Measurements of laminar flame speeds of acetone/methane/air mixtures

The effect of acetone on the laminar flame speed of methane/air mixtures is investigated over a range of stoichiometries at atmospheric pressure and room temperature. The liquid acetone is vaporised and seeded into the methane/air mixture at 5%, 9% and 20% of the total fuel by mole. The experiment is performed using the jet-wall stagnation flame configuration and the particle imaging velocimetry (PIV) technique. Laminar flame speeds are derived by extrapolating the reference flame speed back to zero strain rate. Experimental results are compared to numerically calculated values using a base methane chemical kinetic mechanism (GRI-Mech 3.0) extended with acetone oxidation and pyrolysis reactions from the literature. The experimental results show that acetone addition does not affect the laminar flame speed of methane significantly within the range of concentrations considered, with a stronger effect on the rich range than under fuel-lean conditions, and that the peak laminar flame speed of acetone in air is ~42.5 cm/s at ϕ = 1.2. Simulation results reveal that the most important reactions determining acetone laminar flame speeds are H + O2 → O + OH, OH + CO → H + CO2, HO2 + CH3 → OH + CH3O and H + O2 + H2O → HO2 + H2O. Comparison of the expected disappearance of acetone relative to methane shows that the former is a good fluorescent marker for the latter

The response of stratified swirling flames to acoustic forcing: Experiments and comparison to model

The gradient of local equivalence ratio in reacting mixtures significantly affects the flame structure and their corresponding response to acoustic velocity perturbations. We study the effect of acoustic velocity fluctuations on flames created by two co-annular, swirling streams with different equivalence ratios to simulate the effects of pilot-mains split. The flames are stabilized both by a bluff body and by swirl. The flame responses were measured via chemiluminescence as a function of frequency, in the linear perturbation range. A linearized version of the G-equation model is employed to describe the flame dynamics, combined with effects of axial and azimuthal velocity perturbations downstream of the swirlers. The model accounts for the phase shift between the main acoustic and swirler vortical perturbations, which propagate at different speeds. The very different flame structures generated by different fuel splits lead to different flame responses. Models based on time delay of vortical disturbances are able to capture the behaviour reasonably well for the case of outer fuel enrichment, but offer limited agreement for the case of the inner enriched flame, particularly under higher mean equivalence ratios.The authors acknowledge the support provided by the Cambridge Overseas Trust and China Scholarship Council. Additional funding was provided by Rolls-Royce plc for the initial set up of the experiments.This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.proci.2014.05.04

Uncertainty analysis in Structured Laser Illumination Planar Imaging (SLIPI) applied to non-linear signals: gas-phase phosphor thermometry

Recent studies have used structured laser illumination planar imaging (SLIPI) combined with phosphor thermography to remove multiple scatter effects and near-wall reflections, which lead to biases in temperature measurements and reduced spatial resolution. We show that for the typical non-linear pump-signal range under which thermographic phosphors are used, errors may arise in the reconstruction of the temperature field using SLIPI. In this study, synthetic laser induced phosphorescence (LIP) images are generated numerically by adapting the Synthetic PIV Image Generator (SIG) for the purpose. The simulations are combined with phosphorescent signal yield functions obtained from experimental data to investigate the application of SLIPI to gas-phase phosphor thermography. We conclude that whilst SLIPI is effective in removing scattering noise for phosphors for which the two-colour signal ratio is insensitive to the laser fluence, it creates a bias in the temperature measurement otherwise. We also show that the extent of multiple scatter in LIP images is always overestimated by SLIPI, owning to the non-linear emission behaviour and particle image diffraction.The project has been partly funded by the Universiti Teknologi Malaysia

Conditional analysis of turbulent premixed and stratified flames on local equivalence ratio and progress of reaction

Previous studies on the Cambridge/Sandia stratified burner have produced a comprehensive database of line Rayleigh/Raman/CO LIF measurements of scalars, as well as LDA and PIV measurements of velocity, for flames under non-uniform mixture fraction, under moderate turbulent conditions where the ratio of the turbulent velocity fluctuations to the laminar flame speed is of order 10. In prior work, we applied multiple conditioning methods to demonstrate how local stratification increases the levels of CO and H2, relative to the corresponding turbulent premixed flame, and enhances surface density function (SDF) and scalar dissipation rate of progress of reaction (SDR), based on extent of temperature rise, at a particular location in the flame where the mixing layer and flame brush cross. In the present study, we examine the global features of selected flames at all locations, by obtaining probability density functions (PDFs) for species concentrations, SDRs, and SDFs, conditioned on local equivalence ratio and location in the flame brush throughout the domain. We find that for most cases, species profiles as a function of temperature are well represented by laminar flame relationships at the local equivalence ratio, with some deviations attributable to either differential diffusion near the flame base and local stratification effects further downstream where the flame brush crosses the mixing layer. In particular, CO2 is significantly affected by differential diffusion, and CO and H2 by stratification. However, the stratification effects on the species are relatively minor when conditioned on local equivalence ratio, a simplifying result in the context of modeling. Measurements of the gradient of progress of reaction and scalar dissipation rates, conditioned on local equivalence ratio, show that the thermal zone of the flame is thickened by turbulence: the mean SDF and SDR values are in general lower than those of unstrained laminar flames. The effect is greater under rich conditions, with conditional mean SDR decreasing to less than half of the corresponding laminar value. The extent of flame thickening is the same in the premixed as the stratified case, once the stratified measurements are conditioned on the same equivalence ratio.M. Mustafa Kamal acknowledges funding from University of Engineering and Technology Peshawar (Pakistan). The measurements at Sandia National Labs were sponsored by the United States Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94-AL85000. The authors also thank Dr. Akihiro Hayakawa for his contributions to the laminar flame calculations and Dr. Saravanan Balusamy for his valuable suggestions regarding data processing

A novel FoxD3 gene trap line reveals neural crest precursor movement and a role for FoxD3 in their specification

Neural crest cells migrate extensively and contribute to diverse derivatives, including the craniofacial skeleton, peripheral neurons and glia, and pigment cells. Although several transgenic lines label neural crest subpopulations, few are suited for studying early events in neural crest development. Here, we present a zebrafish gene/protein trap line gt(foxd3-citrine)^(ct110a) that expresses a Citrine fusion protein with FoxD3, a transcription factor expressed in premigratory and migrating neural crest cells. In this novel line, citrine expression exactly parallels endogenous foxd3 expression. High-resolution time-lapse imaging reveals the dynamic phases of precursor and migratory neural crest cell movements from the neural keel stage to times of active cell migration. In addition, Cre-recombination produces a variant line FoxD3-mCherry-pA whose homozygosis generates a FoxD3 mutant. Taking advantage of the endogenously regulated expression of FoxD3-mCherry fusion protein, we directly assess early effects of FoxD3 loss-of-function on specification and morphogenesis of dorsal root ganglia, craniofacial skeleton and melanophores. These novel lines provide new insights and useful new tools for studying specification, migration and differentiation of neural crest cells

Flow Field Results of the Cambridge Stratified Swirl Burner Using Laser Doppler Anemometer

A series of flow fields generated by a turbulent methane/air stratified swirl burner are investigated using laser Doppler anemometer (LDA) to obtain the velocities in axial, radial and tangential directions. All mean and RMS of the LDA velocity results are supplied along with a report describing the experimental methodology, data processing routines and the data format.This work was supported by the China Scholarship Council and Cambridge Overseas Trust

Theory and application of reverberated direct and indirect noise

The generation of a temperature disturbance in a flow is accompanied by the production of acoustic waves (direct noise), and of an entropy perturbation. If this entropy perturbation is accelerated or decelerated (as is the case through a nozzle or flow restriction), additional acoustic waves are generated (indirect noise). Several studies have demonstrated this mechanism in controlled conditions by using Entropy Wave Generators, in which entropy waves are generated and convected through a nozzle, leading to direct and indirect noise. An analytical analysis of the direct and indirect noise produced by the generation and acceleration of entropy waves in a reflective environment is presented. The e ect of reverberation (repeated acoustic reflections) on low-frequency perturbations (characteristic of Entropy Wave Generators) is determined analytically. These results are then implemented in a set of limit cases, showing the limit behaviours of such systems. The analytical model is applied to the case of the Cambridge Entropy Wave Generator experiment, in which entropy waves are generated by an electric heater and accelerated through a subsonic ori ce plate. Due to the clear time separation of direct and indirect noise in the experimental results, direct and indirect noise transfer functions can be extracted from the experimental data for the rst time, and compared directly with existing theoretical models. The backward-propagating indirect noise generated at an ori ce plate is shown to be signi cantly higher than predicted by existing theoretical models for isentropic nozzles.EPSRC EP/K02924X/1

Gas phase Raman spectroscopy of non-reacting flows : comparison between free space and cavity based spontaneous Raman emission

We report on a comparison of free space and Cavity-Enhanced Raman Spectroscopy (CERS) for gas phase measurements of nitrogen and oxygen in ambient air. Real time analysis capabilities, and continuous Raman signals with low power diodes, make the technique noninvasive, affordable, compact and applicable for usage in non-reacting flows. We derive a comprehensive model for estimation of photon emission for both free space and cavity based signals and discuss trade-offs in how to organize the cavity geometry for maximum gain relative to free space. Measurements in both free and cavity configurations are compared to the expected signals, demonstrating the usefulness of the model in predicting amplification. The present results can serve as a quick guide on how to use low power continuous wave lasers in a cavity setup to obtain enhanced laser induced spontaneous Raman scattering.EPSR