1.1 The Research Project
This dissertation examines the aerodynamic effects of Low Pressure Turbine Vane (LPV) clocking in a stage and one half full-scale rotating turbine operating at design corrected conditions. This is the first time this type of work has been done where the effects measured locally on the airfoils have been compared to the overall machine performance. This also represent the first time that highly detailed efficiency changes have been measured using a short-duration facility. This dissertation is part of a large, multi-year and multi-faceted research effort conducted at The Ohio State University (OSU) Gas Turbine Laboratory (GTL) that was designed to provide a variety of experimental results used for the development of advanced research and development design systems. GE Aircraft Engines (GEAE) as part of the GEAE University Strategic Alliance (USA) Program supports this effort. Within GEAE, there is a wide variety of
“customers” for the detailed physical information generated from this experimental program. These range from design support groups requiring applicable data to validate computational models, to engineers trying to understand some of the basic physics
involved in advanced engine concepts. Sometimes the main use of the information is just to answer a question experimentally; “Does this design produce less heat-transfer at the tip?” or “Does this design result in destructive high-cycle fatigue?” Other times it is to verify that a specific manufacturing process could produce an observed behavior in an engine. Another very basic need is to generate a quality data set that matches the proper corrected conditions, with defined control variables becoming the basis for either better understanding of the physics, or just the beginning of an empirical design tool. In this section, the individual goals of the experimental program will be outlined and the
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components of the first phase of this research effort will be discussed which is the basis of this dissertation.
1.1.1 Goals of the experimental program
The research effort with which this dissertation is associated is a complex multi- year program utilizing a modern stage and 1/2 high-pressure turbine for a variety of research purposes. The turbine design is a highly loaded turbine with extremely strong 3D effects on each airfoil row, and an interconnecting “S” duct between the HPB and the LPV. The hardware is configured to make changing components “relatively1” easy in between builds so that different types of hardware configurations can be investigated.
The ultimate goal of this research activity is to obtain experimental results for complex engine geometries operating at design corrected conditions in a fully cooled mode (vane, blade, endwalls, etc.). The current work utilizes the uncooled version of the hardware.
The data from these experiments and the future cooled experiments will be used in one of three ways:
1) As general design code verification. In this case, data is taken in a known configuration and is used to validate design codes used in the design of turbine hardware.
2) As a verification of industrial research codes and techniques. It is not unusual for the industry to have several different types of codes that have similar goals, but perhaps do the task in different ways. There is a steady migration of the industrial research codes into the design code classification. Currently, the industry is capable of a great deal more in their research codes than they are in the design codes, due to the complex nature of these research codes and the amount of computer resources required to make them operate.
3) Finally, as a valid data set from which empirical theories, new models, or just basic knowledge is developed. This is not a small matter, particularly in the area of cooling flow where for many cases it is unclear how the various components of blowing
1 Relatively is a key word. For most facilities, the change of an airfoil or instrumentation on an airfoil requires a complete teardown of the rig. The same is true for this rig, but due to improvements in the overall instrumentation capabilities and rotor wiring, this can be done in a matter of weeks and not months
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ratios, temperature ratios, and measured heat flux relate to overall design properties such as cooling film effectiveness.
The research program was separated into two distinct phases at the beginning, although since then it has developed further. The first stage was the design and
construction of rig hardware and the initial measurement program that utilized uncooled engine hardware. The second stage is the adaptation of the hardware to a fully cooled system with a temperature profile generator, turbulence generator, and traversing rings.
The first phase was designed to obtain data (of which this dissertation is a part) to be used as verification of the design tools and industrial research codes, and as a reference data set for the cooling experiments. The future cooling experiments are designed to provide data sets to be used for the second and third modes (described above).
1.1.2 Components of the first phase of the project
This dissertation, as discussed previously, is concerned with the measurement program associated with the first phase of the research effort: the uncooled experiments.
As described in section 3.1 the OSU GTL Turbine Test Facility (TTF) uses either actual engine parts or “rig” parts, thus the hardware used is the same physical size as engine parts. “Rig” hardware is a term used in the industry to imply that the parts have been modified slightly for their experimental facilities (rigs), which generally operate at slightly different conditions than the real engines. One example is that an uncooled vane ring will often have the vanes rotated slightly so that the mass flow through the uncooled model will match the core mass flow through the cooled engine parts. Another example is that blades might be tipped a little differently so that they run at the same relative gap as an engine operating at high temperatures and speeds. Generally, the differences between “rig” hardware and engine hardware are quite small. Sometimes the TTF will operate a rig configuration that is not an operating engine, but rather a specific piece of hardware designed either to make computational work simpler or so that a large group of industrial government sponsors can share the coordinates and the data [1].
The actual engine or rig hardware is generally mounted in a device that holds the turbine stage. Sometimes the disk may be engine hardware, often it is not since making a heavier disk will reduce the acceleration rate and simplify the measurement program.
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The overall system would be referred to in the engine industry as a rig, but in the academic world, it is often called a model. However, there is nothing small about these models, as they are full sized, high-speed rotating rigs. In this case, the overall “model”
was thirteen ft. long and four ft. in diameter.
For the first phase of the experiments, the uncooled model was heavily
instrumented with both surface pressure and heat-flux sensors on all three airfoil rows (the high pressure vane and blade and the low pressure vane) over three different spans.
There were both types of sensors installed on the endwalls of both vane rings, on the rotor platform and on multiple and differing blade tip configurations. The main goals of the experiments were: (1) to obtain detailed surface pressure distributions as a function of low-vane clocking (more about that later) and as a function of corrected speed and
Reynolds number and (2) to obtain detailed heat-transfer distributions at design point for a range of Reynolds numbers. In addition, research was performed with different shroud configurations to examine their effect on blade unsteady pressure loadings.
Much of the data acquired from this wide variety of experiments will be analyzed and used as part of the work of other OSU GTL students. Some examples are blade tip behavior, heat-flux scaling, three-dimensional effects and performance calculations.
Because of limitations of the experimental apparatus, instrumentation, and data
acquisition channels, the first phase of the experiment (called Build 1) was split into two different experimental assemblies (called entries) that occurred about a year apart. Entry 1 was done primarily to obtain an aerodynamic database and using primarily pressure sensors. These were the experiments that dealt with the clocking data discussed in this dissertation. Entry 2 was performed to obtain a detailed heat transfer database as a function of Reynolds number. To create a consistent database, both the entry 1 and entry 2 data need to be examined together. However, in this dissertation, the entry 2 heat-flux data is presented only in the appendices since the actual data does not directly impact the main dissertation, which is aeroperformance measurement of Low Pressure Turbine Vane (LPV) clocking. Varying the design corrected conditions during this experimental matrix had the effect of making this rig operate like different engines in the field, which is also of great interest.
5 1.2 The Thesis Statement
This dissertation is an experimental examination of LPV clocking on
aeroperformance. These results are critical since it represents the first data published for a high-pressure ratio turbine where clocking effects can be observed both on the local airfoil surface pressures and over the entire machine in terms of measured efficiency differences. The discussion addresses several shortcomings in previous experimental work by more fully documenting the uncertainty analysis, which is so critical to the interpretation of the final results. In addition, it resolves several unresolved issues from past work done at The Ohio State University Gas Turbine Lab (OSU GTL) where clocking effects were observed using one method of investigation, but could not be confirmed using a separate measure from the same data set. The effects of LPV clocking for an uncooled turbine stage are substantial, yielding about a ±2% change based on the relative position of the High Pressure Turbine Vane (HPV) and the LPV. These results are consistent whether one looks at the time averaged data or the time resolved data, or if one looks at the local pressure on the LPV, or the overall machine performance. This work supports conclusions drawn by previous research that suggest that the two primary mechanisms by which LPV clocking operates are the wake propagation throughout the machine, and the decrease in unsteady effects (time-resolved pressure envelopes) which reduce the loses due to reducing the velocities at the LPV and the turbulence generated at the LPV. However, this data set provides an important piece of information which is not accounted for in these models which is the frequency transfer that occurs in the time- resolved data due to clocking which will impact blade designers not so much from an efficiency stand-point, but rather form a life-cycle perspective.
It is important to realize that resolving accurately a ±2% variation in a property requires much higher resolution of the underlying measurements. For a ±2% efficiency measurement to be repeatable and believable requires essentially an order of magnitude increase in the accuracy (0.2%) of the underlying measurements. This was essentially not possible without better understanding of the underlying facility operation,
improvements in the data reduction process, and improvements to the basic
instrumentation. The results of this work not only are significant for the underlying
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results surrounding the effectiveness of clocking in highly radial divergent turbine stages (“S” ducts), but also in the fact that to achieve these measurements required that the short-duration facilities be used in a mode where efficiencies can be measured to this level of precision. This has long been a goal for these types of facilities, but has proved to be an elusive one, and this work represents a milestone in that regard.
To support the LPV clocking results, this dissertation will also address several supporting issues.
1. First, it will be the platform for documenting the main characteristics of the data set for others to use. In this regard, some might be interested in time-average properties, others may be interested in high-cycle fatigue, and still others may be interested in the data for the cooling studies yet to come. The data presented, both aerodynamic and heat-flux that will generally be limited to 50% span, will be based on specific techniques outlined so that others may interpret the data
correctly. In addition, some work will be done to explain how multiple runs with differing instruments can be combined to create one data set. This is critical since this technique is one of the new innovations that allows improvements in the underlying measurements
2. Secondly, the techniques developed with respect to the uncertainty analysis, and the data reduction techniques will be discussed in detail. These are critical since one of the main failure of past published work related to clocking has been in the proper interpretation of the uncertainty analysis
This data set can be used to determine the effect of clocking on aeroperformance in multiple ways: from an overall stage efficiency perspective and based on individual airfoil pressure distributions in both a time resolved and time-averaged reference frame.
It will be demonstrated that these measurements yield similar results. Clearly there are many more topics associated with this measurement program that could be investigated in more detail than will be presented in this dissertation, but those will be left for others to explore. The experimental data acquired in the course of this work has shown that experimentalists using short-duration facilities have progressed to the point where the
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data acquisition/processing and the stability of the facilities allows them to be used as major sources of data for physics-based modeling and code verification. In fact, the major technical questions regarding turbine development (unsteady effects, cooling issues, etc) can only be done in environments where the full time-resolved nature of the flow can be investigated. During the course of this dissertation, the data set quality, particularly with respect to the uncertainty analysis, has quantified how well the design set point can be established in a short-duration facility. The investigations into the effect of LPV clocking have forced the data resolution to new levels that has brought to
conclusion a long-standing process where the short-duration facility has been transformed from a qualitative research tool into a quantitative one. In addition, the ability of the advanced 3D codes to predict the complicated pressure distribution in a 3D turbine are improving, but simplified flow models such as the UNSFLO-2D code (used at the OSU GTL by special arrangement with Rolls-Royce England) can be used to rapidly obtain time-averaged and unsteady aerodynamic and heat-transfer predictions. An example of which is shown in this work.
1.3 Organization of Dissertation
For a work of this size, complexity, and various audiences, one could envision a variety of ways to present the data and to organize the dissertation. A decision has been made to take this work and draw the main connections to the LPV clocking work
throughout the dissertation, and to subjugate some of the supporting information, such as the uncertainty analysis and the heat-flux data, to supporting appendices. This
dissertation is split into two different sections, each being composed of chapters and some supporting appendices. The first section provides the background for those readers who are not familiar with short-duration facilities or the development of this type of time resolved data set. Chapter 2 highlights the current research interests and reviews the current literature with respect to clocking studies. Chapter 3 deals specifically with the main facility (the TTF) and the experimental hardware used in this research. Chapter 4 will finish this section with a general discussion of the description of unsteady data and the main techniques that are used for analysis and data presentation throughout the
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dissertation. Two appendices support this section. Appendix A discusses the
development of the short-duration facility and may be useful to those interested in the connections between the different researchers in this field and the rise in importance of the short-duration facility for turbine research work. Appendix B discusses in more detail some of the topics associated with time-resolved measurements introduced in Chapter 4.
The second section of the dissertation is composed of Chapters 5 and 6, which present the results of this work. Chapter 5 discusses the development of the data sets and will show the time averaged and time resolved pressure distributions at one clocking position. The heat-flux data, which was also taken at this clocking position, is provided in Appendix C. The data presented in Chapter 5 supports the data shown in Chapter 6, which details the changes dues to clocking, and forms the core of the analysis. Chapter 6 also has a supporting appendix (D), which discusses the uncertainty analysis used for presentation of the data.
This system is not quite as linear as some might like. By putting the supporting work in the appendices, the main discussion is not as easily lost in a sea of details.
However, the reader is cautioned that some of the details in these appendices are not just housekeeping items, but are critical to the understanding of the data. An attempt has been made to make the appendices complete chapters so there are a few cases where figures are duplicated in both the chapters and the appendices, as this adds to the readability of the document.
9 CHAPTER 2