Turbine Aerothermodynamics Laboratory - Research
Particulate Deposition
One of the primary goals of current particle deposition research is to develop a physics-based model that, in conjunction with a fluid flow solver, can be used to accurately predict the deposition and rebound behavior of particles in gas turbine environments. Unfortunately, existing physical models for the impact mechanics of particles require mechanical property information about the particulate that is unavailable. The Coefficient of Restitution Test Facility at the Turbine Aerothermodynamics Laboratory, shown below, is designed to provide this information by examining the rebound characteristics of particulate over a wide range of parameters, including velocity, impact angle, flow temperature, and surface temperature.
Particle shadows are recorded with a high speed camera as the particles impact the target surface, as shown in the video below. The images are subsequently processed using an in-house code, yielding trajectory information both pre- and post-impact, as well as size and shape information for each identified particle. The quantity of data generated (around 500-600 thousand rebounds per test series) allows for in-depth statistical analyses of the relative effects of different parameters. This information can then be used as an input to a deposition model by means of generating mechanical properties as a function of temperature, for example.
Funding for coefficient of restitution testing at OSU has been received from several leading corporations in the aircraft engine industry.
The following students work(ed) on this project:
- Steven Whitaker
- Michael Lawrence
In modern gas turbine engines, one of the most common methods used to cool components is to employ a double-wall cooling configuration. This method, typically used for turbine vane and blade cooling, utilizes a combination of impinging jets, through-wall flow, and film cooling to reduce metal temperatures to acceptable levels. Because of the complex nature of these flows, they are susceptible to particulate deposition that reduces the cooling effectiveness of the coolant flow, potentially leading to part failure. At the Turbine Aerothermodynamics Laboratory, a simplified double-wall test article (shown below) is used to evaluate the effects of numerous parameters on the buildup of particulate and subsequent flow blockage development.
Heated flow can be supplied to the test article, which can be placed inside an electric kiln to generate representative metal temperatures. Particulate is delivered to the article via a conveyor system, allowing for precise control over the rate of particle injection. This facility has been instrumental in making several unique discoveries. The most important of these is that the mechanisms responsible for deposition are independent of the rate of particle ingestion, which validates the use of accelerated testing in laboratory environments. Another key observation was the discrete effects small particles have versus larger particles. As shown in the plot below, it was demonstrated that small particles (less than 3 microns) were primarily responsible for deposit buildup, while larger particles (greater than 5 microns) were actually capable of removing existing deposit and limiting the rate of deposit growth. Such information is very beneficial for the purposes of modeling deposition behavior.
Funding for double-wall cooling deposition testing at OSU has been received from various leading corporations in the aircraft engine industry.
The following students work(ed) on this project:
- Steven Whitaker
Around the world gas turbines used for transportation and power generation are more frequently exposed to harsh environments. One concern is the effect of poor air quality on an engine and the subsequent ingestion of particulate such as pollution, sand, and ash. Modern gas turbine engines operate at a temperature above the melting point of the component materials and require advanced cooling techniques to function. Ingested particulate can deposit within the cooling geometry and lower the effective cooling. This in turn can lead to part failure. Combustor liners are susceptible to deposition related part failure and are a component of interest studied in the internal deposition facilities. Test plates, configured in such a way to simulate a combustor liner, are heated in an electric kiln to specific surface temperatures and provided with heated air which acts as a representative coolant. This coolant is seeded with Arizona Road Dust of various sizes via a conveyor belt system which allows for a precise control of mass injection.
Interesting discoveries from these facilities include the influence of plate surface temperature vs coolant temperature, hole size and shape, and flow turning anlge due to hole orientation, on deposition. It was shown in a study that a turning angle of 130 degrees experienced over twice the blockage as a plate with a turning angle of 10 degrees. Although blockage increased with turning angle, total capture efficiency remained independent of turning angle. A companion CFD study was done with particle tracking and an in house deposition model, which showed that the smallest particles deposited in the areas seen experimentally. Current work sponsored by industry leading corporations explores these and other interesting facets of deposition physics.
The photograph below shows what a typical target plate looks like after an impingement cooling deposition test is conducted. The shaded region is the area that is scanned with an optical profilometer for data acquisition. This region was selected due to its periodicity.
After scanning the plate, the data is post-processed with a series of unique Matlab scripts to represent various elements of the deposition results. The figure below demonstrates one of the outputs of this processing procedure. This scan area is divided into a 3 x 5 grid of rectangles based on the impingement site of each jet in the scan area. Inside of each rectangular cell, the pixel with the highest peak is chosen and a horizontal line is drawn through the pixels in the chosen row. The 2nd cone from the right edge in the middle row, when compared with Figure 1, gives the best indication of the accuracy of the scanning technique. The cone shape is preserved and two small, separate formations can be found just left of the main cone in the both the physical and contour images.
The total integrated volume collected on the plate within the scan area is plotted against 5 different test pressures at three temperatures. It is well known that increasing temperature accelerates the deposition process. The results indicate that as temperature is increased deposition decreases linearly, with the exception of low pressure cases as indicated by the spike from 2.35 atm to 0.99 atm at 728 K.
Our lab verifies all of our different experiments using commercial CFD packages. ANSYS Fluent was used to study how the wall shear stress near the impingement locations varies both spatially as well as with changing pressure. The top image corresponds to the CFD generated contours, while the bottom image is a picture of the target plate. In the test images, white regions have dust stuck to the plate, while darker blue regions are bare. The results confirmed our theory that the increase in the flow Reynolds number with increasing pressure is a major contributor to the decrease in deposition at elevated pressures. The thinning boundary layer and increased shear has the effect of carrying dust away from the surface. The CFD indicates a close overlap between the regions affected by shear and the bare regions.
This is only a glimpse into the first publication generated from these experiments. Follow on efforts are needed (and will be taken by our lab) to further explain the physics at work as well as to explore how different boundary conditions influence the process.
Gas turbine engines utilized in power generation as well as aircraft are operated at extremely high temperatures to maximize efficiency. These systems are intricately engineered to survive the harsh conditions they are exposed to in design conditions. The rapid growth of the airline industry in regions such as Africa, China, and the Middle East and the air quality in such regions is a topic of concern for engine manufacturers. Particulate ingestion (sand, dust, salt, or other pollutants) is prevalent in these areas of the world and can cause heavy damage to an engine. Particle impacts can erode the surfaces of the fan and compressor components leading to significant performance degradation and even compressor surge. In the turbine section, temperatures are well above the melting temperatures of the rotor and stator components. Surface particulate deposition affects performance, but it also negatively impacts the cooling effectiveness in this stage. If the cooling passages get restricted by deposits, the parts are at risk of burning through and failing.
The TuRFR (Turbine Reacting Flow Rig) is a state of the art research facility capable of investigating actual turbine nozzle guide vane (NGV) hardware under the harsh conditions of high temperature particle deposition. A description of the TuRFR and the facility’s operational capabilities can be found on the Facilities page.
Research in the TuRFR encompasses multiple disciplines including fluid dynamics, heat transfer, multiphase flows, material science, and impact mechanics. Experiments are performed such that the particulate ingestion exposure over the service life of a NGV is properly simulated in a 3-5 hour experiment. These experiments demonstrate how particulate collects on the surfaces and allow for the investigation of patterns in deposit accumulation. Deposition relations with flow temperature and surface temperature are studied, and infrared imaging reveals the changing cooling effectiveness as deposits form. Of major interest to industry sponsors is the facility’s capability to test various methods for mitigation of deposition such as novel cooling schemes, surface coatings, and purge techniques.
Flow Control
Pulsed Blowing
Flow control is implemented in the form of wall-normal, discrete pulsed blowing near the leading edge of a NACA 643-618 airfoil. At moderate angles of attack, pulsed blowing was found to be capable of introducing disturbances that could be amplified by the natural Kelvin-Helmholtz instability. Capitalizing on natural instabilities can greatly reduce the input energy required for flow control to be effective.
Pulsed-blowing can introduce a wide array of harmonics of the forcing frequency into the flow, some of which may be amplified by natural instabilities. In this case, the pulsed jet introduces frequency content an order of magnitude higher than the fundamental forcing frequency, which gets naturally amplified and then suppresses downstream separation.
At high angles of attack, where the flow is separated near the leading edge, short duration pulses have been found to increase the time-averaged lift coefficient substantially more than long duration pulses, for the same pulsing frequency.This is an attractive finding in the context of energy consumption, and is also somewhat counter-intuitive since large momentum additions are often recorded as most effective. In this case it has been determined that the flowfield response to the actuation event is the source of lift improvement rather than the injection of momentum itself. As such, short pulses create a disturbance and then leave an extended period of “relaxation” time during which the flowfield can be conducive to high lift. However, it has been shown that too much “off-time” between pulses can cause the flowfield to return to its natural, fully separated state.
Three frequencies are studied each with the same short duration jet at the beginning of each period. This allows the effect of extended jet off-times to be studied. For the 20Hz and 10Hz cases, high lift states can be obtained because the flowfield response are not as frequently interrupted by a succeeding jet pulse. In the case of 10Hz, there is an indication that the flowfield is returning to the uncontrolled, fully separated condition for t/T > 0.60.
After the actuation event, the redeveloping shear layer is severed, and a new separation bubble is enclosed. The severed shear layer propagates downstream at roughly the freestream velocity (red vertical line), while the separation bubble reattachment point does so at 1/3 of the freestream velocity (black vertical line).
Large Low-Frequency Oscillations
During some flow control experiments, large low frequency oscillations (LFO) have been observed in the lift force on a NACA 643-618 airfoil near stall. The oscillation frequency is roughly an order of magnitude less that the well-known bluff-body shedding frequency and the deviations from the mean lift force can be as high as 50%. This phenomenon is created most coherently when acoustic forcing is applied globally via a speaker mounted to the wind tunnel wall, and when the acoustic frequency coincides with a transverse test section resonance (this frequency is more than two orders of magnitude higher than the resultant LFO frequency). This resonance mode creates spanwise uniform, airfoil wall-normal velocity perturbations that are believed to be integral to the formation of this self-sustaining flowfield.
The work described here was funded by the Air Force Office of Scientific Research (AFOSR).
The following students work(ed) on this project:
- Kyle Hipp
- Stuart Benton, Ph.D.
Dynamic stall is an airspeed and maneuver limiting event which occurs on helicopter retreating blades at high advance ratios and is associated with aerodynamic flutter and large negative pitch moments. Experimental investigations explore active flow control of an airfoil undergoing periodic pitching motion in a variety of flow conditions including, steady incompressible, steady compressible, and time-varying compressible freestream, representative of a helicopter rotor system in flight. Flow control was achieved through a spanwise row of jets located at 10% chord, oriented normal to the surface, with an effective activated control width of 75% airfoil span. Flow control enhancements evaluated include stall penetration, lift and moment improvements, reduction in negative damping, and flow reattachment angle. Quantitative measurements of lift and moment coefficients were calculated through the integration of airfoil surface pressure taps. Qualitative, time-resolved background oriented schlieren (BOS) supplemented surface pressure measurements to assess spanwise averaged dynamic stall vortex progression as well as shock interaction.
The experimental investigations are conducted in The Ohio State University 6"×22" unsteady transonic dynamic stall facility which is capable of examining pitching and static airfoils in both time-varying and steady freestream. The tunnel has electrically operated rotating mechanisms to oscillate the airfoil pitch and the freestream Mach number, either independently or synchronously, with an adjustable phase difference between the two. The tunnel is always operated in a choked flow condition, with the throat downstream of the test section and can be easily modified for both static and dynamic Mach number with variation of evenly spaced blockage bars in the throat area. Since the test section Mach number is uniquely established by the ratio of choke area and test section area, Reynolds number can be set freely of the Mach number by controlling stagnation pressure.
Vortex Generator Jets (VGJs) delay boundary layer separation, consistently improve cycle average moment, and increased cycle average lift. Additionally, VGJs trigger an earlier flow reattachment, which reduces hysteresis and circuits of clockwise rotation on the CM curve related to negative damping. BOS imagery confirmed the presence of leading edge shock formation and validated VGJ control authority in a highly compressible environment. A comparison of VGJ flow control evaluated on a pitching airfoil in a steady compressible freestream at M=0.4 versus a pitching airfoil in a time-varying compressible freestream at M=0.4+0.07cos(ωt) at matched mean reduced frequency and Reynolds number, experienced similar quantitative improvements. Comparison of BOS imagery reveal the same physical VGJ to shear layer interaction between the steady and time-varying freestream cases. Thus, performance measurements based on active VGJs in a steady compressible freestream provide a good prediction of the expected performance measurements when blowing is applied to an airfoil in a low amplitude, time-varying compressible freestream. At low freestream oscillations, airfoil pitching frequency is the dominant factor influencing VGJ effectiveness.
Funding for dynamic stall research at OSU over the past five years includes:
- Army Research Office (ARO)
- Air Force Research Laboratory (AFRL)
- Leading corporations in the rotorcraft industry
The following students work(ed) on this project:
- Shawn Naigle
- Matthew Frankhouser
Turbine Cooling
The development of additive manufacturing technology has enabled the implementation of innovative cooling architectures inside a turbine blade. Fluidic oscillators are promising devices which can be used for both unsteady impingement and sweeping film cooling applications. The general working principle of these devices is quite simple and involves no moving parts. When a jet enters into the cavity from a pressurized plenum via the inlet nozzle, two opposite vortices begin to form on both sides of the jet. As the intensity of the vortices increase, one vortex becomes dominant. This causes the stream to deflect against the opposite wall and attach to the side wall due to the Coanda effect. This allows a portion of the fluid to enter into the feedback loop which flows back to the control port and causes the stream to detach from the side wall. The stream then switches to the opposite wall and the same process repeats, resulting in an oscillatory fluid motion at the throat. A full scale 3D computational fluid dynamics (CFD) analysis is being performed in order to predict the internal flow and external flow field of the fluidic device. The commercial solver FLUENT is being used for the numerical calculation.
The sweeping jet motion produced by fluidic oscillators could be used as a potential film cooling device for large scale power turbine blades by increasing the spread of film cooling in regions that suffer from poor coverage. These types of holes could be implemented in regions where coverage and uniformity of cooling effectiveness is important. With the increased spread of coolant, fewer holes and less coolant will be required for the same area, reducing coolant consumption. CFD is being used along with experiments as a tool to understand the various physics associated with this technique, and to help rapidly explore the design specification with validated models. For this purpose a computational model of a sweeping jet film cooling configuration using fluidic oscillator has been developed.
Fluidic oscillators are also being used for impingement cooling. The sweeping action of the jet covers a larger area compared to the conventional steady round jet. However, the unsteadiness increases the local turbulence thus enhances the local mixing. Surface curvature has a profound effect as sweeping jet impingement does not show a monotonic behavior with surface curvature. In addition, the sweeping jet impingement exhibits a much more uniform cooling than steady round jets.
This project is funded by a grant from the Department of Energy National Energy Technology Laboratory (DOE-NETL), under the University Turbine Systems Research (UTSR) program.
The following students work(ed) on this project:
- Arif Hossain
- Lucas Agricola
Previous
- Robin Prenter
- Ryan Lundgreen
Film cooling is very commonly used as part of an overall component cooling scheme, and is present on the majority of high pressure turbine nozzle guide vanes. Conventional film cooling consists of coolant being ejected through discrete holes embedded in the part surface, producing a film over the surface with the objective of reducing heat transfer to and protecting the part from the hot gas in the main flow. Discrete film cooling holes provide cooling benefit in a localized region (especially in the lateral direction), and thus are organized into rows along the surface of turbine blades or vanes in an attempt to achieve good coverage and spatial uniformity.
One film cooling configuration that has not been extensively studied is reverse film cooling. Conventional film cooling holes are generally oriented in the direction of the main flow, at some angle α to the surface. With reverse film cooling, the holes are oriented against the main flow, as illustrated below.
Fundamental falt plate studies have shown that reverse film cooling can provide excellent coverage and lateral uniformity downstream of the film cooling holes, with comparable adiabatic effectiveness to the conventional configuration. The figure below compares effectiveness contours between cylindrical film cooling holes in the conventional and reverse orientations, and demonstrates this improved coverage. The reverse coolant jet penetrates into the freestream and is redirected by the main flow, which causes lateral spreading of the coolant. This is believed to be the mechanism for the total coverage observed downstream of the holes.
While the reverse configuration seems to provide a benefit in terms of lateral uniformity and coverage, the reverse jet-crossflow interaction is more vigorous than that of the conventional case. It is thus important to also study the flow field to determine the possible increases in pressure loss and heat transfer to the surface. Particle image velocimetry (PIV) is one of the measurement techniques commonly employed to obtain both flow visualizations and velocity field measurements of this complicated interaction. Below are examples of typical results acquired using this method.
This project is funded by a grant from the Department of Energy National Energy Technology Laboratory (DOE-NETL), under the University Turbine Systems Research (UTSR) program.
The following students work on this project:
- Arif Hossain
- Lucas Agricola
- Robin Prenter