Inga Mahle

Improving the Interaction Between Leakage Flows and Main Flow in a Low Pressure Turbine

A large part of the losses caused by leakage flows through cavities in turbines are mixing losses. They arise when the leakage flow — after passing through the cavity — is re-entering into the mainflow. In the zone of re-entering, the velocity components of the mainflow differ from those of the leakage flow, since the former has passed the precedent airfoil, where it has been accelerated and turned, while the latter has not. This leads to shear stresses which cause increased turbulence and losses. This paper presents a numerical investigation of a device which reduces the mixing losses caused by the leakage flows through inner cavities of a low pressure turbine to 63% of their original value. The device is situated close to the rear openings of the cavities and a large part of the leakage flow is passing through it. The leakage flow is turned and accelerated by the device in a way that brings its velocity components closer to the velocity components of the mainflow. This reduces the mixing losses considerably compared to cavity flows without turning devices. An increase in efficiency of the low pressure turbine of about 0.1% can be noticed. This paper presents numerical results of steady 3D simulations of a three-stage low pressure turbine with a pressure ratio of approximately 3.5. Results with an ideal flow path (no cavities), with inner cavities without turning device and with inner cavities with turning device are compared. Radial distributions of characteristic quantities (turbulent kinetic energy, circumferential velocity etc.) show that these quantities evaluated with cavities with turning device are much closer to the ideal flow path quantities than without. By subtracting the solution with turning device from the one without, the regions where mixing losses are reduced are identified.Copyright

A Low Pressure Turbine at Extreme Off-Design Operation

In a cooperative project between the Institute of Aircraft Propulsion Systems and MTU Aero Engines GmbH, a two-stage low pressure turbine with integrated 3D airfoil and endwall contouring is tested. The experimental data taken in the altitude test-facility study the effect of high incidence in off-design operation. Steady measurements are covering a wide range of Reynolds numbers between 40,000 and 180,000. The results are compared with steady multistage CFD predictions with a focus on the stator rows. A first unsteady simulation is taken into account as well. The CFD simulations include leakage flow paths with disk cavities modeled. Compared to design operation the extreme off-design high-incidence conditions lead to a different flow-field Reynolds number sensitivity. Airfoil lift data reveals changing incidence with Reynolds number of the second stage. Increased leading edge loading of the second vane indicates a strong cross channel pressure gradient in the second stage leading to larger secondary flow regions and a more three-dimensional flow-field. Global characteristics and area traverse data of the second vane are discussed. The unsteady CFD approach indicates improvement in the numerical prediction of the predominating flow-field.

An LP Turbine at Extreme Off-Design Operation

In a cooperative project between the Institute of Aircraft Propulsion Systems (ILA) and MTU Aero Engines GmbH a two-stage low pressure turbine with integrated 3D airfoil and endwall contouring is tested. The experimental data taken in the altitude test-facility study the effect of high incidence in off-design operation. Steady measurements are covering a wide range of Reynolds numbers between 40,000 and 180,000. The results are compared with steady multistage CFD predictions with a focus on the stator rows. A first unsteady simulation is taken into account as well. The CFD simulations include leakage flow paths with disc cavities modeled. Compared to design operation the extreme off-design high-incidence conditions lead to a different flow-field Reynolds number sensitivity. Airfoil lift data reveals changing incidence with Reynolds number of the second stage. Increased leading edge loading of the second vane indicates a strong cross channel pressure gradient in the second stage leading to larger secondary flow regions and a more three-dimensional flow field.Global characteristics and area traverse data of the second vane are discussed. The unsteady CFD approach indicates improvement in the numerical prediction of the predominating flow field.Copyright

Inverse Fin Arrangement in a Low Pressure Turbine to Improve the Interaction Between Shroud Leakage Flows and Main Flow

The paper deals with the geometry of the shroud cavities in low pressure gas turbines and presents a design which helps to reduce the losses that arise when the shroud leakage flows interact with the main flow. The fins in low pressure gas turbines are usually attached to the shroud of the blades. They are therefore rotating while the non-rotating honeycomb or abrasive coating is mounted into the casing. The shroud leakage flow, after passing the rear fin, is decelerated in the rear cavity chamber and enters the main flow path with an axial velocity that is smaller than the axial velocity of the main flow. This difference in axial velocity, together with differences in the circumferential velocity, leads to increased turbulence, mixing losses and an unfavorable incidence of the subsequent vane row in the wall region. Contrarily to the usual configuration, the inverse fins in the turbine presented in the paper are attached to the casing while the honeycomb is mounted onto the rotating blades. This arrangement results in the location of the gap between the fin and the honeycomb being very close to the position of re-entry of the leakage flow into the main flow. Therefore, the leakage flow keeps a high velocity resulting from the narrow fin gap until re-entry which reduces the velocity difference with respect to the main flow. Consequently, the mixing losses and subsequent row losses are reduced. Due to the favorable position of the gap and a particular shaping of the honeycomb, the leakage flow is kept close to the surface of the shroud and enters the main flow with little perturbations. The paper presents numerical results of steady 3D simulations of a three-stage low pressure turbine. Results with an ideal flow path (no cavities), with shroud cavities with conventionally rotating fins and with shroud cavities with inverse fins are compared.Copyright