Learning Forum

[Shawn Ren]

Hello everyone,

I am currently simulating water boiling in a micro tube with low pressure and microgravity.  The inlet radius is 0.5 mm, the near inlet part was set to isotherm wall, followed by > 6mm heating walls. The axial length is about 7 mm, the outlet radius is 0.25 mm.  I ran the case at 10 kPa pressure, -1e-20 m/s2 gravity in X-direction (I’d like to set it to zero, but the boiling model requires it to calculate some parameters like bubble departure diameter), using the 2D axisymmetric condition. The mesh has a structured grid of about 25400 cells and a focused inflation layer in the wall surfaces, I estimated the y plus is about 50. The heat flux of outer and inner walls was set to 75 000 W/m2 and 60 000 W/m2 respectively, it’s supposed to be enough to evaporate all the low mass-flow liquid, so I chose the Critical Heat Flux wall boing model.

For the FLUENT setups, I am using :

 

Gravity On in the -x direction

Multiphase -> Eulerian -> Boiling Model -> Critical Heat Flux

Energy On

SST k-ω model with Low-Re Correction

Phase Interactions:

Forces (I tried just keep Drag (Universal) and Dispersion Force (Burns) because of some ANSYS staff’s answers from this Forum, but it didn’t work)

Drag model -> Ishii

Lift model -> Moraga or tomiyama

Turbulent dispersion force -> lopez-de-bertodano

Surface tension -> Constant -> 0.071 N/m or 0.032 N/m

Heat transfer -> ranz-marshall

Mass transfer -> Boiling from Liquid to Vapor at 318.96 K saturated temperature (10 KPa)

Bubble departure diameter -> Tolubinski-Kostanchuk

Frequency of bubble departure -> Cole

Nucleation site density -> Lemmert-Chawla

Area influence coefficient -> Delvalle-Kenning

Interfacial area -> ia-particle

I defined the fluid properties referring to the water properties corresponding to 10 KPa. I kept the reference temperature by default here, I’m not sure of the exact value I should input.

 

Since I set up this case based on replacing mesh into CHF boiling tutorial VMFL067_FLUENT-1 and modifying boundary conditions, some inlet and outlet profiles like inlet k and ω, outlet k and ω, the outlet backflow temperatures were set by default or in line with the tutorial case. In addition, the inlet mass flow rate is 0.00005 kg/s and outlet gauge pressure is 5400 Pa.

 

For a solver, I tried Coupled with and without Pseudo time, but both eventually got crushed. I also tried a transient solver.

I increased the heating wall heat flux from 10 000 W/m2 gradually, there was no vapor generation with relatively low heat flux. After the heat flux increased to 75 000 W/m2, it seemed OK before the fluid reached the saturation temperature; With further iterations, the mode would diverge, resulting in the discontinuous vapor phase and invalid pressure contours. However, I've seen in previous work that the model achieves convergence but I am not getting the same type of convergence when this high heat flux has been applied.

After liquid water reached saturation temperature

Does anybody know how to achieve convergence for this CHF modeling?

By the way, it would be good if anyone can explain the reference temperature when defining the fluid properties and inlet/outlet turbulence profiles further.

Any help that can be provided is very much appreciated!

[Shawn Ren]

Does anyone have a clue and could you please give me some advice? Thank you in advance!!!