Scale Analysis of Thermocapillary Weld Pool Shape With High Prandtl Number

The molten pool shape and thermocapillary convection during melting or welding of metals or alloys are self-consistently predicted from parametric scale analysis for the first time. Determination of the molten pool shape is crucial due to its close relationship with the strength and properties of the fusion zone. In this work, surface tension coefficient is considered to be negative values, indicating an outward surface flow, whereas high Prandtl number represents the thermal boundary layer thickness to be less than that of momentum. Since Marangoni number is usually very high, the scaling of transport processes is divided into the hot, intermediate and cold corner regions on the flat free surface, boundary layers on the solid-liquid interface and ahead of the melting front. Coupling among distinct regions and thermal and momentum boundary layers, the results find that the width and depth of the pool can be determined as functions of Marangoni, Prandtl, Peclet, Stefan, and beam power numbers. The predictions agree with numerical computations and available experimental data.

Thermoelectric Transport in Sb2Te3/Bi2Te3 Quantum Dot Nanocomposites

We investigate the thermoelectric transport properties of Sb2Te3/Bi2Te3 quantum dot nanocomposites with spherical Sb2Te3 quantum dots arrays embedded in Bi2Te3 matrix through a two-channel transport model. In this model, the transport of quantum-confined electrons through the hopping mechanism is studied by tight-binding model together with Kubo formula and Green’s function method. The formation of minibands due to the quantum confinement and the phonon-bottleneck effect on carrier-phonon scattering are considered. The transport of bulk-like electrons is studied by Boltzmann-transport-equation-based model. We consider the intrinsic carrier scatterings as well as the carrier-interface scattering of these bulk-like electrons. Thermoelectric transport properties are studied with different quantum dot sizes, inter-dot distances, doping concentrations, and temperatures. We find that electrical conductivity and Seebeck coefficient can be enhanced simultaneously in Sb2Te3/Bi2Te3 quantum dot nanocomposites because of the formation of minibands and the phonon-bottleneck effect on carrier-phonon scattering. Our results could shed some light on the design of high-efficiency thermoelectric materials.

Heat Transfer Between Liquid Film Formed on the Inclined Dimpled Surface and Ambient Air

Heat transfer enhancement area attracts the close attention of the researchers and engineers worldwide for the last decades. The most popular techniques nowadays to enhance heat transfer from the surface is to extend it with the fins, studs, etc. or to profile it with the elements of artificial roughness, winglets, dimples, etc. Those types of surface enhancement allow improving the thermal efficiency of the heat transfer equipment with minimal design modification and without significant capital expenses. One of the interesting and promising techniques of the surface profiling is the formation on the surface the arrangement of spherical dimples, which generate intensive vortex structure near the surface, increase flow turbulence and, as a result, enhance heat and mass transfer between a profiled surface and a liquid (or gas) flowing over it [1–3]. In this connection, it is interesting to establish whether surface profiling will also enhance the heat transfer intensity between a liquid film on such a surface and ambient air. Unfortunately, authors were not able to find any publications on this subject in the open domain. At the same time, the investigation of this process could be of great interest for the engineering practice, in particular, for the cooling towers advancement. In the present work, the authors discuss some experimental results obtained for the different profile parameters and flow regimes.

Numerical Analysis on Nanofluid Forced Convection in Ducts With Triangular Cross Sections

Heat transfer of fluids is very important to many industrial heating or cooling equipments. Convective heat transfer can be enhanced passively by changing flow geometry, boundary conditions or by enhancing the thermal conductivity of the working fluids. An innovative way of improving the fluid thermal conductivity is to introduce suspended small solid nanoparticles in the base fluids. In this paper a numerical investigation on laminar forced convection flow of a water–Al2O3 nanofluid in a duct having an equilateral triangular cross section is performed. The hydraulic diameter is set equal to 1.0×10−2 m. A constant and uniform heat flux on the external surfaces has been applied and the single-phase model approach has been employed. The analysis has been run in steady state regime for a nanoparticle size equal to 38 nm, considering different volume particle concentrations. The CFD code Fluent has been employed in order to solve the tri-dimensional numerical model. Results are presented in terms of temperature and velocity distributions, surface shear stress and heat transfer convective coefficient, Nusselt number and required pumping power profiles. Comparison with results related to the fluid dynamic and thermal behaviors in pure water are carried out in order to evaluate the enhancement due to the presence of nanoparticles in terms of volumetric concentration.

The Coupling Effect of Driving Forces to the Anti-Gravity Flow in the Spiral Micro-Channel

In this paper, the anti-gravity flow in the spiral micro-channel on the surface of horizontal tube was visualized by the three-dimensional ultra-microscope system. The coupling relationship between the driving force and the flow was studied by Onsager reciprocal relations. The results show that the formation of the anti-gravity flow in the spiral micro-channel on the surface of horizontal tube is impacted by the combining effect of several factors, such as the capillary pressure, wettability, temperature, and bubbles.

Microstructured Surfaces for Enhanced Pool Boiling Heat Transfer

We experimentally investigated pool boiling on microstructured surfaces which demonstrate high critical heat flux (CHF) by enhancing wettability. The microstructures were designed to provide a wide range of well-defined surface roughness to study roughness-augmented wettability on CHF. A maximum CHF of 196 W/cm2 and heat transfer coefficient (h) greater than 80 kW/m2K were achieved. To explain the experimental results, a model extended from a correlation developed by Kandlikar was developed, which well predicts CHF in the complete wetting regime where the apparent liquid contact angle is zero. The model offers a first step towards understanding complex pool boiling processes and developing models to accurately predict CHF on structured surfaces. The insights gained from this work provide design guidelines for new surface technologies with higher heat removal capability that can be effectively used by industry.

Tip-Sample Heat Transfer in Non-Contact Mode Scanning Thermal Microscopy With Wollaston Microprobes

Scanning thermal microscopy (SThM) is an attractive tool for high spatial resolution thermal characterization with minimal sample preparation.1 SThM measurements are usually performed in contact-mode, which entails multiple tip-sample heat transfer pathways, i.e. across air gap, liquid meniscus, and the solid contact. These hinder the quantification of the sample temperature or thermal properties or result in large uncertainties.2

Wetting Behavior and Drainage of Water Droplets on Microgrooved Brass Surface

In the present study, we report the contact angle hysteresis and drainage behavior of water drops on a number of brass surfaces with parallel microgrooves and compare them to the flat baseline surfaces. Parallel micro-grooves with different groove dimensions are fabricated by micro end-milling process without any modification of the surface chemistry. Advancing and receding contact angles in both parallel and perpendicular direction of the grooves and also the drainage behavior of water droplets on the microgrooved surfaces is found to be considerably affected by change in groove geometry parameters. Very high hysteresis is observed for both low (< 0.2) and high aspect ratio (> 0.7) of grooves and also for surfaces with lower groove spacing due to the droplets being in a Wenzel state. For intermediate aspect ratio (0.25–0.70) and larger spacing of the grooves, droplets remain in a Cassie state and the hysteresis is lower in both directions than that on the flat surfaces. Variation of critical sliding angle (angle at the point of incipient sliding of water droplets due to gravity) with groove geometry is investigated for a range of water droplet volume of 15 to 75 μl. Droplets of all volumes are found to slide much more readily on grooved surfaces than when placed on the flat baseline surfaces. Height and spacing of the grooves are also found to have significant influence on the sliding of the water droplets, as critical inclination angle decreased with groove height and increased with groove spacing. The results from this study may be useful in a broad range of applications where water retention plays an important role.

Thermal Radiation in Silica Aerogel and its Composite Insulation Materials

This paper engages in experimental measurements on thermal radiative transfer in silica aerogel and its composite insulation materials (xonotlite-aerogel composite and ceramic fibre-aerogel composite). The samples of silica aerogel, xonotlite-type calcium silicate, and ceramic fibre insulation materials are all considered as a semi-transparent medium capable of absorbing, emitting and scattering thermal radiation. The spectral transmittances are then measured at different infrared wavelengths ranging from 2.5 to 25μm with a Fourier transform infrared spectrometer (FTIR), and subsequently used to determine the specific spectral extinction coefficient and the specific Rossland mean extinction coefficient of the sample. The radiative conductivities deduced from the overall thermal conductivities measured with the transient hot-strip (THS) method are compared with the predictions from the diffusion approximation by using the measured spectral extinction coefficient. The results show that the spectral extinction coefficients of the samples are strongly dependent on the wavelength, particularly in the short wavelength regime (<10μm). The total Rossland mean extinction coefficients of the samples are all decreasing with the temperature increasing. The radiative conductivities are found almost proportional to the cubic temperature, and decreases as the sample density increases.