INFLUENCE OF STRAIN-RATE AND FUEL DILUTION ON LAMINAR NONPREMIXED HYDROGEN-AIR FLAME STRUCTURE - AN EXPERIMENTAL INVESTIGATION

Temperature and major species concentration profiles are obtained in H -2/N-2 versus air-opposed flow diffusion flames of various fuel jet di lutions and strain rates. Three fuel jet compositions are examined: mo le fractions of 21% H-2 and 79% N-2, an equimolar mixture of H-2 and N -2, and undiluted H-2. A Raman imaging system, capable of providing ti me-averaged linewise measurements of high precision (2%) and high spat ial resolution (160 mu m), is used to obtain the scalar measurements. Laser Doppler velocimetry is used to measure the oxidizer-side axial v elocity gradient, K, ranging up to 3800 s(-1). Mixture fraction and sc alar dissipation rate profiles are derived from the major species conc entration and temperature profiles and clearly show effects of differe ntial diffusion, with deviations among the three elemental species mix ture fraction profiles for all three fuel jet conditions examined. Fla me thickness is experimentally shown to vary linearly with K--1/2. Dif ferences in boundary conditions allow only a qualitative comparison be tween measured peak temperatures and others' numerical results. Howeve r, the comparison generally provides good agreement, especially for th e highly diluted fuel jet case (+/-40 K), though for undiluted fuel je t flames at low values of K the experimental data are consistently hig her (90 to 280 K) than numerical predictions. Also, for the low K undi luted fuel jet flames, the scalar dissipation rate versus mixture frac tion profiles are nonmonotonic near the stoichiometric mixture fractio n and have values there that are over an order of magnitude less than their more diluted counterparts, due to the influence of fuel jet dilu tion upon the relative positions of the stoichiometric location and th e stagnation plane.