Toshiaki Kitagawa

Turbulent burning velocity, burned gas distribution, and associated flame surface definition

Abstract Experimental studies of premixed, turbulent, gaseous explosion flames in a fan-stirred bomb are reported. The turbulence was uniform and isotropic, while changes in the rms turbulent velocity were achieved by changes in the speed of the fans. Central spark ignitions created mean spherical flame propagation. The spatial distributions of burned and unburned gases during the propagation were measured from the Mie scattering of tobacco smoke in a thin planar laser sheet. The plane was located just in front of the central spark gap and was generated by a copper vapor laser operating at a pulse rate of 4.5 kHz. High-speed schlieren images also were captured simultaneously. The distributions of the proportions of burned and unburned gases around circumferences were found for all radii at all stages of the explosion, and mean values of these proportions were derived as a function of the mean flame radius. The flame brush thickness increased with flame radius. The way the turbulent burning velocity is defined depends on the chosen associated flame radius. Various definitions are scrutinized and different flame radii presented, along with the associated turbulent burning velocities. Engulfment and mass turbulent burning velocities are compared. It is shown how the latter might conveniently be obtained from schlieren cine images. In a given explosion, the burning velocity increased with time and radius, as a consequence of the continual broadening of the effective spectrum of turbulence to which the flame was subjected. A decrease in the Markstein number of the mixture increased the turbulent burning velocity.

Study of Thermo-Diffusive Effects on Iso-Octane/Air Flames at Fixed Turbulence Karlovitz Number

Spherically propagating laminar and turbulent flames were studied using iso-octane / air mixtures with and without dilution. The main purpose of this study is to clarify the influence of thermo-diffusive effects on the turbulent flames. In order to examine the thermo-diffusive effects solely by separating them from the effects of flame stretch, turbulent burning velocities were compared at constant flame stretch factors. The mean flame stretch factor acting on turbulent flame front may be represented by the turbulence Karlovitz number. Thus, turbulent explosions were carried out at fixed turbulence Karlovitz numbers. The ratio of turbulent burning velocity to unstretched laminar burning velocity increased with the equivalence ratio for non-diluted mixtures at fixed turbulence Karlovitz numbers. And this ratio for CO2 diluted mixtures was larger than N2 diluted mixtures. The Markstein number that denotes the sensitivity of the flame to thermo-diffusive effects depends on the equivalence ratio and diluents of the mixture. The ratio of turbulent burning velocity to unstretched laminar one increased with decreasing Markstein number. Especially, it changed stepwise around Markstein number of zero. However, the burning velocity ratios did not increase with increasing mixture pressure although the Markstein number decreased with pressure.© 2011 ASME

Effects of Diluent Gases on Combustion of Coal Gasification Gas: Comparison Between N2 and CO2 Dilutions

Combustion properties of coal gasification gas with CO2 dilution were investigated for a newly proposed IGCC power generation system with CO2 capture. In this system, the gasification gas was burned under high CO2 concentration atmosphere. In order to clarify the properties of the flames under such atmosphere, the laminar and turbulent burning velocities were investigated for outwardly propagating stoichiometric H2 /O2 /CO2 and H2 /O2 /N2 flames under the two conditions, 1: the same amount of diluent of CO2 or N2 , 2: the constant flame temperature irrespective of diluents. Under the condition1 of the same amount of diluents, the unstretched laminar burning velocities, ul of CO2 diluted flames were smaller than those of N2 diluted flames. The ratios of the turbulent burning velocity at the flame radius 30mm, utn(30mm) to ul of the CO2 diluted flames were found to be larger than those of N2 diluted flames. Under the condition2 of the constant flame temperature, it was set to 1300, 1500, 1700, and 2135 degrees Celsius. At the flame temperatures except for 2135 degrees Celsius, ul of CO2 diluted flames were slightly smaller than those of N2 diluted flames. The ratios, utn(30mm) / ul of CO2 diluted flames were larger than those of N2 diluted flames. Increase in the turbulence Karlovitz number and decrease in the Markstein number by the CO2 dilution might cause the increase in utn(30mm) / ul of CO2 diluted flames in both conditions.Copyright

Combustion characteristics of transient hydrogen jet and flame propagation into lean region

Combustion characteristics of the transient hydrogen jet were investigated by the experiments. Bulk quenching was found to occur in the periphery of the jet due to the low fuel concentration. Then the flame propagation into the lean region in the jet was investigated. Hydrogen was injected into the lean hydrogen-air mixture charge around the lower flammability limit of the premixed mixture. The flame generated in the fuel jet propagated into the lean mixture charge region as if it had as “inertia”. Then it was quenched in the premixed mixture charge. The flame seems to be supported by the burned region from its back until the decay of this support. In order to investigate this “flame inertia”, the interactions between the rich flame and the lean mixture were investigated by the numerical simulations of counterflow premixed flame. Reaction rate of the fuel in the lean mixture depended on the opposing gas temperature.