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The combustion system must operate over a very wide range of conditions and be capable of starting and acce-lerating the engine. To satisfy the objectives of engine operation, certain design requirements must be met.

Several important design parameters are shown in Table 4.1.

Combustion Efficiency Combustion efficiency, which is defined as the ratio of actual to theoretical heat re-lease, must be as high as possible over the operating range of die engine. Efficiency requirements for a new combustion system design are initially selected by the engine cycle design group and then modified as the com-bustor design progresses to factor in realistic estimates based on a more finalized version of the design.

Total Pressure Loss The total pressure loss of the com-bustion system is defined as the difference between the averaged stream total pressure at the compressor exit station and the turbine inlet station. This loss includes the diffuser total pressure loss, and is usually expressed as a percentage of the compressor exit total pressure, Px3- In general, higher pressure losses result in better combustor performance and the combustor size and weight can be reduced. But, of course, higher pressure losses reduce the engine cycle performance. The pres-sure loss is very nearly proportional to the square of the compressor exit Mach number, and is a weak function of the combustor temperature rise.

Temperature Rise The combustion system temperature rise requirement is determined by the engine cycle de-sign and turbine dede-sign groups. The value, AT, is the difference between the combustion chamber exit temper-ature, T4, and the compressor exit temperature, T3. If the required AT is greater than about 2600 °F, combus-tion efficiency may become less than the chemical effi-ciency due to die adverse effect of the exit temperature profile.

COMBUSTORS AND AUGMENTORS 4-1

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0 .01 . 0 2 . 0 3 .04 . 0 5 . 0 6 . 0 7 . 0 8 . 0 9 .10 .11 . 1 2 .13 . 1 4 FUEL - AIR RATIO. LB/LB OF AIR

Figure 4.1 Combustion Temperature Rise

4-2 COMBUSTORS AND AUGMENTORS

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Figure 4.2 Typical Main Combustor Fiowpath Design

COMBUSTION SYSTEM DESIGN REQUIREMENTS

• High Combustion Efficiency - 77c 2: 99.8%

• Low Total Pressure Losses -A PT/ P T 3 - 4 - 6%

• Uniform Exit Temperature Distribution - P. F. - 0.2

• Altitude Relight Capability - Up To 30,000 Feet

• Short Length - Light Weight

• Long Life

• Low Cost

• Maintainability

• Emissions Requirements

7ao/e 4.1 Combustion System Design Requirements

COMBUSTORS AND AUGMENTORS 4-3

C o m b u s t o r Exit Pattern Factor A combustor exit pat-tern factor requirement is usually established in conjunc-tion with the turbine designers. It is based on past experience with similar combustors and on parametric correlations that have been formulated from a large amount of test data. Pattern factor is a measure of the maximum temperature existing at the combustor exit plane. Pattern factor has a major effect on the life of the turbine nozzle vanes, unless the vanes are designed for stoichiometric temperatures. Many different combustor design parameters have an effect on pattern factor.

C o m b u s t o r Exit Temperature Profile The combustor exit radial temperature profile is defined as the average of all of the circumferential temperature readings at each radial measurement station at the combustor exit plane.

This profile, which is a measure of the temperature ex-perienced by the turbine rotor vanes, is plotted against the radial distance from the turbine inlet root radius to the tip radius position. Combustor exit temperature pro-file factor is the highest temperature of the average tem-perature profile. Temtem-perature profile requirements are

determined by the turbine design group with the close coordination of the combustion system designer, The curve on the left in Figure 4,3 shows a typical desired profile. The curve on the right is the locus of the maxi-mum profile peaks. Any profile, for which the average temperature meets requirements and which falls below the locus of profile peaks, satisfies the combustor exit temperature profile requirements.

Altitude Relight Aircraft engine combustion systems are usually required to have the capability of relighting at specified high altitudes for windmilling conditions with cold, low pressure air and with cold fuel. This quirement is usually defined in the form of a required re-light fre-light condition map. An example of a rere-light map is presented in Figure 4.4. The most difficult light-off conditions are usually encountered at the upper left cor-ner of the map, where the air pressure is lowest and the combustor pressure drop is small. This altitude relight requirement is usually established by the engine project group to meet the requirements of the customer.

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Figure 4.3 Typical Combustor Exit Temperature Profiles

4-4 COMBUSTORS AND AUGMENTORS

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ZERO LOAD AND ZERO ASSIST APPLIED TO THE CORE ENGINE

.4 .5 .6 .7 .8 .9 1.0 FLIGHT MACH NUMBER - MP

Figure 4.4 CF6-80C2 Windmilling Air Start Envelop

Emission Requirements Requirements for the maxi-mum emissions of undesirable atmospheric pollutants from jet engines are established by government agen-cies. The U.S. Environmental Protection Agency estab-lishes emission limits for commercial engines and military agencies establish limits for military engines.

Emission limits are established for smoke, the oxides of nitrogen (which usually reach maximum values at take-off conditions), carbon monoxide, and unburned hydro-carbons (which are usually predominant at ground idle conditions). Since some of the basic combustion system design parameters have a strong effect on emission lev-els, the emission requirements must be considered dur-ing the preliminary design phase of the combustion system.

Space Rate and Aerodynamic Loading Parameters The combustion system space heat release rate is a mea-sure of the concentration of the energy released inside of the combustor. Space rate is calculated as follows:

SR = 3600(FueI Heating Value) *<Fuel/Air)*W3

Py/14.7 *VoIc

where Volc is the interior volume of the combustion liner in cubic feet. In general, combustors for large engines should have relatively low space rates and combustors for small engines can have relatively high space rates.

Space rate is inversely related to residence time, and di-rectly related to temperature rise, AT, as follows: