D. Geyer

Simultaneous PIV/PH-PLIF, Rayleigh thermometry/OH-PLIF and stereo PIV measurements in a low-swirl-flame

The diagnostic techniques for simultaneous velocity and relative OH distribution, simultaneous temperature and relative OH distribution, and three component velocity mapping are described. The data extracted from the measurements include statistical moments for inflow fluid dynamics, temperature, conditional velocities, and scalar flux. The work is a first step in the development of a detailed large eddy simulation (LES) validation database for a turbulent, premixed flame. The low-swirl burner used in this investigation has many of the necessary attributes for LES model validation, including a simplified interior geometry; it operates well into the thin reaction zone for turbulent premixed flames, and flame stabilization is based entirely on the flow field and not on hardware or pilot flames.

A detailed investigation of the stabilization point of lifted turbulent diffusion flames

The stabilization point of lifted turbulent hydrogen diffusion flames is investigated by Raman/Rayleigh/laser-induced fluorescence (LIF) spectroscopy. The stabilization point is determined from simultaneously taken planar laser-induced fluorescence (PLIF) images. It is shown from averaged statistics that lift-off height has negligible influence on the flame length and the far region of the jet. Reactants, premixed downstream of the stabilization point, are rapidly consumed over a very short distance. A new method to generate stabilization point conditioned species and temperature data is proposed and applied to the data. With this method it is possible to describe the surrounding of an observer located at the instantaneous stabilization point. The data are presented by constant contour plots of mixture fraction, species, and temperature in a stabilization point fixed coordinate system. The data obtained by this method are used to assess previously proposed theories on the behavior of lifted turbulent diffusion flames. Experimental findings presented are inconsistent with predictions by the concept of premixed flame stabilization as well as with the flamelet concept. The insensitivity of the spatial location of the stabilization point to the variation of the stoichiometric mixture fraction of the fuels investigated suggests a stabilization mechanism through large-scale turbulent structures. Large-scale structures also explain the existence of products upstream of the stabilization point. The conclusion of this analysis is that large-scale turbulent structures play a dominant role in the stabilization mechanisms of the lifted turbulent diffusion flames, subject to this study.

Investigation of flame structures in turbulent partially premixed counter-flow flames using planar laser-induced fluorescence

This paper reports on investigations of structural turbulent flame characteristics using planar laser-induced fluorescence (PLIF) of hydroxyl (OH) radicals. Turbulent, counter-flowing methane/air flames with different equivalence ratios spanning from premixed to partially premixed are investigated. Additionally, Reynolds numbers of these configurations are varied and reach from stable to extinguishing flames. The main objective of this study is to extract topological quantities from spatial OH distributions such as area and local thickness of the OH layer, flame brush, or contours approximating the profile of the stoichiometric mixture fraction. The post-processing procedure applied to extract these quantities from single-shot OH PLIF images is described. It is shown that OH areas shrink with increasing Reynolds number whereas lengths of stoichiometric contour lines are nearly unaffected. This indicates that flame extinction is driven by large scale eddies rather than small scale wrinkling. In addition to also performed quantitative laser Doppler anemometry and Raman/Rayleigh measurements of the same flames, topological studies are helpful in viewing complex turbulent-chemistry interaction. Latter data are essential for validation purposes of large eddy simulations (LES) that predict transient flame movement and thereby spatial OH distributions.

Advanced Laser Diagnostics for Understanding Turbulent Combustion and Model Validation

This contribution is not an original publication but a report of cumulative work that was carried out within the framework of SFB 568. The work was published in different archival journals and figures and text passages have been taken from different journal articles as indicated by the references. The aim of this report is to present experiments in projects B1 and B3 for improving our understanding in turbulent combustion with a focus of turbulent flow and scalar fields as well as their mutual interactions. The report is restricted to generic gaseous turbulent flames that feature different characteristics important to practical applications. The methods presented here are feasible to study boundary conditions, flow and scalar fields and are based all on interactions between laser light and matter. Following a brief introduction, two target flames are discussed in Sect. 4.2. Sections 4.3 and 4.4 exemplify flow and scalar measurements. Section 4.5 discusses combined scalar/flow measurements that can significantly improve our understanding of turbulence-chemistry interactions. In Sect. 4.6 new developments based on high-repetition-rate imaging are highlighted. These diagnostics complement methods at low repetition rate commonly used to generate an understanding by statistical moments and probability density functions. High repetition rate imaging techniques presently are an emerging field. Although the most recent developments achieved in the funding period of the Collaborative Research Center are included to this report, near-future progress in this field will lead to even more interesting insights into combustion phenomena.