[engr.usmanabdullah]

Hi, I am quite new to Fluent and compressible flow modelling and require your expertise and advice please. Air through a converging nozzle is modelled with annotated BC's. The air properties are Density: Ideal-gas, Cp: constant= 1006.4 J/Kg.K, Viscosity: Sutherland. Solver: pressure-based. Mass and Energy conservation checks are performed to ensure simulation convergence. Despite with multiple iterations, even with density-based solver, there is significantly low temperature (<-70C) and M>1 values at the nozzle outlet, which is quite strange for a subsonic nozzle. I am feeling if I am modelling the air properties not quite right. Any suggestions will be highly appreciated.

[LuckyTran]

Your physical nozzle is purely converging but at the nozzle termination, it is a sudden expansion, which is a kinetic diverging nozzle.

Hence, it should reach the sonic condition and even exceed Mach 1 after the nozzle during the expansion.

However, you can see that the flow in the nozzle is entirely subsonic, so there is no contradiction. You just didn't take into account that there is stuff after the subsonic nozzle.

[engr.usmanabdullah]

Originally Posted by LuckyTran

Your physical nozzle is purely converging but at the nozzle termination, it is a sudden expansion, which is a kinetic diverging nozzle.

Hence, it should reach the sonic condition and even exceed Mach 1 after the nozzle during the expansion.

However, you can see that the flow in the nozzle is entirely subsonic, so there is no contradiction. You just didn't take into account that there is stuff after the subsonic nozzle.

Hi, thanks for responding. Yes I can see there is sudden expansion (Joule thompson effect) just after the nozzle termination. I am trying to model the air flow through this converging nozzle used in Laser cutting machine. However the simulation results are bit unrealistic eg temperature approx -70C. I have defined my inlet condition as 3 atm, 2.9 initial gauge, whereas outlet as P= 0 atm. The operating pressure is 1.
I am slightly confused here either there seem to be something wrong with my BC's (which I have checked multiple times btw), or I am not modelling the required properties of the Air in this simulation. As per my understanding, the Sutherland law
for viscosity along with Ideal-gas density should be sufficient for this simplistic simulation.

[FMDenaro]

Originally Posted by engr.usmanabdullah

Hi, I am quite new to Fluent and compressible flow modelling and require your expertise and advice please. Air through a converging nozzle is modelled with annotated BC's. The air properties are Density: Ideal-gas, Cp: constant= 1006.4 J/Kg.K, Viscosity: Sutherland. Solver: pressure-based. Mass and Energy conservation checks are performed to ensure simulation convergence. Despite with multiple iterations, even with density-based solver, there is significantly low temperature (<-70C) and M>1 values at the nozzle outlet, which is quite strange for a subsonic nozzle. I am feeling if I am modelling the air properties not quite right. Any suggestions will be highly appreciated.

Why don't you start from the theory of quasi-unidimensional flows with area variation to check what happens in a first model?

[engr.usmanabdullah]

apologies, but my knowledge here is very limited. Perhaps I do need to go back and revisit Thermodynamics to calculate this. However, I am concerned that the sudden expansion of air after the converging nozzle could lead to complexities in my analytical calculations.
I believe steering the BC's and defining relevant air properties would be a convenient option for me.

[FMDenaro]

Originally Posted by engr.usmanabdullah

apologies, but my knowledge here is very limited. Perhaps I do need to go back and revisit Thermodynamics to calculate this. However, I am concerned that the sudden expansion of air after the converging nozzle could lead to complexities in my analytical calculations.
I believe steering the BC's and defining relevant air properties would be a convenient option for me.

Start by studying this theory. You can find the theory of quasi one-dimensional flow in classical textbooks of gasdynamic. Search also for "de Laval" nozzle. That will help to check what happens in the converging part of your nozzle.



https://www.grc.nasa.gov/www/k-12/airplane/nozzled.html