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-   -   buoyancy driven flow (http://www.cfd-online.com/Forums/fluent/35242-buoyancy-driven-flow.html)

 ocy December 7, 2004 21:06

buoyancy driven flow

Currently, I am having some difficulties simulating the flow model using FLUENT software.
:
:First, I shall give a brief summary of my project. I am using 2D modeling to simulate heat transfer in a chimney, 50m tall and 1m in diameter. My objective is to find the best design of the chimney to get optimum heat transfer flow inside the chimney.
:
:To save cost, I am looking at the possibility of not using a fan at the base of the chimney and instead, make use of natural convection to enable the hot air at the base of the chimney to flow to the top of the chimney.
:
:Hence, I am using the boussenesq model. The following are my queries,
:
:1) I am setting my gravitational acceleration as -9.81m/se2 in the vertical direction. Am I correct? or should I set it as 0?
:
:2) I am using Pressure Inlet and Pressure Outlet as boundary conditions for my Inflow and Outflow respectively.
:For my pressure inlet, I am setting the total gauge pressure=o, temp= 500k.
:For my pressure outlet, I am setting the gauge pressure=0, backflow temp=300k.
:Operating temp = 500k.
:
:However, the contour of temp profile that i get shows temp of 300k throughout the flow inside the chimney.
:This should not be the case as my initial temp at the base of the chimney is 500k.
:Where have I gone wrong?
:
:3) For all of the above cases, heat flux = 0 to simulate insulation at the walls.
:However, to save cost, I am looking at teh possibility of removing the insulation. How should I change the values of the heat flux to achieve it?

 nabeel mohsin December 8, 2004 06:07

Re: buoyancy driven flow

hi have you enable operating temperature in operating conditions. nabeel

 ocy December 8, 2004 12:17

Re: buoyancy driven flow

hi nabeel, what do you mean by that?

I did set the operating temp as 500k, same as my inflow.

 Evan Rosenbaum December 8, 2004 13:11

Re: buoyancy driven flow

A couple of suggestions.

1. Make sure you are solving the energy equation. 2. Don't use Boussinesq. Try specifying density as a function of temperature or using ideal gas. 3. Why do you think your backflow tempertaure at the outlet should be so low? That gas is much heavier and will flow downward to fill the chimney. Set it to 500K, at least until you get the thing running OK. 4. You refer to 1m in diameter. Are you using axisymmetry?. If so, remember that your axis of symmetry must be the x-axis.

That's all I can think of right now.

 ocy December 8, 2004 19:06

Re: buoyancy driven flow

Hi Evan,

How do I "specifying density as a function of temperature or using ideal gas"? I'm not very gd at FLUENT.

Isn't the backflow temp=ambient temp at the top of the chimney?? If not, how can i specify that the ambient temp outside the chimney is 300k. or is it not neccessary?

I'm not using any axisymmetry. Is is correct?

Hope to hear from you soon. Thanks in advance.

 ocy December 9, 2004 04:59

Re: buoyancy driven flow

Hi Evan,

I tried using ideal-gas + setting backflow temp to 480k. I am getting a much better result. Thanks for your help.

By the way, how can I change the insulation around the wall of the chimney? What kind of value should i input for heat flux?? Thanks

 Evan Rosenbaum December 10, 2004 14:12

Re: buoyancy driven flow

You don't want to specify a heat flux. You probably need to determine an appropriate heat transfer coefficient. You can switch from a fixed flux to a HTC in the boundary conditions panel for the bounding walls.

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