Laser-induced Fluorescence Imaging Of OH Research Articles

Heat release zones in turbulent, moderately dense spray flames of ethanol and biodiesel

This paper extends the application of laser-induced fluorescence imaging of OH & CH2O (LIF-OH-CH2O) to provide a measure of heat release zones in turbulent, moderately dense spray flames of ethanol and biodiesel fuels. A custom filter is used to remove interference from stimulated Raman scattering arising from the liquid fragments. Mie scattering is measured jointly on a separate camera to co-locate the fluid fragments with respect to the reaction zones. It is found that while the overall spray flame structure is similar to that of a turbulent gaseous diffusion flame, additional complexities arise due to the continuous release of fresh fuel vapor from the evaporating fragments. Structures referred to as burning rings of different sizes are observed, and these are ignited by interfacing with a hot edge before they grow, propagate, and burnout. Statistics on the occurrence of these rings, with and without clouds of liquid fragments within them, are presented. These findings are common to both ethanol and biodiesel, although measurements in the latter were affected by strong fluorescence interference from the excited components in the biodiesel blend.

Effects of heat release and imposed bulk strain on alignment of strain rate eigenvectors in turbulent premixed flames

The impact of combustion heat release and bulk compressive strain on local alignment between the flame front normal and strain rate eigenvectors (principal strain rates) are investigated using simultaneous laser-induced fluorescence imaging of OH and tomographic particle image velocimetry in turbulent premixed CH4/O2/N2 counterflow flames with Karlovitz numbers (Ka) of 1.3 and 2.7. The bulk strain rate imposed by the counterflow introduces a preferential alignment of the most compressive principal strain rate, s3, with the flame normal, n. Dilatation induced by heat release acts as a competing mechanism that promotes the alignment of the most extensive principal strain rate, s1, with n. In the counterflow flames, the preferential s3-n alignment prevails and remains dominant across the entire flame. This alignment stands in stark contrast to observations from previous studies in turbulent Bunsen flames or flames in isotropic turbulence, indicating the significance of bulk strain rate in determining local strain-flame alignment. The effects of increasing turbulence intensity on strain rate-flame front alignment are twofold; on the one hand, turbulence diminishes the s3-n preferential alignment that is associated with the bulk strain field by increasing flame surface wrinkling and reducing the tendency of the flame front normal and s3-eigenvectors to align with the axis of the counterflow. On the other hand, turbulence reduces the impact of heat release and enhances preferential s3-n alignment approximately 1 mm ahead of the flame front, reflecting the characteristic alignment of compressive strain and scalar gradients in turbulent non-reacting flows. The effects of bulk strain are also observed in strain rate alignment statistics based on the fluctuating velocity fields, although the impact is less pronounced than for the statistics based on the full velocity fields.As a result of this complex interplay between heat release, turbulence, and bulk strain rate, the flame-tangential strain rate is on average extensive, and the flame-normal strain rate is dominantly compressive except for an approximately 0.8 mm wide region near the flame front where it is extensive due to dilatation. The compressive bulk strain in the counterflow was also shown to compress the length scales over which the strain rate and its alignment are affected by flame heat release. The present finding is important for developing turbulent flame models to accommodate the effect of bulk strain rate that is inherently associated with practical burner geometries, and the length scale dependence of the bulk strain effect could be a consideration for determining the cutoff scale in the context of large-eddy simulations.

Simultaneous multispectral imaging of flame species using Frequency Recognition Algorithm for Multiple Exposures (FRAME)

Imaging the interaction between different combustion species under turbulent flame conditions requires methods that both are extremely fast and provide means to spectrally separate different signals. Current experimental solutions to achieve this often rely on using several cameras that are time-gated and/or equipped with different spectral filters. In this work we explore a technique called Frequency Recognition Algorithm for Multiple Exposures (FRAME) as an alternative solution for instantaneous multispectral imaging of flame species. The method is based on exciting different species with different spatial “codes” and to separate each signal component using a spatial frequency-sensitive lock-in algorithm. This methodology permits the signal from several different species to be recorded at the exact same time with a single camera. Furthermore, since the signals are recognized based on the superimposed spatial codes, there is no need for spectral separation prior to detection. The entire fluorescence envelope from each species can thus, in principle, be detected. In the current work, we present simultaneous planar laser-induced fluorescence imaging of OH and CH2O in a turbulent dimethyl ether (DME)/air flame.

Planar laser-induced fluorescence imaging of OH in a supersonic combustor fueled with ethylene and methane

A supersonic combustor was experimentally investigated using both conventional instrumentation and laser-based diagnostics. Planar laser-induced fluorescence (PLIF) imaging of OH was used in the main section of the combustor to examine flameholding and flame propagation during a series of evaluations at conditions simulating Mach-5.5 flight. Parameters of interest in this study included the angle of the primary fuel injectors, the distribution of fuel throughout the combustor, and the fuel composition. Changes in fuel-injection angle were expected to influence the mixing and combustion processes, and therefore combustor operation. Fuel-distribution variations were expected to modify the flame propagation between flameholding regions. Finally, ethylene and methane were used to examine the suitability of the flameholder designs over a wide range of fuel reactivity. Results suggest that the combustor provides relatively robust flameholding regardless of the fuel used and good flame propagation as long as the fuel distribution provides favorable conditions in the flameholding regions. In addition, the results show that the primary injectors can be useful in controlling certain aspects of combustor operability.

Planar laser-induced-fluorescence imaging measurements of OH and hydrocarbon fuel fragments in high-pressure spray-flame combustion

Planar laser-induced fluorescence images of OH have been obtained in liquid-fueled spray flames burning heptane, ethanol, and methanol over a range of pressures from 0.1 to 1.0 MPa. In addition to the OH fluorescence, a nonresonant fluorescence interference that increased rapidly with pressure was detected. Examination of the spectrum of this interference indicates that it arises from hydrocarbon fuel-fragment species in the fuel-rich zones of the flame. The pressure dependence of the fluorescence signal is examined in both steady-state and time-dependent analyses, and a model for evaluation of pressure effects and quenching variations in quantitative imaging measurements in nonpremixed flame environments is presented. The results indicate that increased combustor pressure results in a rapid rise of the volume fraction of hydrocarbon fragments and a decrease in the OH volume fraction.

Experiments on the structure of an annular compressible reacting shear layer

An axisymmetric burner for the study of compressible reacting shear layers is described, and the ranges of its operating parameters are analyzed. This burner has a central supersonic freejet of resistively heated air, surrounded by a low-speed co-flow of fuel-rich H 2 -air combustion products. Planar laser-induced fluorescence imaging of OH is used to visualize the structure of the annular reacting shear layer in the near field of the jet. Images from two cases with convective Mach numbers of 0.11 and 0.41 are presented and discussed in terms of previously reported results for nonreacting compressible shear layers

Spatially resolved laser-induced fluorescence imaging of OH produced in the oxidation of hydrogen on platinum

The laser-induced fluorescence technique has been used to study OH radicals desorbed from a polycrystalline Pt foil in the H 2 + O 2 reaction. The experiments were performed under steady-state reaction conditions in the pressure range 10–100 mTorr at a catalyst temperature of 1300 K. Concentration profiles of OH radicals as a function of the distance from the catalyst were measured by imaging the fluorescence light with a diode array detector. The measurements in this pressure range support our earlier conclusion that the detected OH radicals are produced by catalytic reactions on the surface (S. Ljungström et al., Surf. Sci. 216, 63 (1989)). No evidence was found for the catalytic promotion of gas-phase reactions, as has been observed in studies of similar reactions with catalytically stabilized thermal burners at higher pressures. Analysis of the measured concentration profiles shows that the desorbed OH radicals can best be descricbed by a cos θ distribution. The effects of gas-phase collisional processes have been investigated by evaluation of the local rotational distribution of the desorbed OH radicals.