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Optical diagnostic techniques are applied to a 21 kW laboratory-scale pulverized coal–methane burner to map the reaction zone during combustion, in mixtures with varying fractions of O_2, N_2 and CO_2. Simultaneous Mie scatter and OH planar laser-induced fluorescence (PLIF) measurements have been carried out to study the effect of the oxidizer/diluent concentrations as well as the coal-loading rate. The spatial distribution of soot is captured using laser-induced incandescence (LII). Additionally, velocity profiles at selected axial locations are measured using the pairwise two-dimensional laser Doppler velocimetry technique. The OH PLIF images capture the reaction zones of pilot methane–air flames and the variation of the coal flame structure under various O_2/CO_2 compositions. Coal particles devolatilize immediately upon crossing the flame interface, so that the Mie scatter signal almost vanishes. Increasing coal-loading rates leads to higher reaction rates and shorter flames. LII measurements show that soot is formed primarily in the wake of remaining coal particles in the product regions. Finally, differences in the mean and RMS velocity field are explained by the combined action of thermal expansion and the changes in particle diameter between reacting and non-reacting flows.

Direct ultraviolet (UV) photoionization enables electrical charging of aerosol nanoparticles with- out relying on the collision of particles and ions. In this work, a low-strength electric field is applied during particle photoionization to capture charge as it is photoemitted from the par- ticles in continuous flow, yielding a novel electrical current measurement. As in conventional photocharging-based measurement devices, a distinct electrical current from the remaining pho- tocharged particles is also measured downstream. The two distinct measured currents are proportional to the total photoelectrically active area of the particles. A three dimensional numerical model for particle and ion (dis)charging and transport is evaluated by comparing simulations of integrated electric currents with those from charged soot particles and ions in an experimental photoionization chamber. The model and experiment show good quantitative agreement for a single empirical constant, KcI, over a range of particle sizes and concentrations providing confidence in the theoretical equations and numerical method used.

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.

© 2017 Elsevier Ltd Rapid compression machines (RCMs) are widely used to acquire experimental insights into fuel autoignition and pollutant formation chemistry, especially at conditions relevant to current and future combustion technologies. RCM studies emphasize important experimental regimes, characterized by low- to intermediate-temperatures (600–1200 K) and moderate to high pressures (5–80 bar). At these conditions, which are directly relevant to modern combustion schemes including low temperature combustion (LTC) for internal combustion engines and dry low emissions (DLE) for gas turbine engines, combustion chemistry exhibits complex and experimentally challenging behaviors such as the chemistry attributed to cool flame behavior and the negative temperature coefficient regime. Challenges for studying this regime include that experimental observations can be more sensitive to coupled physical-chemical processes leading to phenomena such as mixed deflagrative/autoignitive combustion. Experimental strategies which leverage the strengths of RCMs have been developed in recent years to make RCMs particularly well suited for elucidating LTC and DLE chemistry, as well as convolved physical-chemical processes. Specifically, this work presents a review of experimental and computational efforts applying RCMs to study autoignition phenomena, and the insights gained through these efforts. A brief history of RCM development is presented towards the steady imp rovement in design, characterization, instrumentation and data analysis. Novel experimental approaches and measurement techniques, coordinated with computational methods are described which have expanded the utility of RCMs beyond empirical studies of explosion limits to increasingly detailed understanding of autoignition chemistry and the role of physical-chemical interactions. Fundamental insight into the autoignition chemistry of specific fuels is described, demonstrating the extent of knowledge of low-temperature chemistry derived from RCM studies, from simple hydrocarbons to multi-component blends and full-boiling range fuels. Emerging needs and further opportunities are suggested, including investigations of under-explored fuels and the implementation of increasingly higher fidelity diagnostics.

A systematic study of laser-induced thermal-grating scattering (LITGS) using nitric oxide as an absorbing species is presented as a means of thermometry in air-fed combustion. The relative contributions to the scattered signal from degenerate four-wave mixing, DFWM, and from laser-induced thermal-grating scattering, LITGS, are studied in the time domain for NO in N2buffer gas up to 4 bar, using a pulsed laser system to excite the (0,0) γ-bands of NO at 226.21 nm. LITGS signals from combustion-generated NO in a laminar, pre-mixed CH4/O2/N2flame on an in-house constructed slot burner were used to derive temperature values as a function of O2concentration and position in the flame at 1 and 2.5 bar total pressure. Temperature values consistent with the calculated adiabatic flame temperature were derived from averaged LITGS signals over 50–100 single shots at 10 Hz repetition rate in the range 1600–2400 K with a pressure-dependent uncertainty of ± 1.8% at 1 bar to ± 1.4% at 2.5 bar. Based on observed signal-to-noise ratios, the minimum detectable concentration of NO in the flame is estimated to be 80 ppm for a 5 s measurement time at 10 Hz repetition rate.

Quasi-instantaneous Mie scattering measurements were conducted to determine the flame surface location using two orthogonal planes to obtain the 3D orientation and measurement of 3D flame surface density (FSD) on a stabilised piloted Bunsen burner with turbulence levels of 1 to 2.5 times the laminar flame speed. A double-pulsed 527 nm high-frequency laser (part of a high-frequency particle image velocimetry dual-head setup) was split into two separated laser beams through a polarizer to generate laser sheets at 3 kHz on a vertical plane and a horizontal plane. The vertical plane across the centerline of the Bunsen-stabilized flame was kept constant, whilst the height of the horizontal plane was adjusted from the base of the flame for different heights. The flame edge was defined as the location where droplet tracers disappear, and calculated based on the local change in the number density of the flame edge. Projections of the flame normal vector onto two measurement planes were used to calculate directing angles on each plane, and the 3D FSD was estimated based on the measured angles at the intersecting lines of two planes. A comparison of the 2D to 3D FSD magnitudes was made, for cases with and without mean angle corrections. The results show that the 2D FSD approximations are lower than the 3D measurements by a factor of 20-30% if uncorrected, and up to 28% after corrections.

We review the state of the art in measurements and simulations of the behavior of premixed laminar and turbulent flames, subject to differential diffusion, stretch and curvature. The first part of the paper reviews the behavior of premixed laminar flames subject to flow stretch, and how it affects the accuracy of measurements of unstrained laminar flame speeds in stretched and spherically propagating flames. We then examine how flow field stretch and differential diffusion interact with flame propagation, promoting or suppressing the onset of thermodiffusive instabilities. Secondly, we survey the methodology for and results of measurements of turbulent flame speeds in the light of theory, and identify issues of consistency in the definition of mean flame speeds, and their corresponding mean areas. Data for methane at a single operating condition are compared for a range of turbulent conditions, showing that fundamental issues that have yet to be resolved for Bunsen and spherically propagating flames. Finally, we consider how the laminar flame scale response of flames to flow perturbations interacting with differential diffusion leads to very different outcomes to the overall sensitivity of the burning rate to turbulence, according to numerical simulations (DNS). The paper concludes with opportunities for future measurements and model development, including the perennial recommendation for robust archival databases of experimental and DNS results for future testing of models.

Electric charging is one of the essential aerosol dynamic mechanisms and is harnessed for detection, capture and control of ultrafine aerosol particles in a range of devices. For simplicity, charging and transport mechanisms are commonly modelled with zero spatial dimensions (0-D) and averaged properties such as mean charge or mean particle diameter. These models often neglect localised effects of the flow distribution, diffusion, discrete charge states, and particle polydispersity, often proving inadequate to explain experimental data. This work aims to provide an open-source three-dimensional (3-D) aerosol charging and transport model including bipolar and unipolar diffusion charging, and photoelectric charging algorithms for use in detailed design and analyses of aerosol systems. The computational model consists of more than 200 particle transport equations for discrete charge states and polydisperse sizes coupled with ion conservation equations in the framework of OpenFOAM, an open-source computational fluid dynamics platform. Three test cases are introduced to verify implementation of three charging models by comparison with published literature: bipolar and unipolar diffusion charging, and photoelectric charging. Tutorial cases, which model three distinct aerosol sensors, are described and demonstrate the capabilities of the 3-D aerosol charging and transport models within the predetermined flow field. The aerosolChargingFoam code is available at https://openaerosol.sourceforge.io for widespread use and can be further modified under the GNU general public licence. Program summary: Program title: aerosolChargingFoam CPC Library link to program files: https://doi.org/10.17632/4pf4n2vg9f.1 Developer's repository link: https://openaerosol.sourceforge.io/ Licensing provisions: GNU General Public License 3 Programming language: C++ Nature of problem: aerosolChargingFoam solves generalised aerosol electrical charging and transport equations coupled with computational fluid dynamics using the open-source computational platform, OpenFOAM [1]. The electric charging algorithm including unipolar diffusion charging, bipolar diffusion charging, and photoelectric charging is verified with zero-dimensional test cases [2-4] and generalized to three-dimensional monodisperse or polydisperse particle distributions for more than 50 particle charge states. aerosolChargingFoam can be straightforwardly coupled with other existing solvers, which enables computations of complex multi-physics aerosol charging processes in practical conditions. Solution method: aerosolChargingFoam employs an explicit time-stepping for the time-dependent source terms for generalised aerosol charging. The solution methods and schemes provided by OpenFOAM 9 are used for solving the spatial and time derivatives in the transport equations. References: [1] OpenFOAM9, OpenFOAM v9, The OpenFOAM Foundation, https://openfoam.org/. [2] W.A. Hoppel, G.M. Frick, Aerosol Sci. Technol. 12 (3) (1990) 471–496. [3] A. Maisels, F. Jordan, H. Fissan, J. Appl. Phys. 91 (5) (2002) 3377–3383. [4] H. Kaminski, T.A.J. Kuhlbusch, H. Fissan, L. Ravi, H.-G. Horn, H.-S. Han, et al., Aerosol Sci. Technol. 46 (6) (2012) 708–716.

© 2018 The Combustion Institute. Biodiesel and blends with petroleum diesel have shown their potential as renewable alternative fuels for engines, with additional benefits of low particulate matter and low sulfate emissions. In this paper we measure the soot volume fraction produced by three different methyl esters processed biodiesels (extracted from palm (PME), soy (SME) and coconut (CME)), and their blends with petroleum diesel, in a series of co-flow stabilized laminar pool flames, using laser induced-incandescence (LII) and laser extinction optical methods. The soot volume fraction measurement results show that all neat biodiesels produce only up to 33% of the total soot volume compared to pure diesel, and that the total soot volume correlates directly with the degree of unsaturation of the biodiesels. Blending leads to approximately linear behaviour of total soot volume, with a shift in slope with smaller sensitivity towards neat diesel.

Indirect noise generated by the acceleration of synthetic compositional and entropic perturbations through non-isentropic nozzles is measured experimentally. A physics-based analytical low-order model to evaluate the indirect noise generated by non-isentropic compact nozzles is developed and validated with experimental measurements. A one-dimensional model for describing the waves generated by the addition of mass, momentum, energy and species to a steady flow in an entropy and composition wave generator is presented. The transfer functions describing the multiple reflections of acoustic waves in an enclosed environment are derived. This analytical framework allows unambiguous identification and isolation of the experimental direct and indirect noise generated by the injection of helium, methane, argon or carbon dioxide into a flow duct. Experimental data show that entropic and compositional noise make a significant contribution to the overall pressure traces acquired in the entropy generator. Moreover, it is demonstrated that the isentropic modelling assumption is inadequate to capture the experimental behaviour, while the analytical model for non-isentropic nozzles successfully describes the direct and indirect noise transfer functions. The disregard for the compositional contribution and the unjustified use of the isentropic assumption can provide significantly inaccurate noise predictions. This work shows that compositional noise, as well as non-isentropicity in the system, should be considered in future thermoacoustic and combustion noise models.

The present research deals with a hybrid combustion of aluminum/boron dust particles in a heterogeneous quiescent reaction medium with spatially discrete heat sources. A developed thermal model is employed to estimate flame propagation speed in a reaction medium. The burning velocity and minimum ignition energy are studied parametrically as a function of dust concentration and particle diameter for different percentages of boron powder in a hybrid mixture of aluminum/boron dust cloud. The model shows that the addition of boron powder as a component of the mixture decreases the burning rate and causes a higher amount of minimum ignition energy needed for ignition, owing to the role of boron as a heat sink. Comparison of the simulation results with the available experimental data shows that the model captures the flame propagation speed as a function of particle concentration, except at very low concentrations.