Fluid Residence Time
 

Hi, I am trying to find the amount of time water stays in a tank. I defined a volumetric additional variable "residence time", and plot the streamlines from the inlet (colored by residence time). I assumed that the residence time at the outlet is the time it takes water to travel from inlet to outlet.
However, as a check, I also inserted fluid particles with properties of water at the inlet, one-way coupled with the continuous water. I plotted the Particle Traveling Time at the outlet, and they are very different from the residence time.
Can anyone tell me which is the correct one? Did I mis-intrepreted the term residence time? Any advice will be greatly appreciated.

Thanks.

-Eric

I believe residence time will vary to some degree across the inlet, depending on tank geometry and flow conditions. Your thoughts about stream-lines and particle tracking sound appropriate. Are you sure the water and the particles have the same properties? Are you finding an RMS residence time or plotting residence time versus location on the inlet, or some other scheme?

Thanks JDA for replying. I am sure the particles have the properties of water, and I am trying to find the average residence time. Right now, I used the approach where I inject water particles at the inlet, and then Reports->sample and write the data at the outlet boundary to a file, and just average the time and assume that's the average residence time. Do you think it's appropriate?

Thanks again,
Eric

What you have done (streamlines versus water particles) is not equivalent. Streamlines are following the flow perfectly, with no slip. Water particles, even though the bulk fluid is water are treated as particles with a drag law and a mass meaning they will slip relative to the flow. So they will trace different paths and give different results.

The particle approach is wrong. You can correct it be using massless particles. This should give you similar results to streamlines.

Thanks Glenn for your help, I will use massless particles now.

Thanks again to JDA and Glenn for taking their time to reply.

Regards,
Eric1