Learning Forum

[Scott Ormiston]

Here are some details and questions from the researcher in my group who is struggling with the EWF model. Can you please provide suggestions on how to move forward to get a physically realistic converged solution?

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EWF Model Notes

Main problem: Non-physical temperatures (above the inlet temperature) appear in the domain, near the inlet, near the cooled wall on which the film is located. The solution converges with these non-physical temperatures. If the temperature is limited to just above the inlet temperature, the solution does not converge.

Model Options and Setup:

·        Solve Momentum, Energy, Phase Coupling

·        Solving Momentum Equation (not analytical solution)

·        Gravity Force, Surface Shear Force enabled

·        Phase change enabled, film vapor material is water-vapor, film material is water-liquid

Solution Method and Control:

·        Time: Second Order Implicit

·        Continuity, Momentum, Energy: Second Order Upwind

o   Note: First Order Upwind did not improve results

·        Adaptive Time Stepping:

o   Max CFL: 0.05

§  Anything above this is unstable

o   Initial Time Step: 0.0005 [s]

o   Increase Factor: 1.1

o   Decrease Factor: 1.2

o   Subiteration Stop: 1e-08

o   Sub-Iterations: 5-10

·        Maximum Thickness: 0.02 [m]

·        Continuity and Momentum Coupling: Coupled Solution

Wall Film Options:

·        Boundary Type: initial condition (Film Thickness: 0 [m], Film Velocity (X, Y, Z): <0, 0, 0> [m/s], Film Temperature: same is inlet temperature.

·        Flow Momentum coupling engaged

·        Phase Change Enabled, using wall-boundary-layer model

Domain:

Domain is initialized without EWF model engaged, with CFL=200, default under-relaxation then EWF model is turned on and CFL and under-relaxation factors are lowered.

With EWF on:

·        Scheme: SIMPLEC

o   Coupled scheme produces instabilities in film thickness

·        Pressure: Second Order

·        Momentum, TKE, SDR, Species, Energy: Second Order Upwind

·        Pseudo-Time CFL: 1-5 (increasing past 5 produces instabilities in film thickness)

·        Under-relaxation: h20 = 0.99

·        Under-relaxation: energy = 0.2-0.5

o   Higher under-relaxation produces instabilities when the film is forming

o   Under-relaxation of energy can be increased (up to 0.99) after the film thickness has progressed down the entire domain

o   Low under-relaxation (e.g. 0.001-0.1) does not improve convergence of results.

Mesh:

·        Current mesh has wall-adjacent cell size of 1e-04 [m]. K-omega SST model is used.

 

Theory:

In the theory manual, Chapter 18.2.1.6 ‘Coupling of Wall Film with Mixture Species Transport’ indicates the various available transport models. Currently, we are using the wall boundary layer model as it best matches the approach we wish to take.

Chapter 18.2.1.6 redirects us to Chapter 12.9.5.2 ‘Mass Transfer from the Film’ for the wall boundary layer model, which is in the Lagrangian Wall-Film Model Theory chapter. However, the exact subsection dealing with the wall boundary layer model is not indicated.

We believe that Chapter 12.9.5.2.2 Film Condensation is the subsection that corresponds to the wall boundary layer model based on its content. Is this correct?

Chapter 12.9.5.2.2 begins with Fick’s law for determining the condensation rate, however, it then presents a different method where the mass fraction is integrated and mass transfer coefficients are used. For turbulent flow, it is indicated that the mass transfer coefficient is determined via wall functions from equation 4-341 (an equation that gives the law of the wall for species transport).

-        What is the rationale for moving from Fick’s Law to a mass-transfer coefficient approach?

-        How does equation 4-341 determine the film mass-transfer coefficient for turbulent flow? No mass transfer coefficient is evident in the equation.

-        When the domain is turbulent, does the model change its approach when the viscous sublayer is resolved (i.e. wall y+ <=1)?

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