
[Sponsors] 
January 22, 2012, 13:55 
Compressor working as decompressor...

#1 
New Member
Join Date: Apr 2011
Posts: 27
Rep Power: 6 
Hello,
I am modelling compressor, and in contours I see that it works as decompressor: http://i.imgur.com/cQUDM.png I have also local overpressure located at top of screen why? What to do with this case as on screen: http://i.imgur.com/azUUj.png ? 

January 23, 2012, 14:23 

#2  
Senior Member
Join Date: Jun 2011
Posts: 109
Rep Power: 6 
Quote:
Please check your turbulence model, discretization scheme, underrelaxation factors and boundary conditions. 

January 23, 2012, 14:50 

#3 
Senior Member
Real Name :)
Join Date: Jan 2010
Location: United States
Posts: 180
Rep Power: 7 
Specifically, it looks as if your yvelocity is diverging, causing all others to fail. I would suggest:
a) Make a better initial guess for the yvelocity b) Make the underrelaxation factor for momentum smaller. Please let us know what URF's you're using. You might also let us know if there are any textbased errors appearing on the screen; for instance, do you get a "temperature limited to XXXX kelvin" error? ComputerGuy 

January 23, 2012, 14:57 

#4 
New Member
Join Date: Apr 2011
Posts: 27
Rep Power: 6 
So do you suggest to make for example 50m/s at yvelocity to make compressor working easier?
URFs are default: Courant number 5, (sometimes solver makes it lower, to for example 0.05) Modified Turbulent Viscosity 0.8, Turbulent viscosity 1, Solid 1. Sometimes I receive messages about limiting temperature. I made many different simulations and I am not able for now to say when is this. I use as BC to inlet and outlet Modified Turbulent Viscosity 0.001. Gauge, Initial pressure=0 atm at inlet, operating pressure 1 atm, at outlet gauge pressure=1atm. Thank you in advance. 

January 23, 2012, 15:06 

#5  
Senior Member
Real Name :)
Join Date: Jan 2010
Location: United States
Posts: 180
Rep Power: 7 
First question: are you modeling solid formation in the compressor?? It seems as such with your URFs. If you are: solids in fluent are treated as large momentumsinks, and thus it's not surprising that you have convergence errors. I would take smaller time steps for sure, and potentially do some grid refinement.
Next, think about doing 2 things: 1) Make your yvelocity initial guess more reasonable. I don't know if 50 m/s is good for your system, but if it is, then use that. The better the guess you can give fluent to start with, the better the solver will be able to converge. 2) If you're getting temperature errors combined with divergence, think about setting the limits on your solve a little bit tighter. For example, they're probably default which (I think) are: min: 1 kelvin, max: 5000 kelvin. Generally, this isn't a big issue, but for hardtoconverge cases, you can set the limits to be more realistic, especially if you have temperaturedependent material properties or phenomena (solidification). Think about what a reasonable minimum should be...perhaps 250 K? Think about what a max should be...Perhaps 600K in a compressor? I don't know, but it's not likely 5000 K. Play with those two things, and potentially relax momentum a bit (make its factor smaller) and let us know. ComputerGuy Quote:


January 23, 2012, 15:07 

#6  
Senior Member
Join Date: Jun 2011
Posts: 109
Rep Power: 6 
Quote:


January 23, 2012, 15:54 

#7 
New Member
Join Date: Apr 2011
Posts: 27
Rep Power: 6 
I am not modelling solid formation, I have only walls as boundary conditions.
Smaller time steps at Steady Solver? Is it possible? My yvelocity was computed from inlet, when I made BC for inlet: Gauge pressure: 0atm, Initial pressure 0.1atm. I am planning to change to zeros when solution converges and make futher calculations. Is it good idea? I regulated temperature limits to 250K<T<600K. I limited pressure to 5atm max too, because sometimes I got 20100atm. What do you think about Modified Turbulent Viscosity=0.001? Where can I relax momentum? I don't want to make mass flow inlet as BC due to check when solution is converged when continuity equation is fulfilled. 

January 23, 2012, 16:04 

#8  
Senior Member
Real Name :)
Join Date: Jan 2010
Location: United States
Posts: 180
Rep Power: 7 
Tom,
I was confused, as you have a Solid under relaxation factor listed. I thought this must have been for phase change. Time steps in a steadystate solver are not possible. I'll have to think about this a bit more  I don't really understand how you've set up this problem at all. Generally, especially in steadystate modeling, you initialize the solution "close to" the end solution. That is, you want to initialize pressures, velocities, temperatures, etc. within the domain to be what you'd expect them to be. I don't think initializing them, then reinitializing as you're stating, is a good idea in your case. I hope you figure out the problem, because until I look at your case, unfortunately I don't think I can help further. If you need more help, it will be useful to list all of the following: Boundary conditions for fluid inlet and outlet...Pressures, turbulence levels. Fluids used: are they isothermal properties or do they vary with temperature? Compressor blade direction  is it rotating in the proper way, or is it reversed? Initialization conditions: what pressures, temperatures, and velocities are you using? Good luck. ComputerGuy Quote:


January 23, 2012, 16:14 

#9 
New Member
Join Date: Apr 2011
Posts: 27
Rep Power: 6 
I sometimes get areas of local hiperpressures. Like on this picture: http://i.imgur.com/vITQN.png Why is this?
Solver Steady, density based. I use Energy equation, SpalartAllmaras model with default ratios. Air is ideal gas. Rotational speed 100000rpm. At inlet: Gauge pressure: 0atm, Initial pressure: 0.1atm, Modified Turbulent Viscosity=0.001 Operating pressure: 1atm. At outlet: Gauge pressure: 1atm. Backflow Turbulent Viscosity Ratio 5. Velocity calculated at Initialization from inlet by pressure is 45m/s. Formulation Implicit. Flux Type RoeFDS. Solution method is Gradient: GreenGauss Cell Based. Flow: Second Order Upwind. Modified Turbulent Viscosity: First Order Upwind. Very often I get diverged solution after about 4060 iterations. Mass flows aren't going in proper way. (at outlet is constant for all iterations, at inlet is smaller during iterations.) Rotations are in proper way. Actually I try to help the compressor to make flow in correct way I made outlet initial pressure at 0.1atm, what do you think about this? 

January 27, 2012, 07:41 

#10 
New Member
Join Date: Apr 2011
Posts: 27
Rep Power: 6 
Problem solved with limiting temperature and pressures and changing pressure models (cell based, green gauss based etc.).


Thread Tools  
Display Modes  

