Simultaneous images of the CH and OH reaction zones are reported for Intensely Wrinkled nonpremixed flames, to determine whether reaction zones retain their thin laminar flamelet structure or become distributed reaction zones. Intensely Wrinkled Flames (IWFs) were achieved by using a special burner with large coflow air velocities to obtain a normalized turbulence intensity of 3.6, which is 10 times greater than the turbulence intensity within jet flames. The images were used to measure profiles of the flame surface density and the average CH layer thickness; it is argued that these parameters are the ones that should be used to assess new large eddy simulations (LESs), rather than insensitive parameters such as mean concentrations. In the regime of IWFs, the CH reaction zones remained as thin as those measured in laminar jet flames (i.e., less than 1 mm thick) and had the appearance of flamelets. These thin reaction zones were extinguished before they became thickened by intense turbulence, which provides experimental evidence to support laminar flamelet modeling concepts. Shredded flames occurred, within which the reaction zones were short, discontinuous segments, and the degree of flame wrinkling was significantly larger than in jet flames. Shredded flames have not been observed previously. There is no evidence of small-scale wrinkling of the reaction zones at scales less than half the integral scale. The images showed where the instantaneous stoichiometric contour is located, since it exists at the boundary between the CH and OH layers. Flame surface densities were typically 0.3 mm to the negative 1 power.

Explore a thorough overview of the current knowledge, developments and outstanding challenges in turbulent combustion and application.

A methodology for investigating premixed turbulent flames propagating in intense isotropic turbulence has been developed. The burner uses a turbulence generator developed by Videto and Santavicca and the flame is stabilized by weak-swirl generated by air injectors. This set-up produces stable premixed turbulent flames under a wide range of mixture conditions and turbulence intensities. The experiments are designed to investigate systematically the changes in flame structures for conditions which can be classified as wrinkled laminar flames, corrugated flames and flames with distributed reaction zones. Laser Doppler anemometry and Rayleigh scattering techniques are used to determine the turbulence and scalar statistics. In the intense turbulence, the flames are found to produce very little changes in the mean and rams velocities. Their flame speed increase linearly with turbulence intensity as for wrinkled laminar flames. The Rayleigh scattering pdfs for flames within the distributed reaction zone regime are distinctly bimodal. The probabilities of the reacting states (i.e. contributions from within the reaction zone) is not higher than those of wrinkled laminar flame. These results show that there is no drastic changes in flame structures at Karlovitz number close to unity. This suggest that the Klimov-Williams criterion under-predicts the resilience of wrinkled flamelets to intense turbulence.

A work on turbulent premixed combustion is important because of increased concern about the environmental impact of combustion and the search for new combustion concepts and technologies. An improved understanding of lean fuel turbulent premixed flames must play a central role in the fundamental science of these new concepts. Lean premixed flames have the potential to offer ultra-low emission levels, but they are notoriously susceptible to combustion oscillations. Thus, sophisticated control measures are inevitably required. The editors' intent is to set out the modeling aspects in the field of turbulent premixed combustion. Good progress has been made on this topic, and this cohesive volume contains contributions from international experts on various subtopics of the lean premixed flame problem.

The combustion of fossil fuels remains a key technology for the foreseeable future. It is therefore important that we understand the mechanisms of combustion and, in particular, the role of turbulence within this process. Combustion always takes place within a turbulent flow field for two reasons: turbulence increases the mixing process and enhances combustion, but at the same time combustion releases heat which generates flow instability through buoyancy, thus enhancing the transition to turbulence. The four chapters of this book present a thorough introduction to the field of turbulent combustion. After an overview of modeling approaches, the three remaining chapters consider the three distinct cases of premixed, non-premixed, and partially premixed combustion, respectively. This book will be of value to researchers and students of engineering and applied mathematics by demonstrating the current theories of turbulent combustion within a unified presentation of the field.