8. Atomization vs. Vaporization of Fuel in Micro Gas Turbines

Atomization vs. Vaporizationof Fuel in Micro Gas TurbinesDaniel Kutikov and Prof. Yeshayahou LevyFaculty of Aerospace Eng.Technion, Haifa

Impact of fuel delivery systemAtomizationFuel distributionEvaporationMixingIgnitionFlame stabilityCombustion efficiencyDifficulties in small-scale jet engine: Low ignition energy Small combustor volume Simplicity of the atomizer/vaporizer2/20

Existing models for combustion ofliquid fuel• Semi-empirical droplet sizing models for a singleatomizer• Evaporation models for droplets Need for integration of processes and models• CFD involving two-phase flow and evaporation,combustion chemistry, heat transfer Heavy numerical calculations3/20

Research method (cont.)Measurements:• Air & fuel flows• Combustor outlet temperature at 4 points• Combustion gas concentrations at 4 pointsEvaluations:• Stable operational envelope• Combustion Efficiency• Chemical efficiency6/20

Research method (cont.)Combustion efficiencies – definitions:Enthalpy rise through combustorCombustion Efficiency, η(T) =Chemical energy of liquid fuelChemical efficiency, η(Ch) = 1 -Chemical energy of fuel remain in exhaust gasesChemical energy of vaporized liquid fuelUHC CH47/20

Experiment setupAir compressorGas analyzerCO2, CO, UHC,O2, NOX8/20

Experiment setup (cont.)Gas analyzer:CO2, CO, UHC, NOX, O29/20

Experiment setup (cont.)Combustor setup:10/20

Experiment setup (cont.)Blocked combustorNozzle ringOpen combustor11/20

Experiment setup (cont.)12/20

ResultsExit temperature for different air and fuel mass flow ratesT04 =QRf + CPinT03CPg(1 + f)°Cη bf13/20

Results (cont.):combustion efficiencies η(T) for different configurationsηTfCurrentcompressor limitf14/20

Low efficiency:• low inlet temperature• low air pressure Incomplete vaporization Insufficient residence time Insufficient mixingResults (cont.)η = F(tR)Too high O2Too low CO215/20

Results (cont.)ηbDifferences mainly due tounvaporized liquid fuel16/20

Low equivalence ratio:almost completecombustionHigh equivalence ratio:incomplete combustion17/20

ConclusionsAtomizers Wider stable operational range(wider fuel supply ranges: 3 – 6g/sec vs. 0.5 – 2g/sec) Higher fuel flow rates & temperatures at the max efficiency points(4g/sec vs. 1g/sec & 1300K vs. 800K) Simple ignition (electric plug) Faster transient responseX Larger combustor volume required (for evaporation & mixing)X Higher fuel pressure requirementVaporizers Higher efficiency due to longer vaporized fuel flow path(η=0.9 vs. 0.75) (higher combustion pressure would give higher efficiencies)X Complicated ignition (additional gas ignition system required)X Slower transient response (thermal inertia & fuel evaporation time)18/20

GeneralConclusions (Cont.)• Combustion efficiency increases with pressure• Unvaporized fuel affect efficiency values, henceshould be accounted for (η(Ch) vs. η(T))• Direct retrofitting of atomizers instead of vaporizeswiden operational envelope but reducescombustion efficiencies• New designs that can account for the larger volumerequired by atomizer may benefit from itsadvantages19/20

Questions?20/20

21/20

Experiment setup (cont.)Compensation of the radiative heat losses from TC:The model was validated by comparisonof shielded and unshielded thermocouples22/20

Results (cont.)Low efficiency: high UHC valuesf23/20