# Question about the Energy Equation (Turbomachinery)

 Register Blogs Members List Search Today's Posts Mark Forums Read

May 11, 2024, 05:47
Question about the Energy Equation (Turbomachinery)
#1
New Member

Lorenz
Join Date: Jun 2023
Location: Austria
Posts: 9
Rep Power: 2
Hello Foamers,

Maybe someone can clarify or have a suggestion what is wrong with my temperature field. I am solving a compressible flow through a turbomachinery (radial turbine). The model includes a MRF zone, and between the zones a mixing plane treatment is included.
The mesh was generated using Ansys TurboGrid and was transferred to OpenFOAM. The same mesh was used in a CFX simulation to compare results with literature (APU Test Case).

Case Setup:
• Inlet: total pressure|total temperature with pressureVelocityInletOutlet
• Outlet: total pressure|inletOutlet T with pressureVelocityInletOutlet
• Periodics: cyclicAMI
• Interfaces: stator<>rotor, rotor<>diffuser -> mixing plane
• Walls: fixedValue for U, else zeroGradient and wall functions
• komega SST turbulence model, y+ values in a good range
• BC were ramped up
• MRF rotational speed 7508.41 rad/s
• Thermo: psiThermo, sutherland, sensibleEnthalpy, fluid: air

Primarily I am using rhoSimpleFoam to validate the setup and compare the results. Pressure and velocity fields tend towards the same results obtained using CFX. However, the temperature field is fully off. I do not see any cooling inside the turbine, although the pressure and velocity reduce along the flow path.

Energy equation in rhoSimpleFoam
Code:
``` fvScalarMatrix EEqn
(
fvm::div(phi, he)
+ (
he.name() == "e"
? fvc::div(phi, volScalarField("Ekp", 0.5*magSqr(U) + p/rho))
: fvc::div(phi, volScalarField("K", 0.5*magSqr(U)))
)
- fvm::laplacian(turbulence->alphaEff(), he)
==
fvOptions(rho, he)
);```
For my understanding, the kinetic part of the energy equation should reduce along the blade.
In the UEqn the MRF zone is considered adding the Coriolis and centrifugal force to the system using the term
Code:
`MRF.DDt(rho,U)`

OpenFOAM solves in absolute velocity formulation. Therefore, quite high velocities are reached inside the rotational domain. Could this affect the kinetic part of the simulation?

Am I missing some term in the energy equation or the UEqn that reduces the energy in the system due to the MRF zone? Pressure gradients at the blade surfaces do look good, and I see a moment acting on those surfaces (post processed using the force library).

Attached the pressure, velocity, tempreaure and total tempreature field along the flow path of the turbine.

looking forward to the discussion,
Lorenz
Attached Images
 uField.png (126.1 KB, 8 views) pField.png (105.5 KB, 6 views) tField.png (146.6 KB, 5 views) tTotalField.png (118.7 KB, 5 views)
__________________
Lorenz H.

PhD Student
Montanuniversity Leoben

 Tags energy equation, mrf, rhosimplefoam, turbomachery