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November 23, 2014, 10:40 
Difficulty In Setting Boundary Conditions

#1 
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Join Date: Nov 2014
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Hi,
I am a relatively new user of ANSYS. I am using it for my thesis project  " Experimental And Numerical Study Of Thermal Distribution In A Conventional Kitchen." We've made a kitchen model (43cm*48cm*60cm) using transparent celluloid sheet which is approximately a 5 times scale down model of an actual kitchen. We've used voltage variac to regulate the heat delivered by the heater to the kitchen. We set up 8 temperature sensors inside the model & one of them resembles the breathing point of a human being in an actual kitchen. We varied the input & found outputs i.e. breathing point temperature. We created the geometry in ANSYS ICEM CFD & then did meshing. But in CFXPre we were confused about the heater heat transfer condition. The basic settings & Outline are > https://www.dropbox.com/s/l1q7lprwqe...tings.JPG?dl=0 https://www.dropbox.com/s/230uun6090...Contd.JPG?dl=0 https://www.dropbox.com/s/5riza1zcxi...tline.JPG?dl=0 At 1st we gave the condition as surface temp. & ran steady simulation. but for various surface temp. we found the breathing point temp approx 310k every time. then we tried other conditions such as heat transfer coefficient (.2) but the result was same. We couldn't fathom out why this happened every single time.Thanks in advance. Some conditions & diagrams are attached as thumbnails. All suggestions are welcomed. Thanks in advance Md. Moinul Haque Islamic University Of Technology (IUT) Email: mmhpromi@gmail.com Last edited by Moinul Haque; November 24, 2014 at 01:59. 

November 23, 2014, 17:14 

#2 
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Glenn Horrocks
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I cannot see your links. Please just attach them to the post as you did with the other images.
Also, please post your CCL and some images which show the flow you are currently modelling in the box. 

November 25, 2014, 05:15 

#3 
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In this run our desired temp at the breathing point (0.37, 0.236, 0.215) was 315.1K. The actual surface temp. recorded while conducting the exp. was 344˚C . I ran simulation using 344˚C as input but got approx 310K which was 5K below my desired result. Then I ran simulation using 365˚C & 400˚C in pursuit of 315K but unforunately got the same outcome. I have updated the links in the original query and i have attached some images of the ongoing simulation. Please check. Thanks in advance.
Data Sheet > https://www.dropbox.com/s/itt05zvses...Sheet.JPG?dl=0 CCL > 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 = 1.831E05 [kg m^1 s^1] Option = Value END THERMAL CONDUCTIVITY: Option = Value Thermal Conductivity = 2.61E2 [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 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: Default Domain Coord Frame = Coord 0 Domain Type = Fluid Location = AIR BOUNDARY: BOTTOM Boundary Type = WALL Location = BOTTOM BOUNDARY CONDITIONS: HEAT TRANSFER: Heat Transfer Coefficient = 0.2 [W m^2 K^1] Option = Heat Transfer Coefficient Outside Temperature = 34 [C] END MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: DOOR Boundary Type = OPENING Location = DOOR BOUNDARY CONDITIONS: FLOW DIRECTION: Option = Normal to Boundary Condition END FLOW REGIME: Option = Subsonic END HEAT TRANSFER: Opening Temperature = 34 [C] Option = Opening Temperature END MASS AND MOMENTUM: Option = Opening Pressure and Direction Relative Pressure = 0 [kPa] END TURBULENCE: Option = Low Intensity and Eddy Viscosity Ratio END END END BOUNDARY: FAN Boundary Type = OUTLET Location = FAN BOUNDARY CONDITIONS: FLOW REGIME: Option = Subsonic END MASS AND MOMENTUM: Normal Speed = 6.4 [m s^1] Option = Normal Speed END END END BOUNDARY: FRONT Boundary Type = WALL Location = FRONT BOUNDARY CONDITIONS: HEAT TRANSFER: Heat Transfer Coefficient = 0.2 [W m^2 K^1] Option = Heat Transfer Coefficient Outside Temperature = 34 [C] END MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: HEATER Boundary Type = WALL Location = HEATER BOUNDARY CONDITIONS: HEAT TRANSFER: Fixed Temperature = 400 [C] Option = Fixed Temperature END MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: LEFT Boundary Type = WALL Location = LEFT BOUNDARY CONDITIONS: HEAT TRANSFER: Heat Transfer Coefficient = 0.2 [W m^2 K^1] Option = Heat Transfer Coefficient Outside Temperature = 34 [C] END MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: REAR Boundary Type = WALL Location = REAR BOUNDARY CONDITIONS: HEAT TRANSFER: Heat Transfer Coefficient = 0.2 [W m^2 K^1] Option = Heat Transfer Coefficient Outside Temperature = 34 [C] END MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: RIGHT Boundary Type = WALL Location = RIGHT BOUNDARY CONDITIONS: HEAT TRANSFER: Heat Transfer Coefficient = 0.2 [W m^2 K^1] Option = Heat Transfer Coefficient Outside Temperature = 34 [C] END MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END BOUNDARY: SLOT1 Boundary Type = OPENING Location = SLOT1 BOUNDARY CONDITIONS: FLOW DIRECTION: Option = Normal to Boundary Condition END FLOW REGIME: Option = Subsonic END HEAT TRANSFER: Opening Temperature = 34 [C] Option = Opening Temperature END MASS AND MOMENTUM: Option = Opening Pressure and Direction Relative Pressure = 0 [kPa] END TURBULENCE: Option = Low Intensity and Eddy Viscosity Ratio END END END BOUNDARY: SLOT2 Boundary Type = OPENING Location = SLOT2 BOUNDARY CONDITIONS: FLOW DIRECTION: Option = Normal to Boundary Condition END FLOW REGIME: Option = Subsonic END HEAT TRANSFER: Opening Temperature = 34 [C] Option = Opening Temperature END MASS AND MOMENTUM: Option = Opening Pressure and Direction Relative Pressure = 0 [kPa] END TURBULENCE: Option = Low Intensity and Eddy Viscosity Ratio END END END BOUNDARY: TOP Boundary Type = WALL Location = TOP BOUNDARY CONDITIONS: HEAT TRANSFER: Heat Transfer Coefficient = 0.2 [W m^2 K^1] Option = Heat Transfer Coefficient Outside Temperature = 34 [C] END MASS AND MOMENTUM: Option = No Slip Wall END WALL ROUGHNESS: Option = Smooth Wall END END END DOMAIN MODELS: BUOYANCY MODEL: Buoyancy Reference Density = 1.293 [kg m^3] Gravity X Component = 0 [m s^2] Gravity Y Component = 9.8 [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 = Total Energy END THERMAL RADIATION MODEL: Option = None END TURBULENCE MODEL: Option = k epsilon BUOYANCY TURBULENCE: Option = Production and Dissipation END END TURBULENT WALL FUNCTIONS: High Speed Model = Off Option = Scalable END END END OUTPUT CONTROL: MONITOR OBJECTS: MONITOR BALANCES: Option = Full END MONITOR FORCES: Option = Full END MONITOR PARTICLES: Option = Full END MONITOR POINT: TA Cartesian Coordinates = 0.37 [m], 0.555 [m], 0.215 [m] Option = Cartesian Coordinates Output Variables List = Temperature END MONITOR POINT: TB Cartesian Coordinates = 0.37 [m], 0.49 [m], 0.215 [m] Option = Cartesian Coordinates Output Variables List = Temperature END MONITOR POINT: TC Cartesian Coordinates = 0.37 [m], 0.428 [m], 0.215 [m] Option = Cartesian Coordinates Output Variables List = Temperature END MONITOR POINT: TD Cartesian Coordinates = 0.37 [m], 0.364 [m], 0.215 [m] Option = Cartesian Coordinates Output Variables List = Temperature END MONITOR POINT: TE Cartesian Coordinates = 0.37 [m], 0.299 [m], 0.215 [m] Option = Cartesian Coordinates Output Variables List = Temperature END MONITOR POINT: TF Cartesian Coordinates = 0.37 [m], 0.236 [m], 0.215 [m] Option = Cartesian Coordinates Output Variables List = Temperature END MONITOR POINT: TG Cartesian Coordinates = 0.37 [m], 0.17 [m], 0.215 [m] Option = Cartesian Coordinates Output Variables List = Temperature END MONITOR POINT: TH Cartesian Coordinates = 0.37 [m], 0.11 [m], 0.215 [m] Option = Cartesian Coordinates Output Variables List = Temperature END MONITOR RESIDUALS: Option = Full END MONITOR TOTALS: Option = Full END END RESULTS: File Compression Level = Default Option = Standard END END SOLVER CONTROL: Turbulence Numerics = High Resolution ADVECTION SCHEME: Option = High Resolution END CONVERGENCE CONTROL: Length Scale Option = Conservative Maximum Number of Iterations = 800 Minimum Number of Iterations = 1 Timescale Control = Auto Timescale Timescale Factor = 1.0 END CONVERGENCE CRITERIA: Residual Target = 0.000000001 Residual Type = RMS END DYNAMIC MODEL CONTROL: Global Dynamic Model Control = On END END END COMMAND FILE: Version = 14.0 Results Version = 14.0 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: gypsydanger Remote Host Name = GYPSYDANGER Host Architecture String = winntamd64 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 = kway Option = MeTiS Partition Size Rule = Automatic END END RUN DEFINITION: Run Mode = Full Solver Input File = D:\ANSYS Work \ Files\Urban_kitchen\TRIAL_400_wall_34_htc_.2_total _6.4.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 

November 25, 2014, 05:59 

#4 
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Glenn Horrocks
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Some comments:
* Simulations with strong buoyancy effects like this are unlikely to be steady state. I am surprised you managed to get it to converge. * Your heat transfer coefficients are very low, much lower than is normal for natural convection. Why have you used such low values? * Please show an image of your mesh, and images of the results of the simulation (eg temperature and velocity cross sections) 

November 25, 2014, 17:30 

#5 
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Dear Mr. Horrocks,
I am really grateful for your kind response. Actually this is my only chance to publish a research paper in my undergrad level which seems pretty impossible right now and I would really, really appreciate your kind assistance here. Pardon me if I have asked too much. I have given you below the link of the geometry file, mesh file, definition file and a result file after reaching convergence. I think it will be better if you see the boundary conditions we have used in the for the simulation. Geometry File Link> https://www.dropbox.com/s/8xfefelmvs...METRY.uns?dl=0 Definition File Link> https://www.dropbox.com/s/mxlydta48d...l_6.4.def?dl=0 Result File Link> https://www.dropbox.com/s/5jlmdh9kuj...4_001.res?dl=0 Overview Of The Experiment : Celluloid (cellulose nitride) sheet made rectangular kitchen model. Dimension is 48x43x60. One door on the wall, two slots in a crisscross position in the roof (for the movement of the sensor array), One ventilation fan, and one cylindrical heater resembling heat source in real kitchen. We started the heater at a definite wattage which gave a definite surface temp at heater surface, started the fan at a definite speed (measured by anemometer). There is a series of temperature sensor hanging in front of the heater, vertically from the slots in the roof that measures temp. at 8 positions. Now we only took final temp. at breathing point after 40 or 50 minutes when the temp. of that point became steady. Now in simulation we tried to do the same thing, but temp varied almost 56 degree from that of experimental. We changed many parameters in the CFXPRE, even changed the heater surface temp widely. But WHATEVER the condition is, after achieving convergence, final temp is ALWAYS around 310 K. And moreover the value of the HTC was wrong. .2 is the value of thermal conductivity of acrylic sheet. I know HTC is to be found from this equation > U = 1 / (1 / hA + dxw / k + 1 / hB) but i am having trouble finding out the hA & hB value as both sides of the sheet have air of approximately same temperature. Celluloid sheet being an insulator, negligible amount of heat would’ve passed through it. For this reason we considered the heat transfer through the walls to be adiabatic in some simulations but found the same result. I have run the simulation with varying parameters like total energythermal energy, adiabatic nonadiabatic , various heater surface tempheat flux, buoyant nonbuoyant etc. but each and every case gave same result which is very abnormal I think. May be the mesh is problematic. By the way, we have used meshdensities around active areas like fan, heater , slots etc. Please help me in this regard at your convenience. Thanks in advance. 

Tags 
boundary condition, breathing point, same output 
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