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mehrdadeng September 6, 2009 06:37

natural convection
 
hi
how can simulate natural convection of air in closed room that its floor has heated at constant temprature?

ghorrocks September 6, 2009 07:56

Have a look in the tutorial manuals for examples of how to model natural convection.

rogbrito September 6, 2009 09:18

Please take a look on Tutorial 17.

http://www.4shared.com/file/72198900...tionpart1.html

http://www.4shared.com/file/72199871...tionpart2.html

Or these files (to download):

http://www.4shared.com/file/52194930...rogeriow1.html

http://www.4shared.com/file/52205921...rogeriow2.html

http://www.4shared.com/file/52185334...rogeriow3.html

with kind regards,
Rogerio.


Quote:

Originally Posted by mehrdadeng (Post 228652)
hi
how can simulate natural convection of air in closed room that its floor has heated at constant temprature?


mehrdadeng September 7, 2009 08:42

thanks rogerio and glen

mehrdadeng September 12, 2009 05:08

1 Attachment(s)
why temperature contour is this?

ghorrocks September 12, 2009 08:37

Without a description of what you are doing there is no way anybody can help you. Steady or transient? Is it converged? (almost certainly it is not converged) Have you checked the physics, eg that gravity and buoyancy is correctly set?

The variations you are seeing are 0.1K, which are pretty small. What is the driving temperatures?

Attesz February 23, 2011 10:56

Natural Convection Convergence Problem
 
3 Attachment(s)
Hi CFX users,

I'm simulating a heat convection in a room. There is a stove in the middle, which recirculates and heats the air.

My solution after some checks does not converge fully.

Of course, I'm using Buoyancy Model, the directions of gravity is set right. I've defned an inlet on the stove with a boundary energy source of 4kW, and a massflow of 0.06 kg/s. I'v defined at the bottom of the stove an outlet with average static pressure of 0 Pa (relative to 1 atm reference pressure). I'm using SST k-w model, Total Energy/ Thermal Energy heat transfer models. Both Automatic Timescale and Physical Timescal don't have a significant effect for the convergence.

Attachment 6603

Attachment 6604

Attachment 6605

Thanks in advance for any suggestions!

I post the out file:

+--------------------------------------------------------------------+
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: Legter
Coord Frame = Coord 0
Domain Type = Fluid
Location = LEGTER
BOUNDARY: Furdo_falak
Boundary Type = WALL
Location = FURDO
BOUNDARY CONDITIONS:
HEAT TRANSFER:
Option = Adiabatic
END
MASS AND MOMENTUM:
Option = No Slip Wall
END
WALL ROUGHNESS:
Option = Smooth Wall
END
END
END
BOUNDARY: Inlet
Boundary Type = INLET
Location = INLET
BOUNDARY CONDITIONS:
FLOW DIRECTION:
Option = Normal to Boundary Condition
END
FLOW REGIME:
Option = Subsonic
END
HEAT TRANSFER:
Option = Total Temperature
Total Temperature = volumeAve(Total Temperature )@Legter
END
MASS AND MOMENTUM:
Mass Flow Rate = 0.06 [kg s^-1]
Option = Mass Flow Rate
END
TURBULENCE:
Option = Medium Intensity and Eddy Viscosity Ratio
END
END
BOUNDARY SOURCE:
SOURCES:
EQUATION SOURCE: energy
Option = Total Source
Total Source = 4000 [kg m^2 s^-3]
END
END
END
END
BOUNDARY: Kazan_falak
Boundary Type = WALL
Location = KAZAN_FALAK
BOUNDARY CONDITIONS:
HEAT TRANSFER:
Option = Adiabatic
END
MASS AND MOMENTUM:
Option = No Slip Wall
END
WALL ROUGHNESS:
Option = Smooth Wall
END
END
END
BOUNDARY: Oldalfalak
Boundary Type = WALL
Location = FALAK
BOUNDARY CONDITIONS:
HEAT TRANSFER:
Heat Transfer Coefficient = 2 [W m^-2 K^-1]
Option = Heat Transfer Coefficient
Outside Temperature = -10 [C]
END
MASS AND MOMENTUM:
Option = No Slip Wall
END
WALL ROUGHNESS:
Option = Smooth Wall
END
END
END
BOUNDARY: Outlet
Boundary Type = OUTLET
Location = OUTLET
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: Padlo
Boundary Type = WALL
Location = PADLO
BOUNDARY CONDITIONS:
HEAT TRANSFER:
Heat Transfer Coefficient = 1 [W m^-2 K^-1]
Option = Heat Transfer Coefficient
Outside Temperature = 10 [C]
END
MASS AND MOMENTUM:
Option = No Slip Wall
END
WALL ROUGHNESS:
Option = Smooth Wall
END
END
END
BOUNDARY: Plafon
Boundary Type = WALL
Location = PLAFON
BOUNDARY CONDITIONS:
HEAT TRANSFER:
Heat Transfer Coefficient = 2 [W m^-2 K^-1]
Option = Heat Transfer Coefficient
Outside Temperature = 10 [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 = 1.2 [kg m^-3]
Gravity X Component = 0 [m s^-2]
Gravity Y Component = 0 [m s^-2]
Gravity Z Component = -9.8182 [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 = SST
BUOYANCY TURBULENCE:
Option = Production and Dissipation
END
END
TURBULENT WALL FUNCTIONS:
Option = Automatic
END
END
INITIALISATION:
Option = Automatic
INITIAL CONDITIONS:
Velocity Type = Cartesian
CARTESIAN VELOCITY COMPONENTS:
Option = Automatic with Value
U = 0 [m s^-1]
V = 0 [m s^-1]
W = 0 [m s^-1]
END
STATIC PRESSURE:
Option = Automatic with Value
Relative Pressure = 0 [Pa]
END
TEMPERATURE:
Option = Automatic with Value
Temperature = 19 [C]
END
TURBULENCE INITIAL CONDITIONS:
Option = Low Intensity and Eddy Viscosity Ratio
END
END
END
END
OUTPUT CONTROL:
MONITOR OBJECTS:
MONITOR BALANCES:
Option = Full
END
MONITOR FORCES:
Option = Full
END
MONITOR PARTICLES:
Option = Full
END
MONITOR POINT: Homerseklet
Cartesian Coordinates = 2.5 [m], 5 [m], 1.5 [m]
Domain Name = Legter
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:
Maximum Number of Iterations = 1000
Minimum Number of Iterations = 1
Physical Timescale = 1 [s]
Timescale Control = Physical Timescale
END
CONVERGENCE CRITERIA:
Residual Target = 1e-04
Residual Type = MAX
END
DYNAMIC MODEL CONTROL:
Global Dynamic Model Control = Yes
END
END
END
COMMAND FILE:
Version = 12.1
Results Version = 12.1
END
SIMULATION CONTROL:
EXECUTION CONTROL:
EXECUTABLE SELECTION:
Double Precision = Off
END
INTERPOLATOR STEP CONTROL:
Runtime Priority = Standard
MEMORY CONTROL:
Memory Allocation Factor = 2
END
END
PARALLEL HOST LIBRARY:
HOST DEFINITION: ***********
Host Architecture String = linux-amd64
Installation Root = ******************
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 = 2
END
PARTITIONING TYPE:
MeTiS Type = k-way
Option = MeTiS
Partition Size Rule = Automatic
Partition Weight Factors = 0.12500, 0.12500, 0.12500, 0.12500, \
0.12500, 0.12500, 0.12500, 0.12500
END
END
RUN DEFINITION:
Run Mode = Full
Solver Input File = \
/************************m0_06_4kW_totalenergy.def
INITIAL VALUES SPECIFICATION:
INITIAL VALUES CONTROL:
Continue History From = Initial Values 1
Use Mesh From = Initial Values
END
INITIAL VALUES: Initial Values 1
File Name = \
****************************szt_m0_06_4kW_001.res
Option = Results File
END
END
END
SOLVER STEP CONTROL:
Runtime Priority = Standard
MEMORY CONTROL:
Memory Allocation Factor = 2
END
PARALLEL ENVIRONMENT:
Number of Processes = 8
Start Method = HP MPI Local Parallel
Parallel Host List = ********
END
END
END
END

ghorrocks February 23, 2011 17:52

I do not think your plan of using an inlet and an outlet with a heat source is numerically stable. Especially as you are just using the average temperature of the whole domain. I would either use a periodic pair with a heat source or join the inlet and outlet up and use a heat and momentum source.

Attesz February 24, 2011 06:10

Quote:

Originally Posted by ghorrocks (Post 296620)
I do not think your plan of using an inlet and an outlet with a heat source is numerically stable..

Yes this seems to be right...

Quote:

Originally Posted by ghorrocks (Post 296620)
Especially as you are just using the average temperature of the whole domain...

I've used a constant temperature with no effect, so it is not problem.

Quote:

Originally Posted by ghorrocks (Post 296620)
would either use a periodic pair with a heat source or join the inlet and outlet up and use a heat and momentum source.

What do you mean on periodic pair?

Momentum source can be defined only in subdomains. I've planned to do this but I have to model a little domain in the stove for the inlet and an other for the outlet or am I wrong? In this case I have to define the opposite value of the kinetic energy on the inlet/outlet sides, or what do you mean to join the inlet and outlet up? On that boundaries, only heat source can be defined. Or should I define Continuity Source with temperatures velocities etc?

Thanks for wasting your time for this :o

Best Regards,
Attila

ghorrocks February 24, 2011 20:13

Quote:

I've used a constant temperature with no effect, so it is not problem.
Yes it is a problem. The only temperature which is important is the temperature entering the heater, but the averaging function includes the temeprature from away from the heater. This does not sound like a good idea to me.

A periodic pair is a type of interface condition. Look under interfaces.

You have the wrong idea on sources. A source is simply where you add some momentum or heat to a region. You can use this to drive flow or heat it up. You do not define inlets and outlets.

I think for what you are trying to do the periodic pair is a god approach. Try that one first before a momentum source.

Attesz February 25, 2011 06:25

1 Attachment(s)
Okay I know Periodic Interface types. The heater has a fan at the inlet, so I've defined energy source and continuity source with 0 mass flow and specified velocity at the periodic side corresponding to the inlet. Am I correct doing this?

Thanks in advance!

Attachment 6669


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