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January 3, 2011, 10:03 
Out of my depth with CFX and ANSYS

#1 
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Bobby
Join Date: Dec 2010
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Hi
I'm hoping someone will be able to help with a project I am trying to run. I want to model heat transfer in a room. I have my own geometry but I cant get that to work, thats another problem for the moment. Within the room there is essentially a water pipe absorbing/cooling the air. To begin with, can someone just confirm whether I am on the right lines in my methods to model this. I was thinking to use fluent or cfx to model the fluid flow in the pipe which would give me an effective thermal conductivity (which I could assume constant) for the pipe in the room which I would model using CFX? I'm pretty new to all this so apologies if this isn't clear. Can answer any questions if it helps. I've done many tutorials on CFX and used FLUENT before but it doesnt seem to help that much when you have a problem of your own!? Thanks for help 

January 3, 2011, 21:48 

#2 
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Bobby,
If I understand correctly:


January 4, 2011, 07:17 

#3  
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Bobby
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Quote:
1it is a model 1646x1646x1646 (mm) 2can I not enable buoyancy within the model and apply g in the z direction. I did a tutorial using this method. Where would I obtain such data? Slight adaption to the problem/another scenario. Modelling just a room with a heater in it that has a given heat flux. I think I have managed to set this up in CFX. Chosen a domain with air in it, set an initial T for the air. If the walls are adiabatic, and I run it for long enough, it should just get hotter and hotter right? My issues is what convergence criteria/monitors would I set for a problem like this? How long would it need to be run for? Thanks Thanks Last edited by tomcatbobby; January 4, 2011 at 12:18. 

January 4, 2011, 23:09 

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If you're modeling natural convection, turn on the Boussinesq model for more rapid convergence (than with density as f(T,P)). See the following:
Steps in Solving BuoyancyDriven Flow Problems It'll only be valid for relatively small changes in temperature, however (i.e., thermal expansion coefficient * dT << 1). For the case you describe, with adiabatic walls and a constant internal heat source, the room will continue to get warmer and warmer: in  out + generation = accumulation 0  0 + heat flux = temperature rise Boussinesq will break down (be invalid) over very large temperature swings, but you can always get the density of air as a function of temperature and put it in the materials panel. It'll work the same. See the link for for detail. What you're describing is inherently unsteady (the temperature never plateaus), and thus you'll have to run for an infinite period of time. If you're only interested in how the room warms and the velocity of the air within the room changes, choose a time step described in the link above (see equation 13.222) and run the simulation in transient mode. Stop whenever you get bored! If you have some heat loss through the walls, or flow out of the domain such that you might achieve a steady state, simply monitor the total heat flux out of the domain and the heat flux out of your heater. The sum will be zero at steady state. The standard convergence criteria will do. The Fluent manual suggests solving a "steady state" solution as a large number of transient steps; in the link provided >5000 time steps is suggested. ComputerGuy 

January 10, 2011, 17:11 

#5 
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Bobby
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Hi
Got simulation working in CFX which is good. The choice of an appropriate timestep confused me as I dont know the time constant and have never quite understood length scales? The simulation is not right though as within the setup I cannot deselect the heater 'face' as a wall. This is a problem as it includes it in the boundary condition for adiabatic wall? Any ideas? Also, when in the CFX post, I would like to view a video of the velocity vectors coloured by temperature to observe the natural convection in the room over time but Im not able to select this? I'll post the last two questions in the CFX forum as well as you said you can't speak for it as much as FLUENT. Thanks for you help 

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