[Achilleas]

Hello all,


I am modeling an axial-flow turbine, known as Wells turbine, using CFX as a solver. It is composed of a rotor with untwisted airfoil blades of symmetrical cross section radially set at 90 degrees stagger angle. I have 4 blades but if we'll take into account the axisymmetric geometry of the turbine, it is not necessary to mesh the whole geometry of the turbine but only one quarter of an overall domain. Periodical boundary conditions are imposed on the neighboring surfaces of the turbine. My model consists of three domains. "Inlet" and "Outlet" domains are stationary while "Rotor" domain is rotating with some angular velocity (i.e. 2000, 3000, 4000 rpm). All of them are fluid domains with air at 25 oC as the fluid. I am using "k-omega SST" as turbulence model setting advanced turbulence controls such as curvature correction and production limiter. Reference pressure is 1 atm. At the inlet I input a normal speed and at the outlet I have Static Pressure with relative pressure at 0 atm. I am quite confident that the generated mesh has a valid quality. The elements amount to approximately 11 millions with the smallest size at the rotor domain. Frozen rotor interfaces connect the domains. No-slip boundary conditions are imposed on the domain walls. In the middle part of the domain, I have used either numerical cells which are rotating with the blades of the turbine with constant angular speed or numerical cells which are stationary. The analysis type is "Steady state". I should say that I have completed series of simulations in which I was changing the angular velocity. In every different angular velocity I was studying the behavior of the turbine in different inlet velocities, in order to be able to compare the results with those which will arise in the long term by a transient analysis. The solution has converged at all cases.


The problem that I encounter regard the value of torque mainly at high inlet velocities. The highest inlet velocity that I am studying is 17.5 m/s. In those high velocities the airflow becomes detached from the blade. However, the value of torque which has came of the funtion “torque_z()@Whole_Blade”, is greater than values where flow seperation doesn't occur and I was expecting that I would have the peak in a diagram Torque-φ. Is my statement logical? What I am doing wrong at this case? I thought that the k-omega SST turbulence model wasn't suitable for higher inlet velocities. In fact, SST model is applicable for low Reynold numbers. So, it is the most correct choice for my case. However, I have checked a lot of available turbulence models. Also, I have checked an even more dence mence. The problem still remains.


mesh.jpg

setup.jpg


Thank you in advance for your assistance,
Achilleas