[kaegen]

Hi,

I am a student at NTU. Presently I am doing a Fluent simulation on choked flow in orifice to investiagate how discharge coefficient(Cd)of co2 varies with pressure difference. To get Cd, I need the mass flow rate. The geometry is a circular pipe 60mm in diameter and 142mm long, with a orifice(13.8mm) in the middle.

I have tried pressure-based solver with k-epsilon model and attained convergence. I use constant density. But it seemed not able to simulate choked flow as my mass flow rate keeps on increasing with pressure difference. I use inlet pressure of 1000kpa. The outlet pressure varies from 900kpa to 100kpa.

Next, I tried density-based solver, implicit and explicit, ROE-FDS and was not able to get any convergence. I use ideal gas as density. I have tried decreasing courant number and relaxation factor which fail to converge as well. Most of the time I get floating point error.

Questions
1. Do I have to play with adapt/gradient settings? If so, how to I determine the threshold and the max/min no. of cells.

2. I use turbulent intensity % and hydraulic diameter settings for inlet and outlet. Is the H.diameter the actual diameter of the inlet and outlet pipe diameter or are they the diameters of the orifice in the pipe?

3. I understand that density changes with pressure. Is it inaccurate that I use const density for co2 for my prssure-based simulation? If I use ideal gas as density, the mass flow rate attained is way too high.

Any help from anybody is greatly appreciated!!