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The molten pool shape and transport variables induced by thermocapillary force during welding (or melting) of workpieces, such as pure iron, titanium, high speed steel and stainless steel alloys, can be self-consistently predicted from scale analysis. Determination of the molten pool shape and transport variables is crucial due to their close relationship with the microstructures, strength and properties of the fusion zone. In this study, the pool excludes a strongly wavy bottom and the surface velocity profile has two peaks valid for Prandtl numbers lying between 0.3 and unity. The surface tension coefficient is negative and suitable for all pure liquid metals and alloys containing minor surface active solutes, giving rise to an outward surface flow. In view of high Marangoni number, the domain of scaling is divided into the hot and cold corner regions, boundary layers on the solid–liquid interface and ahead of the melting front. The results find that the width and depth of the pool, peak and secondary peak surface velocities, and maximum temperatures in the hot and cold corner regions can be explicitly and separately determined as functions of working variables, or Marangoni, Prandtl, Peclet, Stefan, and beam power numbers and solid-to-liquid thermal conductivity ratio. The scaled results agree with numerical data and available experimental data. This work has academic and practical importance. Successful scaling not only reveals physical mechanisms, but also provides quantitative predictions of the fusion zone shapes and transport variables prior to melting or welding.

Critical heat flux (CHF) and pressure drop of subcooled flow boiling are measured for a microchannel heat sink containing 75 parallel 100 μm × 200 μm structured surface channels. The heated surface is made of a Cu metal sheet with/without 2 μm thickness diamond film. Tests and measurements are conducted with de-ionized water, de-ionized water +1 vol.% MCNT additive solution, and FC-72 fluids over a mass velocity range of 820–1600 kg/m2 s, with inlet temperatures of 15(8.6)°C, 25(13.6)°C, 44(24.6)°C, and 64(36.6)°C for DI water (FC-72), and heat fluxes up to 600 W/cm2. The CHF of subcooled flow boiling of the test fluids in the microchannels is measured parametrically. The two-phase pressure drop is also measured. Both CHF and the two-phase friction factor correlation for one-side heating with two other side-structured surface microchannels are proposed and developed in terms of the relevant parameters.

The effects of Prandtl number on two-dimensional thermocapillary convection and molten pool shape induced by negative surface tension coefficient in welding or melting with a time-dependent and distributed incident flux are numerically predicted in this study. This work is also applicable for predicting the quasi-steady three-dimensional thermocapillary convection. In this model, the time-dependent incident flux is specified as a function of scanning speed and energy distribution parameter. The computed flow patterns and molten pool shapes under the flat free surface are found to have distinct regions for different Marangoni and Prandtl numbers. Prandtl number indicates the thickness ratio between momentum and thermal boundary layers, whereas Marangoni number with negative surface tension coefficient induces an outward surface flow. Rather than the enhanced pool depth resulted from melting from an induced vortex cell near the bottom in the case of Prandtl number much less than unity, the molten pool shape for Prandtl number much greater than unity can produce a thin and narrow edge. The variations of Peclet number and dimensionless beam power with flow and temperature fields and fusion zone shapes are similar. The dimensionless peak surface speed versus product of Marangoni and Prandtl number, which involve predicted peak speed and temperature and molten pool width on the surface, agree with scale analysis and experimental data provided in the literature.

Experiments were performed to study the heat transfer characteristics of channel flows of deionized (DI) water, methanol, 50 wt% DI water/50 wt% methanol mixture, and ethanol solution in asymmetrically (one sided heating) heated rectangular microchannels with an aspect ratio (H/W) of 0.56 and the corresponding hydraulic diameters ( D ) of 129 μm at 5 ⩽ Re ⩽ 240. Local heat transfer coefficients distribution were recorded with both isothermal (273 K) and isoflux (12.6, 18.1, 32.3, 50.5 mW/mm 2 ) heating. The influences of test liquid mass flow rates, test fluids, heating condition (isothermal vs isoflux), and surface condition (hydrophilic vs hydrophobic) on heat transfer behavior were examined. Thermal entrance length were also found and correlated in terms of the relevant parameters.

An experimental and theoretical study of low Reynolds number compressible gas flow in a microchannel is presented. Nitrogen gas was used. The channel was microfabricated on an oxidized silicon wafer and was 50 μm deep, 200 μm wide and 24,000 μm long. The Knudsen number ranged from 0.001 to 0.02. Pressure drop at inlet and exit of the channel were measured and friction factor constant ratios were calculated at different mass flow rates in terms of Re. The results were found in good agreement with those predicted by analytical solutions in which a 2-D continuous flow model with first slip boundary conditions are employed and solved by a perturbation method with a proposed new complete momentum accommodation coefficient σ. Consequently, using slip boundary conditions, one can well predict the mass flow rate as well as inlet/exit pressure drop and friction factor constant ratio for a three-dimensional physical system.