Time needed to make the flow steady again
 

Hi all,
I want to simulate a transient water vapor pipe flow that the mass flow inlet is changed at a certain time. I have done the simulation with the boundary condition: mass-flow-inlet using UDF to change the mass flow, and pressure outlet. I want to know the time that needed to make the mass flow at the outlet is equal again with the inlet. I have plotted the mass flow at the outlet at each time, but the time that needed is smaller than it should. The average velocity is around 20 m/s and the length of the pipe is 100 m, so the time needed should be around 5 s, but based on my plot, it is only 0.3 s. What's wrong with my simulation?
Thank you

Compressible or incompressible
 

Are you modeling the vapor as compressible phase? If not, then the results are as expected. For an incompressible fluid, speed of sound is ideally infinite, i.e., any disturbance is transferred almost immediately. Now, a tool does not know that but it also does not know the speed at which it is to be transferred. Hence, if the objective is to predict a behavior, such as, gas filling in a container, transient flow boundary, etc., fluid has to be considered compressible.

Hi Vinerm
The vapor in my simulation was compressible. I used real gas Redlich Kwong for the density of the fluid

Check SoS
 

Check the Speed of Sound for your system then. Information does not travel at the speed of flow rather at the speed of sound. Take ratio of length and SoS and that should give you an estimate of time required.

How to check the speed of sound of my system?

Fluent calculates it
 

This is calculated by Fluent and you can check it in Contour plots or use volume reporting. This is available under Properties. You can also check Mach number under Velocity and then take ratio of velocity and Mach number.

Ok I know that the mach number is around 0.05 and I can calculate the speed of sound. But how can I know that the fluid travel at the speed of sound?

Fluid Velocity
 

Fluid travels at the speed you want it to. That's as per the conditions specified at the boundary. If Mach is 0.05, then fluid is traveling at 5% of the speed of sound. But the information in the fluid travels at the speed of sound and not at the speed of flow. If you change pressure at any boundary, it would be felt at another point depending upon the speed of sound and not based upon the speed of fluid. Otherwise, the information will never travel through a static fluid. Do note, this is true only for pressure and not for temperature or species.

Sorry, I can't understand what you mean, can you explain it more?

Article
 

Though you can go through a lot of literature on compressible flow but I just searched and found this one to be rather simple one for explanation of what is happening in your case

http://thermopedia.com/content/646/