Arthur H. Lefebvre

GAS TURBINE COMBUSTION—Alternative Fuels and Emissions

Basic Considerations Introduction Early Combustor Developments Basic Design Features Combustor Requirements Combustor Types Diffuser Primary Zone Intermediate Zone Dilution Zone Fuel Preparation Wall Cooling Combustors for Low Emissions Combustors for Small Engines Industrial Chambers Combustion Fundamentals Introduction Classification of Flames Physics or Chemistry? Flammability Limits Global Reaction-Rate Theory Laminar Premixed Flames Laminar Diffusion Flames Turbulent Premixed Flames Flame Propagation in Heterogeneous Mixtures of Fuel Drops, Fuel Vapor, and Air Droplet and Spray Evaporation Ignition Theory Spontaneous Ignition Flashback Stoichiometry Adiabatic Flame Temperature Diffusers Introduction Diffuser Geometry Flow Regimes Performance Criteria Performance Effect of Inlet Flow Conditions Design Considerations Numerical Simulations Aerodynamics Introduction Reference Quantities Pressure-Loss Parameters Relationship between Size and Pressure Loss Flow in the Annulus Flow through Liner Holes Jet Trajectories Jet Mixing Temperature Traverse Quality Dilution Zone Design Correlation of Pattern Factor Data Rig Testing for Pattern Factor Swirler Aerodynamics Axial Swirlers Radial Swirlers Flat Vanes versus Curved Vanes Combustion Performance Introduction Combustion Efficiency Reaction-Controlled Systems Mixing-Controlled Systems Evaporation-Controlled Systems Reaction- and Evaporation-Controlled Systems Flame Stabilization Bluff-Body Flameholders Mechanisms of Flame Stabilization Flame Stabilization in Combustion Chambers Ignition Assessment of Ignition Performance Spark Ignition Other Forms of Ignition Factors Influencing Ignition Performance The Ignition Process Methods of Improving Ignition Performance Fuel Injection Basic Processes in Atomization Classical Mechanism of Jet and Sheet Breakup Prompt Atomization Classical or Prompt? Drop-Size Distributions Atomizer Requirements Pressure Atomizers Rotary Atomizers Air-Assist Atomizers Airblast Atomizers Effervescent Atomizers Vaporizers Fuel Nozzle Coking Gas Injection Equations for Mean Drop Size SMD Equations for Pressure Atomizers SMD Equations for Twin-Fluid Atomizers SMD Equations for Prompt Atomization Internal Flow Characteristics Flow Number Discharge Coefficient Spray Cone Angle Radial Fuel Distribution Circumferential Fuel Distribution Combustion Noise Introduction Direct Combustion Noise Combustion Instabilities Control of Combustion Instabilities Modeling of Combustion Instabilities Heat Transfer Introduction Heat-Transfer Processes Internal Radiation External Radiation Internal Convection External Convection Calculation of Uncooled Liner Temperature Film Cooling Correlation of Film-Cooling Data Practical Applications of Transpiration Cooling Advanced Wall-Cooling Methods Augmented Cold-Side Convection Thermal Barrier Coatings Materials Liner Failure Modes Emissions Introduction Concerns Regulations Mechanisms of Pollutant Formation Pollutants Reduction in Conventional Combustors Pollutants Reduction by Control of Flame Temperature Dry Low-Oxides of Nitrogen Combustors Lean Premix Prevaporize Combustion Rich-Burn, Quick-Quench, Lean-Burn Combustor Catalytic Combustion Correlation and Modeling of Oxides of Nitrogen and Carbon Monoxide Emissions Concluding Remarks Alternative Fuels Introduction Types of Hydrocarbons Production of Liquid Fuels Fuel Properties Combustion Properties of Fuels Classification of Liquid Fuels Classification of Gaseous Fuels Alternative Fuels Synthetic Fuels Index References appear at the end of each chapter.

Gas Turbine Combustion : Alternative Fuels and Emissions, Third Edition

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Spray characteristics of aerated-liquid pressure atomizers

The atomizing performance of a novel type of injector is investigated. Essentially, the injector comprises a plain-orifice atomizer with means for injecting air or gas into the liquid upstream of the final discharge orifice. Measurements of mean drop size and drop-size distribution are made using a Malvern spray analyzer. The liquid employed is water, and all tests are carried out at normal atmospheric pressure. Water injection pressures are varied from 34.5 to 690 kPa (5 to 100 psid) and air/liquid ratios from 0.002 to 0.022 by mass. The results obtained with injector orifice diameters of 0.8,1.6, and 2.44 mm show that good atomization can be achieved using only small amounts of atomizing gas at injection pressures down to 34.5 kPa (5 psi). The system appears to have considerable potential for practical applications in which the available gas and liquid pressures are low and in which small holes and flow passages cannot be employed owing to the risk of plugging by contaminants in the liquid.

The influence of spark discharge characteristics on minimum ignition energy in flowing gases

Abstract The electrical aspects of spark ignition in flowing combustible mixtures have been investigated in a specially designed, closed circuit wind tunnel in which a fan was used to drive the gas through a 9 cm square working section at various levels of pressure and at velocities up to 100 m/sec. The turbulence intensity in the ignition zone was varied between 1% and 15%. Turbulence scales ranged from 0.2 cm to 0.8 cm. The methods employed in the generation and measurement of turbulence have been fully described elsewhere [5]. The ignition unit supplied capacitance sparks of “rectangular” form whose energy and duration could be varied independently. The optimum spark duration for minimum ignition energy was found to be independent of turbulence intensity, but to vary appreciably with pressure, velocity and mixture strength. Measurement of the energy released during a spark discharge showed that it was linearly proportional to gap width and increased slightly with increase in pressure and velocity. The energy required to effect spark ignition was reduced by the use of electrode materials having low conductivity and low boiling point, and also by locating the spark electrodes with their axes parallel to the direction of the flow.

Spontaneous ignition characteristics of gaseous hydrocarbon-air mixtures

Experiments are conducted to determine the spontaneous ignition delay times of gaseous propane, kerosine vapor, and n-heptane vapor in mixtures with air, and oxygen-enriched air, at atmospheric pressure. Over a range of equivalence ratios from 0.2 to 0.8 it is found that ignition delay times are sensibly independent of fuel concentration. However, the results indicate a strong dependence of delay times on oxygen concentration. The experimental data for kerosine and propane demonstrate very close agreement with the results obtained previously by Mullins and Lezberg respectively.

Mean drop sizes from pressure-swirl nozzles

A study of the factors governing the atomization process in pressure-swirl nozzles is presented. Extensive measurements of mean drop size are conducted on six simplex nozzles of different sizes and spray-cone angles. The liquids employed are water, diesel oil, and several blends of diesel oil with polybutene. These liquids provide a range of viscosity from 3 to 18 X 10~6 m2/s (3-18 cs), and a range of surface tension from 0.027 to 0.0734 kg/s2 (27-73.4 dyne/cm). The results are used to substantiate an equation for mean drop size derived from basic considerations of the hydrodynamic and aerodynamic processes that govern the atomization processes in pressureswirl nozzles. A very satisfactory correlation is demonstrated between predictions based on this equation and the actual measured values of mean drop size. Nomenclature A,B = constants, Eq. (16) Aa = air core area, m2 A0 = discharge orifice area, m2 Ap = swirl chamber port area, m2 Ds = swirl chamber diameter, m d0 = liquid orifice diameter, m m =mass flow rate, kg/s P = pressure, Pa AP = pressure differential, Pa Re = Reynolds number SMD = Sauter mean diameter, m t - film thickness in final orifice, m ts = liquid sheet thickness after exit from nozzle, m U = velocity, m/s We = Weber number X =Aa/A0 6 = spray cone half-angle, deg fj. = dynamic viscosity, kg/ms v = kinematic viscosity, m2/s p = density, kg/m3 a - surface tension, kg/s2 Subscripts A =air F = fuel L = liquid R = air relative to liquid

Ignition and flame quenching of flowing heterogeneous fuel-air mixtures

Abstract A model is described for the ignition of heterogeneous fuel-air mixtures that assumes that mixing rates and chemical kinetics are infinitely fast and that the sole criterion for successful ignition is an adequate concentration of fuel vapor in the ignition zone. From analysis of the relevant heat and mass-transfer processes involved, expressions for quenching distance and minimum ignition energy are derived that have general application to both quiescent and flowing mixtures. In the present investigation attention is focused on flowing mixtures, and minimum ignition energies are measured over wide ranges of pressure, velocity, turbulence intensity, equivalence ratio, mean drop size, and fuel volatility. The results obtained show very satisfactory agreement with corresponding predicted values, thus confirming the basic premise of the model.

The influence of flow parameters on minimum ignition energy and quenching distance

Experiments have been carried out on the effects of pressure, velocity, mixture strength, turbulence intensity and turbulence scale on minimum ignition energy and quenching distance. Tests were conducted at room temperature in a specially designed closed-circuit tunnel in which a fan was used to drive propane/air mixtures at subatmospheric pressures through a 9 cm square working section at velocities up to 50 m/sec. Performated located at the upstream end of the woring section provided near-isotropic turbulence in the ignition zone ranging from 1 to 22 percent in intensity, with values of turbulence scale up to 0.8 cm. Ignition was effected using capacitance sparks whose energy and duration could be varied independently. The results of these tests showed that rectangular, arc-type sparks of 60 μsec duration gave lower than previously reported values of ignition energy for both stagnant and flowing mixtures. It was found that both quenching distance and minimum ignition energy increased with (a) increase in velocity, (b) reduction in pressure, (c) departures from stoichiometric fuel/air ratio, and (d) increase in turbulence intensity. Increase in turbulence scale either raised or lowered ignition enegy, depending on the level of tubulence intensity. Equations based on an idealized model of the ignition process satisfactorily predicted all the experimental data on minimum ignition energy.

Studies on Aerated-Liquid Atomization

The atomizing performance of an aerated-liquid atomizer operating under conditions of bubbly flow is investigated. The device tested consists of a plain-orifice atomizer with provision for injecting air or gas through a porous tube into a flowing liquid stream. Measurements of mean drop size and drop-size distribution are made using a light diffraction technique. Water injection pressures are varied from 173 to 690 kPa (25 to 100 psid) and gas/liquid ratios from 0.001 to 0.05 by mass. The results obtained show that good atomization can be achieved using only small amounts of atomizing gas at injection pressures as low as 173 kPa (25 psid). Moreover, atomization quality appears to be largely independent of the size of the nozzle discharge orifice. However, the bubbly flow mechanism of atomization is limited to low gas/liquid mass ratios, the actual value depending on the injection pressure. High injection pressures permit high values of air/liquid ratio.

Flame propagation in heterogeneous mixtures of fuel drops and air

An experimental study is conducted on the influence of fuel chemistry on the flame speeds of flowing mixtures of fuel drops in air at atmospheric pressure. Air is supplied at room temperature to a 10 cm2 test section which is fitted with quartz windows to provide optical access to the flame. Sixty-four evenly spaced airblast atomizers ensure a uniform fuel distribution in the mixture entering the flame zone. Variation in mean fuel drop size is accomplished by varying the air flow rate to the airblast atomizers. Schlieren pictures of the flame provide the basic data for the measurement of flame speed, using the angle method. The fuels employed include a conventional No. 2 fuel oil, plus various blends of JP7 with stocks containing single-ring and multiring aromatics. For all fuels the measured flame speed is found to be inversely proportional to SMD above some critical size, indicating that in this range of large drop sizes evaporation rates are controlling to flame speed. The fuels exhibiting the highest flame speeds are those containing multiring aromatics. This is attributed to the higher radiative heat flux emanating from their soot-bearing flames, which enhances the rate of evaporation of the fuel drops approaching the flame front.