Gas Turbine Lab: Research
Research
The GTL has a wide array of unique experimental facilities complemented by extensive computational tools.
- Aerodynamic and heat transfer experimental facilities range from the large Turbine Test Facility, capable of matching design-corrected turbine operating conditions, down to the Small Calibration Facility.
- Aeromechanics and structural dynamics experimental facilities can hold a variety of size test articles from up to 30 inches in diameter in the Compressor Spin Pit, to 60 inches in diameter in the Large Spin Pit to 168 inches in diameter in the Large Fan Spin Tank.
In addition, the lab makes use of computational fluid dynamics codes such as FINE/Turbo, FINE/OPEN, STAR-CD, TURBO, the finite element tools ANSYS, LS-DYNA, and many other proprietary LabView and Matlab tools.
Aerodynamics and Heat Transfer Research
The Gas Turbine Laboratory has active research programs investigating the aerodynamics and heat transfer of a variety of different situations:
- Full-stage Turbine. These experiments utilize actual engine hardware to investigate the flow physics for a turbine operating at conditions that match key non-dimensional parameters. Improvements in film-cooling coverage and disk cavity cooling can enable higher turbine inlet temperatures and correspondingly higher efficiencies.
- Internal Cooling Passages. A significant fraction of the heat removed from a high-pressure turbine blade is extracted through the coolant channels inside the blade. These programs work to better understand the flow inside these channels for a rotating blade.
- Flat Plate. Transient flat plate experiments provide a simple case for computational comparison and for developing new experimental techniques that can be used in the other facilities.
Every research program performed at the Gas Turbine Laboratory also has a computational component to guide experimental design, aid in data interpretation, and examine new computational techniques.
Accordions
The hot section of an operating turbine is one of the more complicated flow environments associated with any practical machine; the flow is always unsteady, it may be transonic, it is three dimensional, and it is subject to strong body forces. Further, the interaction of these factors means that to obtain realistic information about the flow behavior inside an engine, all of these factors must be replicated. The Turbine Test Facility (TTF) is a short-duration facility designed to do just that. It is capable of matching stage pressure ratio, flow function, corrected speed, Mach number, Reynolds number, and other important non-dimensional parameters--both with and without cooling flows.
The Turbine Test Facility was designed and constructed at the Calspan Corporation in 1983 to produce conditions representative of the inlet to a turbine stage. In 1995, the researchers who designed, constructed, and operated this facility moved to The Ohio State University to form the Gas Turbine Laboratory (GTL), and they purchased the facility from Calspan. It now resides in the 12,000 square foot GTL at Don Scott Airport in Columbus, Ohio.
The TTF consists of a 100 foot long shock tunnel attached to a 32 foot long by 9 foot diameter dump tank in which the turbine rig is mounted. It can operate in either shock or blowdown mode depending on the test conditions needed. Shock tunnel mode creates the appropriate temperature and velocity conditions, while blowdown mode only generates the desired velocity.
A full-scale turbine rig is mounted inside the dump tank. These rigs are built using real engine hardware and are heavily instrumented. High-speed data is collected on the fixed shrouds and stators as well as the rotating blades to provide a good characterization of the unsteady effects that are a critical part of any engine's operation. Past experimental programs have included pressure, temperature (static and total), heat flux, and strain measurements. In addition, rotational encoders are used to precisely track the speed of the rotor. The rotor's acceleration can then be combined with its rotational inertia to provide aeroperformance measurements.
Data produced using the TTF at Calspan and at OSU has been instrumental in developing industry understanding of the flow physics of an uncooled turbine and the computational fluid dynamics codes used to model them. In 2006, Tallman et. al, came to the conclusion that computational codes had reached the point they were able to accurately predict uncooled flows and the data available was sufficient for further refinement and ready to tackle the added complexity of film-cooled turbines(GT2006-90927). Significant changes were required to prepare the TTF for film-cooled experiments including the integration of a coolant supply, the construction of a large combustor emulator to heat the gas even in blowdown mode, the development of advanced instrumentation, and the expansion of the Main Data Acquisition System.
The GTL ran the first experiments using a fully film-cooled stage operating at design corrected conditions in 2005. The results of these experiments as well as further discussion of the facility improvements are published in GT2006-90966 and GT2006-90968. Much was learned about how to run film-cooled design-corrected experiments, and these lessons have been applied to subsequent research programs. Since those initial experiments, measurement programs have been performed for three additional cooled turbines, and a fourth turbine is under development.
Comparisons with CFD have shown that the introduction of cooling makes the prediction of fluid temperature and heat-flux much more difficult. This is due in part to the multi-scale nature of cooling simulations, since the CFD must capture the large scale unsteady flow features as well as their interactions with hundreds of small cooling jets. In addition, cooled experiments introduce new challenges in determining the appropriate boundary conditions and require much more detailed measurements.
The goal of this research is to better understand heat transfer to the coolant flowing through the internal serpentine passages of a high-pressure turbine blade. Heat transfer within these passages provides a significant fraction of the overall blade cooling, and a variety of techniques are used to enhance it such as the use of turbulators, pin-fin arrays, and unique turn geometries. However, accurate (and timely) prediction of the heat transfer induced by these features remains a difficult challenge.
This experimental program provides data for a scaled-up blade geometry for appropriate ranges of Reynolds Number and Rotation Number. Matching these parameters requires that the entire test section be able to rotate at speeds up to 3,000 RPM. Comparisons for rotating and stationary situations, a variety of Reynolds Numbers, and different passage aspect ratios will help provide a more complete understanding of the governing flow physics.
In addition to providing valuable measurements, this experiment has also been a technology driver. Preparing the test section to rotate at high-speed has required the development of new micro-electronics, structurally integrated circuitry, and mechanical assembly techniques. Research in this area will continue to require the advancement of new measurement techniques to provide higher resolution data in a challenging environment.
Transient flat plate experiments are conducted in the Small Calibration Facility to explore new experimental techniques in a simpler environment before testing them in the Turbine Test Facility.
Recent experimental and computational programs have examined the influence of film cooling for a hole pattern representative of the pressure surface of a high-pressure turbine blade. In addition for providing new insight into the interactions among multiple rows of cooling holes, this experiment also helped validate a new method of injecting cooling streams into a transient facility that validate a new method of injecting cooling streams into a transient facility that has since been applied in the TTF. | |
Experimental method development has recently included film cooling effectiveness measurements using a time accurate, temperature compensated pressure sensitive paint (PSP) technique. PSP provides a full coverage method and can be used to suppliment heat flux guage measurements with both time accurate jet location and jet behavior characteristics. |
While the GTL is primarily focused on experimental work, it is important to pair this with computations to extend the interpretation of the data and develop more accurate predictive tools. Ultimately, the way that lessons from the experimental data will be incorporated into new engine designs is through the improvement of the computational design tools. This means that work comparing predictions to data and identifying where the predictions succeed and, more importantly, where they fail, is a critical component of the goal to improve operational engine performance. Because very few other experimental facilities are able to capture many of the unsteady interactions of a rotating turbine, few other predictions of these interactions can be validated against experimental data.
Computational fluid dynamics (CFD) models are generated for the full-stage turbines, internal flow passages, and flat plate experiments using Numeca's FINE/Turbo or FINE/Open as well as Fluent and CFX.
Aeromechanics and Structural Dynamics Research
The Gas Turbine Laboratory has active experimental and computational research programs investigating aeromechanics and structural dynamics focused on turbomachinery applications:
- Tip Rubs
- Blade Damping, Mistuning, and Excitation Studies
- Foreign Object Ingestion
- Mistuning and Multi-Stage Turbomachinery Modeling
- Nonlinear Modeling in Turbomachinery
- Miscellaneous Nonlinear Dynamics
The GTL has three spin facilities (CSPF, LSPF, LFST) used to experimentally investigate the effect of blade tip-rubs, blade damping, and a variety of other questions. Each facility is highly configurable to accomodate a range of hardware (bladed disks up to 14 ft in diameter) and research interests. Additionally, new modeling techniques are being developed continuously and these techniques are being validated with experiments when possible.
Accordions
Blade tip rubs have been explored at the GTL in a vacuum at room temperature in both the Compressor Spin Pit Facility (CSPF) and the Large Spin Pit Facility (LSPF). The previous work in this area has been published in a variety of journals and conferences.
Among the many phenomena that can be investigated in the CSPF and LSPF are asymmetric and full rotor clearance closure for actual engine stages. In single blade asymmetric clearance closure investigations, a rapid contact of the bladed rotor with its housing is simulated as would occur in an abrupt flight maneuver. In full rotor clearance closure, an extended contact of the bladed rotor with its housing is simulated as could be experienced in some flight phases due to temporary thermal imbalances in different parts of the engine.
Blade-to-case rub can degrade the performance of jet engines through the introduction of high amplitude shaft vibration and severe blade/seal wear. It can even lead to catastrophic failure of the whole engine in the worst occurrence. Aerodynamic requirements dictate that engines operate with the minimum blade tip clearances that are mechanically practical. However, the smaller the blade tip clearances, the higher the possibility for blade-to-case rubbing during operation. Complex blade-casing rub-in-systems are used to improve the tip clearance and maintain greater gap uniformity over the life of the engine. Typically a rub-in-system in the compressor section of the engine may consist of a specific circumferential area of the metal alloy case shaped to accept coatings of materials selected for in-service wear and, when required, fire shielding interactions. In the experiments at the GTL, single blade rubs have obtained the same wear pattern in the facility as observed in field occurrences. Detailed measurements of blade stresses and casing forces have been obtained and modeling is in progress to improve predictions of the structural modal response.
Blade damping, mistuning and excitation studies have been performed at the GTL in the past and are currently underway. Some details of past and current studies have been published. These studies can take place in any of the three spin facilities:
The facilities are modified as necessary to accommodate the specific test article (i.e., size of system, mounting attachments) and program requirements (i.e., modes of interest, operating speed). Air-jet excitation systems have successfully been used to force the modes of interest. While light probe systems and strain gauges have been used to obtain the primary data for these studies. Work to integrate the experimental and computational components of this work is now underway.
Foreign object debris (FOD) ingestion into aircraft engines is a serious concern. The most studied and researched FOD are birds and there are current regulations that engine manufacturers must follow and their engines must be certified to withstand. The GTL has studied other FOD including volcanic ash and unmanned aerial vehicles and these are active areas of research. The initial studies have been published.
The current focus of our work is the development of the appropriate component models and computational modelling. Future experimental work in this area could include full scale ingestions tests in the Large Fan Spin Tank (LFST), if appropriate modifications to the current facility are completed.
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A variety of new mistuning and multi-stage modeling tools are being developed at the GTL and can be found in the open literature. These modeling tools are being incorporated with the mistuning and damping experiments for validation at actual operating speeds.
The multi-stage modeling approach is based on using single sector models via cyclic analysis with a modified Craig-Bampton component mode synthesis to project the interface motion directly onto a set of Fourier constraint modes (FCMs) and enforce compatibility.
The multi-stage modeling has been efficiently combined with:
- component mode mistuning (CMM) for small mistuning
- pristine rogue interface modal expansion (PRIME) for large mistuning
- X-Xr for cracked bladed disks
- aeroelastic matrices
As well as any combination of the above modeling methods. A key attribute of all these methods is that they require only sector-level models and calculations to construct the reduced order models, which greatly improves the computational efficiency.
In addition to the multi-stage modeling, parametric reduced order models (PROMs) have also been developed to simultaneously allow for changes to mistuning and rotational speed effects in the reduced space.
Nonlinearity plays an important role in the dynamics of turbomachinery. Past published research at the GTL has focused on the nonlinearity caused by the intermitent contact along crack surfaces. The work has focused on:
- Efficiently creating reduced order models (ROMs) of bladed disks with cracks using sector level models even when the cyclic symmetry is lost
- Efficiently modeling the nonlinear dynamics of the reduced system
A current area of active research both computationally and experimentally is the nonlinear dynamics caused by friction dampers (e.g., underplatform dampers, shrouds, snubbers, ring dampers).
A variety of nonlinear dynamics research has been published for various applications including:
- Forecasting bifurcations
- Nonlinear parametric reduced order models
- Damage detection
- System identification
- Sensing
Current modeling work is focused on efficiently predicting the dynamics in nonlinear systems with contact, gaps, prestress and friction nonlinearities. These new techniques will have applications in a variety of fields. In the turbomachinery field they will help in friction damper design, understanding rub phenomena, and the dynamics of bladed disks with cracks.
More details on this research can be found at the Nonlinear Dynamics and Vibration Lab website.
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