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 uday February 1, 2010 14:40

reverse flows

hello friends,
Iam simulating a CI engine manifold.

For this manifolds there are one inlet and two outlets.Iam using velocity inlet and presssure outlet boundary conditions.

For outlets the pressures are below atmp pressures means suction pressures. (I tried by taking guage and absolute pressures but)

When iam running the calculation one of the outlet is becoming the inlet and reverse flows are occureing in the so many faces.

Please tell me what is the reson for this problem i tried in so many ways but iam unable to find out the root of the problem.

 -mAx- February 2, 2010 03:02

Let pressure outlet to 0. The pressure distribution will be calculated from your geometry and your masflow (velocity inlet).
Once it has converged, check the pressure at your inlet, it will be your pressure drop (dp = pressure_inlet-pressure-outlet)

 uday February 2, 2010 07:02

Thanks for u r replay max ,

But i didn't get u ,can u please tell me how we control the reverse flows at manifold outlets ( in these outlets one of the outlet is changing in to inlet i think the reason for this is reverse flows ).

can u please give me the suggestion to avoid my problem.

 -mAx- February 2, 2010 07:41

Your outlet cannot change to inlet from itself.
What is the computed pressure at your inlet?

 uday February 2, 2010 09:39

Thanks for u r replay,

The inlet pressure is around 2bar and at the outlets the pressures are changing from suction to atmospheric (like 89456 to 101325 pascals).

I think due to these suction pressures at outlets the atmospheric air entering in to outlets (reverse flows are occurring at outlet faces) and that outlet is becoming in to inlet. Is this thought is correct.

How can avoid these reverse flows on outlet faces.

 -mAx- February 2, 2010 09:53

Are you working with transient solver?

 uday February 2, 2010 10:06

Yes i am working with transient solver

 -mAx- February 2, 2010 10:35

And I assume you are varying the value of your pressure outlet with time.
If your pressure of 2 bar at your inlet is in accordance with what you are expecting, you can try followings:
*switch velocity-inlet to pressure inlet
*fix the pressure outlet to 0Pa
*Create the profile for your inlet, with the right values, ie: 2e5 Pa - the fluctuations you wanted at your outlet
*Check the massflow, it should be in accordance with your velocity inlet

 uday February 3, 2010 04:04

Thanks for u r replay,

And i think u r suggestions will definitely work for one outlet but ,

I have two outlets, and there pressures are changing with respect to time and two outlets have different pressure values.

Then how would i Create the profile for inlet, with the right values, ie: 2e5 Pa - the fluctuations you wanted at your outlet

 -mAx- February 3, 2010 05:30

ah.. your outlets don't have the same value.
So it won't work.
Did you try to compute a steady state scenario, to see if your backflow occures?

 uday February 3, 2010 06:24

Thank u for u r replay,

yes i tried steady state at that time there is no reserve flow problem.

when am using transient conditions in this some pressure are below atmp(suction) i think thats why these reverse flows are occurring is this is right.

 -mAx- February 3, 2010 07:06

Try to plot the massflow rates at each outlet depending with iterations

 uday February 3, 2010 07:45

mass imbalance is 3.45*10^-7

OK i will try to plot the mass flow rates at each outlet depending with iterations but

How we will avoid this reverse flow problem by mass flow rates.

 -mAx- February 3, 2010 09:25

This is not a method for avoiding backflows... but just to be sure that the massflow rates converged between each time step

 uday February 4, 2010 06:19

Thank u for u r replay max,

I am sure that mass flow rates are converging in every timesteps because my simulation time is small 0.069 and time step size is 0.000236.

I am using pressure based solver,

For tubulence K-epsilon standard ,material is air, for inlet velocity inlet and for outlets pressure outlets BC are using.

back flow specification direction is normal to boundary.

For turbulence specification i am using turbulence intensity and hydraulic dia are using.

is there any thing wrong in my procedure.

 -mAx- February 4, 2010 06:34

I assume all is ok.
If your mass imbalance is around 1e-7, then what flows in... flows out. And you only have reversed flow at some faces. YOu can extrude your inlets to set the outlets far from the suctions area.

 uday February 4, 2010 09:08

Actually i didn't get u,

If i extrude inlets to set the outlets far from the suction area .how reverse flows are reduced.means suction pressure are outlets of the geometry.

and if i extrude inlets the manifold design will change.

 -mAx- February 5, 2010 01:58

sorry I wanted to say: "extrude your outlets"

 uday February 5, 2010 11:40

Thanks for u replay,

I will try with u r suggestion.

 Vijay April 12, 2012 12:37

I also need help

I am also faced with same issue. Did you solve it ?

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