On Laminar Flame Speed Extrapolation Uncertainty And Turbulent Flame Speed Scaling Of Expanding Flames

Turbulent premixed combustion is a widely adopted low-pollution combustion technology in energy and power device systems,in which the flame speeds of hydrocarbon fuels are essential for flame stabilization and combustion stability.In this dissertation,experimental studies of expanding flames in a constant-pressure apparatus,together with theoretical and numerical analyses,have been performed to investigate the flame propagation properties of representative fuels and reduce their extrapolation uncertainties of laminar flame speeds.The flame propagation properties of low-carbon emission ammonia/methane fuel blends were studied with flame chemistry being analyzed.Furthermore,turbulent flame speeds of ethylene/air and ammonia/methane/air were experimentally measured under different levels of turbulence intensities to study the scaling laws of turbulent flame speeds in different combustion regimes.First,for thick and unstable laminar flames with a narrow range of applicable data,modified extrapolation models accounting for finite flame structure were derived to improve the accuracy of laminar flame speed measurement.The modified models show improved accuracy in terms of laminar flame speed and Markstein length extrapolations.Subsequently,for stable laminar flames with slow burning-velocities,a modified laminar flame speed measurement method was proposed which accounts for the flame deformation due to buoyancy.The modified method shows robustness and improved accuracy for the flame speed extrapolation within various Markstein lengths,extrapolation data ranges,and flame deformation extents.Furthermore,the modified method was experimentally applied to determine the laminar flame speeds of the low carbon emission ammonia/methane fuel blends.A linear correlation between the flame speed and methane content in the fuel blends was identified.The flammability limits were also experimentally measured for these mixtures.Results show that both the upper and lower flammability limits vary nearly linearly with the methane volume fraction from 0.1 to 0.7(0.1 to 0.9)at 1.0(5.0)atm.The flame chemistry of ammonia/methane/air mixtures was also studied.It is found that the(H,OH)and(H,OH,CH₃)radicals play important roles in the propagation of lean and rich mixtures,respectively.Furthermore,linear correlations between the laminar flame speed and the maximum value of the radical concentration(H+OH)/([H][O₂]-[H][CH₃])for fuel-lean/rich flames were observed for the ammonia/methane/air mixtures with the methane volume fraction of fuel blends ranging from 30%to 70%at elevated pressures(1.0?15.0atm)and various initial temperatures(298?448 K).Finally,as for turbulent premixed flames,the particle image velocimetry(PIV)system was used to calibrate the cold flow field in the constant-pressure expanding spherical flame apparatus.Results show that the turbulent flow field is approximately homogeneous and isotropic,and the turbulent fluctuation velocity varies from 0.70 m/s to 1.77 m/s,which has a linear relationship with the fan speed.The integral length scale is about 10 mm,which hardly changes with the fan speed.Then,ethylene/air and ammonia/methane/air turbulent flame speeds were measured in the turbulent Reynolds number (?) range of 29?2706 and 69?2560,respectively,where a new turbulent flame speed scaling correlation applicable in both the corrugated flame and the thin reaction zone regimes was proposed.The new model accounts for the geometry and history effects of the interaction between turbulence and flame propagation.For ethylene/air and ammonia/methane/air turbulent flame speeds,the mean absolute percentage error(MAPE)between the experimental and predictive values is 7.8%and8.5%,respectively,which shows higher accuracy compared to other existing scaling models.