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Difficulty In Setting Boundary Conditions

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Old   November 23, 2014, 11:40
Question Difficulty In Setting Boundary Conditions
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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 CFX-Pre 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 co-efficient (.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)
E-mail: mmhpromi@gmail.com
Attached Images
File Type: jpg Kitchen Model.JPG (36.3 KB, 30 views)
File Type: jpg Kitchen Geometry.JPG (33.6 KB, 38 views)
File Type: jpg Heat Transfer Cond. 1.JPG (31.6 KB, 30 views)
File Type: jpg Heat Transfer Cond. 2.JPG (34.7 KB, 24 views)
File Type: jpg Heat Transfer Cond. 3.JPG (32.5 KB, 23 views)

Last edited by Moinul Haque; November 24, 2014 at 02:59.
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Old   November 23, 2014, 18:14
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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.
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Old   November 25, 2014, 06:15
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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 = 1.831E-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
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 = GYPSY-DANGER
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 = 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
Attached Images
File Type: jpg Result After 186 Iteration.jpg (60.3 KB, 9 views)
File Type: jpg Result After 504 Iteration.jpg (59.3 KB, 7 views)
File Type: jpg Result After 604 Iteration.jpg (56.9 KB, 6 views)
File Type: jpg Result After 700 Iteration.jpg (55.0 KB, 10 views)
File Type: jpg Result After 800 Iteration (Final Result).jpg (40.5 KB, 11 views)
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Old   November 25, 2014, 06:59
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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)
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Old   November 25, 2014, 18:30
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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 criss-cross 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 5-6 degree from that of experimental. We changed many parameters in the CFX-PRE, 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 energy-thermal energy, adiabatic- non-adiabatic , various heater surface temp-heat flux, buoyant- non-buoyant 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 mesh-densities around active areas like fan, heater , slots etc.

Please help me in this regard at your convenience. Thanks in advance.
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