Mass Imbalance (mass inflow & outflow is not equal)
 

Hi everyone.

i am trying to simulate fine powder flow in a screw conveyor. as it is the transient simulation, the problem i am facing is in mass conservation. in cfx solver manager i plotted the monitors for mass flow at inlet and outlet and monitor for imbalance also. i wanna ask you guys that

1: flow at outlet is negative is it due to that fluid is leaving the domain or it means something else?

2: mass flow at outlet is larger than inlet and keeps on increasing why is that?

while the imbalance of fluid in domain ranges between 2 to -2 %. initially imbalance was high for few timesteps but then it came in this range.

the thing which is confusing me is that when imbalance is small then how can outlet flow be much larger than the inlet?

ideally, you should aim for an imbalance less than 1%.

as i said imbalance is in range of 2 to -2. its not always 2 or -2. normally its value remain close to 1. sometimes it jumps to 2.
but the confusing thing is that outflow is greater than inflow. simulation is running and also converging on each time step. outflow keeps on increasing. it violates general physics. how can this be possible?

If you only have one flow boundary condition then any flow in or out of the boundary in a steady state run is an imbalance of 100%. Even if the flow is a tiny, tiny numerical noise size - it is 100% imbalance so the imbalance is 100%.

This is why imbalance is not always used as a convergence criteria. It is not always appropriate.

So have a look to see if you are actually seeing irrelevant numerical noise, and whether you need an imbalance convergence check anyway.

thanks for replying.

i didn't get your point. would you please elaborate a little bit.

i am using opening type inlet and outlet.
inlet is in the direction of gravity so opening type is used. for outlet if i use outlet type it gives error that "a wall has been placed on area" i tried to move the outlet far away (domain length 450 mm & outlet is moved 100 mm away) but error persist, so i had to use opening type outlet boundary condition.

inlet mass flow is constant while outlet is larger than inlet and increasing. simulation runs smoothly solution converges on each timestep and i am not getting any error.

You mentioned it is a transient problem. Have you checked the monitors for the "Neg Accumulation" term ?

If that term is 0, and you still get mass flow at the outlet very different that mass flow at the inlet, you got a point to contact ANSYS CFX support. Otherwise, does the Neg Accumulation corresponds to the differences you observed ?

Hope the above helps,

when i plotted the monitor for neg accumulation it shows negative values. should it be positive or negative. what does this means if values are negative.