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June 12, 2017, 09:04 
Multiphase flow in a pipe with a CHT

#1 
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Hello everybody
I'm trying to simulate the condensation process in a pipe. The coolant flows around the pipe in a cooling annulus. ==> (Multiphase flow in the pipe + CHT) * 3 Domains: cooling annulus, condenser pipe and steel pipe. the unsteady simulations haven't worked so far (convergence problems). The steady ones gave odd results. It seems there is no flow inside the pipe, even y+ isn't defined in the domain where condensation is supposed to find place. It would be of great help if anyone of you guys can see where the problem is. In the attachement u'll find the results for the velocity. Here is my setup for the Condenser domain: FLOW: Flow Analysis 1 SOLUTION UNITS: Angle Units = [rad] Length Units = [m] Mass Units = [kg] Solid Angle Units = [sr] Temperature Units = [K] Time Units = [s] END ANALYSIS TYPE: Option = Steady State EXTERNAL SOLVER COUPLING: Option = None END END DOMAIN: Condenser Coord Frame = Coord 0 Domain Type = Fluid Location = B38 BOUNDARY: Default Fluid Solid Interface Side 1 Boundary Type = INTERFACE Location = F47.38 BOUNDARY CONDITIONS: HEAT TRANSFER: Option = Conservative Interface Flux END MASS AND MOMENTUM: Option = No Slip Wall END WALL CONTACT MODEL: Option = Use Volume Fraction END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: Inlet Boundary Type = INLET Location = InletCondenderTube BOUNDARY CONDITIONS: FLOW DIRECTION: Option = Normal to Boundary Condition END FLOW REGIME: Option = Subsonic END HEAT TRANSFER: Option = Static Temperature Static Temperature = 145.3 [C] END MASS AND MOMENTUM: Mass Flow Rate = 0.0166861111 [kg s^1] Option = Bulk Mass Flow Rate END TURBULENCE: Option = Medium Intensity and Eddy Viscosity Ratio END END FLUID: Humid Air BOUNDARY CONDITIONS: COMPONENT: H2O GAS Mass Fraction = 0.8524 Option = Mass Fraction END VOLUME FRACTION: Option = Value Volume Fraction = 1 END END END FLUID: Liquid BOUNDARY CONDITIONS: VOLUME FRACTION: Option = Value Volume Fraction = 0 END END END END BOUNDARY: Outlet Boundary Type = OUTLET Location = F39.38 BOUNDARY CONDITIONS: FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Option = Average Static Pressure Pressure Profile Blend = 0.05 Relative Pressure = 0 [Pa] END PRESSURE AVERAGING: Option = Average Over Whole Outlet END END END BOUNDARY: Symmetry_Condenser Boundary Type = SYMMETRY Location = F43.38,F46.38 END DOMAIN MODELS: BUOYANCY MODEL: Buoyancy Reference Density = 0.46 [kg m^3] Gravity X Component = 0 [m s^2] Gravity Y Component = 9.81 [m s^2] Gravity Z Component = 0 [m s^2] Option = Buoyant BUOYANCY REFERENCE LOCATION: Option = Automatic END END DOMAIN MOTION: Option = Stationary END MESH DEFORMATION: Option = None END REFERENCE PRESSURE: Reference Pressure = 413.1 [kPa] END END FLUID DEFINITION: Humid Air Material = Gas Mixture Option = Material Library MORPHOLOGY: Option = Continuous Fluid END END FLUID DEFINITION: Liquid Material = Liquid Option = Material Library MORPHOLOGY: Option = Continuous Fluid END 

June 12, 2017, 09:42 

#2 
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Unfortunately, you are setting up the problem the most difficult way to solve it.
Your simulation should be setup as a single phase flow in each fluid domain, plus a solid domain for the pipe. However, to do so you must enable beta features and disable the Constant Domain Physics. Unfortunately, you must be careful from here on as the software does not do a bunch of physics checks for you. 

June 12, 2017, 10:04 

#3 
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Thank you Opaque for your reponse
I actually did that. I defined 3 domains: Condenser is where condensation takes place (with a subdomain for condensation and another one for evaporation), Cooling Jacket where the coolant flows, and a solid domain for the Tube. So Condenser is the only Multiphase domain. The setup I attached is only for Condenser. What bothers me is that it seems that the fluid doesn't even flow through the pipe (picture CFD post). The results for velocity are weird. Y+ is Undefined in this domain (why??). Something is sooo wrong and I can't figure out the problem. 

June 12, 2017, 10:22 

#4 
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Perhaps you have already done the following, but good to point out anyways.
Since a beta feature is being used, I would step carefully in the model setup. Have you already run the single phase flow case ? That is, coolant fluid in the coolant jacket domain, and say "humid mixture" in the "condenser/evaporator" domain w/o any thermodynamic phase change active ? No need for multiphase flow anywhere. Once that case is running properly, I would move into the next level of complexity. Also, have you used the equilibrium phase change model already ? It is meant for condensation using a single phase fluid. No need for multiphase flow. 

June 12, 2017, 12:15 

#5 
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I will try that out!
Thanks a lot for your help 

Tags 
cfx & fluent, cht problem, condensation model, multiphase flow 
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