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Planar, two-dimensional, two-colour, time-resolved laser-induced incandescence (2D-2C-TiRe-LII) is employed to investigate the formation of soot in a standard methane laminar diffusion flame for the first time. The work builds on previous developments in 2D LII by time-resolving the signal decay by shifting the gating time of ICCD cameras. The two-colour configuration allows application of the technique to extract the initial temperature of soot particles immediately after the laser pulse, instead of relying on estimates of the soot absorption coefficient, thus allowing higher accuracy in the determination of very small particle sizes produced in methane flames, using a minimum error approach. The technique is combined with a previous work on Continuous Wave Laser Cavity Extinction (CW-LCE) to allow absolute measurements of sub-ppm low soot volume fractions with very small particle sizes.

In this study, the generation of soot from the combustion of biodiesels and their blend with fossil diesel are investigated under laminar pool flame and prevaporised diffusion jet flame conditions. Neat biodiesels from eight feedstocks including carotene palm, palm, soy, coconut, rice bran, waste vegetable oil, duck, and goose, alongside two methyl esters of methyl laureate and methyl myristate are blended with fossil diesel at 20% volumetric intervals to form 51 different blends. The soot volume fractions for the combustion of all blends are determined using continuous-wave laser cavity extinction (CW-LCE) calibrated 2D laser-induced incandescence (2D-LII) technique. Results show that biodiesels with higher degree of saturation produce less soot, as reflected in both laminar pool and prevaroprised diffusion jet flames. The experimental data is further analysed using design of experiments (DOE) methodology. The mixture design model (MDM) following the simplex design is used to focus on the soot generating effects arising from the chemical compositions by classifying the chemical components into hydrocarbons and saturated, monounsaturated and polyunsaturated fatty acid methyl esters (FAME). The results from the Cox response trace plot associated with DOE elucidate the effects of the individual chemical group within the fuels. Both laminar flame types for the all 51 biodiesel-diesel blends exhibit the same trends with differing magnitudes. Saturated FAME has the largest singular effect on soot generation followed by hydrocarbons. However, when only neat biodiesels are considered, the laminar pool flame and laminar diffusion jet flames show differing trends for all the chemical groups. Monounsaturated FAME and saturated FAME have the greatest influence on neat biodiesel-generated soot for laminar pool flame and laminar diffusion jet flame, respectively. From the mixture design model, an optimum synthetic biodiesel mixture containing 16.1%, 34.6% and 49.3% of saturated, monounsaturated and polyunsaturated methyl esters is recommended for the lowest soot generation.

This study examines the flow field dynamics of bluff-body stabilized swirling and non-swirling flames produced from the Cambridge/Sandia Stratified Swirl Burner. This burner has been used in previous studies as a benchmark for high-resolution scalar and velocity measurements and for validating numerical models. The burner was designed to create reacting flow conditions that are representative of turbulent flows in modern combustion systems, including sufficiently high turbulence levels, and to operate under both premixed and stratified conditions. High-speed stereoscopic particle image velocimetry was used to acquire time-resolved velocity data for a series of turbulent methane/air flames at both premixed and stratified conditions. We employ the multi-scale proper orthogonal decomposition (mPOD) to identify the main flow patterns in the velocity field and isolate coherent structures linked to various flow instabilities. The results show that the most energetic structures in the flow are consistent with the Bénard–von Kármán (BVK) instability due to the presence of the bluff-body and the Kelvin–Helmholtz (KH) instability caused by the shear layer between the inner and the outer flow. In both the swirling and non-swirling cases, the BVK is suppressed by the combustion, except for the most stratified swirling case. Moreover, the results show that combustion does not affect the KH instability because the shear layer does not coincide with the flame position.

Biodiesel and blends with petroleum diesel are promising renewable alternative fuels for engines. In the present study, the soot concentration generated from four biodiesels, two pure methyl esters, and their blends with petroleum diesel are measured in a series of fully pre-vapourised co-flow diffusion flames. The experimental measurements are conducted using planar laser induced-incandescence (LII) and laser extinction optical methods. The results show that the maximum local soot volume fractions of neat biodiesels are 24.4% - 41.2% of pure diesel, whereas the mean soot volume fraction of neat biodiesel cases was measured as 11.3% - 21.3% of pure diesel. The addition of biodiesel to diesel not only reduces the number of inception particles, but also inhibits their surface growth. The discretised population balance modelling of a complete set of soot processes is employed to compute the 2D soot volume fraction and size distribution across the tested flames. The results show that the model also demonstrates a reduction of both soot volume fraction and primary particle size by adding biodiesel fuels. However, it is not possible to clearly determine which factors are responsible for the reduction from the comparison alone. Moreover, analysis of the discrepancies between numerical and experimental results for diesel and low-blending cases offers an insight for the refinement of soot formation modelling of combustion with large-molecule fuels.

This work investigates a novel heat-driven multi-stage thermoacoustic cooler that can satisfy cooling requirements in the applications of natural gas liquefaction and hightemperature superconductivity. The proposed system consists of a compressor, multiple thermoacoustic units (engines and coolers) coupled by piston-cylinder assemblies. The acoustic power input by the compressor is successively multiplied in the thermoacoustic engine units, and the amplified acoustic power is then consumed to produce cooling power in the thermoacoustic cooler units. The proposed system overcomes the limitations of the traditional thermoacoustic systems owing to high efficiency, compact size, and ease of control. Analyses are first performed to explore the influence of the number of stages. The design method of the pistons is presented based on acoustic impedance matching principle. Based on the optimized system, simulations are then conducted to investigate the axial distribution of the key parameters, which can explain the reason for improved thermodynamic performance. At heating and cooling temperatures of 873 K and 130 K, the system achieves a cooling power of 2.1 kW and a thermal-to-cooling relative Carnot efficiency of 23%. This represents significant increases by over 60% in efficiency and 80% in cooling capacity when compared to existing systems. Simulations further demonstrate how controlling the input acoustic power and frequency via the compressor enables control of the system under various conditions. Further discussions are made considering a potential combined cooling and power system, indicating a thermal-cooling-electricity efficiency of 34% without any external electric power required for the compressor.

The soot volume fraction (SVF) of waste cooking oil (WCO) biodiesel and blends was quantified and compared under the same total carbon flow rate via two experimental setups, namely prevaporised diffusion jet flames and pool flames using extinction calibrated laser induced-incandescence (LII). The spatial SVF distribution shows that for diesel-rich fuels, soot formation peaks near the flame and is convected downstream, whereas biodiesel flames show a more evenly distributed SVF at the flame center region. An increase in biodiesel fraction in diesel results in a reduced propensity for soot, as evident in both pool and vapour flames. Comparison of the radial profiles of SVF along the centerline shows broader SVF profiles for pool flames, reflecting the longer residence times for soot diffusion and growth compared to vapour flames, which reflected the lower mass flux for the pool burner. The total soot produced from pool flames was found to be higher than vapour flame by a factor of two for the same fuel mass consumption rate. WCO biodiesel exhibited the lowest total SVF value regardless of flame type owing to the combined effects of lack of aromatic compounds and fuel chemistry. The soot primary particle sizes produced by WCO biodiesel show lower mean diameter values by a factor of approximately 1.5 compared to diesel-produced soot. The pool flames produced carbon particulates of larger mean diameter by around 22% and 8% for diesel and WCO biodiesel, respectively, relative to the counterpart vapour flames, as a result of extended soot surface growth period.

In this study, we investigate the e ffect of the presence of fuel droplets, their size and concentration, on stretched laminar flame speeds. We consider premixed strained methane/air mixtures, with the addition of small acetone droplets, and compare the flame velocity eld behavior to that of the fully vaporized mixture. An impinging stagnation flame con figuration is used, to which a narrowly distributed polydisperse mist of acetone droplets is added. Total acetone molar concentrations between 9% and 20% per mole of methane are used, corresponding to 18.6% and 41.4% of the total fuel energy. The Sauter Mean Diameter (SMD) of acetone droplets is varied from 1.0 to 4.7 m by carefully tuning the air ow rate passing through an atomizer. The droplet size distribution is characterized by a Phase Doppler Anamometry (PDA) system at the outlet of the burner. The flame propagation speed is measured using Particle Image Velocimetry (PIV) for overall equivalence ratios ranging from 0.8 to 1.4 at various strain rates, and the result is compared with a reference case in which acetone was fully vaporized. Unlike the fully vaporized flame, a two-stage reaction flame structure is observed for all droplet cases: a blue premixed flame front followed by a reddish luminous zone. Comparison of the results between gas-only and droplet-laden cases shows that the mean reference burning velocity of the mixture is signi cantly enhanced when droplets are present under rich cases, whereas the opposite is true for stoichiometric and lean cases. The mean droplet size also changes the relationship between flame speed and strain rate, especially for rich cases. The result suggests that with typical conditions found in laminar strained flames, even for the finest droplets that may have been vaporized before reaching the flame front, the resulting inhomogeneities may lead the flame to behaves di fferently from the well-premixed gaseous counterpart. Simulations at similar conditions are performed using a two-phase counter flow flame model to compare with experimental data. Model results of reference velocities do not compare well with observations, and the possible reasons for this behaviour are discussed, including the di culties in determining the pre-vaporization process and thus the boundary conditions, as well as the fi delity of the current point-source based 1D model.

The application of laser diagnostics to high pressure combustion phenomena is particularly challenging, especially in practical combustors such as rocket motors. In this study, temperature measurements using Laser Induced Thermal Grating Spectroscopy (LITGS) are demonstrated in oxygen enriched CH4/O2/N2premixed laminar flames at pressures up to 1.0 MPa. We use a previously developed OH absorption LITGS technique to determine product gas temperatures from 0.3 to 1.0 MPa, for both high temperature oxygen-enriched and pure-oxygen flames, for measurements up to 3000 K. Further, we demonstrate how it is necessary to correct the measurements for the local absorption of laser light to obtain accurate temperatures, and offer a technique for producing the correction by using different laser energies. Once the correction is applied, we demonstrate that the measurements at 0.5 MPa are within 1.6% of the adiabatic non-strained flame temperatures, with a standard deviation of about 160 K, thus offering a competitive method for the challenging conditions at high pressures and temperatures. The values obtained at derived temperatures at 1.0 MPa were lower than the adiabatic unstrained flame temperatures, which could possibly be attributed to loss mechanisms.

Biodiesel and blends with petroleum diesel are promising renewable alternative fuels for engines. In the present study, the soot concentration generated from four biodiesels, two pure methyl esters, and their blends with petroleum diesel are measured in a series of fully pre-vapourised co-flow diffusion flames. The experimental measurements are conducted using planar laser induced-incandescence (LII) and laser extinction optical methods. The results show that the maximum local soot volume fractions of neat biodiesels are 24.4% - 41.2% of pure diesel, whereas the mean soot volume fraction of neat biodiesel cases was measured as 11.3% - 21.3% of pure diesel. The addition of biodiesel to diesel not only reduces the number of inception particles, but also inhibits their surface growth. The discretised population balance modelling of a complete set of soot processes is employed to compute the 2D soot volume fraction and size distribution across the tested flames. The results show that the model also demonstrates a reduction of both soot volume fraction and primary particle size by adding biodiesel fuels. However, it is not possible to clearly determine which factors are responsible for the reduction from the comparison alone. Moreover, analysis of the discrepancies between numerical and experimental results for diesel and low-blending cases offers an insight for the refinement of soot formation modelling of combustion with large-molecule fuels.

A small-scale, heat-driven cooling system is required for on-site liquefaction of unconventional natural gas in a distributed station. To meet such demands, we propose a highly efficient heat-driven thermoacoustic cryocooler. This paper presents the experimental results of the proposed system, which is optimized based on previous theoretical analysis. Firstly, we compare two high-temperature heat exchangers with similar heat transfer effectiveness but different flow uniformity. The experimental results show that the heat exchanger with uniform flow can improve system efficiency by 28%. Experimental investigations are then carried out to understand the effect of operating temperatures on system performance. Later, the performance of the system operating at variable heating temperatures is studied. Finally, the reasons for the discrepancy between experiments and calculations are discussed. The experimental results show that the proposed thermoacoustically-driven cryocooler can achieve an exergy efficiency of 10 % and a cooling power of 378 W at a heating temperature of 730 K and a cooling temperature of 130 K. This represents a 25% improvement in efficiency compared to the previous record-holder thermoacoustic system.

Recovering low-grade heat from renewable energy sources and waste heat is crucial for improving energy utilizing efficiency as well as reducing CO2 emissions. Conventional thermoacoustically-driven refrigerators have a high onset temperature and low cooling efficiency, which significant limit their capacity for low-grade heat utilization. This paper investigates a novel thermoacoustically-driven refrigerator with gas-liquid resonators which enable a lower onset temperature and better cooling performance for harvesting low-grade heat. Theoretical analyses were performed on multi-stage systems to explore the onset characteristics and steady performance. Onset characteristics analysis was conducted by using a transfer matrix method. The effects of mean pressure, liquid volume ratio and the expected liquid mechanical damping coefficient on the onset temperature difference and working frequency were studied for systems with different numbers of stages. A comparison of system onset performance was made with conventional systems containing a gas-only resonator. The research illustrated that for a mean pressure of 1 MPa, the proposed system can significantly reduce the onset temperature difference from 144.1 K to below 35.5 K. In addition, an analysis was then conducted to study the parametric sensitivity of the thermodynamic performance. Calculation results show that the proposed system can achieve a baseline cooling power of 2.7 kW and a thermal-to-cooling efficiency of 0.67 at a heating temperature of 420 K and a cooling temperature of 270 K. This represents significant increases by a factor of 5.6 in cooling power and 1.5 in efficiency from a gas-only to a gas-liquid resonator.

© 2020 Elsevier B.V. We explore the production of single-walled carbon nanotubes (CNTs) in a stream surrounded by rich premixed laminar H2/air flames using a feedstock containing ethanol and ferrocene. The as-produced nanomaterials were characterised by Raman spectroscopy, transmission electron microscopy, scanning electron microscopy and X-ray diffraction. A formation window of equivalence ratios of 1.00–1.20 was identified, and single-walled CNT bundles with individual CNTs of an average diameter of 1 nm were observed. The formation of CNTs was accompanied by the production of highly crystalline Fe3O4 nanoparticles of a size of 20–100 nm. The investigation of the limiting factors for the CNT synthesis was carried out systematically, assisted by numerical modelling. We conclude that the key factors affecting CNT synthesis are the surrounding flame temperatures, and the concentration of carbon available for CNT nucleation.

Multi-pass cavity line-of-sight extinction (MPC-LOSE) and laser- induced incandescence (LII) techniques are deployed to measure the soot volume fraction in a series of nitrogen-diluted flames, which produce only ppm volume mass fractions of soot. The separate suppression effects on soot formation of direct fuel dilution and indirect effects of temperature and res- idence time are interpreted by using a numerically calculated flow velocity and temperature field using a one-step fast chemistry model. The experi- mentally determined rate of soot formation is shown to obey approximately the same function of the local temperature for all dilution cases. The results show that a simple one-step reaction model using previously measured acti- vation energies can account for the dilution effect with good accuracy. The results show that the direct effect of dilution on concentration is comparable to the effects of changing the temperature estimated local temperature and residence time.

Low-cost ultrafine aerosol sensors are experimentally calibrated with controlled aerosol sources to provide metrics such as surface area, lung-deposited surface area, mean particle size and/or total concentration from one or more electrical current measurements. However, an aerosol with a large standard deviation in particle size provides a significantly different signal from a monodisperse aerosol with the same median particle size. In this paper, we investigate the effect of particle polydispersity on measurements in devices which employ unipolar charging. The conservation equations are solved for particle/ion charging and transport (convection, diffusion and electrical transport) in laminar, steady-state, incompressible flow. Lognormal particle size distributions are represented by over coupled conservation equations for multiple size bins and discrete charge states and solved numerically for the first time. Modelling results show that integrated electrical current from a polydisperse particle distribution can be represented by a monodisperse distribution characterised by the count mean diameter ( ; unipolar diffusion charging) or diameter of the average surface ( ; photoelectric charging) and total concentration, for a large range of particle distributions and operating conditions offering a convenient simplification for the interpretation of ultrafine particle measurements. The simplification reduces the number of simultaneous conservation equations required, thereby reducing computation time by up to 57 times for a polydisperse particle distribution represented by 16 discrete size bins. The method of analysis is useful to both users and developers of low-cost ultrafine particle sensors to understand the effect of particle polydispersity on measurements.

Laser-Induced Grating Spectroscopy (LIGS) is applied to premixed CH4/ air laminar flat flames under operating pressures of 1 to 6 bar. For the first time, temperature and water concentration have been acquired simultaneously in a reacting flow environment using LIGS. A 1064 nm pulsed laser is used as pump to generate a temporary stationary intensity grating in the probe volume. Water molecules in the flame products absorb the laser energy and generate a thermal grating if sufficiently high energies are delivered by the laser pulses, here more than 100 mJ per pulse. Such energies allow the electric field to polarize the dielectric medium, resulting in a detectable electrostrictive grating as well. This creates LIGS signals containing both the electrostrictive and the thermal contributions. The local speed of sound is derived from the oscillation frequency of LIGS signals, which can be accurately measured from the single shot power spectrum. Data show that the ratio between the electrostrictive and the thermal peak intensities is an indicator of the local water concentration. The measured values of speed of sound, temperature, and water concentration in the flames examined compare favorably with flame simulations with Chemkin, showing an estimated accuracy of 0.5 to 2.5% and a precision of 1.4-2%. These results confirm the potential for 1064-nm LIGS-based thermometry for high-precision temperature measurements of combustion processes.

The knowledge of the acoustic and entropic transfer functions at the boundaries of combustors is crucial to understand the fate of flame-generated pressure perturbations and to predict and prevent the emergence of combustion instabilities. Traditional models often rely on the isentropic assumption for nozzle guide vanes. In real systems, however, pressure losses and local flow recirculations may occur, as evidenced by drops in the static pressure. In this work we relax the isentropic assumption and derive a parametric model to predict the acoustic and entropic transfer functions of generalised convergent-divergent nozzles with subsonic-to-sonic throat conditions in the low frequency domain. By tuning two parameters, this model can retrieve the impedance of three limit cases known from the literature: the isentropic nozzle, the orifice plate and the convergent nozzle duct termination. The generalised model also includes the conversion of entropy to sound through orifice plates and non-isentropic nozzles, yet to be considered in the literature. These analytical results are then compared with the experimental data acquired in the Cambridge Entropy Generator. The comparison highlights the need to correctly account for the losses in the system to properly explain the transfer functions of nozzles, as isentropic predictions differ substantially from the acquired experimental data., Qualcomm

Pressure perturbations arise in combustors from direct noise related to the change in density in flames, as well as the indirect (entropic) noise associated with the acceleration of non-homogeneous regions of flow through nozzles. In this paper we review our recent work on quantifying the relative contributions of direct and indirect noise generated from perturbations in temperature and composition, and the resulting transmitted and reflected pressure perturbations. We show that (a) isentropic models are inadequate to capture the acoustic and entropic transfer functions across a nozzle; (b) corrections to non-isentropic behaviour are possible using existing models for orifices using a single parameter accounting for losses; (c) the behaviour of low frequency entropic noise generated in a chamber can be entirely accounted for when reverberation is taken into account; and (d) indirect noise due to compositional fluctuations can be as large as entropic noise arising from temperature fluctuations. The findings have implications for both the study of entropy noise in model systems, as well as for understanding how to separate the origins of noise in practical systems. In particular, the role of compositional noise in gas turbines (regarding for example the role of cooling in rich-quench-lean turbines) and the role of non-isentropic effects is not accounted for in current models, and should be revisited in the light of current findings., QUALCOMM

Pressure fluctuations associated with combustion, particularly in gas turbines, are often linked to either the direct gas expansion due to heat release (direct noise) or the passage of temperature or composition non-uniformities through an outlet gas nozzle (indirect noise). Experiments using the Cambridge Entropy Generator (CEG) have been able to separate and measure the contributions between direct and indirect noise in a system with well controlled boundary and operating conditions, suitable for direct comparison with models. The CEG consists of a tube through which air flows at a controlled rate. Temperature variations are generated by Joule heating of a thin wire grid, and are accelerated through an orifice plate operated at sub- or supercritical conditions, with pressure fluctuations measured upstream. Current prediction models assume a 1D propagation of the entropy wave, where the input is the measured temperature. The present work provides a compressible, unsteady RANS simulation for the unsteady temperature and pressure distribution of the CEG experiments, including advection and dispersion of entropy spots. The implementation of the model in OpenFOAM shows that it is possible to capture the behaviour of the experiments, including the evolution of the temperature in time and space, as well as the acoustic signature for different boundary conditions. The model further highlights some of the limitations in representing entropy spots as one-dimensional waves., EPSRC, Rolls-Royce

We use recurrence analysis to investigate the forced synchronization of a self-excited thermoacoustic system. The system consists of a swirl-stabilized turbulent premixed flame in an open-ended duct. We apply periodic acoustic forcing to this system at different amplitudes and frequencies around its natural self-excited frequency, and examine its response via unsteady pressure measurements. On increasing the forcing amplitude, we observe two bifurcations: from a periodic limit cycle (unforced) to quasiperiodicity (weak forcing) and then to lock-in (strong forcing). To analyse these bifurcations, we use cross-recurrence plots (CRPs) of the unsteady pressure and acoustic forcing. We find that the different time scales characterizing the quasiperiodicity and the transition to lock-in appear as distinct structures in the CRPs. We then examine those structures using cross recurrence quantification analysis (CRQA) and find that their recurrence quantities change even before the system transitions to lock-in. This shows that CRPs and CRQA can be used as alternative nonlinear tools to study forced synchronization in thermoacoustic systems, complementing classical linear tools such as spectral analysis., EPSRC

The application of laser diagnostics to high pressure combustion phenomena is particularly challenging, especially in practical combustors such as rocket motors. In this study, temperature measurements using Laser Induced Thermal Grating Spectroscopy (LITGS) are demonstrated in oxygen enriched CH4/O2/N2premixed laminar flames at pressures up to 1.0 MPa. We use a previously developed OH absorption LITGS technique to determine product gas temperatures from 0.3 to 1.0 MPa, for both high temperature oxygen-enriched and pure-oxygen flames, for measurements up to 3000 K. Further, we demonstrate how it is necessary to correct the measurements for the local absorption of laser light to obtain accurate temperatures, and offer a technique for producing the correction by using different laser energies. Once the correction is applied, we demonstrate that the measurements at 0.5 MPa are within 1.6% of the adiabatic non-strained flame temperatures, with a standard deviation of about 160 K, thus offering a competitive method for the challenging conditions at high pressures and temperatures. The values obtained at derived temperatures at 1.0 MPa were lower than the adiabatic unstrained flame temperatures, which could possibly be attributed to loss mechanisms.

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This work presents simultaneous 2D temperature and velocity measurements on a heated jet using two non-intensified cameras to realize thermographic PIV. In contrast to previous studies which use separate PIV cameras and LIP cameras, the present experiment uses only a double-pulsed UV laser and two low-speed CCD cameras running in double-frame mode, greatly simplifying the setup. The intensity ratio is calculated based on the image pair recorded at two spectral lines for thermography, while the cross-correlation is performed over the two consecutive frames for PIV. A method is proposed to correct for the effects of non-uniform spatial distribution of laser fluence on the intensity ratio, by including the laser fluence into the calibration function. The laser sheet energy profiles are measured and the intensity ratio is translated into temperature according to the local laser fluence and two-colour ratio. The temperature accuracy using this technique is estimated as 3 K at 410 K, by comparing the mean temperature field with the result provided by a thermocouple. Simultaneous 2D temperature and velocity fields are presented for a simple heated jet, demonstrating the expected similarities. The demonstration shows the potential of thermographic PIV in the investigation fundamental problems on turbulent flows, and shows that the technique can be improved through the availability of a dual-cavity UV laser.

The ability to obtain laser based quantitative temperature measurements is particularly important in high pressure environments. In this study, quantitative temperature measurements using Laser Induced Thermal Grating Spectroscopy (LITGS) were employed for a toluene vapor/air jet varying the toluene concentration and pump laser power. In LITGS, the gas temperature at the crossing point of two pump laser beams can be derived from the oscillation frequency of the LITGS signal and, in principle, the signal at high pressure is larger than that at atmospheric pressure. Thus, LITGS is a promising laser diagnostic method for high pressure environments. LITGS signal was acquired for toluene vapor/air mixtures at atmospheric pressure and room temperature and it was found that the derived temperature is influenced by the energy of incident pump laser, showing an increase with an increase in the pump energy. A semi-empirical equation showing the relationship between the relative temperature rise, pump energy and mole fraction was then derived. The experimental results match the linear semi-empirical equation., This study was supported by the Strategic International Multiple Collaborative Research Project of the Japan Society for the Promotion of Science (JSPS).

The spray combustion characteristics of rapeseed biodiesel/methyl esters (RME) and 50% RME/diesel blend were investigated and compared with conventional diesel fuel, using a model swirl flame burner. The detailed database with well-characterised boundary conditions can be used as validation targets for flame modelling. An airblast, swirl-atomized liquid fuel spray was surrounded by air preheated to 350°C at atmospheric pressure. The reacting droplet distribution within the flame was determined using phase Doppler particle anemometry. For both diesel and RME, peak droplet concentrations are found on the outside of the flame region, with large droplets migrating to the outside via swirl, and smaller droplets located around the centreline region. However, droplet concentrations and sizes are larger for RME, indicating a longer droplet evaporation timescale. This delayed droplet vaporisation leads to a different reaction zone relative to diesel, with an extended core reaction. In spite of the longer reaction zone, RME flames displayed no sign of visible soot radiation, unlike the case of diesel spray flame. Blending 50% RME with diesel results in significant reduction in soot radiation. Finally, RME emits 22% on average lower NOx emissions compared to diesel under lean burning conditions.

Laser diagnostics for quantitative measurements of temperature are important in high pressure and high temperature environments. In this study, Laser Induced Thermal Grating Spectroscopy (LITGS) is employed for oxygen enriched CH$_{4}$/O$_{2}$/N$_{2}$ laminar flames at elevated pressure. The wavelength of the pump beam was set for OH excitation. LITGS is a promising laser diagnostic for quantitative measurements of temperature at high pressure because the magnitude of LITGS signal increases with pressure due to higher quench rates and stronger density perturbations in more dense mixtures. As a result, LITGS signal can be successfully acquired at 0.5 MPa for various equivalence ratios. The derived temperature was close to the adiabatic flame temperatures for each condition and the maximum derived temperature was higher than 2600 K. Therefore, LITGS is also a promising diagnostic for quantitative temperature measurements in high temperature flames, such as rocket motors., This study was supported by the Collaborative Research Project of Institute of Fluid Science, Tohoku University. SL was supported by a DTA from EPSRC in the UK and grant EP/K02924X/1.

Simultaneous Rayleigh scattering and OH-PLIF imaging measurements of temperature and OH were used to investigate the properties of turbulent premixed flames, including the nature of the 2D thermal structures and scalar dissipation rate in the Cambridge/Sandia swirling bluff body stabilized flames, with and without the effect of swirl. Swirl creates enhanced turbulence as well as outer flow entrainment, and disrupts the pre-flame zone significantly, whilst the high temperature reaction zone as marked by OH remains relatively intact. In particular, the temperature at the location of maximum OH gradient shows very low variance across the flame region. The 2D image analysis of OH and temperature shows that the corresponding 2D gradients are aligned up to a distance of half the laminar flame thickness away from the flame front, deviating significantly in the case of swirling flames beyond that region. As in previous investigations in diffusion flames, the mean width of the observed thermal structures increases from 300 to 600 microns near the flame, with a main mode around the laminar flame thermal width in the unswirled case. The correlation between 2D thermal fluctuation gradients and variance extracted from the images shows a direct proportionality, with a slope which agrees well with theory in the region of high turbulence away from the base. At the base of the flame where turbulence is low, the local scalar dissipation becomes a function of the local temperature via the thermal diffusivity., The Leverhulme Trust funded the collaboration through an International Network grant for Stratified Flames. This work at Sandia was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences.

Planar two-dimensional two-colour time-resolved laser-induced incandescence (2D-2C-TiRe-LII) is employed to investigate soot formation in a standard ethylene laminar di usion ame. The time resolution of the 2D LII signal is realised by shifting the delay time of ICCD cameras. The two-colour con guration is applied to measure the peak temperature Tp of soot particles immediately after the laser pulse rather than using the energy balance to compute Tp. The Sauter mean diameter D32 and the corresponding distribution width parameter of the measured soot particles is extracted by using an error minimisation method. The method shows that a range of possible geometry mean particle diameters and corresponding distribution width parameters are also possible solutions, and further information on realisable particle size distribution widths is necessary to narrow down the extracted diameter.

Environmental and economic concerns have pushed aeronautical authorities to set stringent environmental regulations on fuel consumption, noise production, and pollutant emission. Engine manufacturers are developing novel staged injection concepts to ensure their respect. The injection staging creates a fuel-air mixture stratification involving new combustion processes not fully understood. This paper presents the experimental investigation of NO production for known swirled and / or stratified lean premixed flames. The fuel staging parameter defined as the stratification ratio is studied for values of 1, 2, and 3, while the swirl fractions are 0, 25 and 33%, changing the flowfield from non-swirling cond itions to high swirl numbers (up to 0.55). The implementation of simultaneous OH- and NOPLIF imaging techniques is achieved using high energy pulsed laser systems, able, for instance, to deliver 30 mJ/ pulse around the 226-nm UV wavelength for NO excitation. OH-PLIF is used to characterize the flame structure through the commonly extracted curvature, and also through the measurement of the flame thickness. These results show to be more accurate than thickness obtained from temperature profiles measured by Raman/ Rayleigh laser diagnostics. NO-PLIF is used to quantify the pollutant concentration. To this end, preliminary work was done to select the Q1 (29.5) transition as it the least temperature dependent excitation scheme with high fluorescence levels. After realizing a specific calibration of the NO-PLIF technique, the stud ied flames presented concentrations ranging from traces (20 ppm) to high levels (230 ppm). Further analysis of these results reveals that for high stratification ratios the prompt NO is favored and is responsible for the elevated level of NO pollutant.