Direct numerical simulation is used to investigate effects of heat release and compressibility on mixing-layer turbulence during a period of self-similarity. Temporally evolving mixing layers are analysed at convective Mach numbers between 0.15 and 1.1 and in a Reynolds number range of 15000 to 35000 based on vorticity thickness. The turbulence inhibiting effects of heat release are traced back to mean density variations using an analysis of the fluctuating pressure field based on a Green's function.
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.
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.
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