Scott C. Morris

Turbine Tip Clearance Flow Control using Plasma Actuators

The tip clearance gap leakage ∞ow is of continuing concern in reducing e‐ciency losses that occur within turbines. Active ∞ow control using a blade-tip-mounted unsteady plasma actuator was implemented in a low pressure linear turbine cascade. Downstream ∞ow velocity and pressure were acquired using a flve-hole probe to document changes in leakage vortex size and strength. Reynolds numbers of 5£10 4 and 10 5 for tip gaps of 4% and 1.56% of axial chord were examined for unactuated and actuated cases. Due to the large ∞ow angles seen in the leakage vortex at a 4% gap size, the probe was unable to acquire the downstream pressure because the calibration region of the probe was exceeded. The extensive three-dimensionality of the ∞owfleld in the tip region has proven di‐cult to measure for low velocities. However, for the smaller 1.56% gap, the leakage vortex size is reduced over the 4% gap, which allows for measurement of actuator efiectiveness on the downstream ∞owfleld. Here, actuation achieved a 29.5% reduction in maximum pressure loss at an axial chord Reynolds number of 10 5 , while at a Reynolds number equal to 5 £ 10 4 , there was a 14.7% reduction. These results are dependent on the unsteady frequency the actuator, although these results show a signiflcant beneflcial change in the tip gap ∞ow.

Tip Clearance Control Using Plasma Actuators

Abstract : This report documents experimental results from a series of experiments using active flow control to improve the performance and efficiency of turbine tip clearance flows. The flow control was based on plasma actuators. Two experimental facilities were used as part of this research project. The first was a low-speed, large-scale turbine cascade using Pak-B turbine blades. The second was a smaller scale, high-speed cascade with low aspect ratio. The active flow control was design to act as a plasma squealer. Specifically, the plasma force was oriented perpendicular to the camber line in order to resist the leakage flow through the tip gap. Both steady and unsteady forcing was used as a method for manipulating natural jet-like instabilities that existed as the leakage flow separated in the tip gap. The results have indicated that the unsteady plasma forcing was as effective as the passive squealer geometry in reducing the losses that occurred in the wake of the blade tip.

Turbine Blade Tip Leakage Flow Control by Partial Squealer Tip and Plasma Actuators

In order to further the understanding of turbine tip leakage and passage ∞ow mechanisms for undesirable entropy production, an experiment was conducted in a linear cascade at the Hessert Laboratory. Blade surface and tip endwall static pressure, total pressure loss, and wake vorticity measurements were taken to document the efiects of upstream axial Reynolds number, 1:0 £ 10 5 < Re2 < 5:0 £ 10 5 , and tip gap height, 0:015 < g=cx < 0:05 for ∞at and partial, suction-side squealer tip geometries, as well as active ∞ow control using a plasma actuator situated on the blade tip. Interaction of tip leakage and passage vortices proved critical, and an inverse relationship was observed between the two structures in terms of streamwise vorticity, core total pressure loss, and the vortex size denoted by the i‚2 criteria. When a squealer tip is added, the results indicate an efiective reduction in gap size, and thus a reduction in loss. However, the high loss region in the gap disappears, indicating that the squealer traps the air in the region. When a plasma actuator is used to control the ∞ow, the efiect depends strongly on the unsteady frequency at which it is run. An efiect is seen in the downstream loss, however the high loss region in the gap appears unafiected, indicating that this method does not trap air in the tip gap.

Turbine Blade Tip Leakage Flow Control: Thick/Thin Blade Effects

An experiment was conducted in a linear cascade of Pak-B blades for an exit Mach number of 0.3 to simulate the flow in the tip-gap region of a low pressure turbine blade row. The experiment focused on the independent effects of thickness-to-gap and gapto-chord ratios on the tip-gap flow behavior. Two extreme gap-to-chord ratios of 5% and 8% were chosen, for which four thickness-to-gap ratios were examined. The flow was documented through blade-tip and end-wall static pressure measurements, and downstream total pressure loss coefficients. Additionally, surface flowvisualization was performed on the blade tip end for a greater understanding of the gap-flow behavior. The response of the flow to passive flow control using a partial suction-side squealer tip at each of the thickness-togap and gap-to-chord cases was documented. The intention was to examine any sensitivity of the flow to the gap-to-chord ratio that might be attributed to the thickness-to-gap ratio in a manner that can be categorized as “thick” and “thin” blade behavior. For this the focus was on possible changes in the size and location of a separation and re-attachment lines on the blade tip end. Defining such features in these flows is important to our goal of active tip-gap flow control using plasma actuators because they are highly receptivity to unsteady forcing.

Tip Clearance Flow Control in a Linear Turbine Cascade using Plasma Actuation

This research examines the use of passive and active on-blade flow control to reduce the unwanted losses associated with the blade tip clearance flow in a stationary, rectilinear turbine cascade. A SDBD plasma actuator and a passive partial suction-side squealer were tested over a Reynolds number range from 5.3×10 4 to 1.03×10 5 at a fixed tip clearance of 2.18 percent of axial chord. The pla sma actuator was designed to mimic the beneficial effects of the suction-side squealer ti p, while removing the negative aspects of the passive squealer design, including blade degradation flow recircul ation or potential blade-wall contact. The flowfield was documented with five-hole-probe measurements at 1 axial chord downstream of the test blade and within the clearance using wall pressure taps located on the endwall opposite the blade tip. These tests allowed the loss associated with the flow and the change in this loss with a pplied flow control to be recorded. The plasma actuator caused an improvement in the downstream flow, with a reduction in the total pressure loss coefficient within the tip leakage vortex ranging between 2% to 12%, depending on Reynolds number, while the passive squealer showed a change of approximately 40%. On the endwall within the clearance, the plasma actuator generated a 19% peak increase in wall static pressure while the passive squealer caused a maximum increase of 52%. These results show that the plasma actuator was able to favorably mitigate the adverse effects of the tip clearance flow in a similar manner as the squealer tip, withou t the drawbacks of the passive squealer method. Although less effective than the squealer tip, the positive results of the plasma actuator show that this type of flow control is suitable as a means of reducing the tip clearan ce flow loss.

Control of a Turbine Tip Leakage Vortex Using Casing Vortex Generators

An experiment was conducted in a linear cascade of Pak-B blades to simulate the ∞ow in the tip-gap region of a low pressure turbine blade row, and investigate the sensitivity of the ∞ow to casing-mounted passive and active vortex generators. The inlet Reynolds number was 5£10 5 corresponding to an inlet Mach number of 0.2 and an exit Mach number of 0.3. The gap-to-chord ratio was 5%. The ∞ow was documented using blade-tip static pressure measurements and downstream total pressure loss coe‐cients. Additionally, surface ∞ow visualization was performed on the cascade end-wall for a greater understanding of the gap∞ow behavior. These measurement techniques were used to investigate the response of the ∞ow to passive ∞ow control using vortex generators located on the cascade end-wall. The vortex generators were designed to produce vorticity of opposite sign of the tip-leakage vortex. Vortex generators that were of the height and roughly half the height of the clearance gap were investigated. Additionally, two placements of the vortex generator were investigated: on the wall directly across from the trailing edge of the blade and approximately 0.25cx upstream of the trailing edge. All investigated placements and sizes of the passive vortex generators reduced the total pressure loss associated with the tip leakage vortex. A 25% reduction was achieved with both heights of the vortex generators when placed at the upstream location. When placed at the trailing edge, the shorter vortex generator resulted in a 15% reduction of total pressure loss associated with the tip leakage vortex, and the taller vortex generator resulted in a 20% reduction. The sensitivity of the ∞ow to the passive devices was used to determine placement and design of a plasma actuator on the end-wall as an active ∞ow control mechanism. Three plasma actuators were investigated, one of which resulted in a 7% decrease in total pressure loss associated with the tip leakage vortex.

A Transonic Axial Compressor Facility for Fundamental Research and Flow Control Development

A single-stage transonic axial compressor facility has been constructed at the University of Notre Dame. The initial blading consists of inlet guide vanes followed by a rotor and stator row. The stage has a design pressure ratio of 1.55 at a corrected mass ∞ow rate of 9.97 kg/s. The design blade tip speed is 352 m/s and the rotor tip relative Mach number at design is 1.27. The casing outer diameter is 0.457 m. Efiorts were made to ensure that the blade design is comparable to that found in the the current generation of aero-gas turbine engines. The compressor stage and facility were designed to withstand operation of the compressor in surge and stall. In addition, the facility is equipped with custom designed active magnetic bearings for active whirl actuation. These features, along with the substantial optical access provided by the casing design, make the facility ideal for detailed studies of the blade passage ∞ow during and after stall inception. Baseline performance data in the form of pressure ratio characteristics are presented along with a brief description of the stall behavior of the stage. The operation of the magnetic bearing system and its expected utility as both a research tool and an actuator for stability control are also discussed.

A Primitive Variable Central Flux Scheme for All Mach Number Flows

A new finite volume solver for all Mach number flows suitable for the solution on unstructured meshes with the collocated arrangement of variables is presented. The solver is based on the primitive variable equations. The newly proposed method utilizes the fractional step method that resembles the projection methods used in incompressible flow simulations. The proposed solver is suitable for the computations ranging from incompressible to supersonic flows. A notable characteristic of the newly proposed method is that the new formulation of the compressible pressure equation contains the pressure Laplacian term pre-multiplied by the time step size. Unlike the incompressible projection methods, the new pressure equation is derived from the discrete form of the continuity equation, resulting in the equation that has the potential (elliptic) and the hyperbolic (convective) parts of the pressure field separated. Central flux formulation is used for the approximation of numeric fluxes, resulting in small numerical dissipation thus making it suitable for the high fidelity computations including direct and large eddy simulations. A set of validation problems is presented in this work demonstrating the main characteristics of the newly proposed solver.

Numerical Investigations of Slot-type Film Cooling of HPT Rotor for Small Engines

Two dimensional (2D) and three dimension (3D) numerical simulations and 3D structural analysis were used to understand aero-dynamical, thermal mechanical and structural behaviors of the slot-type film cooling of a relatively small turbine rotor blade. Different blowing ratios were examined. The mechanisms of the performance of slot-type film cooling in both 2D and 3D conditions were analyzed in detail, based on the numerical results. Overall, the slot-type film cooling had much better cooling effects over the discrete-hole film cooling by avoiding the 3D flow near the coolant jet due to the non-uniformity of the coolant jet in the third dimension and the discontinuity of the jetting area, which brought the hot gas of the mainstream flow to the blade surface and worsened the cooling effect. The rotating effects were considered. Regions on the rotor blade where the Coriolis effect should be taken account were provided according to the operating conditions and the real flow field. The structural reliability based on the 3D finite element modeling (FEM) was examined, and was proved to be feasible. Take the advantage of the model design and manufacturing technique, reconsidering the slot-type film cooling could help further improve the turbine cooling effectiveness.

Analysis of Stable Rotating Stall in a High Speed Axial Compressor

Experimental studies of transonic rotating stall phenomena have used measurements of unsteady casing pressure to infer spatial and temporal scales associated with part-annulus stall cells. These scales are often described with characteristics such as cell counts and rotating speeds derived from either visual inspection of pressure signals or a discrete Fourier series of the circumferential distribution of pressure. The present work utilized experimental measurements from a new, modern, transonic compressor experiment in order to investigate the physics of post-stall transients in a compressor with a low hub-tip ratio. The results of this paper will show that the stall cells were highly random in nature, with spatial correlations that were less than a full revolution and temporal convergence times of greater than ten revolutions. This limited the utility of the Fourier analysis in understanding the spatial structure. The data reduction methods provide a framework for characterizing the flow physics and compression system dynamics of post-stall compressor performance.