EXPERIMENTAL INVESTIGATION OF TURBULENT PLANAR INHOMOGENEOUS JET FLAMES USING RAYLEIGH SCATTERING TECHNIQUE

Abstract

Nowadays, modern combustion systems are facing challenges in terms of operating conditions, stability and emissions. The mode of turbulent inhomogeneous combustion covers a wide range of inhomogeneity between the fully premixed and non-premixed combustion mode and thus a wide operation range can be guaranteed. In addition, the combustion emissions can be easily controlled in inhomogeneous combustion without any further degradation in the performance. Recent studies in literature discussed that the combustion stability and the produced emissions are strongly related to the mixing field of the developed flames owing to the fact that the mixing field represents the inlet boundary conditions for the flames. Accordingly, understanding the mixing field of turbulent flames and its impact on the flame stability is an important and crucial concept in combustion systems design and operation in order to achieve efficient and intense combustion, minimum pollutant emissions and reliable operation. Accordingly, the mixing field is investigated in a concentric flow slot burner (CFSB), inspired by the Wolfhard-Parker slot burner and developed by Mansour, to create turbulent planar inhomogeneous flames. The mixing field measurements are conducted in non-reacting jets at the burner exit using highly resolved Rayleigh scattering technique with more than 290 mixing cases of turbulent flames covering wide ranges of mixing level, Reynolds number, equivalence ratio, fuel type and air to fuel velocity ratio. The current burner’s design can control precisely the level of mixture inhomogeneity and thus be able to create a wide range of flames between fully premixed and non-premixed. The mixing field is analyzed statistically by calculating the probability density function of the mixture fraction and its two-dimensional gradient in order to classify the field in the partially premixed regime diagram. In addition, the stability characteristics of the flames generated in this burner are measured at different velocities of co-flow air to couple the mixing field data at different mixing levels with the flame stability. The data show that the structure of the mixing field is greatly enhanced with increasing the normalized mixing length and the velocity ratio between the air and the fuel since an obvious transformation from bimodal structure of the PDF of the mixture fraction to single peak is achieved with increasing the mixture homogeneity. On the other hand, increasing the Reynolds number within the turbulent regime has insignificant effect on the mixing field. The trend of data, resulted from the statistical analysis, in the range of mixture fraction versus the normalized mixing length is used to correlate the data and couple the stability characteristics and the flame structure with the structure of the mixing field. Besides, several trends can be generated in order to discuss the flame stability, burner design, flame structure, and pollution control based on the level and nature of the mixing. Regarding the stability characteristics, a bell shape trend with optimum peak point is observed for the blow out Reynolds number at constant equivalence ratio. The blowout limit is increased with increasing the equivalence ratio. Besides, the data show higher stability in case of turbulent flames with inhomogeneous mixture compared to the fully premixed and non-premixed flames. Furthermore, the blow out limit is significantly increased with increasing the velocity of the co-flow air. The optimum stability of these flames with compositionally inhomogeneous mixtures is achieved at certain mixing level which is located in the regime diagram between the minimum and maximum flammability limits, where the flames are expected to be more stable with probably more stratified mixtures. This indicates