measuring gas holdup
 

I want to simulate a bubble column in cfx. How can I measure gas holdup in cfx post?please help me.
thanks.

very simple. you must to monitoring the gas volume fraction. If you are modelling air -water, the air is a dispersed phase. so you shold measure the air volume fraction. I suggest you measure time average gas holdup :):)

how to measure the time average gas holdup.

you need to créate a transient simulation and add in the output a aritmetic average to account for thr "Time or Trnavg" for each timestep

There is a neat way of doing this which does not require a transient simulation :). You create an additional variable with units of time. You make the variable have a convection equation (but not diffusion), and start at the inlet with time = 0. You then put a source term on additional variable on the entire domain of 1 [1/s].

This creates an additional variable which moves around with the fluid and the source term makes it increment by time. By the time it reaches the outlet it has incremented to the residence time.

(I may have details of the implementation wrong, hopefully somebody will correct me if this is the case. But I know the approach works.)

Hi, I think the additional variable helps if you want to track the particles as a residence time distribution, but if you need to know the "gas volumen fraction(time averaged) I suggest create TRNAVG variables in the output or create a PERL routine instead automatic aritmetic average in each timesteps.....regards

Air-water bubble column
 

How to ensure the accurate time average volume fraction in an air-water bubble column. I have monitored the volume weighted average of gas fraction and after the transient run of 60 seconds still the volume fraction is increasing. Please guide me, up to how much time I should run the simulation and how to ensure the converged volume weighted average of the gas fraction?

Why don't you just keep running the simulation and find out?