Based on the entropy generation concept of thermodynamics, this paper estabfished a general theoretical model for the analysis of entropy generation to optimize fins, in which the minimum entropy generation was select...Based on the entropy generation concept of thermodynamics, this paper estabfished a general theoretical model for the analysis of entropy generation to optimize fins, in which the minimum entropy generation was selected as the object to be studied. The irreversibility due to heat transfer and friction was taken into account so that the minimum entropy generation number has been analyzed with respect to second law of thermodynamics in the forced cross-flow. The optimum dimensions of cylinder pins were discussed. It＇s found that the minimum entropy generation number depends on parameters related to the fluid and fin physical parameters. Varlatioms of the minimum entropy generation number with different parameters were analyzed.展开更多
In ambient condition,capillary forces are the major contributors to the adhesive forces between the tip of an atomic force microscope(AFM) and the sample.In general,capillary forces are thought to be related to water ...In ambient condition,capillary forces are the major contributors to the adhesive forces between the tip of an atomic force microscope(AFM) and the sample.In general,capillary forces are thought to be related to water film thickness,contact time and relative humidity and so on.In this paper,an original analysis regarding the liquid bridge,based on the surface and interface thermodynamic theory,is proposed.The cases covered in the study include the capillary forces and temperature of liquid bridge for quickly drawn liquid bridge,and for nonvolatile liquid bridge.The study results show that variation in temperature may occur in the liquid bridge when it is stretched.展开更多
Dihedral fullerenes are thermodynamically stable molecules with Dnd or Dnh symmetry. Based on experimental findings, two series of dihedral fullerenes with five-fold （C5） and six-fold （C6） symmetry have been studi...Dihedral fullerenes are thermodynamically stable molecules with Dnd or Dnh symmetry. Based on experimental findings, two series of dihedral fullerenes with five-fold （C5） and six-fold （C6） symmetry have been studied using density functional theory （DFT）. The DFT calculations showed that for both series the stabilities increased with increasing fullerene size. Structural analyses indicated that the stabilities are related to specific local geometries. In the case of the more abundant C5 series, the presence of approximately planar pentagons and hexagons on the top bowl favors their formation. That is to say, those fuller- enes with small dihedral angles within the polygons are readily formed, because planar hexagons lead to strengthened conjuga- tion which lowers average bonding energies （ABE） and increases thermodynamic stabilities. Non-planar hexagons at equatorial positions in tube-shaped fullerenes have an adverse effect on the conjugation and inhibit their formation. Calculations also demonstrated that fullerenes in the two series, including C50（D5h）, C60（O6h）, C80（O5d）, C96（D6d）, Cllo（D5h）, and Cl20（D5d）, have thermodynamically stable triplet structures with strong conjugation. The calculated IR and 13C NMR spectra of the fullerenes show some similarities and regular trends due to their homogenous structures. The electronic structures indicate that short dou- ble bonds in hexagons with high electron occupancies are readily attacked by electrophilic agents and can also be coordinated by transition metals. Mechanistic discussions suggested that C2 additions and C2 losses constitute reversible processes at high temperature and C2 additions in pentagonal fusions are crucial to the kinetics of the curvature of structures. C3 additions lead to the formation of large fullerenes of other types.展开更多
文摘Based on the entropy generation concept of thermodynamics, this paper estabfished a general theoretical model for the analysis of entropy generation to optimize fins, in which the minimum entropy generation was selected as the object to be studied. The irreversibility due to heat transfer and friction was taken into account so that the minimum entropy generation number has been analyzed with respect to second law of thermodynamics in the forced cross-flow. The optimum dimensions of cylinder pins were discussed. It＇s found that the minimum entropy generation number depends on parameters related to the fluid and fin physical parameters. Varlatioms of the minimum entropy generation number with different parameters were analyzed.
基金supported by the National Natural Science Foundation of China(Grant No.11072024)the Young Scholars Fund of Beijing University of Chemical Technology(Grant No.QN0716)
文摘In ambient condition,capillary forces are the major contributors to the adhesive forces between the tip of an atomic force microscope(AFM) and the sample.In general,capillary forces are thought to be related to water film thickness,contact time and relative humidity and so on.In this paper,an original analysis regarding the liquid bridge,based on the surface and interface thermodynamic theory,is proposed.The cases covered in the study include the capillary forces and temperature of liquid bridge for quickly drawn liquid bridge,and for nonvolatile liquid bridge.The study results show that variation in temperature may occur in the liquid bridge when it is stretched.
文摘Dihedral fullerenes are thermodynamically stable molecules with Dnd or Dnh symmetry. Based on experimental findings, two series of dihedral fullerenes with five-fold （C5） and six-fold （C6） symmetry have been studied using density functional theory （DFT）. The DFT calculations showed that for both series the stabilities increased with increasing fullerene size. Structural analyses indicated that the stabilities are related to specific local geometries. In the case of the more abundant C5 series, the presence of approximately planar pentagons and hexagons on the top bowl favors their formation. That is to say, those fuller- enes with small dihedral angles within the polygons are readily formed, because planar hexagons lead to strengthened conjuga- tion which lowers average bonding energies （ABE） and increases thermodynamic stabilities. Non-planar hexagons at equatorial positions in tube-shaped fullerenes have an adverse effect on the conjugation and inhibit their formation. Calculations also demonstrated that fullerenes in the two series, including C50（D5h）, C60（O6h）, C80（O5d）, C96（D6d）, Cllo（D5h）, and Cl20（D5d）, have thermodynamically stable triplet structures with strong conjugation. The calculated IR and 13C NMR spectra of the fullerenes show some similarities and regular trends due to their homogenous structures. The electronic structures indicate that short dou- ble bonds in hexagons with high electron occupancies are readily attacked by electrophilic agents and can also be coordinated by transition metals. Mechanistic discussions suggested that C2 additions and C2 losses constitute reversible processes at high temperature and C2 additions in pentagonal fusions are crucial to the kinetics of the curvature of structures. C3 additions lead to the formation of large fullerenes of other types.
