Exhaust Pressure in a Gas Turbine Exhaust Duct

[Anonymous]

Hello,

I am trying to determine the outlet pressure on a gas turbine using a particular ducting design. I have used the following approach:

1) Set pressure at outlet surface to 0 psig (atmosphere).

2) Set inlet mass flow rate (155 lbm/s) and temperature (1051 degrees F) according to the engine (GE LM2500) specifications.

3) Run simulation.

4) Use the Wall Calculator to determine the DP from inlet to outlet; this is my outlet pressure.

All runs smoothly, but the results seem unrealistically small (0-1 psid). I am an engineer by education, but not so smart on fluids. At this point I am using air as a medium for simplicity, but may change that once I feel like the results make sense. Given the relatively low velocities, I am assuming incompressible flow.

I have the feeling that I may be doing something wrong. The CFZ file is attached; any help would be very much appreciated.

Thanks,

Kerry

[Jon.Wilde · Accepted solution]

Hi,

It is better to use the bulk calculator near the inlet and outlet, the wall calc is really for heat transfer.

You need an extension on the inlet though, the length should be 5x the diameter. Otherwise it would be pretty tricky to capture the pressure here.

Then look at mesh refinement, as a basic setup we should have 4-5 elements across a gap, here there is only  2 at the outlet, which is not quite enough. More mesh will yield a more accurate result, this is almost always true until we reach a point of mesh independence where the mesh is so fine, more makes no difference.

You might also like to try Advection Scheme 5, at least compare it to 1. It will likely be a little more accurate also.

Kind regards,

Jon

[Anonymous]

Jon,

Thanks for the reply. I'll give all of those a try. A related question: a plane that is aligned to the turbine outlet (the recessed circle at the bottom of the duct) will also cut through part of the ducting above (see picture below). I was concerned that this makes the summary statistics for that surface non-representative of the turbine outlet itself. Is there a way to restrict the summary to just a part of the plane?

Thanks again,

Kerry

[Jon.Wilde]

Hi,

No problem. You need to extend that inlet still 🙂

Within the bulk calc you should have the option to choose the surface where you want to calculate results, CFD will automatically split them for you.

Thanks,

Jon

[Anonymous]

Jon,

I implemented all your recommendations, including increasing the mesh to five elements across on the outlet. The results are much more intuitive now.

Just as a check, I modified the original design by choking it off a bit at the outlet. The inlet pressure increased as expected.

CFZ file attached for anyone interested.

Thanks again,

Kerry

[Jon.Wilde]

Hi Kerry,

Good to know! I appreciate this was just a quick test but to avoid avoid errors - if you do drop the outlet area down like that, it is always good to add a further extension 🙂 Nice looking image too!

OK if I mark this as solved?

Kind regards,

Jon

[Anonymous]

Jon,

I marked your original reply as the solution.

Thanks again,
Kerry

[Anonymous]

A follow-on question:

To analyze the INLET of a similar turbine, should an inlet extension be placed at the machine end as well? This surface would theoretically only have a mass flow boundary condition, if that makes a difference. Boundary conditions for atmospheric pressure and some assumed temperature would apply to the "open" end.

Thanks,

Kerry

[Jon.Wilde]

Hi Kerry,

The idea of an inlet extension is to provide an opportunity for flow to develop before it enters the area of interest.

With an outlet extension, the goal is to low the flow settle, any recirculation to complete and ideally become laminar before exiting - so we avoid recirculation over the outlet. This does not change with swapping the boundary conditions. So if you don't have a uniform cross section before the outlet, extend it.

Does that make sense?

Kind regards,

Jon

[Anonymous]

Jon,

Thanks for the clarification.

Kerry

[Anonymous]

Please MR Jon can you help  me I have a primary level question "does improper combustion causes exhaust duct pressure(EXHAUST MANI FOLD PRESSURE) in turbines" and why

[Anonymous]

m at: shoaibullah101@gmail.com