Douglas A. Greenhalgh

The development and structure of flame instabilities and cellularity at low Markstein numbers in explosions

Abstract Flame instabilities and the formation of cellular structures during spherical gaseous explosions have been studied experimentally using natural light and schlieren high-speed cine photography, as well as single-shot planar laser-induced fluorescence (PLIF) from the OH radical. High-pressure, rich-hydrocarbon and lean-hydrogen flames at low Markstein numbers were employed. Ranges of unstable wavelengths have been identified as a function of Markstein and Peclet numbers. The cine photography enables the dynamics of cell growth and fissioning to be studied and qualitatively interpreted, in terms of flame stretch rates and thermodiffusion. The PLIF technique enabled unstable wavelengths to be measured and flame fracture at negatively stretched cracks to be observed. A cascade of unstable wavelengths terminates in a cellular structure. This structure appears at a second critical Peclet number. The smaller cells are continually destabilizing and restabilizing. As they increase in size, the localized stretch rate on the cell surface decreases and the cell becomes unstable. It restabilizes by fissioning into smaller cells with higher localized stretch rates. The cells are bounded by cracks in regions of negative curvature. At sufficiently small Markstein numbers the cracks are fractured. The results are interpreted within the theoretical framework of the stability analysis of Bechtold and Matalon.

Experimental and numerical studies of a triple flame

The structure of a laminar triple flame is investigated both experimentally and numerically. The distribution of OH radicals in the flame is mapped using Laser Induced Fluorescence. The structure of the velocity field is mapped using Particle Imaging Velocimetry. To obtain detailed information on both the fields of velocity and scalars, the flame is numerically simulated. In the simulations, gas expansion is taken into account and the full Navier-Stokes equations are solved. Numerical results obtained with global one-step chemistry and, alternatively, with a detailed chemical kinetic mechanism are presented. The experimental findings are compared with the numerical results.

Wrinkling and curvature of laminar and turbulent premixed flames

Abstract Premixed iso-octane and methane-air flames have been ignited in a fan stirred bomb in laminar conditions and turbulent flow fields at 1 and 5 bar. Sheet images of the flames were captured using LIF of OH. In spherically expanding laminar flames, the shape of cusps in the flame surface was shown to change from a dent for flames with positive Markstein numbers to a Huygen type cusp at lower Markstein numbers and finally complete quench was observed at the cusp tip on flames with negative Markstein numbers. The curvatures of turbulent flame edges were calculated and pdf’s generated. The pdf’s were symmetrical about a mean of zero, as the turbulence intensity was increased the pdf’s broadened and became flatter. Turbulent rich iso-octane-air flames (φ = 1.4) exhibited areas of quench in the flame front, the distance between areas of quench was shown to increase as the turbulence intensity was raised. The 5 bar flames exhibited higher curvature than those at 1 bar. The influence of laminar flame and turbulent flow properties on the curvature and hence flame wrinkling were investigated.

Influence of refractive index matching on the photon diffuse reflectance.

Photon migration in a randomly inhomogeneous, highly scattering and absorbing semi-infinite medium with a plane boundary is considered by a Monte Carlo (MC) technique. The employed MC technique combines the statistical weight scheme and real photon paths simulation, allowing the exclusion of the energy conservation problem. The internal reflection of the scattered radiation on the medium interface is taken into account by allowing the trajectories of photon packets to be split into reflected and transmitted parts. The spatial photon sensitivity profile (SPSP), spatially resolved diffuse reflectance and angular and spatial photon detector weight distributions are considered in terms of Fresnels reflection/refraction on the boundary of the medium. The effect of the refractive index match is predicted correctly by the MC method and by the diffusion approximation. The results demonstrate that matching of the refractive index of the medium significantly improves the contrast and spatial resolution of the spatial photon sensitivity profile (SPSP). The results of simulation of the spatially resolved diffuse reflectance agree well with the results predicted by the diffusion approximation and the experimental results reported earlier.

The application of coherent anti-Stokes Raman scattering to turbulent combustion thermometry

Abstract Coherent anti-Stokes Raman scattering (CARS) has been used for noninvasive temperature measurements in a 1,5-sector BS360 gas turbine chamber. Both instantaneous and average CARS spectra have been recorded and temperatures calculated from the shapes of nitrogen Q-branch CARS spectra. Small samples of instantaneous temperature measurements, approximately 30, have been used in two instances to illustrate the potential of CARS for rms temperature determination inside primary combustion zones.

The structure and stability of the laminar counter-flow partially premixed methane/air triple flame

Abstract The flame stability map defining the regime of existence of a counter-flowing laminar partially premixed methane–air triple flame has been determined using OH planar laser-induced fluorescence (PLIF). The stability limits were determined through the observation of flame merging and flame extinction, a function of rich and lean equivalence ratios, and mean axial strain rate. Relatively quantitative OH species profiles and Rayleigh scattering profiles have been measured for three flame conditions. Axial flow velocity profiles, and nozzle exit velocity profiles have been determined for two of the three conditions using 1-D laser Doppler velocimetry (LDV). The diffusion flame extinction axial velocity profile has been measured, and the local extinction axial strain rate has been determined to be 710 s−1.

Amending of fluorescence sensor signal localization in human skin by matching of the refractive index.

Fluorescence diagnostic techniques are notable amongst many other optical methods because they offer high sensitivity and noninvasive measurement of tissue properties. However, a combination of multiple scattering and physical heterogeneity of biological tissues hampers interpretation of the fluorescence measurements. Analyses of the spatial distribution of endogenous and exogenous fluorophores excitation within tissues and their contribution to the detected signal localization are essential for many applications. We have developed a novel Monte Carlo technique that gives a graphical perception of how the excitation and fluorescence detected signal are localized in tissues. Our model takes into account the spatial distribution of fluorophores, the variation of concentrations and quantum yield. We demonstrate that matching the refractive indices of the ambient medium and topical skin layer improves spatial localization of the detected fluorescence signal within the tissues.

A closed form solution for the cars intensity convolution

Abstract An analytic solution for the “cross-coherence” terms in the CARS intensity convolution is given. The closed form expression is valid for gaussian pump laser intensity profiles and for overlapping spectral lines. The expression makes possible very rapid calculations of the full CARS intensity convolution integral.

Degenerate four-wave mixing in nitrogen dioxide: Application to combustion diagnostics

Single shot degenerate four wave mixing (DFWM) images of the distribution of nitrogen dioxide (NO2) doped into a propane/air flame at concentrations of the order of 7000 ppm have been obtained. These images indicate the relative concentration of NO2 in different parts of the flame with an estimated spatial resolution of 150 μm.Initial experiments were performed using NO2 in a glass cell with nitrogen buffer gas. DFWM signals were generated using both the frequency doubled output of a pulsed ND:YAG laser and the tunable blue output of an excimer pumped dye laser. The signal was investigated as a function of laser power, NO2 concentration and buffer gas pressure. In addition, spectra of NO2 in the region 450 to 480 nm were obtained.Signals were then sought in both a cold air/NO2 gas flow and an ignited mixture of propane and air seeded with NO2, using a DFWM imaging geometry. The resulting images from the flame demonstrate the disappearance of the NO2 molecules in the flame interaction zone.

Monte Carlo simulation of coherent effects in multiple scattering

Using a combination of the stochastic Monte Carlo technique and the iteration procedure of the solution to the Bethe–Salpeter equation, it has been shown that the simulation of the optical path of a photon packet undergoing an nth scattering event directly corresponds to the nth–order ladder diagram contribution. In this paper, the Monte Carlo technique is generalized for the simulation of the coherent back–scattering and temporal correlation function of optical radiation scattered within the randomly inhomogeneous turbid medium. The results of simulation demonstrate a good agreement with the diffusing wave theory and experimental results.