Turbine Aerothermodynamics Laboratory - Facilities

The low-speed windtunnel at the ARC (Fig. 1) is a blower type tunnel which features two test sections (each 0.38m x 0.38m). Included is the capability of increasing free-stream turbulence levels as high as 12% and a built in heater to be used for heat transfer applications. A variety of experimental studies have been performed on this facility covering the topics of boundary-layer interactions with free-stream turbulence and surface roughness, film-cooling effectiveness, and applications of active flow control to boundary layer separation and secondary flows. The tunnel is capable of velocities up to 30 m/s with free-stream turbulence levels less than 0.4% in a clean configuration.

Flow diagnostic capabilities include two- and three-component particle image velocimetry, infrared thermography, single- and double-element hot-film anemometry, and a variety of pressure transducers and probes including a bank of 32 pressure transducers that can be used to obtain rapid measurements of airfoil pressure distributions. Flow visualization using smoke wires, tufts, and oil have been used in the past. 

Internal Deposition Facilities

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.

A schematic of the more small-scale internal deposition testing facility is shown in the image below. The test section portion is modular, allowing for various other cooling geometries to be studied such as effusion cooling common in combustor liners. The flow can be heated either with the use of an in-line heater or by utilizing electric heating tape wraps depending upon the desired temperature range for a given experiment.

High Pressure Facility

Outside of full engine testing, very few facilities have the capability of running at elevated pressures, let alone pressures representative of the overall pressure ratio of an aero engine. The TuRFR and internal deposition facilities exhaust to ambient pressure. In these facilities, cooling geometry mechanisms are operated by matching the pressure ratio and fluid temperatures which therefore means Mach number is matched. However, since the absolute pressure is not matched, these test facilities operate with a substantially lower fluid density than in an actual engine. This has direct implications to particle deposition testing. Higher fluid density means higher particle Reynolds numbers which in turn yields a lower effective Stokes number. The Stokes number describes the ability of a particle to follow flow streamlines; a Stokes number St<<1 implies the particle follows the streamlines well, while a Stokes number St>>1 implies the particle follows a more ballistic trajectory. In addition, higher density flow yields greater wall shear stress and heat transfer which effect particle sticking mechanics.

In an effort to study the effects pressure has on particle deposition, a high-pressure test facility was constructed for the Turbine Aerothermodynamics Laboratory at the ARC. Shown in the schematic below, the facility is capable of operating at up to 18 atmospheres and flow temperature up to 1,200 °F and is used to study deposit formation on an impingement plate test coupon. Testing has found that deposition at higher pressure changes the deposit structure as well as reduces the volume of captured deposit.

Coefficient of Restitution Facility

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 image 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.

The demand for higher fuel efficiency and reduced noise continues to push the trend of increasingly larger bypass ratio turbofan engines with increasingly higher turbine inlet temperature. These trends in aircraft propulsion have levied increasing demands on the turbine (hot) section of modern turbofan engines and on the low-pressure turbine efficiency and work output. Additionally, growth markets for turbofan engines have moved from North America and Europe to the Middle and Far East. Air quality concerns in these new markets are presenting increasing hardship for turbines as airborne deposits collect in cooling passages in the gas turbine and reduce performance. A turbine accelerated deposition facility was commissioned to address the issue of harsh operating environments and high temperature.

A unique Turbine Reacting Flow Rig (TuRFR) was constructed for the Turbine Aerothermodynamics Laboratory at the ARC in 2008 that simulates the flow exiting a gas turbine combustor as it impacts the first stage turbine nozzle guide vanes (NGVs). This early facility is capable of providing main gas path flow temperatures up to 2,100° F with an inlet Mach number of 0.1 and coolant temperatures up to 1,000° F. Airborne particulate can be added to the main gas path and/or the coolant flow to simulate in an experimental environment the deposition build-up observed on NGVs in the field.  The spatial temperature distribution in the inlet plane can be modified to simulate various pattern factors typical of modern engines. Optical access allows for video footage of deposition growth as well as surface temperature measurement with infrared imaging. Temperature, velocity, and pressure probes can be traversed across both the inlet and exit planes.

Design and construction of an upgraded and world class “TuRFR II” facility was completed in the summer of 2015 representing a $2M infrastructure investment at the ARC. This impressive facility simulates a more modern engine environment, capable of operating at significantly higher temperatures and mass flow rates. An industrial burner, capable of temperatures up to 3,000° F, provides heated airflow to the NGVs downstream. A double walled construction with liner cooling permits the test section to operate in excess of 2,600° F while internal cooling airflow to the engine parts is provided at up to 1,200° F. TuRFR II shares the testing capabilities of its predecessor, including: particulate injection options, inlet flow pattern factor variations, optical access for infrared thermography, and inlet and exit plane instrumented traverse access. The modular nozzle box test section allows for experiments to be designed for a large array of current engine nozzle guide vane hardware. The TuRFR II facility is employed in exciting research sponsored by various industry leaders.