Effects of Off-Design Operating Conditions on the Blade Row Interaction in a HP Turbine Stage

An extensive experimental analysis on the subject of the unsteady periodic flow in a highly subsonic HP turbine stage has been carried out at the Laboratorio di Fluidodinamica delle Macchine (LFM) of the Politecnico di Milano (Italy). In this paper the blade row interaction is progressively enforced by increasing the stator and rotor blade loading and by reducing the stator-rotor axial gap from 100% (very large to smooth the rotor inlet unsteadiness) to 35% (design configuration) of the stator axial chord. The time-averaged three-dimensional flow field in the stator-rotor gap was investigated by means of a conventional five-hole probe for the nominal (0°) and an highly positive (+22°) stator incidences. The evolution of the viscous flow structures downstream of the stator is presented to characterize the rotor incoming flow. The blade row interaction was evaluated on the basis of unsteady aerodynamic measurements at the rotor exit, performed with a fast-response aerodynamic pressure probe. Results show a strong dependence of the time-averaged and phase-resolved flow field and of the stage performance on the stator incidence. The structure of the vortex-blade interaction changes significantly as the magnitude of the rotor inlet vortices increases, and very different residual traces of the stator secondary flows are found downstream of the rotor. On the contrary, the increase of rotor loading enhances the unsteadiness in the rotor secondary flows but has a little effect on the vortex-vortex interaction. For the large axial gap, a reduction of stator-related effects at the rotor exit is encountered when the stator incidence is increased as a result of the different mixing rate within the cascade gap.

CFD Study of HPT Blade Cooling Flow Supply Systems

This work summarizes the results of the CFD analyses to investigate the effect of the geometrical parameters for a typical coverplate-disk cavity and blade broach system also known as the blade cooling flow supply system. A turbofan high pressure turbine was used as the test vehicle for this investigation. The main objective was to explore potential improvements in engine SFC (aerodynamic performance) by reducing the parasitic work while minimizing the impact on the factors that affect the durability of the turbine blades; feed pressure, temperature, and mass flow. Various tangential on-board injection (TOBI) blade cooling flow supply systems were considered: i) Phase 1 compared the radial TOBI and axial TOBI, ii) Phase 2 compared coverplate-disk cavity shapes, and iii) Phase 3 compared blade broach shapes. The in-house CFD code NS3D was used for the analyses. Compared to the radial TOBI, the axial TOBI has a positive impact on the parasitic work (lower) and blade feed temperature (lower) while it has a negative impact on the blade feed pressure (lower). Further, the coverplate-disk cavity shapes investigated had no significant impact on the parasitic work, blade feed pressure, and blade feed temperature. The CFD solutions show that the major portion of the parasitic work is due to flow turning at the broach entrance. Finally, reducing the blade broach cross-section by sloping up the lower wall has no significant impact on the parasitic work and blade feed temperature but a negative impact on the blade feed pressure and mass flow. Modifying the broach pressure side wall shape is preferred among the blade broach geometries investigated. Future work to improve the CFD analysis consists of performing unsteady analyses to better capture the vortex flow in the blade broach, and including upstream stationary components with either iterative boundary condition modeling or an unsteady multi-stage approach.

Some Aspects of CFD Modeling in the Analysis of a Low-Pressure Steam Turbine

A three-dimensional, multistage, Navier-Stokes solver is applied to the numerical investigation of a four stage low-pressure steam turbine. The thermodynamic behavior of the wet steam is reproduced by adopting a real-gas model, based on the use of gas property tables. Geometrical features and flow-path details consistent with the actual turbine geometry, such as cavity purge flows, shroud leakage flows and partspan snubbers, are accounted for, and their impact on the turbine performance is discussed. These details are included in the analysis using simple models, which prevent a considerable growth of the computational cost and make the overall procedure attractive as a design tool for industrial purposes. Shroud leakage flows are modeled by means of suitable endwall boundary conditions, based on coupled sources and sinks, while body forces are applied to simulate the presence of the damping wires on the blades. In this work a detailed description of these models is provided, and the results of computations are compared with experimental measurements.

Axis-Asymmetric Profiled Endwall Design by Using Multiobjective Optimisation Linked With 3D RANS-Flow-Simulations

Secondary flow loss in modern axial compressors is considered to be the prime reason for the reduction of overall isentropic efficiency in these engine components. This paper presents a new methodology to diminish blade secondary loss and endwall loss by an axis-asymmetric modification of endwalls using an automated multiobjective optimizer in conjunction with 3D-RANS-flow-simulations. In order to obtain a favorable design for a wide operating range, the most important operating-points are considered in the optimization process. The existing multiobjective optimization package is enhanced by implementation of DLR’s in-house 3D-flow-solver TRACE. A straightforward stator optimization was performed for a 3D-process-chain test run. Finally, the novel endwall design technique is introduced and the first optimization results and further studies are discussed.

The Effect of Vane Clocking on the Unsteady Flowfield in a One and a Half Stage Transonic Turbine

The current paper presents the results of numerical and experimental clocking investigations performed in a high-pressure transonic turbine with a downstream vane row. The objective was a detailed analysis of shock and wake interactions in such a 1.5 stage machine while clocking the vanes. Therefore a transient 3D-Navier Stokes calculation was done for two clocking positions and the three dimensional results are compared with Laser-Doppler-Velocimetry measurements at midspan. Additionally the second vane was equipped with fast response pressure transducers to record the instantaneous surface pressure for 20 different clocking positions at midspan.

The Influence of Shroud and Cavity Geometry on Turbine Performance — An Experimental and Computational Study: Part II — Exit Cavity Geometry

The geometry of the exit shroud cavity where the rotor shroud leakage flow re-enters the main passage flow is very important due to the dominant influence of the leakage flow on the aerodynamics of low aspect ratio turbines. The work presented in this paper investigates, both experimentally and numerically, possibilities for the control of shroud leakage flow by modifications to the exit shroud cavity. The processes through which the leakage flow affects the mainstream aerodynamics identified in the first part of this study were used to develop promising strategies for reducing the influence of shroud leakage flow. The experimental program of this study was conducted on a three-stage model air turbine, which was extensively supported by CFD analysis. Three different concepts for shroud leakage flow control in the exit cavity were analysed and tested: a) profiled exit cavity downstream end-wall, b) axial deflector, and c) radial deflector concept. Reductions in aerodynamic losses associated with shroud leakage were achieved by controlling the position and direction at which the leakage jet re-enters the mainstream when it leaves the exit shroud cavity. Suggestions are made for an optimum shroud and cavity geometry.

Analytical Modeling of Turbine Cascade Leading Edge Heat Transfer Using Skin Friction and Pressure Measurements

The flow near the leading edge stagnation-line of a plane turbine cascade airfoil is analyzed using measurements, analytical modeling, and computational fluid dynamics (CFD) modeling. New measurements of skin friction and pressure indicate that the aerodynamics of the leading edge are well described by an exact analytical solution for stagnation-point or Hiemenz flow. The skin friction measurements indicate the extent over which the analytical model applies. Based on measurements from an earlier study, the highest heat transfer levels occur along the leading edge stagnation-line. The same parameters that characterize Hiemenz flow also characterize a stagnation-point potential flow, which is used to accurately predict the heat transfer levels along the stagnation-line. CFD analysis indicates that pressure predictions are better than skin friction predictions for characterizing the analytical modeling that is used for more accurate heat transfer evaluation. This provides an approach for predicting the peak heat transfer coefficient in a cascade based only upon surface static pressure calculations.

Influence of Hot Streak Temperature Ratio on Low Pressure Stage of a Vaneless Counter-Rotating Turbine

In order to explore the influence of hot streak temperature ratio on low pressure stage of a Vaneless Counter-Rotating Turbine, three-dimensional multiblade row unsteady Navier-Stokes simulations have been performed. The predicted results show that hot streaks are not mixed out by the time they reach the exit of the high pressure turbine rotor. The separation of colder and hotter fluids is observed at the inlet of the low pressure turbine rotor. After making interactions with the inner-extending shock wave and outer-extending shock wave in the high pressure turbine rotor, the hotter fluid migrates towards the pressure surface of the low pressure turbine rotor, and the most of colder fluid migrates to the suction surface of the low pressure turbine rotor. The migrating characteristics of the hot streaks are predominated by the secondary flow in the low pressure turbine rotor. The effect of buoyancy on the hotter fluid is very weak in the low pressure turbine rotor. The results also indicate that the secondary flow intensifies in the low pressure turbine rotor when the hot streak temperature ratio is increased. The effects of the hot streak temperature ratio on the relative Mach number and the relative flow angle at the inlet of the low pressure turbine rotor are very remarkable. The isentropic efficiency of the Vaneless Counter-Rotating Turbine decreases as the hot streak temperature ratio is increased.

Active Control of Combustion Instability Using Symmetric and Asymmetric Premix Fuel Modulation

Active instability control was applied to an atmospheric swirl-stabilized premixed combustor using open loop and closed loop control schemes. Actuation was realised by two on-off valves allowing for symmetric and asymmetric modulation of the premix fuel flow while maintaining constant time averaged overall fuel mass flow. Pressure and heat release fluctuations in the combustor as well as NOx, CO and CO2 emissions in the exhaust were recorded. In the open loop circuit the heat release response of the flame was first investigated during stable combustion. For symmetric fuel modulation the dominant frequency in the heat release response was the modulation frequency, while for asymmetric modulation it was its first harmonic. In stable open loop control a reduction of NOx emissions due to fuel modulation of up to 19% was recorded. In the closed loop mode phase-shift control was applied while triggering the valves at the dominant oscillation frequency as well as at its second subharmonic. Both, open and closed loop control schemes were able to successfully control a low-frequency combustion instability, while showing only a small increase in NOx emissions compared to, for example, secondary fuel modulation. Using premixed open loop fuel modulation, attenuation was best when modulating the fuel at frequencies different from the dominant instability frequency and its subharmonic. The performance of asymmetric fuel modulation was generally slightly better than for symmetric modulation in terms of suppression levels as well as emissions. Suppression of the instability’s pressure rms level of up to 15.7 dB was recorded.

Optimization of Highly Loaded Fan Rotor Based on Throughflow Model

This paper presents an optimization method for fan/compressor which couples throughflow model solving axisymmetric Euler equations with adaptive simulated annealing (ASA) algorithm. One of the advantages of this optimization method is that it spends much less time than 3D optimization due to the rapid solving of throughflow model. In addition, the optimization space is quite extensive because more design variables can be adjusted in throughflow phase, such as swirl distribution, hub curve and sweep. To validate this optimization method, a highly loaded fan rotor with pressure ratio of 3.06 as a baseline is optimized. During the optimization process, the objective function is total pressure ratio, moreover, mass flow and efficiency are selected as the constraint conditions. Three important design variables including swirl distribution, hub curve and sweep are parameterized using Bezier curve, and then optimized in throughflow model independently, finally the optimum designs are validated using 3D viscous CFD solver. It is shown that pressure ratio and rotor loading can be improved further through optimizing swirl distribution, however, hub and sweep curves take more effects on mass flow and efficiency respectively. The optimization results demonstrate the advantage and feasibility of this optimization method.