Mass flow imbalance in transient state

Ton · June 1, 2009, 02:54

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

June 1, 2009, 02:54	Mass flow imbalance in transient state	#1
Ton New Member Join Date: Apr 2009 Posts: 7 Rep Power: 17	Hi, I tested a 2D pipe for transient analysis. BC. is as follows: Inlet, velocity,24sin(wt+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

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June 1, 2009, 19:28		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Hi, Is your simulation adequately converged? Are the fluids compressible? Glenn Horrocks

June 1, 2009, 21:54		#3
Ton New Member Join Date: Apr 2009 Posts: 7 Rep Power: 17	Hi Glenn The material is helium . Every timestep loops 5. i think the timestep is adequate (one period has 400 timesteps). Don

June 2, 2009, 02:05		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	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