# Simulation of a Subsonic diffuser

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February 2, 2017, 10:15
Simulation of a Subsonic diffuser
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Saibal
Join Date: Feb 2017
Posts: 3
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Hello friends,
I am doing a simulation on sub-sonic diffuser(Divergent nozzle) using ansys fluent, I am not able to get the exact result what it has to come actually.
As it is known to all of us that during subsonic flow diverging cross-section will be acting as diffuser, where velocity will decrease at the outlet and simultaneously pressure will increase.But, in my case I am getting a reverse result. Pressure is decreasing and velocity is increasing.
I am completely describing my problem details:
For the case of diffuser:
Inlet Diameter-55 mm.
Outlet diameter-110 mm.
Inlet pressure- 410043 Pascal
Inlet temperature-483 K.
Inlet gas-Air
Inlet Density-2.9563 kg/m3
Inlet velocity- 397 m/s.
As the temperature at the inlet is 483K, Mach number at the inlet is 0.9 as the velocity of sound at inlet is around 440 m/s.
During problem solving through Fluent,
I have used P inlet as the inlet boundary condition.Gauge total pressure is 693400Pascal as I found it from isentropic table corresponding to M=0.9.
Initial gauge pressure is 410043Pascal and Inlet temperature is 483K.
For the outlet case. I have used P outlet as the boundary condition and the gauge pressure at outlet is 101325 Pascal.
When I did not get the result, I changed the P outlet gauge pressure value to a higher pressure value than the Pinlet, but at that time also I found errors like reverse flow condition and divergence of the solution etc.
In the solution method I used Implicit+AUSM second order upwind scheme.
After solving this problem, I found the solution as velocity is reaching to 550m/s and the pressure is decreasing simultaneously.

Please suggest me where actually I am doing mistake, so the result is not coming properly.
Attached Images
 FFF-4.2_pressure.jpg (55.4 KB, 11 views) FFF-4.2_velocity.jpg (56.7 KB, 10 views)