Laseroptical investigation of highly preheated combustion with strong exhaust gas recirculation

Laseroptical measurements using OH laser-induced predissociative fluorescence and Rayleigh thermometry were carried out in a combustion chamber with highly preheated air and strong exhaust gas recirculation. Under these conditions, combustion takes place without any luminousity and is referred to as flameless oxidation. It is realized by a configuration with exhaust gas recirculation that is entrained by air nozzles operating as a jet pump. This leads to low temperatures even at significant air preheating so that thermal NO formation is largely suppressed. In the optically accessible combustion chamber, the reaction zones of flameless oxidation were visualized, and burning and mixing regions were identified. The instantaneous flame structure is compared to measurements in turbulent premixed flames. Only disconnected reaction zones due to strong flame stretch resulting from high flow velocities were observed. In the flowfield ahead of the burner, fresh gas was mixed with the hot exhaust gases. The fresh gas was diluted and heated up simultaneously. Flameless oxidation could only be observed if the temperature of the unburnt mixture was above 950 K. The temperature rise between the unburnt and burnt side of the reaction zones varied between 200 and 400 K, depending on the dilution of the unburnt mixture with exhaust gases. This temperature rise was compared with a theoretical analysis of a temperature equation for the well-stirred reactor. It can be shown that flameless oxidation takes place in the well-stirred reactor regime. Maximum local temperatures below 1650 K were measured. The OH concentration in the combustion zones of flameless oxidation is lower than in non-preheated undiluted turbulent premixed flames.