[hhhhe]

Hi,

I am trying to evaluate a convergent-divergent nozzle at zero pressure atmosphere.

The problem I have is that the total pressure at the nozzle exit is not the same as the inlet total pressure. It should be the same and constant outside the boundary layer. I am modeling a small thruster with pressure boundary inlet and outlet conditions.

model:
Axisymmetric 2D, density-based, energy equation: on, Laminar flow (Re small and a really small thruster).

Inlet: 15 bar, 1874 K
Outlet: 0 bar? (vacum atmosphere) and 2.73 K

I will post pictures to simplify the problem.

The first picture is describing my problem. it is the total pressure at the nozzle exit. so position 0 is the center of the nozzle exit and then it follows vertically along the nozzle exit to the nozzle wall at 3.5 mm (0.0035 m).
So at the centerline, the total pressure should be constant at 15 bar outside the boundary layer. Which it is not.

Second picture illustrates my Fluent configuration
pressure drop.JPG

configuration.JPG

[LuckyTran]

There should be a total pressure drop across the Mach disc near the nozzle throat, unless your nozzle isn't a nozzle.

As for your other inquiry: Far from the centerline the total pressure will approach the ambient total pressure of 0 Pa. Imagine you extend your wall all the way up to infinity. The total pressure at the edge of the universe won't be the total pressure of your nozzle. And this has nothing to do with boundary layers. It's not a drop in total pressure due to boundary layers, it's a drop in total pressure due to the flow there not being at all related to the nozzle discharge plume.

[hhhhe]

Thank you for your response! Could you please explain why that is? Shouldn't the total pressure always be constant if no losses occur?

[hhhhe]

Maybe I explained the picture a little carelessly. So the position at 0 is the center of the nozzle exit. The further out to the right of the "x-axis", you evaluate the total pressure closer to the wall of the nozzle outlet. at position 35 mm you are at the wall of the nozzle and inside the boundary layer.

From what I understand, boundary layer causes losses of total pressure.

[LuckyTran]

I already figured that out.

Total pressure is constant along a streamline if there are no losses. Traversing along the exit plane is not along a streamline. The actual boundary layer of the wall is probably the very last two points on your plot.

Like I said... take a total pressure probe to the edge of the universe, you won't measure 15 bar there.

Additionally, there's a huge total pressure loss in a sudden expansion, which is what you have at the nozzle exit.


I don't know what makes you think you can look at this flow system and think "it must be lossless."

[hhhhe]

The boundary layer is very thick, at least 20 % of the exit 2d "area". This is a really small nozzle with a laminar flow. So the losses of the boundary layer should take up a big chunk of that plot (figure)

[LuckyTran]

yawnnnnnnnnnnn