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Research Papers

Léonard Thomas, Gicquel Laurent Y. M., Gourdain Nicolas, et al. Steady/Unsteady Reynolds-Averaged Navier–Stokes and Large Eddy Simulations of a Turbine Blade at High Subsonic Outlet Mach Number J. Turbomach. 137, 041001 (2014) (10 pages);   Paper No: TURBO-10-1188;   doi:10.1115/1.4028493

Reynolds-averaged Navier–Stokes (RANS), unsteady RANS (URANS), and large eddy simulation (LES) numerical approaches are clear candidates for the understanding of turbine blade flows. For such blades, the flow unsteady nature appears critical in certain situations and URANS or LES should provide more physical understanding as illustrated here for a laboratory high outlet subsonic Mach blade specifically designed to ease numerical validation. Although RANS offers good estimates of the mean isentropic Mach number and boundary layer thickness, LES and URANS are the only approaches that reproduce the trailing edge flow. URANS predicts the mean trailing edge wake but only LES offers a detailed view of the flow. Indeed, LESs identify flow phenomena in agreement with the experiment, with sound waves emitted from the trailing edge separation point that propagate upstream and interact with the lower blade suction side.

Marn Andreas, Broszat Dominik, Selic Thorsten, et al. Comparison of the Aerodynamics of Acoustically Designed Exit Guide Vanes and a State-of-the-Art Exit Guide Vane J. Turbomach. 137, 041002 (2014) (10 pages);   Paper No: TURBO-14-1135;   doi:10.1115/1.4028457

Within previous EU projects, possible modifications to the engine architecture have been investigated, which would allow for an optimized aerodynamic or acoustic design of the exit guide vanes (EGVs) of the turbine exit casing (TEC). However, the engine weight should not be increased and the aerodynamic performance must be at least the same. This paper compares a state-of-the art TEC (reference TEC) with typical EGVs with an acoustically optimized TEC configuration for the engine operating point approach. It is shown that a reduction in sound power level for the fundamental tone (one blade passing frequency (BPF)) for this acoustically important operating point can be achieved. It is also shown that the weight of the acoustically optimized EGVs (only bladings considered) is almost equal to the reference TEC, but a reduction in engine length can be achieved. Measurements were conducted in the subsonic test turbine facility (STTF) at the Institute for Thermal Turbomachinery and Machine Dynamics, Graz University of Technology. The inlet guide vanes (IGVs), the low pressure turbine (LPT) stage, and the EGVs have been designed by MTU Aero Engines.

Habib Alexander J., Barton Jeffery L., Mathison Randall M., et al. Wireless Telemetric Data Acquisition and Real-Time Control for a High Measurement-Density Internal Heat Transfer Experiment J. Turbomach. 137, 041003 (2014) (11 pages);   Paper No: TURBO-14-1176;   doi:10.1115/1.4028329

This paper describes a wireless data transmission system for a large-scale rotating experiment to investigate the heat transfer in a three-passage serpentine test section. Patterned after the NASA HOST program, the current experiment extends the data set to larger aspect ratios including 1:2, 1:4, and 1:6. As with HOST, heat transfer is measured using the heated segments technique, and the serpentine test section spins at rotation numbers representative of engine conditions. Rotating experiments are essential for capturing the representative operating conditions and complicated flow physics that must be understood to advance internal cooling technology for high aspect ratio configurations. There are challenges associated with controlling the operating parameters and collecting accurate data for high measurement-density rotating experiments. This experiment requires that 140 copper panels be held at a constant temperature by independently controlling and recording the power supplied to a separate heater on each panel. This means there must be 140 temperature measurements, 140 pairs of heater power leads, enough power to drive all of these heaters, and data recording capacity left over to measure fluid temperatures and pressures. Traditional methods of transferring rotating signals to the stationary frame of reference (like slip rings) are widely implemented but have practical limitations in the quantity of transferrable signals and the electrical current capacity of the individual channels. Alternatively, wireless transmission techniques were first developed decades ago, but their practical use has been limited by onboard power delivery requirements and cost. This paper describes the development of a new data transmission and control system that takes advantage of improvements in inexpensive electronics to create a battery-powered and microprocessor controlled system for acquisition, storage, control, and wireless communication. These components are assembled as an integral part of the rotating mechanical hardware. By handling high-fidelity microcircuit signal conditioning, data acquisition, feedback control, and data storage in the rotating frame and transmitting the results wirelessly, this system provides high measurement density and active feedback control that would have been impractical with a conventional slip-ring approach. The design and construction of the wireless control system for one full sidewall of the three-serpentine passage is described in detail. Its capability and functionality is demonstrated with operational data. It will be demonstrated that while all of the components in this system are readily available, the unique combination of this technology opens up a new world of measurement capabilities.

Martel Carlos, Corral Roque, Ivaturi Rahul. Flutter Amplitude Saturation by Nonlinear Friction Forces: Reduced Model Verification J. Turbomach. 137, 041004 (2014) (8 pages);   Paper No: TURBO-14-1178;   doi:10.1115/1.4028443

The computation of the final, friction saturated limit cycle oscillation amplitude of an aerodynamically unstable bladed-disk in a realistic configuration is a formidable numerical task. In spite of the large numerical cost and complexity of the simulations, the output of the system is not that complex: it typically consists of an aeroelastically unstable traveling wave (TW), which oscillates at the elastic modal frequency and exhibits a modulation in a much longer time scale. This slow time modulation over the purely elastic oscillation is due to both the small aerodynamic effects and the small nonlinear friction forces. The correct computation of these two small effects is crucial to determine the final amplitude of the flutter vibration, which basically results from its balance. In this work, we apply asymptotic techniques to consistently derive, from a bladed-disk model, a reduced order model that gives only the time evolution on the slow modulation, filtering out the fast elastic oscillation. This reduced model is numerically integrated with very low computational cost, and we quantitatively compare its results with those from the bladed-disk model. The analysis of the friction saturation of the flutter instability also allows us to conclude that: (i) the final states are always nonlinearly saturated TW; (ii) depending on the initial conditions, there are several different nonlinear TWs that can end up being a final state; and (iii) the possible final TWs are only the more flutter prone ones.

Laveau Benoit, Abhari Reza S., Crawford Michael E., et al. High Resolution Heat Transfer Measurements on the Stator Endwall of an Axial Turbine J. Turbomach. 137, 041005 (2014) (10 pages);   Paper No: TURBO-14-1195;   doi:10.1115/1.4028431

In order to continue increasing the efficiency of gas turbines, an important effort is made on the thermal management of the turbine stage. In particular, understanding and accurately estimating the thermal loads in a vane passage is of primary interest to engine designers looking to optimize the cooling requirements and ensure the integrity of the components. This paper focuses on the measurement of endwall heat transfer in a vane passage with a three-dimensional (3D) airfoil shape and cylindrical endwalls. It also presents a comparison with predictions performed using an in-house developed Reynolds-Averaged Navier–Stokes (RANS) solver featuring a specific treatment of the numerical smoothing using a flow adaptive scheme. The measurements have been performed in a steady state axial turbine facility on a novel platform developed for heat transfer measurements and integrated to the nozzle guide vane (NGV) row of the turbine. A quasi-isothermal boundary condition is used to obtain both the heat transfer coefficient and the adiabatic wall temperature within a single measurement day. The surface temperature is measured using infrared thermography through small view ports. The infrared camera is mounted on a robot arm with six degrees of freedom to provide high resolution surface temperature and a full coverage of the vane passage. The paper presents results from experiments with two different flow conditions obtained by varying the mass flow through the turbine: measurements at the design point ($ReCax=7.2×105$) and at a reduced mass flow rate ($ReCax=5.2×105$). The heat transfer quantities, namely the heat transfer coefficient and the adiabatic wall temperature, are derived from measurements at 14 different isothermal temperatures. The experimental data are supplemented with numerical predictions that are deduced from a set of adiabatic and diabatic simulations. In addition, the predicted flow field in the passage is used to highlight the link between the heat transfer patterns measured and the vortical structures present in the passage.

Regina K., Kalfas A. I., Abhari R. S. Experimental Investigation of Purge Flow Effects on a High Pressure Turbine Stage J. Turbomach. 137, 041006 (2014) (8 pages);   Paper No: TURBO-14-1197;   doi:10.1115/1.4028432

In the present paper, an experimental investigation of the effects of rim seal purge flow on the performance of a highly loaded axial turbine stage is presented. The test configuration consists of a one-and-a-half stage, unshrouded, turbine, with a blading representative of high pressure (HP) gas turbines. Efficiency measurements for various purge flow injection levels have been carried out with pneumatic probes at the exit of the rotor and show a reduction of isentropic total-to-total efficiency of 0.8% per percent of injected mass flow. For three purge flow conditions, the unsteady aerodynamic flow field at rotor inlet and rotor exit has been measured with the in-house developed fast response aerodynamic probe (FRAP). The time-resolved data show the unsteady interaction of the purge flow with the secondary flows of the main flow and the impact on the radial displacement of the rotor hub passage vortex (HPV). Steady measurements at off-design conditions show the impact of the rotor incidence and of the stage flow factor on the resulting stage efficiency and the radial displacement of the rotor HPV. A comparison of the effect of purge flow and of the off-design conditions on the rotor incidence and stage flow factor shows that the detrimental effect of the purge flow on the stage efficiency caused by the radial displacement of the rotor HPV is dominated by the increase of stage flow factor in the hub region rather than by the increase of negative rotor incidence.

Tan David, Li Yuanchao, Wilkes Ian, et al. Visualization and Time-Resolved Particle Image Velocimetry Measurements of the Flow in the Tip Region of a Subsonic Compressor Rotor J. Turbomach. 137, 041007 (2014) (11 pages);   Paper No: TURBO-14-1205;   doi:10.1115/1.4028433

A new optically index matched facility has been constructed to investigate tip flows in compressor-like settings. The blades of the one and a half stage compressor have the same geometry, but lower aspect ratio as the inlet guide vanes (IGVs) and the first stage of the low-speed axial compressor (LSAC) facility at NASA Glenn. With transparent blades and casings, the new setup enables unobstructed velocity measurements at any point within the tip region and is designed to facilitate direct measurements of effects of casing treatments on the flow structure. We start with a smooth endwall casing. High speed movies of cavitation and time-resolved PIV measurements have been used to characterize the location, trajectory, and behavior of the tip leakage vortex (TLV) for two flow rates, the lower one representing prestall conditions. Results of both methods show consistent trends. As the flow rate is reduced, TLV rollup occurs further upstream, and its initial orientation becomes more circumferential. At prestall conditions, the TLV is initially aligned slightly upstream of the rotor passage, and subsequently forced downstream. Within the passage, the TLV breaks up into a large number of vortex fragments, which occupy a broad area. Consequently, the cavitation in the TLV core disappears. With decreasing flow rate, this phenomenon becomes more abrupt, occurs further upstream, and the fragments occupy a larger area.

Saddoughi S., Bennett G., Boespflug M., et al. Experimental Investigation of Tip Clearance Flow in a Transonic Compressor With and Without Plasma Actuators J. Turbomach. 137, 041008 (2014) (10 pages);   Paper No: TURBO-14-1207;   doi:10.1115/1.4028444

Blade tip losses represent a major performance penalty in low aspect ratio transonic compressors. This paper reports on the experimental evaluation of the impact of tip clearance with and without plasma actuator flow control on performance of an U.S. Air Force-designed low aspect ratio, high radius ratio single-stage transonic compressor rig. The detailed stage performance measurements without flow control at three clearance levels, classified as small, medium, and large, are presented. At design-speed, increasing the clearance from small to medium resulted in a stage peak efficiency drop of almost six points with another four point drop in efficiency with the large clearance (LC). Comparison of the speed lines at high-speed show significantly lower pressure rise with increasing tip clearance, the compressor losing 8% stall margin (SM) with medium clearance (MC) and an additional 1% with the LC. Comparison of the stage exit radial profiles of total pressure and adiabatic efficiency at both part-speed and design-speed and with throttling are presented. Tip clearance flow-control was investigated using dielectric barrier discharge (DBD) type plasma actuators. The plasma actuators were placed on the casing wall upstream of the rotor leading edge and the compressor mapped from part-speed to high-speed at three clearances with both axial and skewed configurations at six different frequency levels. The plasma actuators did not impact steady state performance. A maximum SM improvement of 4% was recorded in this test series. The LC configuration benefited the most with the plasma actuators. Increased voltage provided more SM improvement. Plasma actuator power requirements were almost halved going from continuous operation to pulsed plasma. Most of the improvement with the plasma actuators is attributed to the reduction in unsteadiness of the tip clearance vortex near-stall resulting in additional reduction in flow prior to stall.