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 April 8, 2015, 19:45 Questions about buoyancy driven flow #1 New Member   Mechaniac Join Date: Dec 2014 Posts: 7 Rep Power: 10 I'm doing a flow diagnostic for a combustion lab. A ventilating hood is suspended above the burner and the hood is connected to a pipe systems where there is a fan at the alternative end. The hood is like a rectangular nozzle that its smaller cross section is connected to the pipe line as mentioned before. The measured velocity at this cross section is at an average of 5.45m/s. If the burned gases can successfully reach to the top of the hood(the smaller cross section), they will be ventilated. It seems that at working condition, the burned gas may escape to the surroundings rather than go up to the top of the hood and then be sucked off. I'm performing a 2-D axisymmetric simulation now, but doubts arose frequently. Provided by inlet velocity 10m/s at 800K The simulation is carried out with B.Cs as following: velocity inlet: 10m/s, turbulence parameter: intensity 5%, length scale 0.07*D=0.0007m, thermal:800K Wall-hood: no slip Wall-free: no shear, slip wall pressure outlet: gauge pressure 0 Backflow direction: normal to the boundary average pressure specification: ticked Target mass flow rate: 0.054398 (0.1m diameter with the measure velocity(5.45m/s) Turbulence: backflow turbulence intensity: 5% and backflow hydraulic diameter: 0.1m thermal: backflow total temperature: 500k Are there some mistakes in the boundary conditions? Because there were terrible reversed flow at the pressure-outlet and I tried to extend the exit but to no avail. Could you pls give me some suggestions on this? And I intended to use Boussinesq Model to account for buoyancy effect; however, in the user's guide, it states that the limitation of this model is large temperature difference. In my case, is it with a large temperature difference? If this model can be used, how can I determined those inputs for Boussinesq parameters, namely the operating temperature, operating density, thermal expansion coefficient. Moreover, is this operating density the same as that in Material section where the thermal expansion coefficient can be inputted. I will be so grateful if you could help me on this!!!!!! And if anything I did not specify or not clearly stated, pls let me know.

 April 9, 2015, 14:07 #2 Member   nm Join Date: Mar 2013 Posts: 99 Rep Power: 12 have a screenshot? it's hard to understand your problem from a textwall.

April 9, 2015, 14:08
#3
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nm
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Quote:
 average pressure specification: ticked Target mass flow rate: 0.054398 (0.1m diameter with the measure velocity(5.45m/s)

That, from my experience can cause convergence issues. Why can't you use a massflow outlet instead?

April 9, 2015, 18:04
#4
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Quote:
 Originally Posted by nvarma have a screenshot? it's hard to understand your problem from a textwall.

Hi, here are two pictures of the schematic drawing of this simulation. Hope these could help you understand the scenario. It does give me the convergence problems; the problems of reversed flow at the pressure outlet is quite prominent; this continued to happen until hundreds of iterations

Thank you very much!

April 9, 2015, 18:05
#5
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Quote:
 Originally Posted by nvarma have a screenshot? it's hard to understand your problem from a textwall.

the inserted images seemed not working, here are the links

 April 10, 2015, 09:54 #6 Member   nm Join Date: Mar 2013 Posts: 99 Rep Power: 12 I have a few suggestions/ questions. 1.I would use a velocity inlet and set the components in the outward direction. You can also try to use an exhaust fan/outflow vent to reduce the backflow. 2. Is the ventilation pipeline open to the atmosphere? I see there's a fan in between in that case what pressure do you set for the pressure outlet? that might be incorrect. There might be a lower pressure inside the vent due to the suction from the fan. 3. How did you calculate the target massflow rate? Did you take the lower density into account while doing so? 4. Run the calculation with gravity set to .0981. This will lower the Rayleigh number and let you get an intermediate solution. Then increase gravity and re run it. Good luck.

April 10, 2015, 11:53
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Quote:
 Originally Posted by nvarma I have a few suggestions/ questions. 1.I would use a velocity inlet and set the components in the outward direction. You can also try to use an exhaust fan/outflow vent to reduce the backflow. 2. Is the ventilation pipeline open to the atmosphere? I see there's a fan in between in that case what pressure do you set for the pressure outlet? that might be incorrect. There might be a lower pressure inside the vent due to the suction from the fan. 3. How did you calculate the target massflow rate? Did you take the lower density into account while doing so? 4. Run the calculation with gravity set to .0981. This will lower the Rayleigh number and let you get an intermediate solution. Then increase gravity and re run it. Good luck.
Thank you very much!
Those are very much of my concerns now, but those concerns cannot be sorted out with my limited knowledge.
1. yes, I used a velocity inlet with 10m/s and the direction is normal to the boundary( is this the same as the outward direction you stated?
I will try to use exhaust fan/outflow vent
2. yes, it opens to the atmosphere; at the very end of the pipeline there is a fan providing suction; As for the simulation, the outlet was at some distance upstream of the fan in the pipe. I set the gauge pressure to 0 so far. I will reduce the gauge pressure and give a go
3. For the target mass flow rate
I used hotwire velocimetry to measure the velocity along the cross section at 5 evenly distributed points; Someone suggested me not to used the mean velocity but the bulk velocity when working out the mass flow rate.
No, I did not take the lower density into account( could you pls specify the lower density for me?)
4. Ok, I will try! and let you know what happens

 April 10, 2015, 14:46 #8 Member   nm Join Date: Mar 2013 Posts: 99 Rep Power: 12 where did you get the inlet velocity(10m/s) from? Since you don't know the density at exhaust, I don't know how you calculated the massflow rate. But that's likely to be inaccurate if you use the density at colder temperatures. Try using pressure outlet without target massflow rate. we cannot specify that, it would be part of the solution. Use an outlet bc which doesn't require massflow rate. Is there anyway you can measure the pressure inside the exit port? How far is the fan from your model outlet?

 April 10, 2015, 18:47 #9 New Member   Mechaniac Join Date: Dec 2014 Posts: 7 Rep Power: 10 Thank you very much for your inputs!! My professor told me so; this is maximum inlet velocity when fluid comes out of the burner nozzle. He asked me to start the simulation with this. I roughly calculated the mass flow rate using 1.225 for air; just to see if it works. Technically, it can be measured but could take some efforts, which my professor will not pay for that much of attentions or money. The pipes are suspended near the ceiling and I reckon from the connection of the hood the the pipe to the fan could be as long as 5m( roughly).

 April 11, 2015, 18:56 #10 New Member   Mechaniac Join Date: Dec 2014 Posts: 7 Rep Power: 10 Hi, do you think I need to use the target mass flow rate for the simulation even though it did bring me troubles? If I could give a correct mass flow rate? As you suggested, I turned off the target mass flow rate, the reversed flow did disappeared after several hundreds of iterations. I've found some suggestions from the user's guide and I quote:' By default, ANSYS FLUENT will compute the operating density by averaging over all cells. In some cases, you may obtain better results if you explicitly specify the operating density instead of having the solver compute it for you. For example, if you are solving a natural-convection problem with a pressure boundary, it is important to understand that the pressure you are specifying is Ps' in Equation (Ps' =Ps-rho0*g*x) (p. 750). Although you will know the actual pressure Ps, you will need to know the operating density rho0 in order to determine Ps' from Ps . Therefore, you should explicitly specify the operating density rather than use the computed average. The specified value should, however, be representative of the average value. ' Does this suit my case?

 Tags 2-d axisymmetric, buoyancy, incompressible flow