A STUDY OF LAMINAR AND TURBULENT BURNING VELOCITIES

Abstract

An apparatus for measuring laminar and turbulent burning velocities in constant volume spherical and cylindrical combustion chambers was designed and fabricated. Simultaneous records of flame growth and pressure rise with time were obta-ined for initially quiescent as well as turbulent methane-air mixtures ignited centrally in a spherical combustion vessel. For quiescent mixtures the records were obtained with varying initial pressures from 0.5 to 4.0 atm, equivalence ratio from 0.8 to 1.2 at ambient temperatures. An analytical method is • suggested to compute the spatial speeds and rates of pressure rise from the rb-t and P-t records. A computerized method to compute equilibrium state of the mixture following constant pressure or constant volume combustion assuming the products of combustion to con-sist of 18 chemical species has been presented. Laminar burning velocities have been computed by six different equations, mainly of Rallis and Rallis and Tremeer's, using P-t and rb-t records simultaneously or individually. A comparison of the laminar burning velocity values obtained by these equations is presented. A modified form of the Rallis and Tremeer equation is proposed which uses P-t record alone and yield the laminar burning veloci-ties comparable with those given by their own equations. Various correlations showing the dependence of laminar burning velocity on mixture composition, pressure and temperature have been presented. An equation for predicting the laminar burning velocity and its dependence on all the three parameters above has been presented for the first time and is as follows: For cl); 1.0 SL = (-417.9008+1286.8339/Y-1195.7535/Y.2+360.0358/93-15XYaag 1K T 1.68/ fy (3i0) For y 1.0 SL = - (-417.9008+1286.8339/Y-1195.7535/Y2+360.0358/Y3-15xy loge) x... 1.68 Arg (Tu/300) The above equation is' for methane-air mixtures for pressures 1-8 atm, equivalence ratios 0.8 to 1.2 and tem-peratures 300-600-K. The values obtained by this equation compares with present experimental results within 9.0 per cent and within 4.5 to 34 per_cent with the experimental results of previous investigators; these values also compare well with those predicted by the theoretical equation of Semenov and the approximate equation given by Zeldovich Frank-Kamene-tsky provided that these equations are modified to account for the pressure dependence by multiplying them by a factor p0.7265 Turbulent burning velocities inside a closed vessel have been computed with the use of spatial speeds ratio of initially turbulent mixture to initially quiescent mixture,assuming the same expansion ratio for both at the same pressure. The two records have been obtained for one atmosphere and with varying equivalence ratio from 0.8 to 1.2. The dependence of turbulent burning velocity on intensity of turbulence has been studied qualitatively. Turbulent burning velocity increases with increase in turbulence intensity. Turbulent burning velocity of butane-air mixtures have been determined by nozzle burner method with varying intensity of turbulence and approach stream velocity. Various screens have been employed to generate turbulence. The turbu-lence intensity, as measured with hot-wire anemometer,appears to decrease with increase in flow velocity. Various turbulence criteria have been used to correlate the turbulent burning velocity but the turbulent Reynolds number criterion seems to be indicative of a correlation independent of fuel, fuel-air ratio and type and size of the burner used. The correlation obtained with turbulent Reynolds number criterion-is ST = 1 - 0.94544x10- 0.36881x10-4R2A SL