Application Of Laser Diagnostic Techniques In Hydrocarbon Combustion Measurements At Atmosphere And Elevated Pressures

The products of fossil fuel gasification have great variation in composition because of the diversity of source materials and their composition as well as variations in the gasification process itself.Generally,the main components of gasification gas are combustible gases like CH4,C₂H₆,H₂,CO,and inert gases like N₂ and CO₂.Combustion devices used for gasification gas must be adaptable to a wide variation in combustion characteristics,changing proportion of combustible components,and low calorific value caused by the presence of inert gases.This has become a prominent challenge for clean fossil fuel utilization.Continuing research on hydrocarbon combustion characteristics is necessary for the further improvement of new equipment designs and combustion technologies,that are based on an analytical understanding ofrelevant combustion phenomena.This work employed non-intrusive,high resolution advanced laser diagnostic technologies to quantitatively investigate and visualize combustion characteristics of hydrocarbon fuels in combustion environments which directly relevant to industrial applications,i.e.,high temperature,elevated pressure and high Reynolds number.First,planar laser-induced fluorescence（PLIF）was used to visualize CH radical distribution in laminar conical flames operating in a custom-built 50 atm high pressure vessel.The effects of environment pressure（1-7 atm）,equivalence ratio（0.9-1.4）,fuel composition and gas supply speed on CH formation and distribution were analyzed.The CH signal was used to locate the conical flame front,and the elevated pressure laminar flame speed was calculated from the ratio of the unburned gas flow rate and the flame surface area defined by the rotational projection of the flame front.Results demonstrated that a gas supply speed of 5 times the laminar flame speed was necessary for high quality measurements.The flame speeds of CH4/H₂/C₂H₆-air mixtures were measured at various system pressures and equivalence ratios.Empirical formulas were defined for CH4-air flames and low H₂ proportion syngas-air flames,respectively.Laminar flame speeds calculated by the empirical formulas were within 3.6%of the flame speed measurements made using spherically expanding flames.Second,Laser-induced saturated fluorescence（LSF）was applied,to measure the NO mole fraction in the flue gas of a hydrocarbon flame,which is of great value for industrial applications in developing reduction strategies.Calibrated measurements were used to validate four common chemical kinetic mechanisms used for computer modeling of NOx production.The fuels used were based on typical bituminous coal-derived syngas compositions;investigations were made of NOx production versus variation in CH4 ratio,H₂-CO ratio,inert gases ratio and equivalence ratio.CH4 strongly promoted NO formation,leading to a bimodal distribution with maxima atφ=1.0 and 1.3 in the CH4-air flames.H₂-CO flames produced a single peak near stoichiometry.H₂ has lower NO production than CO on a per mole basis,but much higher emission on mass basis.Addition of either N₂ or CO₂ inert gases reduced NO emission,with CO₂ having greater inhibition than N₂.All mechanisms gave acceptable predictions at fuel lean and stoichiometric cases,but deviated excessively from experimental data at fuel rich conditions.Additionally,all mechanisms were weak at predicting syngas fuel cases and prompt NOx production;further measurements and mechanism optimizations are needed before these conditions can be accurately predicted by computer models.The third activity was the use PLIF of OH and CH radicals to visualize turbulent combustion.Experimental conditions ranged from laminar flow to weak turbulence,and the fuel composition was varied at each flow condition.CH distribution broadening was captured in transitional Reynolds number conditions,confirming Lipatnikov’s speculation.CH-PLIF measurements suggest a correspondence of stretch rate to CH fluorescence intensity,raising the possibility of calculating a 2D index of stretch from the PLIF measurement.Addition of H₂ moves the OH distribution upstream and CH distribution downstream.Finally,the feasibility of applying PLIF in elevated pressure turbulent flames was assessed.CH₂O,NO,OH and CH radical fluorescence each decreased with increased pressure,necessitating the use of higher power lasers in these investigations.