[Hello_Fluent]

Hello friends,
I am trying to simulate a subsonic diffuser by ANSYS 14.5 _ FLUENT. My Inlet mach number is 0.9 and Inlet gas is Air. I want to increase pressure at the outlet of a diffuser. So I have added a diverging area diffuser at the end of a pipe through which the flow is taking place. But in my result instead of rise in pressure and decreasing in velocity, reverse is taking place. Pressure is decreasing and velocity is increasing. Please help me out which boundary condition and solver I have to choose to obtain the exact result.

Please give some suggestion about simulation of compressible flow in a diffuser.

[Kushal Puri]

Originally Posted by Hello_Fluent

Hello friends,
I am trying to simulate a subsonic diffuser by ANSYS 14.5 _ FLUENT. My Inlet mach number is 0.9 and Inlet gas is Air. I want to increase pressure at the outlet of a diffuser. So I have added a diverging area diffuser at the end of a pipe through which the flow is taking place. But in my result instead of rise in pressure and decreasing in velocity, reverse is taking place. Pressure is decreasing and velocity is increasing. Please help me out which boundary condition and solver I have to choose to obtain the exact result.

Please give some suggestion about simulation of compressible flow in a diffuser.

What are the boundary conditions you are using?

[Hello_Fluent]

Thank you for the reply.

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, because 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 boundary condition, I have used P outlet as the boundary condition and the gauge pressure at outlet is 101325 Pascal.
Operating condition pressure as 0 pascal.
When I did not get the result, I changed the P outlet gauge pressure value to a higher pressure value than the Pinlet as I obtain that value from analytical calculation, 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.

Please suggest me the appropriate procedure for solving a diverging diffuser.

Thank You

[waseeqsiddiqui]

Please help

[Kushal Puri]

Originally Posted by waseeqsiddiqui

Please help

You can use a mass flow inlet and provide the pressure value in supersonic gauge pressure and also use coupled solver. Exred the outlet domain more. And increase also the outlet pressure..
I

[waseeqsiddiqui]

Originally Posted by Kushal Puri

You can use a mass flow inlet and provide the pressure value in supersonic gauge pressure and also use coupled solver. Exred the outlet domain more. And increase also the outlet pressure..
I

Could you tell me what the Guage pressure, initial supersonic Guage pressure in pressure inlet and outlet mean?
In terms of static pressure total pressure etc.

[waseeqsiddiqui]

Originally Posted by Kushal Puri

You can use a mass flow inlet and provide the pressure value in supersonic gauge pressure and also use coupled solver. Exred the outlet domain more. And increase also the outlet pressure..
I

So I specify the theoretically evaluated static pressure at outlet or any certain value that is higher than inlet?

[LuckyTran]

At outlets: you need to specify the static pressure

Originally Posted by waseeqsiddiqui

Could you tell me what the Guage pressure, initial supersonic Guage pressure in pressure inlet and outlet mean? In terms of static pressure total pressure etc.

At inlets: you specify the total pressure if you use the pressure inlet BC or the mass-flow rate if you use the mass flow rate BC. So you take your value for the total pressure and put it into the box labeled

Gauge Total Pressure (pascal)

If your inlet is subsonic you are done. If the inlet is supersonic, then you need to specify (in addition to the total pressure or mass flow rate) the static pressure. You take your static pressure and put it into the box labeled:

Supersonic/initial Gauge Pressure (pascal)

If your inlet is subsonic, this input has no effect and is ignored. You can put in any value and it won't matter if your inlet is subsonic. This box has another purpose during initialization and that's why it is labeled supersonic/initial. It actually has two purposes.

Almost all pressures in Fluent are gauge pressures, so that the letters Gauge appear everywhere for redundancy.

[waseeqsiddiqui]

Originally Posted by LuckyTran

At outlets: you need to specify the static pressure





At inlets: you specify the total pressure if you use the pressure inlet BC or the mass-flow rate if you use the mass flow rate BC. So you take your value for the total pressure and put it into the box labeled

Gauge Total Pressure (pascal)

If your inlet is subsonic you are done. If the inlet is supersonic, then you need to specify (in addition to the total pressure or mass flow rate) the static pressure. You take your static pressure and put it into the box labeled:

Supersonic/initial Gauge Pressure (pascal)

If your inlet is subsonic, this input has no effect and is ignored. You can put in any value and it won't matter if your inlet is subsonic. This box has another purpose during initialization and that's why it is labeled supersonic/initial. It actually has two purposes.

Almost all pressures in Fluent are gauge pressures, so that the letters Gauge appear everywhere for redundancy.

Always a pleasure to hear it from you.