Chih-Yi Lin

Liquid flow in a micro-channel

An experimental study with MPIV (micro particle image velocimetry) flow visualization of incompressible liquid flow in a microchannel is presented. Deionized water served as a working medium. The channel was microfabricated in a PMMA substrate using an excimer laser, and was 115 µm deep, 200 µm wide and 24 000 µm long with a hydraulic diameter of about 146 µm. The pressure drop between the inlet and the outlet of the duct as well as mass flow rates were measured and friction factors were calculated for different Reynolds numbers. Data were discussed and compared with those of previous investigations of similar studies. Moreover, time evolutions of the microflow at the middle of the microchannel at Re = 50, 100, 470 and 900 were photographed. Hydrodynamic entry length correlations were found for both laminar and turbulent flow in the present microchannel.

Scaling Weld or Melt Pool Shape Affected by Thermocapillary Convection With High Prandtl Numbers

The molten pool shape and thermocapillary convection during melting or welding of metals or alloys are self-consistently predicted from scale analysis. Determination of the molten pool shape and transport variables is crucial due to their close relationship with the strength and properties of the fusion zone. In this work, surface tension coefficient is considered to be negative, indicating an outward surface flow, whereas high Prandtl number represents a reduced thickness of the thermal boundary layer compared to that of the momentum boundary layer. Since the Marangoni number is usually very high, the domain of scaling is divided into hot, intermediate and cold corner regions, boundary layers along the solid‐liquid interface and ahead of the melting front. The results show that the width and depth of the pool, peak and secondary 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. The scaled results agree with numerical results and available experimental data. [DOI: 10.1115/1.4005206]

Scaling Thermocapillary Surface Velocity in Weld Pool

In this study, the peak surface velocity driven by thermocapillary force in melting or welding pool irradiated by a distributed low-power-density beam is determined from a scale analysis. In view of different distances of diffusion between momentum and energy, the effects of Prandtl number on surface velocity are of interest. A low-power-density-beam heating implies no deep and narrow cavity (or keyhole) taking place in the pool. The results find that the peak surface velocity is proportional to the first power and 2/3 power of the surface tension coefficient or Marangoni number for high and low Prandtl number, respectively. The free surface velocity is determined by Prandtl and Marangoni numbers for given dimensionless beam power and Peclet numbers. The predictions agree with numerical computations.Copyright

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.Copyright

Dsmc Simulation for constant-wall-temperature Heat Transfer in a Short Parallel Plate Microchannel

The heat transfer characteristics of low speed gas flows through a short parallel plate microchannel (L/D(subscript h)=6) are examined using the direct simulation Monte Carlo (DSMC) method. Computations were carried out for nitrogen, argon, and helium gas. Micro pressure driven flows are simulated with the inlet value of the Knudsen numbers ranging from 0.09 to 0.2. The effects of varying wall temperature, pressure, inlet flow and gas transport properties on the wall heat transfer and velocity distribution were examined. Heat transfer from the channel was also calculated and compared with those of previous studies. Molecular diffusion dominates over both slip and transition flow regimes. Finally, the averaged Nusselt number ((average)Nu) was correlated in simple form of the averaged Peclet number ((average)Pe and Knudsen number ((average)Kn ) in the transition flow regime.

Gaseous Slip Flow in a Micro-Channel

An experimental and theoretical study of low Reynolds number compressible gas flow in a micro channel is presented. Nitrogen gas was used. The channel was microfabricated on silicon wafers and were 50 μm deep, 200 μm wide and 24000 μm long. The Knudsen number ranged from 0.001 to 0.02. Pressure drop were measured at different mass flow rates in terms of Re and 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 perturbation methods.Copyright