluminum tubes of 0.68mm inner diameter were fabricated with a two step procedure of chemical etching and then surface chemical modification.A kind of micro-nanometric hierarchical structure was formed on the surface,which could trap air serving as the slip agent.Heat transfer and fluid flow of deioned water flowing laminary in the super-hydrophobic/hydrophilic microchannels were studied experimentally.The results showed that the air-layer existing in the micro-nano hierarchical structures of the super-hydrophobic surface decreased flow resistance evidently but decreased heat transfer coefficient only a little,which was still higher than the superficial heat transfer coefficient while considering the heat conduction of stationary nanolayer of air.It was supposed that eddy flow was generated in the micro-nano bubbles by the slip flow of water on the tube wall,which enhanced the heat transfer.
基金supported by the National Basic Research Program of China (Grant Nos. 2010CB951801 and 2006CB400500)
文摘Here we report a multiyear study on the surface roughness length and bulk transfer coefficients over the degraded grassland and cropland surfaces in a semiarid area of China. Eddy covariance measurement and the meteorological profile observation data were used to analyze characteristics of these parameters on the diurnal, seasonal, and annual scales. Significant seasonal and annual variations of the aerodynamic roughness length are observed over the two surfaces. A large variation of kB-1 is measured during the day. Both kB-1 and the bulk transfer coefficients exhibit significant seasonal and annual variations. During the growing season (May to October), average Cd and Ch are 3.1×10-3 and 2.5×10-3 over the degrade grassland surface, and 4.7×10-3 and 3.1×10-3 over the cropland surface respectively. During the non-growing season, average Cd and Ch are 2.3×10-3 and 2.0×10-3 over the degrade grassland surface, and 2.9×10-3 and 2.2×10-3 over the cropland surface respectively.
