Thermally driven bubble evolution at a heater wire in water characterized by high-speed transmission electron microscopy

Using in situ high-speed transmission electron microscopy, we investigate the nucleation and thermocapillary migration of microbubbles in water.

Effect of Densification Conditions on Physical Quality of Densified Sorghum Straw Briquette Fuel

An experiment was conducted on densification processing of sorghum straw stalks with self-made steel molds on a hydraulic universal test platform. The influences of different densification conditions on physical quality of briquette fuel are investigated, including pressures, temperatures, moisture contents and straw preheating method. Moreover, the relationship between molding temperature and inner stress decreasing rate of straw briquette at holding pressure stage is also analyzed. The result shows that the suitable value of moisture content, temperature and molding pressure for straw compression process is 20%, 90°C and at least 50MPa, respectively. The phenomenon of uneven temperature distribution in straw briquette keeps in existence even in the case of microwave preheating. However, in contrast with the very low central temperature in straw briquette when preheating by electric heater wire, the phenomenon of local overheating caused by microwave preheating may be helpful to straw molding process.

An Experimental Study of Surface Tension-Dependent Pool Boiling Characteristics of Carbon Nanotubes-Nanofluids

This paper reports an experimental investigation of the pool boiling heat transfer characteristics of single-walled carbon nanotubes (SWCNTs)-nanofluids. Two main characteristics were studied to identify their influence on boiling heat transfer: one is the surface tension through the addition of surfactant and the other is the chemical treatment of nanotubes sidewalls (i.e. oxidized and untreated sidewalls). A Transmission Electron Microscope was used to study the morphology of the functionalized nanotubes and their deposition on heater wire. The maximum enhancement of both the critical and burnout heat fluxes of this nanofluid over those of the pure deionized water are found to be 492% and 265%, respectively at a surfactant to carbon nanotubes concentration ratio of 1:5. This indicates that high enhancement of heat flux is possible and would depend on the concentration of the surfactants. Present results also demonstrate that CNT-nanofluids in a pool boiling environment can extend the saturated boiling regime and the burnout of the heated surface. The burnout heat flux is found to be a strong function of the relaxation of nanofluid surface tension with the base fluid. Based on the best fit of experimental data, an empirical correlation between the burnout heat flux of nanofluid and its relaxation of surface tension is introduced.

Electric Field Effects on the Stability of Vapor Microlayers: Enhancement of Heat Transfer

The effect of dc electric fields on destabilization of a vapor microlayer formed during film boiling at various subcooling levels is investigated. High voltage electric fields up to 2000 volts were applied between a 127 μm heater wire and a screen electrode that is concentrically placed at a radius of 25 mm. The qmax and qmin heat fluxes were also measured for the various subcooling and electric field strengths. Up to 50% increase in the qmax and the qmin heat fluxes were observed when using the electric fields in this range of experimental parameters. The relationship among subcooling level for a given fluid, the heat flux level, and the electric field strength required to reach the qmin condition is of interest. The preliminary experimental results and the bubble departure and transition boiling patterns resulting from destabilization of the vapor microlayer are discussed.