1/34 UNIVERSITY OF SALENTO – SCHOOL OF INDUSTRIAL ENGINEERING DEPT. OF ENGINEERING FOR INNOVATION – Lecce-Brindisi (Italy) MASTER OF SCIENCE IN AEROSPACE ENGINEERING MASTER OF SCIENCE IN AEROSPACE ENGINEERING PROPULSION AND COMBUSTION PROPULSION AND COMBUSTION COMBUSTION SYSTEM COMBUSTION SYSTEM Chap. 9 – TURBO-MACHINERY DESIGN Chap. 9 – TURBO-MACHINERY DESIGN LECTURE NOTES AVAILABLE ON LECTURE NOTES AVAILABLE ON http://www.ingegneria.unisalento.it/scheda_personale/-/people/antonio.ficarella/materiale Prof. Eng. Antonio Ficarella University of Salento - [email protected]REVIEW R00 DATE 04/11/2013 FILE propTMDcombsysR00.odp RESPONSIBLE Antonio Ficarella [email protected]DISTRIBUTION LIST REPLACE MODIFICATIONS REPLACED BY
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UNIVERSITY OF SALENTO – SCHOOL OF INDUSTRIAL ENGINEERINGDEPT. OF ENGINEERING FOR INNOVATION – Lecce-Brindisi (Italy)
MASTER OF SCIENCE IN AEROSPACE ENGINEERINGMASTER OF SCIENCE IN AEROSPACE ENGINEERING
PROPULSION AND COMBUSTIONPROPULSION AND COMBUSTION
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INTRODUCTION
Both physical and chemical aspects are embraced during combustion. This subject of physics includes mass and heat transfer, thermodynamics, and gas and fluid dynamics; while chemistry influences pollutant emission among the products of combustion, the heat release rate, and radiation properties of the flame at high temperatures.
In aviation applications the chemical process also impacts lean light-off and flameout limits at high altitudes.
Flames may be categorized as premixed type when the fuel and air are mixed before combustion and as diffusion type when the two components are diffused within the flame zone. The two flame types may also be described as laminar or turbulent, depending on the flow velocity.
When burning liquid fuels, complete vaporization may not take place before entering the flame zone, resulting in a diffusion flame burning of fuel droplets superimposed on a premixed turbulent flame zone.
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Annular combustors have the liner inside an annular casing to give a compact design and a clean aerodynamic flow path with little loss in pressure. But the absence of radial load carrying members generates buckling problems in the outer liner.
Multican configurations with a group of between 6 and 10 tubular liners arranged inside a single annular casing combine the compactness of an annular chamber with the mechanical strength of the tubular type.
Besides the pressure differential on either side, the liner must have thermal resistance to withstand continuous and cyclic operation at elevated temperatures.
With conventional combustors, any modifications to alleviate the generation of smoke and oxides of nitrogen (NOx, most notably nitrogen oxide and nitrogen dioxide) will invariably result in increased levels of emission of carbon monoxide (CO) and unburned hydrocarbons (UHCs).
Regulating the amount of air entering the primary combustion zone through a variable geometry mechanism has been successful to a considerable extent in overcoming this problem.
This feature is also helpful in initiating the combustion process during light-up.
The variable geometry arrangement for controlling the flow of air calls for a complex control and feedback mechanism, adding to the cost and weight while raising reliability questions.
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The concept of staged combustion attempts to achieve the same objectives by using two or more separate zones, each designed specifically to optimize certain features of combustion.
In the lightly loaded primary first zone, high combustion efficiency and minimized CO and UHC production are achieved by operating at a relatively high equivalence ratio Φ of around 0.8, where equivalence ratio is defined by the actual fuel ratio divided by the stoichiometric fuel ratio.
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FUELS FOR VARIOUS APPLICATIONS
JP-4 and JP-5 are used widely by the U.S. military for turbojet operations. JP-4 is a naptha fuel with vapor pressure of about 2.5 psi and aromatic content under 25 percent. JP-5 is a blended kerosene fuel, has a flash point of 140°F, and freezing point of −51°F.
Airplanes operating at supersonic speed up to 3.5 Mach require thermally stable fuels. JP-7 and JP-8 provide varying degrees of flash and freezing points, thermal stability, aromatic content, and flame luminosity. Commercial airlines rely on ASTM jet aviation turbine fuel. This is also a kerosene-based fuel with a flash point of 110°F and freezing point of −36°F.
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Atomization of liquid fuels into droplets with extensive surface area is necessary for ignition and combustion of liquid fuels, since they are not sufficiently volatile to produce vapors. The rate of evaporation is enhanced by reducing the size of the droplets. Forcing the fuel under pressure through an orifice aids in atomization.
Another concept calls for the fuel to flow at lower pressure over a lip in a high-velocity air stream, causing atomization by the air as it enters the combustion zone. In this form of air-blast atomizers the airflow pattern controls the distribution of the fuel to reduce soot and exhaust smoke formation. However, the system has limited stability limits, and at startup a lack of air velocity leads to poor atomization.
The problem may be controlled by combining the features of a pilot pressure-swirl atomizer to obtain easy light-up and improved stability limits.
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8/34UNIVERSITY OF SALENTO – DEPT. OF ENGINEERING FOR INNOVATION – Lecce-Brindisi (Italy)
Storage, handling, metering, atomization, and combustion are affected by a fuel’s properties, and are discussed at length in ASTM specifications D-2880 and D-1655.
Flash point provides an indication of the maximum temperature at which a fuel may be safely stored and handled without risking a fire hazard.
The pour point determines the lowest temperature at which oil may be stored and still flow under gravity.
Water and sediments in oil foul the fuel handling equipment, and are also detrimental to the turbine’s fuel system.
The presence of carbon residue in a fuel approximates formation of carbon deposits in the combustor.
Viscosity of a fluid determines how easily it flows and atomizes in the nozzles.
Specific gravity does not directly affect the combustion of a fuel oil, but plays a role in the weight–volume relation and heating-value calculations.
Reduced hydrogen in a fuel enhances its aromatic content, resulting in soot formation and flame radiation in the combustor.
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COMBUSTION PRINCIPLES
The rate at which a combustion wave travels as a plane flame front in a direction normal to its surface through the flammable mixture is controlled by flame radiation
The chemical reaction rate determines the laminar flame speed in premixed systems, which is governed by the equivalence ratio, temperature, and pressure.
Turbulence appreciably increases the flame speed, but the manner and extent of its influence is not well understood. Increase in the burn rate may be attributed to the wrinkling of the flame front by the turbulence by enlarging the specific surface area
Heterogeneous mixtures of fuel drops, fuel vapor, and air experience increased flame speed with reduced mean drop size
The ignition of combustible mixtures relies on a transient ignition source, usually an electric spark, to supply sufficient energy to create a volume of hot gas that just satisfies the necessary and sufficient condition for propagation, so that the rate of heat generation exceeds the rate of heat loss.
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Premixed combustion systems also experience flashback when the flame travels upstream from the combustion zone into the premixing part of the combustor.
The adiabatic temperature of the flame, attained when the energy liberated by the chemical reaction is fully used to heat the products, plays a substantial role in the rate of reaction. In practice, however, some heat is lost by radiation and convection.
Reduction of NOx emissions to meet regulatory levels may be obtained by lowering the reaction temperature, which in turn calls for operating the combustor under fuel-lean conditions.
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COMBUSTOR DESIGNS AND SELECTION
Reduced pressure losses because of the relatively straight flow path of the gases and compactness in design make the annular combustor suitable for aviation engines.
The number, diameter, and location of the holes is optimized to bring down NOx emissions. An annular vortex generator is mounted in the combustor cap around the fuel nozzle.
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Regression analysis shows that NOx emissions from the standard liner are proportional to P0.55 and for the new design are P0.58
steam injection - catalytic combustor
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CONTROL OF POLLUTANTS
Inadequate residence time in the primary zone, improper mixing of air and fuel, and chilling of the products of combustion by cooling air near the liner walls are also responsible for incomplete combustion of the fuel and high concentrations of CO.
Soot in the form of a fine powder is produced when recirculating burned gases move back toward the flame close to the spray. Local regions of the fuel vapor then are surrounded by oxygen-deficient gases at elevated temperature, and emerge from the combustor as smoke
Soot and smoke formation becomes more pronounced at higher pressures. The chemical reaction rate tends to rise with increase in pressure, causing combustion to initiate earlier and a greater portion of the fuel to burn in the region near the spray.
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Production of NOx occurs mostly in flame zones where the temperature exceeds 1850 K.
Air temperature at the inlet to the combustor plays a role, since it affects the flame temperature.
Larger residence time within the combustion chamber tends to increase NOx emissions, except when the rate of formation is low due to considerably lean mixtures.
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Gas pressure (P) increase tends to raise the formation of NOx in conventional combustor designs
Emission of NO increases with the size of the droplets at low equivalence ratio, mostly because local burn regions of high temperature develop around the larger drops.
Reduction in the levels of CO and UHC can be achieved through more complete combustion of the fuel
Recirculation of the airflow to obtain better mixing of fuel and air with an equivalence ratio of 0.8 in the primary zone helps toward this objective.
Restricting the amount of air for cooling of the liner surface in the primary zone also can be effective in reducing CO and UHC.
Improved fuel atomization and increasing residence time in the primary zone are also factors for enhancing combustion efficiency.
Soot and its accompanying smoke can be controlled by inserting more air in the primary zone to obtain better mixing. Exhaust smoke will then diminish by preventing formation of fuel-rich regions in the flame.
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NOX FORMATION
A combustion model is required to describe the interaction between turbulence and chemistry to understand the formation of nitrogen oxides.
combustion in diffusion flames occurs in thin layers in the vicinity of nearly stoichiometric zones, where the gas temperature and the concentration of radicals are mainly high
In the flamelet model, the only parameter responsible for turbulence-chemistry interaction is the scalar dissipation, N
Chemical calculations are detached from the hydrodynamics
mixture fraction Z
The resulting data are then used for modeling of formation of nitrogen oxides in the postprocessing mode.
Turbulent stresses are approximated using appropriate viscosity values. A
fast (equilibrium) chemistry assumption together with a probability density function for the mixture fraction is used to determine heat release.
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p(Z, N) is the joint probability density function of the mixture fraction and scalar dissipation
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EFFECTS OF SWIRL
Stabilization is achieved by transporting hot and chemically active combustion species from the downstream region to the root of the flame, creating a thermal nonuniformity between the recirculating hot gases and the cooler gases flowing from upstream of the swirler and the flame zone.
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DRY LOW NOX COMBUSTION SYSTEM
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ACOUSTIC RESONANCE
Combustion of the air and fuel mixture is accompanied by noise
Combustion noise can become detrimental when instabilities arising in the burning process couple with acoustic modes inside the chamber.
The natural frequencies of the combustor can be excited by resonant pressure waves in the main gas flow
Sustained oscillating phenomena due to a higher level of mixing of the fuel and air prior to combustion lead to engine noise and vibration problems.
Rapid changes in air and fuel supply and aerodynamic disturbances may lead to the instability because of a sequence of extinction and reignition of the flame in parts of the combustor.
If the heat release rate does not take place uniformly and periodic spikes occur, acoustic waves of the same frequency may be expected in the combustion zone.
Variation in heat release results from changes in flame structure produced by acoustic pressure disturbances. Time delay between pressure disturbance and heat-release variation determines the phase and, consequently, the stability of the system.
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ACTIVE COMBUSTION INSTABILITY CONTROL
Combustion instabilities are difficult to predict analytically in the design phase for all operating conditions due to the complex geometry of the system.
Noticeable humming caused by self-excited vibrations can occur during shop tests in the premixed mode operation of the turbine.
Pressure oscillations may exceed unacceptable levels
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THERMAL PROTECTION OF COMBUSTOR LINER
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STRUCTURAL DESIGN FOR DYNAMIC PRESSURE
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