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-   -   Mass flow imbalance in transient state (https://www.cfd-online.com/Forums/cfx/65001-mass-flow-imbalance-transient-state.html)

Ton June 1, 2009 02:54

Mass flow imbalance in transient state
 
Hi,

I tested a 2D pipe for transient analysis. BC. is as follows: Inlet, velocity,24*sin(w*t+pi/2),static temperature,900C; Outlet, static pressure,1atm. Solve control is as follows: frenquence,63HZ; total time,5 period;timestep,period/400. The turbulence model is SST.

The problem is as follows:
When i set the wall as adiabatic,the massFlow() @inlet =massFlow()@outlet,but when i set the wall as HTC=200[wm^-2 K^-1],outside temp.=50C, the massflow()@inlet is not equal to massFlow()@outlet and the massFlow()@outlet becomes higher with periods. I can't find the reason.

Don

ghorrocks June 1, 2009 19:28

Hi,

Is your simulation adequately converged? Are the fluids compressible?

Glenn Horrocks

Ton June 1, 2009 21:54

Hi Glenn

The material is helium . Every timestep loops 5. i think the timestep is adequate (one period has 400 timesteps).

Don

ghorrocks June 2, 2009 02:05

Hi,

Well in that case the outlet and inlet mass flow rates can be different due to transient effects, compounded by temperature changing the density. I but you have some sort of acoustic waves in the thing.

And unless you have done a sensitivity analysis to show your timestep is adequately small then don't assume so. If acoustic waves are occuring then you have to worry about the compressibility and they requrie much smaller timesteps again.

Glenn Horrocks


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