Thermocapillary Convection During Laser Surface Heating (Surface Tension)
Chan, Cholik
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Permalink
https://hdl.handle.net/2142/70136
Description
Title
Thermocapillary Convection During Laser Surface Heating (Surface Tension)
Author(s)
Chan, Cholik
Issue Date
1986
Department of Study
Mechanical Engineering
Discipline
Mechanical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Mechanical
Abstract
The fluid flow and the heat transfer during laser surface heating are studied. Surface tension gradient is identified as the dominant force from an order of magnitude consideration. Mathematical models for the thermocapillary convection within the molten pool are thus formulated. The models are solved numerically. A transient two-dimensional model, which gives a qualitative understanding of the flow field and the heat transfer in the spanwise direction, is first solved. It is found that the recirculating velocity is much higher than the scanning speed. A perturbation solution is therefore sought for the three-dimensional case. The basic solution corresponds to the stationary axisymmetric case. The scanning case is obtained by perturbing the basic solution with the scanning speed. Numerical solutions are obtained. The effects of the thermocapillary convection in the melt pool on pool geometry, cooling rate and solute redistribution are presented and discussed. The results are limited for slow scanning speeds. Experiments were performed to examine the validity of the numerical results. A stagnation flow analysis, which describes the dominant and common feature of thermocapillary flow in the central region under an intense non-uniform heat flux, is also presented. This model provides the understanding of the physics of the thermocapillary convection in this region. It clarifies the scaling of this type of problem. Explicit formulas are obtained for the viscous and thermal length scales, the velocity scale and the temperature scale. These formulas depend on the Prandtl number implicitly. Two asymptotic limits, small and large Prandtl numbers, are analyzed. The results provide explicit Prandtl number dependences of all the scalings in the respective ranges. Algebraic expressions for the prediction of various physical quantities such as maximum temperature, velocity, shear stress, and temperature gradient are presented. The results of the stagnation flow analysis are compared with the numerical solutions previously obtained. It is found that the stagnation flow analysis gives good approximation of various quantities such as maximum temperature and velocity.
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