Introduction

As discussed in previous chapters, a fan or compressor is the first rotating component that the fluid encounters. A cross-sectional view of a compressor for a simple, single-shaft turbojet is shown in Figure 6.1. The basic function of a compressor is to impart kinetic energy to the working fluid (air) by means of some rotating blades and then to convert the increase in energy to an increase in total pressure, which is needed by the combustor. The limits of operation of an engine are often dictated by a compressor, as is discussed in this chapter. Furthermore, the design of an efficient axial flow fan or compressor remains such a complex process that the success or failure of an engine often revolves around the design of a compressor. Many fundamental and advanced design details are available in Cumpsty (1988), Hawthorne (1964), Horlock (1958), Howell (1945a, 1945b) and Johnsen and Bullock (1965). Rhie et al. (1998), LeJambre et al. (1998), Adamczyk (2000), and Elmendorf et al. (1998) demonstrate how modern computational fluid dynamic (CFD) tools can effectively be used for the complex three-dimensional analysis and design of compressors.

Compressors were the main stumbling block of the early engines and the primary cause of the delays in the development of jet engines for World War II. Dunham (2000) and Meher-Homji (1996, 1997a, 1999) present interesting historical perspectives and technical information on early compressor development.

Introduction

In the previous seven chapters, much emphasis has been placed on the analysis and design of individual components of gas turbines. On the other hand, in Chapters 2 and 3 cycle analyses are presented for ideal and nonideal engines as a whole in which the different components are integrated into a system. However, in Chapter 3, component efficiencies and some characteristics are assumed or assigned a priori for the overall cycle analyses. In general, however, the previous seven chapters demonstrate, through the use of either theoretical analyses or empirical characteristic curves (or “maps”), that component efficiencies and other operating characteristics change significantly at different conditions–for example, at different flow rates and rotational speeds.

To understand the overall effects of changing operating conditions, one can consider an engine initially at some steady-state operating condition. However, as the fuel injection rate in the burner is changed, the turbine inlet temperature and pressure are changed. Thus, the turbine will change rotational speeds. Because the turbine and compressor are on the same shaft, however, this change in rotational speed in turn changes the ingested mass flow rate and pressure ratio developed by the compressor, which influences the burner inlet pressure and the turbine inlet pressure, and so on. Eventually, the engine will again reach a different steady-state operating condition.

My goal with this project is to repay the gas turbine industry for the rewarding profession it has provided for me over the course of more than three decades. At this point in my career, student education is a real passion for me and this book is one way I can archive and share experiences with students. I have written this text thinking back to what I would have liked as an undergraduate student nearly 40 years ago. Thus, this work has been tailored to be a very student friendly text.

This book is intended to serve primarily as an introductory text in air-breathing jet propulsion. It is directed at upper-level undergraduate students in mechanical and aerospace engineering. A basic understanding of fluid mechanics, gas dynamics, and thermodynamics is presumed; however, thermodynamics is reviewed, and an appendix on gas dynamics is included for reference. Although the work is entitled Jet Propulsion, it can well be used to understand the fundamentals of “aeroderivative” ground- or marine-based gas turbines such as those used for marine propulsion, ground transportation, or power generation. Although turbomachinery is not the primary target of the text, it is the book's secondary focus, and thus the fundamentals of, and some advanced topics in, compressors and turbines are also covered.

This text covers the basic operating principles of jet engines and gas turbines. Both the fundamental mathematics and hardware are addressed.

Introduction

Chapter 2 has included a review of the ideal thermodynamic processes of different components, ideal cycle analyses for various types of engines, and a discussion of performance trends. All of the components were assumed to operate without any losses, and the gas was assumed to be perfect and to have constant specific heats throughout the entire engine. The objectives of this chapter are to relax these assumptions, to explain the physical conditions that lead to losses, and to review the nonideal thermodynamic processes from, for example, Keenan (1970) or Wark and Richards (1999) and the gas dynamic processes from, for example, Anderson (1982), Zucrow and Hoffman (1976), or Shapiro (1953). Efficiency levels and losses are included for the different components so that more realistic predictions can be made for overall engine performance. Even though most components operate with individually with relatively high efficiencies (upwards from 90%), when all the components are coupled the overall engine performance can be reduced drastically. However, in general, the performance trends do not change. Also, note that simple single- or two-term expressions are used to model the losses in each component in this chapter for simplicity. Each component is covered separately and in detail. At this stage these loss terms are specified a priori even though, in a real engine, the different component losses are dependent on the engine operating point and are thus dependent on each other.

Introduction

The burner and afterburner are the only components through which energy is added to the engine. That is, in these two components the total temperature of the gas increases. For the primary burner, a part of this energy is used by the turbine to drive the compressor, and the other part is left to generate a high-velocity gas from the nozzle, generating thrust. For the afterburner, all of this energy increase is used to generate an increase of the fluid enthalpy and consequently a higher velocity gas from the nozzle, generating more thrust. As a result of these direct impacts, efficient operation of these components is necessary for the overall efficiency of the engine. However, several complex considerations must be realized in the design of either of these components, and some of the more advanced topics are summarized by Lefebvre (1983) and Peters (1988). Furthermore, Malecki et al. (2001) demonstrate the application of CFD predictions to modern combustor design.

The following are essential considerations in the design of a burner:

1. A major objective is complete combustion or fuel will be wasted.

2. Minimal total pressure loss is another important design goal. As discussed in Section 9.2, these first two objectives are in direct conflict.

3. All of the combustion must take place in the combustor and not the turbine, or the turbine life will be reduced.

4. […]

Introduction

As discussed in Chapter 6, the basic operating principle of a compressor is to impart kinetic energy to the working fluid by the means of some rotating blades and then to convert the increase in energy to an increase in total pressure. Axial flow compressors are covered in Chapter 6. These compressors are used on large engines and gas turbines. However, for small engines – particularly turboshafts and turboprops – centrifugal (or radial) compressors are used.

These compressors have higher single-stage pressure ratios than axial compressors (typically 2 to 4 compared with ~1.25). As a result, centrifugal compressors have lower cross-sectional flow areas per mass flow rate than do axial compressors. They also have larger diameters but shorter lengths per unit mass flow rate than do axial compressors. However, the rotating element (impeller) of the compressor is an integral unit of blades and a disk, and thus if one blade is damaged the entire unit is replaced. The weights of centrifugal units are approximately the same as for axial compressors used for the same application. They, however, demonstrate the characteristic of lower flow rates. As a result, they are physically small units with low flow rates, which makes them ideal for helicopter and small aircraft applications.

Introduction

Two remaining components that affect the overall performance of a turbofan engine are the bypass duct and mixer, as shown in Figure 10.1. These are relatively simple compared with the other components but should be included because they both generate losses in total pressure. Because the length-to-flow-width ratio of the bypass duct is moderate, the duct can incur significant losses. Also, it is desirable to have a uniform temperature gas entering the afterburner or nozzle so that these components operate near peak efficiency. Mixing of two fluid streams at different temperatures is a highly irreversible process, and a mixer consists of three-dimensional vanes in both the radial and circumferential (annular) directions. Thus, with good mixing of the low-temperature bypassed air and high-temperature primary air, further significant losses can occur. Owing to the temperatures exiting from the turbine, mixers are generally fabricated from a nickel-based alloy. This chapter covers total pressure losses in these two components.

Total Pressure Losses

Three irreversible mechanisms exist for pressure drops and total pressure losses to occur in ducts and mixers. The first is frictional flow in the duct primarily due to the viscous effects in the boundary layer. The second is the irreversible mixing process of two gas streams with different properties in the mixer.

History of Propulsion Devices and Turbomachines

Manmade propulsion devices have existed for many centuries, and natural devices have developed through evolution. Most modern engines and gas turbines have one common denominator: compressors and turbines or “turbomachines.” Several of the early turbomachines and propulsive devices will be described in this brief introduction before modern engines are considered. Included are some familiar names not usually associated with turbomachines or propulsion. Many of the manmade devices were developed by trial and error and represent early attempts at design engineering, and yet some were quite sophisticated for their time. Wilson (1982), Billington (1996), ASME (1997), Engeda (1998), St. Peter (1999), and others all present very interesting introductions to some of this history supplemented by photographs.

One of the earliest manmade turbomachines was the aeolipile of Heron (often called “Hero” of Alexandria), as shown in Figure 1.1. This device was conceived around 100 B.C. It operated with a plenum chamber filled with water, which was heated to a boiling condition. The steam was fed through tubes to a sphere mounted on a hollow shaft. Two exhaust nozzles located on opposite sides of the sphere and pointing in opposite directions were used to direct the steam with high velocity and rotate the sphere with torque (from the moment of momentum) around an axis – a reaction machine. By attaching ropes to the axial shaft, Heron used the developed power to perform tasks such as opening temple doors.

Introduction

As discussed in Chapters 1 to 3, the purpose of the turbine is to extract energy from the fluid to drive the compressive devices. The actual operation of the turbine is in some respects similar to, but opposite that, of the compressor. That is, energy is extracted from the fluid and the pressure and temperature drop through the turbine. Typically, 70 to 80 percent of the enthalpy increase from the burner is used by the turbine to drive the compressor. The remainder is used to generate thrust in the nozzle.

Two major differences are apparent between compressors and turbines, however. First, the pressure decreases through a turbine. In compressors, an adverse pressure gradient is present, which has the potential to cause blade stall. Such problems do not occur in a turbine, and turbine efficiency is typically higher than compressor efficiency. Also, because of this favorable pressure gradient, fewer stages are required for a turbine than for a compressor. As a result, the aerodynamic loading per stage is higher for a turbine than for a compressor. However, the inlet temperature is very high and limits the operation of the turbine. Thus, aerodynamically, the turbine is somewhat easier to design but is structurally more difficult.