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January 11, 2016, 01:55 |
Problem in setting Boundary Condition
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#1 |
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Prathamesh Phadke
Join Date: Jan 2016
Location: India
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I am simulating the flow of gas closed in a pressure vessel due to cooling (Natural Convection). I am attaching the images of my geometry and mesh. I have chosen Steel as Solid Domain and Air Ideal Gas as the Fluid. In the Fluid Model, Buoyancy is switched on. I want to give the boundary condition that the cooler absorbs 9W of power at 80K at the Cold End. How can I apply this boundary condition?
I have done one simulation without giving the cooling power as the input and setting the boundary condition at the cold end as Isothermal 80K wall. Kindly help me. CCL of simulation without cooling power: LIBRARY: MATERIAL: Air Ideal Gas Material Description = Air Ideal Gas (constant Cp) Material Group = Air Data, Calorically Perfect Ideal Gases Option = Pure Substance Thermodynamic State = Gas PROPERTIES: Option = General Material EQUATION OF STATE: Molar Mass = 28.96 [kg kmol^-1] Option = Ideal Gas END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 1.0044E+03 [J kg^-1 K^-1] Specific Heat Type = Constant Pressure END REFERENCE STATE: Option = Specified Point Reference Pressure = 1 [atm] Reference Specific Enthalpy = 0. [J/kg] Reference Specific Entropy = 0. [J/kg/K] Reference Temperature = 25 [C] END DYNAMIC VISCOSITY: Dynamic Viscosity = 1.831E-05 [kg m^-1 s^-1] Option = Value END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 2.61E-2 [W m^-1 K^-1] END ABSORPTION COEFFICIENT: Absorption Coefficient = 0.01 [m^-1] Option = Value END SCATTERING COEFFICIENT: Option = Value Scattering Coefficient = 0.0 [m^-1] END REFRACTIVE INDEX: Option = Value Refractive Index = 1.0 [m m^-1] END END END MATERIAL: Steel Material Group = CHT Solids, Particle Solids Option = Pure Substance Thermodynamic State = Solid PROPERTIES: Option = General Material EQUATION OF STATE: Density = 7854 [kg m^-3] Molar Mass = 55.85 [kg kmol^-1] Option = Value END SPECIFIC HEAT CAPACITY: Option = Value Specific Heat Capacity = 4.34E+02 [J kg^-1 K^-1] END REFERENCE STATE: Option = Specified Point Reference Specific Enthalpy = 0 [J/kg] Reference Specific Entropy = 0 [J/kg/K] Reference Temperature = 25 [C] END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 60.5 [W m^-1 K^-1] END END END END 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: Cryocooler Coord Frame = Coord 0 Domain Type = Solid Location = CRYOCOOLER BOUNDARY: Cold End Boundary Type = WALL Location = CRYOCOOLER_COLD_END_2 BOUNDARY CONDITIONS: HEAT TRANSFER: Fixed Temperature = 80 [K] Option = Fixed Temperature END END END BOUNDARY: Cryocooler Default Boundary Type = WALL Location = CRYOCOOLER_INNER_WALL BOUNDARY CONDITIONS: HEAT TRANSFER: Option = Adiabatic END END END BOUNDARY: Default Fluid Solid Interface in Cryocooler Side 1 Boundary Type = INTERFACE Location = CRYOCOOLER_OUTER_WALL_1 BOUNDARY CONDITIONS: HEAT TRANSFER: Option = Conservative Interface Flux END END END BOUNDARY: Upper End Boundary Type = WALL Location = CRYOCOOLER_TOP_END_2 BOUNDARY CONDITIONS: HEAT TRANSFER: Fixed Temperature = 300 [K] Option = Fixed Temperature END END END DOMAIN MODELS: DOMAIN MOTION: Option = Stationary END MESH DEFORMATION: Option = None END END SOLID DEFINITION: Solid 1 Material = Steel Option = Material Library MORPHOLOGY: Option = Continuous Solid END END SOLID MODELS: HEAT TRANSFER MODEL: Option = Thermal Energy END THERMAL RADIATION MODEL: Option = None END END END DOMAIN: Fluid Coord Frame = Coord 0 Domain Type = Fluid Location = FLUID BOUNDARY: Default Fluid Solid Interface in Fluid Side 1 Boundary Type = INTERFACE Location = CRYOCOOLER_OUTER_WALL_2,Primitive 2D,Primitive 2D \ B,Primitive 2D D,Primitive 2D N,Primitive 2D O,Primitive 2D \ P,Primitive 2D Q BOUNDARY CONDITIONS: HEAT TRANSFER: Option = Conservative Interface Flux END MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: Fluid Default Boundary Type = WALL Location = CRYOCOOLER_COLD_END_1 BOUNDARY CONDITIONS: HEAT TRANSFER: Fixed Temperature = 80 [K] Option = Fixed Temperature END MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END DOMAIN MODELS: BUOYANCY MODEL: Buoyancy Reference Density = 1.1416 [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 = 1 [atm] END END FLUID DEFINITION: Fluid 1 Material = Air Ideal Gas Option = Material Library MORPHOLOGY: Option = Continuous Fluid END END FLUID MODELS: COMBUSTION MODEL: Option = None END HEAT TRANSFER MODEL: Option = Thermal Energy END THERMAL RADIATION MODEL: Option = None END TURBULENCE MODEL: Option = k epsilon BUOYANCY TURBULENCE: Option = None END END TURBULENT WALL FUNCTIONS: Option = Scalable END END END DOMAIN: Metal Coord Frame = Coord 0 Domain Type = Solid Location = METAL BOUNDARY: Default Fluid Solid Interface in Metal Side 1 Boundary Type = INTERFACE Location = Primitive 2D A,Primitive 2D C,Primitive 2D G,Primitive 2D \ J,Primitive 2D K,Primitive 2D L,Primitive 2D M BOUNDARY CONDITIONS: HEAT TRANSFER: Option = Conservative Interface Flux END END END BOUNDARY: Metal Default Boundary Type = WALL Location = Primitive 2D H,Primitive 2D I BOUNDARY CONDITIONS: HEAT TRANSFER: Option = Adiabatic END END END BOUNDARY: Walls Boundary Type = WALL Location = OUTER_WALLS BOUNDARY CONDITIONS: HEAT TRANSFER: Fixed Temperature = 300 [K] Option = Fixed Temperature END END END DOMAIN MODELS: DOMAIN MOTION: Option = Stationary END MESH DEFORMATION: Option = None END END SOLID DEFINITION: Solid 1 Material = Steel Option = Material Library MORPHOLOGY: Option = Continuous Solid END END SOLID MODELS: HEAT TRANSFER MODEL: Option = Thermal Energy END THERMAL RADIATION MODEL: Option = None END END END DOMAIN INTERFACE: Default Fluid Solid Interface Boundary List1 = Default Fluid Solid Interface in Cryocooler Side \ 1,Default Fluid Solid Interface in Metal Side 1 Boundary List2 = Default Fluid Solid Interface in Fluid Side 1 Interface Type = Fluid Solid INTERFACE MODELS: Option = General Connection FRAME CHANGE: Option = None END PITCH CHANGE: Option = None END END MESH CONNECTION: Option = GGI END END OUTPUT CONTROL: RESULTS: File Compression Level = Default Option = Standard END END SOLVER CONTROL: Turbulence Numerics = First Order ADVECTION SCHEME: Option = High Resolution END CONVERGENCE CONTROL: Length Scale Option = Conservative Maximum Number of Iterations = 500 Minimum Number of Iterations = 1 Solid Timescale Control = Auto Timescale Timescale Control = Auto Timescale Timescale Factor = 1.0 END CONVERGENCE CRITERIA: Residual Target = 1.E-4 Residual Type = RMS END DYNAMIC MODEL CONTROL: Global Dynamic Model Control = On END END END COMMAND FILE: Version = 15.0 Results Version = 15.0 END SIMULATION CONTROL: EXECUTION CONTROL: EXECUTABLE SELECTION: Double Precision = Off END INTERPOLATOR STEP CONTROL: Runtime Priority = Standard MEMORY CONTROL: Memory Allocation Factor = 1.0 END END PARALLEL HOST LIBRARY: HOST DEFINITION: prathameshpc Remote Host Name = PRATHAMESH-PC Host Architecture String = winnt-amd64 Installation Root = C:\Program Files\ANSYS Inc\v%v\CFX END END PARTITIONER STEP CONTROL: Multidomain Option = Independent Partitioning Runtime Priority = Standard EXECUTABLE SELECTION: Use Large Problem Partitioner = Off END MEMORY CONTROL: Memory Allocation Factor = 1.0 END PARTITIONING TYPE: MeTiS Type = k-way Option = MeTiS Partition Size Rule = Automatic END END RUN DEFINITION: Run Mode = Full Solver Input File = E:\Study\M.Tech. Project\10-12-2015 \ Simulation\Simulation 3.0.0\Simulation 3.0.0.def END SOLVER STEP CONTROL: Runtime Priority = Standard MEMORY CONTROL: Memory Allocation Factor = 1.0 END PARALLEL ENVIRONMENT: Number of Processes = 1 Start Method = Serial END END END END |
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January 11, 2016, 05:56 |
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#2 |
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Glenn Horrocks
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A good clear question. Thank you for spending the time to write a good question.
You are modelling this as a steady state simulation. If you wish to do this you will need to use a solid time scale factor to accelerate convergence in the solid domain. But you will probably have problems with this and will require a transient simulation to get convergence. Natural convection simulations usually do not have a steady state answer (even if the conditions are steady state). Also: You are using air but say you are modelling nitrogren. The difference is small but it is easy to correct so you might as well. Use the molecular weight, Cp and thermal conductivity of nitrogen. Also note that as you are at cryogenic temperatures the room temperature values in CFX by default will be a long way off. Also note that steel material properties vary quite a bit over these temperature ranges too. As the steel ranges in temperature from 80K to 300K you probably want a variable properties model to be accurate over this wide range. You are using k-e turbulence model. I would recommend SST as the general purpose turbulence model instead of k-e. But it is your choice, there probably won't be much difference. You are using a thermal energy model for the fluid. This will not take into account effects do to the gas expanding and contracting. In other words, to model the fluid as an ideal gas you will need to use the "Total Energy" heat transfer model. What controls the pressure of this device? Is it kept at 1 bar pressure, or does it increase or decrease from there as the device is sealed and the gas wants to expand or contract? |
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January 11, 2016, 06:34 |
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#3 | ||
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Prathamesh Phadke
Join Date: Jan 2016
Location: India
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Thanks for the reply!
Quote:
Quote:
From your reply I got the following which I had not considered previously: 1. To model the properties of Steel as temperature dependent 2. To use SST model for turbulence 3. To use Total energy model for fluid 4. To Model the problem as a transient problem. I am attaching the temperature contour and Velocity Vector plot I got in the previous simulation. I will make the modifications suggested by you and again run the simulation and check whether the results vary. The problem is I expected the temperatures to be much lower (just by intuition) which they are not. So I am rechecking by boundary conditions. Because one thing which I did not include in the previous simulation was the cooling power of the cooler (9 W at 80K). I dont know how to apply this boundary condition at the cold end. Thanks for your help in advance!! |
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January 11, 2016, 16:17 |
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#4 |
Senior Member
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Regarding your concern with the boundary condition for the cooler. Mathematically, it is not possible to enforce both conditions simultaneously for the energy equation. Either you model using a temperature specified condition, say 80 [K], or heat flux specified, say 9 [W] / area()@boundary.
Once either of the two cases has converged, you can check if the non-enforced condition has been met, or how far off the conditions are from the expected value. Hope the above helps, |
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January 11, 2016, 17:07 |
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#5 |
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Glenn Horrocks
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And your mesh has no inflation layers are these will be necessary accurate results. But I would do all the basic model development on the coarse mesh you already have, and once that is working well you can refine your mesh to get the accuracy you require.
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January 11, 2016, 17:27 |
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#6 |
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Erik
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Looks like you are doing a cryocooler's cold head? And have been told 9W at 80K is the operational point?
These Cryocoolers have a performance curve, for example, it may do 10W at 90K, and 8W at 70K. What I've done in the past is put this curve in as a function of temperature. If you find this curve, You can write an expression that describes its cooling power and input it. You can do a convective boundary condition with a heat transfer coefficient that is a function of temperature, or a heat flux or anything else that accurately represents it. |
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January 11, 2016, 23:29 |
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#7 |
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Prathamesh Phadke
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Thank you Opaque for your insight. I will apply the boundary condition as suggested by you and evcelica.
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January 11, 2016, 23:35 |
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#8 |
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Prathamesh Phadke
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Thanks for your suggestion. That is what I was planning. To get proper results on coarse mesh first then go for accuracy. I still dont know how to create inflation layers properly. But I will read about it and try it once my initial settings of the problem are correct.
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January 11, 2016, 23:47 |
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#9 | |
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Prathamesh Phadke
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Quote:
What I did not get is - I dont know the value of heat transfer coefficient or any equations which can be applied at the cold end of the cryocooler. So can you guide me about it? (I dont want the exact equation but a little guidance about applying the convective boundary condition) Thanks for your help |
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January 12, 2016, 04:05 |
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#10 |
Super Moderator
Glenn Horrocks
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You do not need to define a heat transfer boundary condition at the cooler. As this is a solid/fluid simulation the simulation will automatically work out the heat transfer conditions for the interface. This assumes the cooler acts on the solid domain, not the fluid domain.
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January 12, 2016, 04:13 |
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#11 | |
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Prathamesh Phadke
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Quote:
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January 12, 2016, 04:17 |
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#12 |
Super Moderator
Glenn Horrocks
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Yes, that is what I am talking about. The cooler acts on the steel and the heat is conducted through the steel from the hot fluid to the cooler. In this case you do not need to define a boundary condition at the steel/fluid interface as it is an internal interface.
The cooler face on the steel should probably be modelled as a heat flux (an option under wall boundaries), with a performance curve against temperature as previously mentioned. |
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January 12, 2016, 04:39 |
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#13 |
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Prathamesh Phadke
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Ok. I understood now. Thanks a lot. I will do the same.
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boundary condition, cfx, natural convection |
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