Fluent Melting and Solidification

Topics: Thermodynamics, Fundamental physics concepts, Phase transition Pages: 110 (27177 words) Published: February 6, 2013
Chapter 8. Phase Change Simulations
This chapter describes the phase change model available in FLUENT and the commands you use to set up a phase change problem. Information is organized into the following sections: Section 8.1 : Section 8.2 : Section 8.3 : Section 8.4 : Overview of Phase Change Modeling Phase Change Modeling Theory User Inputs for the Phase Change Model Solution Strategies for Phase Change Problems

8.1 Overview of Phase Change Modeling in FLUENT
FLUENT can be used to solve uid ow problems involving phase

change taking place at one temperature e.g., in pure metals or over a range of temperature e.g., in binary alloys. Instead of explicitly tracking the liquid-solid front as the phase change occurs, which requires a moving mesh methodology, an enthalpy-porosity formulation is used where the ow and enthalpy equations are solved with extra source terms on the xed grid. Marangoni shear, due to the variation of surface tension with temperature, is important in many industrial uid ow situations involving phase change. The phase change model in FLUENT provides the ability to specify the Marangoni gradient at a sloping surface, as well as an arbitrary shear at a boundary coinciding with one of the curvilinear grid lines. The model also allows you to specify the convective heat transfer, radiation, and heat ux at a wall as piecewise linear pro les, polynomials, or harmonic functions. FLUENT provides the following phase change modeling options: Calculation of liquid-solid phase change in pure metals as well as in binary alloys. Modeling of continuous casting processes i.e., pulling" of solid material out of the domain.


Chapter 8 | Phase Change Simulations
Ability to specify an arbitrary shear at a curved boundary as a piecewise linear pro le or polynomial in terms of one of the Cartesian coordinates. Modeling of Marangoni convection due to the variation of surface tension with temperature. Modeling of the thermal contact resistance between frozen material and the wall e.g., due to an air gap. Ability to patch a momentum source in each Cartesian direction and or a heat source to simulate magnetic force elds or heat generation in the domain, for example. Display and patching of latent heat content, pull velocities in continuous casting and other relevant variables.

These modeling capabilities allow FLUENT to simulate a wide range of phase change problems including melting, solidi cation, crystal growth and continuous casting. The physical equations used for these phase change calculations are described in the following sections. Limitations of the As mentioned above, the phase change formulation in FLUENT Phase Change can be used to model the melting freezing of pure materials, as Model well as alloys. The liquid fraction versus temperature relationship used in FLUENT is the lever rule|i.e., a linear relationship Equation 8.2-3. Other relationships are possible 124 , but not available in FLUENT. The following FLUENT features cannot be used in conjunction with the phase change model: Radiation Combustion Speci ed periodic mass ow Cylindrical velocities

Overview of Phase In order for you to enable the phase change model, the energy equaChange Modeling tion must be active. You are then required to supply additional Procedures physical constants pertaining to the phase change problem liquidus and solidus temperature, latent heat of freezing, etc.. You may invoke one of the enhanced boundary conditions that are available c Fluent Inc. May 10, 1997

8.2 Phase Change Modeling Theory


in the phase change model applied shear, free surface with surface tension gradient, or mixed heat transfer. Before solving the coupled uid ow and heat transfer problem, you should patch an initial temperature and latent heat distribution or solve the steady conduction problem as an initial condition. The coupled problem can then be solved as either steady or unsteady. Because of the nonlinear...
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