Yacai Hu

The interface effect of carbon nanotube suspension on the thermal performance of a two-phase closed thermosyphon

An aqueous solution of carbon nanotubes, treated by a concentrated nitric/sulfuric acid mixture to disentangle the nanotubes, was utilized as the working medium in a two-phase closed thermosyphon to investigate its performance. In comparison with the thermosyphon filled with a distilled water medium, the one filled with carbon nanotube suspension has a high evaporation section wall temperature, incipience temperature, and excursion, as well as thermal resistance. The carbon nantotubes’ nanofluid deteriorates the performance of the gravity-assisted heat pipe. Measurements employing the maximum bubble pressure method demonstrate that suspending carbon nanotubes in bulk water gives rise to increased surface tension. In addition, the contact angle of suspension obtained with the sessile drop method on a copper plate is much smaller than that of water. Alterations of solid-liquid-vapor interfacial properties, arising from the addition of carbon nanotubes, change the boiling mechanism and thus deteriorate the b...

Characteristic boiling curve of carbon nanotube nanofluid as determined by the transient calorimeter technique

Nickel-plated copper sphere is employed as the transient calorimeter to explore the boiling heat transfer characteristics of carbon nanotube (CNT) nanofluid. As compared to water, aqueous gum arabic (GA) solution has an enhanced critical heat flux (CHF), transition boiling, and minimum heat flux in film boiling (Leidenfrost point). CNT nanofluid has higher CHF than GA solution. Better wettability and deposits on the heating surface distort the characteristic boiling curve.

Experimental Measurements of Thermal Conductivity of Wood Species in China: Effects of Density, Temperature, and Moisture Content

Experimental measurements of thermal conductivity of wood were performed using the heat flow meter and transient plane source technique. The specimens were prepared from five species of both softwoods and hardwoods widely available and used in China, with a wide range of density and moisture content. The transverse thermal conductivity of ovendry specimens is presented as a function of density and temperature up to 90°C and is compared with that along the grain direction for two select species. The influence of moisture content up to 23 percent, which is below the typical fiber saturation point of wood, on the transverse thermal conductivity is presented as well. It is shown that the transverse thermal conductivity of wood increases with density, temperature, and moisture content. Linear correlating equations are proposed in terms of these factors.

A Parametric Study of Prandtl Number Effects on Laminar Natural Convection Heat Transfer From a Horizontal Circular Cylinder to Its Coaxial Triangular Enclosure

A parametric study of Prandtl number effects on laminar natural convection heat transfer in a horizontal equilateral triangular cylinder with a coaxial circular cylinder is conducted. The Prandtl number is varied over a wide range from 10−2 to 105, which corresponds to a variety of working fluids. The governing equations with the Boussinesq approximation for buoyancy are iteratively solved using the finite volume approach. It is shown that the flow patterns and temperature distributions are unique for low-Prandtl-number fluids (Pr ≤ 0.1), and are nearly independent of Prandtl number when Pr ≥ 0.7. In addition, the inclination angle of the triangular enclosure is found to noticeably affect the variations of the local Nusselt number, and to have insignificant influence on the average Nusselt numbers for low Rayleigh numbers when Pr ≥ 0.7.

Thermal Conductivity Enhancement of Ethylene Glycol-Based Suspensions in the Presence of Silver Nanoparticles of Various Shapes

In this technical brief, the effect of adding silver (Ag) nanoparticles of various shapes on the thermal conductivity enhancement of ethylene glycol (EG)-based suspensions was investigated experimentally. These included Ag nanospheres (Ag NSs), Ag nanowires (Ag NWs), and Ag nanoflakes (Ag NFs). Measurements of the thermal conductivity of the suspensions were performed from 10 to 30 °C at an increment of 5 °C. It was shown that the thermal conductivity of the EG-based suspensions increases with raising the temperature. The Ag NWs of a high aspect ratio (∼500) caused greatest relative enhancement up to 15.6% at the highest loading of nearly 0.1 vol. %, whereas the other two shapes of nanoparticles, Ag NSs and Ag NFs with much smaller aspect ratios, only led to enhancements up to 5%. The formation of a network of Ag NWs that facilitates heat conduction was likely responsible for their better performance. The relative enhancement was also predicted by the Hamilton-Crosser model that takes the particle shape effect into consideration. It was shown that the predictions far underestimate the thermal conductivity enhancements but are qualitatively consistent with their shape dependence. As a penalty, however, the presence of Ag NWs was shown to give rise to significant increase in the viscosity of the EG-based suspensions.

Enhanced Critical Heat Flux During Quenching of Extremely Dilute Aqueous Colloidal Suspensions With Graphene Oxide Nanosheets

In this Technical Brief, we report on preliminary results of an experimental investigation of quenching of aqueous colloidal suspensions with graphene oxide nanosheets (GONs). Extremely dilute suspensions with only 0.0001% and 0.0002% (in mass fraction) of GONs were studied and their critical heat fluxes (CHF) during quenching were determined to increase markedly by 13.2% and 25.0%, respectively, as compared to that of pure water. Such efficient CHF enhancement was interpreted to be caused primarily by the improved wettability of the quenched surfaces, due to deposition of the fish-scale-shaped GONs resulting in self-assembly quasi-ordered microscale morphologies.

The consistency of product design and brand image

Through the analysis of consumers, products and brand image, this paper intercept product design and brand image, analysis brand image by understanding consumer motivation, product design processes and brand development, summarize design methods suitable for brand products, so as to provide some inspiration and ideas for product design study.

An Experimental Study of Thermal Performance of a Two-Phase Loop Thermosyphon (TPLT)-Based Steam Generator: Effects of Thermal Boundary Conditions

For the Parabolic trough collector (PTC) system, thermal boundary condition of the receiver (or heating section) is important for the thermal optimization. In this work, effects of thermal boundary on thermal performance of the two-phase loop thermosyphon (TPLT) natural circulation PTC system was investigated experimentally. Three kinds of thermal boundary heating conditions (upper and lower half, and whole circular heated) and two filling ratios (FR = 0.6, 1.2) were adopted in this paper. The results show that half heating condition can improve heat transfer performance in receiver and system thermal resistance. But the preferred half heating boundary was varied as the filling ratio was changed. However, a lower thermal efficiency was observed for the partly heating boundary conditions. For a low heat flux condition in this work, the effects of thermal boundary on flow instability were not obvious, especially for the bigger filling ratio condition.Copyright

Enhanced Thermal Conductivity of Ethylene Glycol-Based Suspensions in the Presence of Silver Nanoparticles of Various Sizes and Shapes

Engineered suspensions in the presence of highly-conductive nanoparticles, coined as nanofluids, have been studied extensively as a novel family of advanced heat transfer fluids. Attention has been paid primarily to the enhanced thermal conductivity of the suspensions that depends significantly on the material, size, shape, dispersion and loading of the nanoparticles. In this paper, the effects of adding silver (Ag) nanoparticles of various sizes and shapes on the thermal conductivity of ethylene glycol (EG)-based suspensions were investigated experimentally. These included Ag nanospheres (Ag NSs), Ag nanowires (Ag NWs) and Ag nanoflakes (Ag NFs). The suspensions were prepared at concentrations of 1, 5 and 10 mg/mL. The size and shape of the various Ag nanoparticles were observed by means of microscopy techniques. The dispersion and stability of the suspensions were also inspected. Measurements of the thermal conductivity of the suspensions were performed on a Hot Disk Thermal Constants Analyzer, which is based on the transient plane source technique, at elevated temperatures from 10 to 30 °C at an increment of 5 °C. It was shown that the thermal conductivity of the EG-based suspensions increases with raising the temperature. The Ag NWs of a very high aspect ratio (∼400) caused greatest relative enhancement up to 15.6% at the highest loading of 10 mg/mL (∼0.1 vol.%). The other two types of nanoparticles, Ag NSs and Ag NFs with much smaller aspect ratios, only led to enhancements up to 5%. The formation of a network of Ag NWs that facilitates heat conduction was likely responsible for their better performance. In addition, the relative enhancement was predicted by the Hamilton-Crosser (H-C) equation that takes the shape effect of the particles into consideration. It was shown that the predictions far underestimate the thermal conductivity enhancements but are qualitatively consistent with their shape dependence.Copyright

A Numerical Investigation of Constrained Melting of Nanostructure-Enhanced Phase Change Materials in a Rectangular Cavity Heated From Below

A numerical study of constrained melting of nanostructure-enhanced phase change materials (NEPCM) consisting of eicosane and various loadings of CNTs in a rectangular cavity heated from below was performed. Assuming that the NEPCM are single-phase PCMs with homogeneous thermophysical properties, the problem was solved using a finite volume method based on the enthalpy-porosity scheme for solid-liquid phase change. The effective thermophysical properties of NEPCM were predicted using the mixture models and empirical equation with respect to the loading of CNTs. Three nominal Grashof numbers corresponding to three sizes of the cavity were considered. Evolutions of the constrained melting processes were presented by means of snapshots of the temperature contour at representative time instants. The melting rates and local heat transfer along the heated bottom were compared quantitatively based on the variations of the instantaneous liquid fraction and average Nusselt number over the bottom during melting, respectively. It was shown that at a given size of the cavity, melting was expedited as more CNTs were introduced. The expediting of melting was mainly attributed to the enhanced thermal conductivity and lowering of latent heat of fusion of NEPCM. The inclusion of CNTs, however, increases considerably the viscosity of melted NEPCM, which in turn leads to less significant natural convection effect during melting. As a result, increase of loading of CNTs was shown to lead to two competing effects. The feasibility of NEPCM in melting is justified when the enhanced heat conduction overweighs the suppressed natural convection.Copyright