Mitch Wolff

Reynolds Number Effects on the Secondary Flow of Profile Contoured Low Pressure Turbines

Low pressure turbine profiles with high aerodynamic loading can suffer from poor midspan performance at lower Reynolds number. Studies have shown that forward loading can mitigate the low Reynolds number lapse in performance at midspan, but concerns remain about increased secondary loss from front loading profiles. The effect of Reynolds number on low pressure turbine secondary flow losses are considered here. The front loaded L2F profile is studied experimentally in a low speed linear cascade. Several different geometric contours are used to modify the shape of the blade near the endwall, which decreases passage total pressure loss. Profile contouring is accomplished by placing an endwall glove over the blade at the blade to endwall junction. Performance of each profile is compared versus Reynolds number and boundary layer parameters. Translation of dominate secondary loss flow features were tracked using surface oil flow visualization, and their movement in the passage is related to changes in passage total pressure loss. Measurements showed that the lift off-line of the passage vortex moved downstream as Reynolds number decreased, and passage total pressure loss increased.

PIV Investigation of a Highly-Loaded LPT Blade Using a Curved Laser-Sheet

Low Reynolds number flow around a highly loaded LPT blade (the L1A) was investigated by Particle Image Velocimetry (PIV) at the blade midspan. PIV data was acquired in both the blade normal and spanwise direction planes. Measurements in the spanwise direction planes were taken near the airfoil suction surface using a curved lasersheet that closely matched the curvature of the aft portion of the blade profile. Images in the blade normal plane over a range of Reynolds numbers clearly show the shear layer and transition length, while images in the spanwise direction plane show the three-dimensional nature of the transition region. The experiment was performed on a linear cascade of seven blades mounted in the Air Force Research Lab Low Speed Wind Tunnel facility. Images were taken at two different freestream turbulence levels across a Reynolds number range between 25,000 and 125,000.

The Effect of Profile Contouring on Secondary Flow Structures in Low Pressure Turbines

Research aimed at reducing weight and improving efficiency of low pressure turbines has led to the design of highly loaded blades. These blades typically have higher endwall losses due to increased blade spacing and higher pressure gradients that strengthen secondary vortex structures. The present study focuses on the effects of profile contouring spanwise blade shape variations used to manipulate dominant vortex structures such as the passage vortex and suction side horseshoe vortex. With the goal of improved efficiency from reducing total pressure loss across the passage, the study aims to determine the effect of various profile contours on the secondary flow field and underlying physics. Experiments were conducted in the Air Force Research Laboratory Low Speed Wind Tunnel Facility at Wright-Patterson Air Force Base. Data includes surveys of total pressure, velocity, and surface flow visualizations through the passage. Measurements are focused on the aft portion of the passage where the passage vortex interacts with the blade suction surface. These measurements confirmed correlations between losses and Reynolds shear stress components. Deformation work is also seen to correlate well with total pressure loss.

Experimental Comparison of DBD Plasma Actuators for Low Reynolds Number Separation Control

This paper presents experimental work comparing several Dielectric Barrier Discharge (DBD) plasma actuator configurations for low Reynolds number separation control. Actuators studied here are being investigated for use in a closed loop separation control system. The plasma actuators were fabricated in the U.S. Air Force Research Laboratory Propulsion Directorate’s thin film laboratory and applied to a low Reynolds number airfoil that exhibits similar suction surface behavior to those observed on Low Pressure (LP) Turbine blades. In addition to typical asymmetric arrangements producing downstream jets, one electrode configurations was designed to produce an array of off axis jets, and one produced a spanwise array of linear vertical jets in order to generate vorticity and improved boundary layer to freestream mixing. The actuators were installed on an airfoil and their performance compared by flow visualization, surface stress sensitive film (S3F), and drag measurements. The experimental data provides a clear picture of the potential utility of each design. Experiments were carried out at four Reynolds numbers, 1.4 × 105, 1.0 × 105, 6.0 × 104, and 5.0 × 104 at a-1.5 deg angle of attack. Data was taken at the AFRL Propulsion Directorate’s Low Speed Wind Tunnel (LSWT) facility.

Investigation of Losses on a Highly Loaded Low Pressure Turbine Blade with Unsteady Wakes

Unsteady flow and its effects on the boundary layers of a low pressure turbine blade is complex in nature. The flow encountered in a low pressure turbine contains unstructured freestream turbulence as well as structured periodic perturbations caused by upstream vane row wake shedding. Researchers have shown that these conditions strongly influence turbine blade performance and boundary layer separation, especially at low Reynolds numbers. In order to simulate these realistic engine conditions and to study the effects of periodic unsteadiness, a moving bar wake generator has been designed and characterized for use in the Air Force Research Labs (AFRL) low speed wind tunnel. The layout is similar to other traditional squirrel cage designs, however, the entire wake generator, with the exception of the motor, is enclosed inside the wind tunnel up-stream of a linear cascade. A unique aspect of the wake generator design is the inclusion of a delay loop in the wake generator track. The delay loop avoids backwards running secondary wakes that are caused by the wake generating mechanism making a reverse traverse across the wind tunnel during each cycle. The effect of the periodic disturbances on turbine blade performance has been investigated at low Reynolds numbers (Re < 5 x 10) using the highly loaded, AFRL designed L1A low pressure turbine profile. The momentum deficit along with wake width and peak velocity deficit were used to quantify the wakes shed from the wake generating device. These properties along with the integrated wake total pressure loss were then used to quantify the profile loss of the L1A in the presence of unsteady wakes.

Experimental Investigation of a High-Lift Low-Pressure Turbine Suction Surface

This work employs a shear and stress sensitive film (S3F) to investigate the suction surface flow features associated with a higher-lift low-pressure turbine airfoil (L2F). Well-behaved higher-lift low-pressure turbine designs suffer from an inability to accurately predict the transition location above the suction surface, and the separation onset locations obtained with the S3F sensor herein allow the validation of the separated-flow transition model used in the L2F design cycle. Improvements to the S3F measurement technique are explained in this work, and results are compared over a range of Reynolds numbers at 3.3 % freestream turbulence including skin friction measurements at the trailing edge of the airfoil. Results demonstrate an improvement to the S3F data reduction process by accounting for the tunnel and model vibration, which will allow a greater range of sensor application.

Liquefied Natural Gas as the Next Aviation Fuel

Military aircraft will have advanced capability in maneuverability, weaponry and surveillance while maintaining a low detectability. These advanced capabilities introduce more challenges to the energy management system by increasing both electrical and thermal loads and reducing the options for heat rejection. The aerospace industry has been investigating alternative fuels for aviation fuel supply for environmental, economic and security benefits. Most of the fuels investigated and tested were derived from biomass or other forms of organic supply and waste. Biomass fuels can be interchangeable with jet fuels, but typically do not offer any technical advantages unless modified to withstand higher temperatures. Another fuel option is liquefied natural gas (LNG). At first glance, LNG has environmental, economic and security benefits, but with technical challenges. Technical challenges such as low energy density (MJ/m 3 ) and thermal stability at standard atmospheric conditions have limited the research and development efforts due to hurdles in implementation, such as complexity in fuel tanks, storage, transportation and energy storage density. When considering the aircraft as an entire system and not just focusing on the fuel management system, LNG can provide some benefits. LNG has at least ten times more thermal heat sink potential when compared to jet fuels. A quarter of the heat sink potential is at a sink temperature of 112K. Utilizing thermal cooling capabilities of LNG has the potential to reform the way aircraft are designed. LNG offers a compact and efficient thermal management solution which can reduce subsystem size and weight while reducing energy consumption. The increase in thermal management capabilities will open the door to integrating new technologies on aircraft such as high energy pulsed systems. The work presented in this paper evaluates some of the major challenges and benefits of using LNG as an aviation fuel. Nomenclature

Various Integrated Aerospace Systems utilizing a Cryogenic Fluid

As aircraft systems become more advanced, their power demands and heat generation will increase, while at the same time the limit of how much heat an aircraft can dissipate using current technology is being reached. Compounding this problem is the addition of High Energy Pulsed Systems (HEPS) to an aircraft’s already considerable thermal and power load. This device has the potential to more than double the thermal load on the aircraft. Using an innovative solution of cryogenically cooling the HEPS, this proposed system has the potential to both manage the thermal loads and power demands of a HEPS using Liquefied Natural Gas (LNG) as a coolant. The LNG will be used to thermally manage the HEPS by using it as an evaporator, then, since the LNG is combustible, it will be burned in a power plant to generate power for the system. In order to analyze this system conceptually and determine its feasibility, a MatLab/Simulink model has been constructed and used to compare several different configurations to find the advantages and disadvantages of each system. These cases include: (1) using a micro gas turbine, which can be sized using various criteria, (2) using a solid oxide fuel cell gas turbine hybrid (SOFC/GT) and (3) integrating the HEPS system into the aircrafts own cooling and power generation systems. The mass and volume of the comparative systems are analyzed. Using estimates for each of the major components of the system, the mass and volume were found as a function of operating time. For the same operating time, the integrated system had the smallest mass and volume, since it had no power plant, followed by the SOFC/GT. The systems using a more traditional gas turbine were scaled to either generate enough power for the HEPS or to generate maximum power from the LNG. The minimalized gas turbine had a mass and volume on par with the SOFC/GT and the maximized gas turbine being far heavier and larger than any of the other systems.

Surface Stress Sensitive Film as a Separation Control Sensor

Low Reynolds number boundary layer separation causes reduced aerodynamic performance in a variety of applications such as MAVs, UAVs, and turbomachinery. The inclusion of a boundary layer separation control system offers a way to improve efficiency in conditions that would otherwise result in poor performance. Many effective passive and active boundary layer control methods exist. Active methods offer the ability to turn on, off, or adjust parameters of the flow control system with either an open loop or closed loop control strategy using sensors. This research investigates the use of a unique sensor called Surface Stress Sensitive Film (S3F) in a closed loop, low Reynolds number separation control system. S3F is an elastic film that responds to flow pressure gradients and shear stress along its wetted surface, allowing optical measurement of wall pressure and skin friction. The S3F sensor was integrated into the curved airfoil surface so as not to perturb the boundary layer. In this proof of concept investigation the S3F image signal was acquired via high speed interface and analyzed using an off board control system. The S3F displacement signal was used directly in a closed loop separation control system to drive a Dielectric Barrier Discharge (DBD) plasma actuator used to control laminar boundary layer separation on an Eppler 387 airfoil over a range of low Reynolds numbers. Operation of the plasma actuator resulted in a 33% reduction in section drag coefficient and reattachment of an otherwise separated boundary layer. A simple On/Off controller and Proportional Integral (PI) controller were used to close the control loop.

An adaptive neuromorphic chip for augmentative control of air breathing jet turbine engines

Continuous Time Recurrent Neural Network Evolvable Hardware (CTRNN-EH) has been proposed as an enabling control technology for electromechanical devices. In addition to being able to learn control laws tabula rasa, CTRNNs can learn how to augment existing, trusted, controllers to add new capabilities without breaking existing operation. The ability to augment would be most useful in situations in which significant patching of existing controllers is needed to address contingencies not seen at design time and in which traditional design processes might be too slow to deliver quickly. In this paper, we will discuss the use of CTRNN-EH to augment a standard FADEC controller for an air-breathing jet turbine engine. We will show how we were able to extend the FADEC to properly control thrust under unusual loading conditions that were not considered at design time. Following, we will discuss future applications.