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`978-0-521-76405-6 - Gas Turbine Emissions
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`PART 1
`
`OVERVIEW AND KEY ISSUES
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`Cambridge University Press
`978-0-521-76405-6 - Gas Turbine Emissions
`Edited by Timothy C. Lieuwen and Vigor Yang
`Excerpt
`More information
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`978-0-521-76405-6 — Gas Turbine Emissions
`Edited by Timothy C. Lieuwen and Vigor Yang
`Excerpt
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`Aero Gas Turbine Combustion: Metrics,
`
`Constraints, and System Interactions
`
`Randal G. McKinney and James B. Hoke
`
`1.1 Introduction
`
`The aircraft gas turbine engine is a complex machine using advanced technology
`
`from many engineering disciplines such as aerodynamics, materials science, combus-
`
`tion, mechanical design, and manufacturing engineering. In the very early days of
`
`gas turbines, the combustor section was frequently the most challenging (Golley,
`
`Whittle, and Gunston, 1987). Although the industry’s capability to design combus-
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`tors has greatly improved, they remain an important design challenge.
`
`This chapter will describe how the combustor interacts with the rest of the
`engine and flight vehicle by describing the relationship between attributes of
`
`the engine and the resulting requirements for the combustor. Emissions, a major
`
`engine performance characteristic that relies heavily on combustor design, will be
`
`introduced here with more detail found in following chapters. The wide range of
`operating conditions a combustor must meet as engine thrust varies, which is a
`
`major challenge for combustor design, will also be described. Last, the relationship
`
`between combustor exit temperature distribution and turbine section durability
`will be discussed.
`
`1.2 Overview of Selected Aircraft and Engine Requirements and their
`
`Relation to combustor Requirements
`
`Aircraft gas turbine engines have been used in many different sizes of aircraft since
`
`their introduction in the 1940s. Small aircraft such as single-engine turboprops use
`
`engines of low shaft horsepower, which are of small physical size. Business jets and
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`smaller passenger aircraft may use turbojets or turbofans with thrust in the range of
`
`several thousand pounds, usually with two engines per aircraft. The other extreme
`includes four-engine aircraft with turbofan engine thrusts as high as seventy thousand
`
`pounds and very large twin-engine aircraft with thrust per engine in the one hundred
`
`thousand pound class These thrust designs are also physically very large, with fan
`
`diameters over 100 inches. In all of these applications, the engine system imposes a
`common set of requirements upon the combustor, as summarized in Table 1.1.
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`978-0-521-76405-6 — Gas Turbine Emissions
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`Aero GT Combustion
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`Table 1.1. Engine system-level requirements and supporting combustor characteristics
`
`Engine requirement
`
`Combustor characteristic
`
`Optimize fuel consumption
`Meet emissions requirements
`Wide range of thrust
`Ground and altitude starting
`Turbine durability
`Overhaul and repair cost
`
`High combustion efficiency and low combustor pressure loss
`Minimize emissions and smoke
`Good combustion stability over entire operating range
`Easy to ignite and propagate flame
`Good combustor exit temperature distribution
`Meet required combustor life by managing metal temperatures
`and stresses
`
`‘?>
`\
`.2’
`
`Altitude relight
`- (T)
`and smmng
`
`.
`Exit temperature
`
`Figure 1.1. Combustor performance requirements are interrelated.
`
`As shown in Figure 1.1, these requirements are interdependent. Years of design
`and development within the industry have produced successive designs that improve
`
`upon all of the requirements concurrently. Although emissions are the focus of this
`
`text, each of these other requirements interacts with the emissions constraints and
`
`will be introduced briefly.
`
`1.3 Combustor Effects on Engine Fuel Consumption
`
`Gas turbine engines are Brayton cycle devices. An ideal version of such a cycle com-
`prises isentropic compression, addition of heat at constant pressure, and isentropic
`
`expansion through the turbine. Figure 1.2 is a simplified schematic of the effect of
`
`such a cycle on the pressures and temperatures in the engine. In real engines, all
`
`of the processes incur some loss of performance versus the ideal, manifested as a
`stagnation pressure loss in the combustor. Combustion systems incur pressure losses
`
`because of flow diffusion and turning, jet mixing, and Rayleigh losses during heat
`
`addition (Lefebvre and Ballal, 2010). However, at most power conditions, the em-
`
`ciency with which the fuel chemical energy is converted into thermal energy is very
`
`high, typically greater than 99.9 percent. “Low” levels of 98 to 99.5 percent can be
`seen at low—power levels. In general, though, the combustion system is a small para-
`
`sitic effect on overall fuel consumption.
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`978-0-521-76405-6 - Gas Turbine Emissions
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`1.4 Fundamentals of Emissions Formation
`
`5
`
`Fan flow —> '
`
`Thrust
`",7 m» Fan
`C079 Power to operate
`
`fan + some thrust Core flow
`Temperature - — Pressure — Temperature
`
`Pressure
`
`Figure 1.2. Summary of component characteristics.
`
`1.4 Fundamentals of Emissions Formation
`
`The pollutants emitted by engines that are of most interest are carbon monoxide
`
`(CO), unburned hydrocarbons (UHC), nitric oxides (NO,), and particulate matter
`
`(PM or smoke). At low-power conditions, the inlet combustor pressure and temper-
`ature are relatively low, and reaction rates for kerosene-type fuels are low. Liquid
`
`fuel must be atomized, evaporated, and combusted, with sufficient residence time
`
`at high enough temperatures to convert the fuel into CO2. If the flow field permits
`
`fuel vapor to exit the combustor without any reaction, or, if partially reacted to spe-
`cies of lower molecular weights, UHC will be present. If a portion of the flow field
`
`subjects a reacting mixture to a premature decrease in temperature via mixing with
`
`cold airstreams, these incomplete or quenched reactions lead to the production of
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`CO, as detailed in Chapter 7.
`At high power conditions, high air pressures and temperatures lead to fast reac-
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`tions, with the result that CO and UHC are nearly zero. At these elevated tempera-
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`tures, emissions of NO, and PM become more prevalent. NO, can be formed through
`
`several processes, but the dominant pathway is thermal NO,, as described by the
`
`extended Zeldovich mechanism, also detailed in Chapter 7.
`
`02:20
`N,+O=NO+N
`
`N+U2=NU+O
`N+OH=NO+H
`
`The formation rate is exponentially related to the temperature in the flame, peaking
`
`near stoichiometric conditions. Thermal NO, emissions can be reduced by limiting
`the time the flow spends at the high temperature and/or by reducing the maximum
`
`temperatures seen in the flame via stoichiometry control. Other NO, formation
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`978-0-521-76405-6 — Gas Turbine Emissions
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`6
`
`Aero GT Combustion
`
`mechanisms, such as NO, formed in the flame zone itself, are also described in
`
`Chapter 7, but are negligible for aircraft engines.
`When fuel-rich regions of the combustor flow exist at high pressures and tem-
`
`peratures, the formation of small particles of carbon can occur. These carbon par-
`
`ticles result from complex chemical processes and undergo multiple processes within
`
`the combustor such as surface growth, agglomeration, and oxidation prior to leaving
`
`the combustor, as detailed in Chapter 5.These particles pass through the turbine and
`exit the engine in the exhaust.When the concentration of the particles in the exhaust
`
`is high enough to be visible, as was often the case in early gas turbines, it is referred
`
`to as smoke or soot. Recently, the more general term particulate matter (PM) has
`been used to describe this emission. Modern engine smoke levels are invisible but
`
`still possess large quantities of very small soot particles and aerosol soot precursors
`
`(see Chapter 5) at the exhaust. Emerging research on the effect of PM on health
`and climate focuses more attention on measuring, modeling, and understanding the
`processes governing PM production.
`
`These relationships between engine power conditions and emissions production
`
`lead to the behavior shown in Figure 1.3. As shown in the figure, levels of UHC and
`
`C0 are highest at low power and drop quickly with increasing thrust. Conversely,
`NO, and PM increase with engine power and are typically maximized at maximum
`
`power. Chapters 5 and 7 discuss these emissions formation processes in more detail.
`
`1.5 Effect of Range of Thrust and Starting Conditions
`on the combustor
`
`Flight gas turbine engines must provide a range of thrust and thrust response to
`
`power the aircraft mission. Missions vary depending on the aircraft application.
`Commercial aircraft and military transports have similar missions. Military fighters
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`and other specialized aircraft can have very different missions because their use is
`
`not exclusively for the transport of payload between two points. Design require-
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`ments are also very different for commercial and military applications. Military
`fighter engines are often designed for maximized thrust developed per unit weight
`
`so that the maneuverability of the aircraft is maximized. Military fighter engines also
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`fly at a wide range of thrust throughout the flight envelope and must undergo fre-
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`quent rapid thrust transients Typically, commercial engines are designed for maxi-
`
`mum fuel efficiency per unit thrust. They fly at high altitude to achieve the best
`fuel efficiency and often do not have to endure the aggressive and numerous thrust
`
`transients of military fighter engines. Engine combustors must operate stably and
`
`efficiently over the full range of operating conditions, and must reliably relight if an
`engine shutdown or flameout should occur in flight.
`
`1.5.1 Engine Mission Characteristics
`
`A typical commercial engine mission consists of ground starting, taxi, takeoff, climb
`
`to altitude, cruise, deceleration to flight idle and descent, approach, touchdown,
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`978-0-521-76405-6 - Gas Turbine Emissions
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`Excerpt
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`1.5 Effect of Range of Thrust and Starting Conditions on the Combustor
`
`7
`
`O!0
`
`0|0
`
`
`
`ElorSmokeNumber
`
`°*3O 20
`
`10
`
`0
`
`0
`
`:
`
`z
`
`*
`20000
`
`60000
`40000
`Thrust
`
`80000
`
`1 00000
`
`Figure 1.3. Emissions versus power level for the PW4084.
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`thrust reverse, and taxi in. The extremes in combustor operating conditions drive the
`
`overall design approach. The combustor must meet performance, operability, and
`
`emissions metrics over the full range of operation. In order to do so, it must operate
`
`at the following extremes:
`
`1. Minimum fuel—air ratio — This occurs during decelerations from high power to
`
`low power. Flight decelerations normally occur when descending from high alti-
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`tude cruise and during approach throttle movements. They can also occur in
`
`emergencies. Minimum fuel—air ratio typically depends on the thrust decay rate,
`as the time response of the engine turbomachinery that governs the airflow is
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`much longer than that of the fuel flow. Risk of weak extinction (flameout) is
`
`highest during decelerations.
`
`2. Minimum operating temperatures and pressures — These occur at flight and
`ground idle conditions. Low pressure and temperature challenges combustion
`
`efficiency due to slower fuel vaporization and chemical kinetics.
`
`3. High operating temperatures and pressures — These occur at takeoff, climb,
`thrust reverse, and cruise conditions These conditions result in the bulk of NO,
`
`formation and the most severe liner metal temperatures.
`
`4. Ignition conditions — Ignition normally occurs on the ground but also occasion-
`
`ally in flight. Ignition is required at near surrounding ambient pressure and
`
`temperature. High altitude and extremely cold conditions are typically the most
`challenging to achieve ignition, flame propagation, and flame stabilization.These
`
`conditions lead to low temperature (-40°F) and pressure (4 psia at 35,000 ft.)
`combustor inlet conditions.
`
`Thus, the combustor design must meet the performance, emissions, and durability
`requirements at low- and high-power operations without compromising stability
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`978-0-521-76405-6 — Gas Turbine Emissions
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`Excerpt
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`Aero GT Combustion
`
`
`
`Figure 1.4. (a) Can-annular combustor (Pratt & Whitney JT8D-200); (b) RQL annular com-
`bustor (IAE V2500).
`
`and ignition. This requires favorable combustion fuel-air stoichiometry to meet
`requirements at all operating conditions. Two principal approaches have been used
`
`to achieve stoichiometry control in the industry. The first, fixed geometry without
`
`fuel staging, is the most common approach and is in the large majority of engines in
`
`service. These systems have all fuel injectors operating at all conditions. The second
`approach controls local fuel-air ratio through fuel staging. In these systems, not all
`
`fuel injectors operate at low power. This enables more active control of the local
`combustion fuel-air ratio.
`
`1.5.2 Fixed-Geometry Rich-Quench-Lean (RQL) Combustors
`
`Fixed-geometry combustors have been used in the gas turbine industry since its
`
`inception. Early designs used multiple cans in a circumferential array. The cans
`
`transitioned through an annular duct to the turbine (Figure 1.4a). Later designs
`used an annular duct geometry that allowed for reduced overall length and weight
`
`(Figure 1.4b). Annular combustors also have reduced liner surface area relative to
`
`can-annular combustors and therefore use less cooling. All designs use multiple fuel
`injectors to provide spray atomization and fuel-air mixing. Achieving good atomiza-
`
`tion and fuel-air mixing is critical for efficient combustion, low emissions, and good
`
`temperature uniformity into the turbine. Normally, the fuel is injected in the front end
`
`of the combustor and flow recirculation is created to provide a stabilization region
`
`for the combustion process. This is typically accomplished with air swirlers, which
`leads to vortex breakdown and flow recirculation. The stabilization zone promotes
`
`recirculation of hot product gases forward to the incoming fuel spray, thereby pro-
`
`viding a continuous ignition source and faster fuel droplet evaporation. Accelerated
`droplet evaporation is critical to high-efficiency combustion at low-power conditions,
`
`when low air inlet temperatures are insufficient to provide fast enough evaporation.
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`978-0-521-76405-6 - Gas Turbine Emissions
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`Excerpt
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`1.5 Effect of Range of Thrust and Starting Conditions on the Combustor
`
`9
`
`Steady state
`fuel-air ratio
`
`
`
`Pressure
`
`Transient deoel
`fuel-air ratio
`
`5
`Q‘;
`5O
`E.:
`
`§3
`
`-g
`8
`
`Idle
`
`Thrust
`
`Take-off
`
`Idle
`
`Thrust
`
`Take-ofl
`
`Figure 1.5. Combustor operating conditions.
`
`If continuous ignition is not provided at low power, the vaporization and reaction
`times can exceed the combustor residence time and flameout occurs.
`
`The airflow distribution in a fixed-geometry combustor must be selected to
`
`achieve both low- and high-power performance requirements. Conditions at the
`
`combustor inlet vary significantly between low-power idle and high-power takeoff
`
`conditions.At idle, inlet temperature, pressure, and global fuel-air ratio are relatively
`
`low. At takeoff, the opposite is true (Figure 1.5). The operating temperatures and
`pressures are largely a function of the engine thermodynamic cycle; therefore the
`
`most significant parameter for the combustor designer to consider is the fuel-air
`
`ratio. Because air is introduced in stages along the length, the designer can tailor
`the airflow distribution to achieve key performance metrics. This creates a distri-
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`bution in fuel-air ratio along the length of the combustor, leading to variations in
`
`local temperature as power level is adjusted. The difference in fuel-air ratio between
`
`high-power takeoff and low-power deceleration and idle conditions is critical because
`it determines the range of local fuel-air ratio in the front end of the combustor. For
`
`most modern gas turbines, the difference is large enough that the front end operates
`
`fuel rich (f/a > 0.068 for jet fuel) at takeoff conditions. Consequently, fixed-geometry
`
`combustors are referred to as rich-buming or rich-quench-lean (RQL) designs. This
`
`refers to the rich front-end fuel-air ratio that is diluted (quenched) by additional
`airflow in the downstream section of the combustor to reach the fuel—lean conditions
`
`at the combustor exit. The RQL-type design has several advantages and challenges,
`
`which are discussed later in this chapter.
`As previously described, the challenges at low power are combustion efficiency
`
`and stability. The local fuel-air ratio in the RQL combustor front end at idle is
`
`designed to generate high recirculating gas temperatures (Figure 1.6). Therefore, the
`
`local fuel-air ratio should be near the stoichiometric (f/a ~.068 for jet fuel) fuel-air
`ratio to achieve high combustion efficiency. High combustion efficiency minimizes
`unburned hydrocarbon and carbon monoxide emissions that predominate at idle.
`
`Some increase in NO, emissions is generated by the hot front end, but emissions at
`
`idle are not significant when compared to high power. By designing for near stoichio-
`metric conditions at idle, stability can be ensured at deceleration conditions, where
`
`minimum fuel-air ratio occurs. If the minimum fuel-air ratio during deceleration is
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`Aero GT Combustion
`
` CO consumed
`
`______ _-T__mresno»a
`temperature
`
`Turbine
`inlet
`
`O
`
`3
`E0
`2;,‘
`«I
`0
`
` Compressor
`exit
`
`
`Gas residence time in combustor
`
`Figure 1.6. Combustor at low power.
`
`not more than one-third below idle fuel-air ratio, the local fuel-air ratio in the front
`
`end is maintained above the weak extinction limit and flameout is avoided. Limiting
`
`of minimum deceleration fuel-air ratio is accomplished by the engine control and
`
`controls the maximum thrust decay rate for the engine transient.
`At high-power conditions, the principal emissions challenges are NO, and
`
`smoke. The RQL combustor axial temperature distribution at high power is depicted
`
`in Figure 1.7. The front end is fuel rich and consequently has lower flame tempera-
`
`tures. The dilution or quench region is characterized by peak gas temperatures as
`the fuel-rich mixture transitions through stoichiometric fuel-air ratio to the fuel-lean
`conditions at the combustor exit. In the front end, smoke is formed due to the com-
`bustion at fuel-rich conditions. Some of the smoke formed in the front end is oxidized
`
`in the high-temperature, oxygen-rich quench region.Tl1us, the front-end airflow level
`
`must be set with understanding of the formation and oxidation processes. The NO,
`emissions are formed in both the front end and quench regions at high power. NO,
`
`formation is exponentially a function of gas temperature, but also depends on the
`
`residence time at the local temperature. The highest rate of formation occurs in the
`
`quench region because it is the region where peak temperatures occur. However,
`time at peak temperature in the quench region is relatively short due to high mixing
`
`rates In contrast, the formation of NO, in the front end is not negligible because it
`
`has relatively longer residence time due to the flow recirculation. The presence of
`
`cooling flow in the front end also leads to NO, formation when it interacts with the
`fuel-rich gas mixture.
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