Ignition of uniform droplet-laden weakly turbulent flows following a laser spark

The forced ignition process has a stochastic nature, which can be intensi ed due to turbulence and mixture uctuations. Although fuel droplets represent strong inhomogeneities which are generally detrimental to ignition, the presence of small droplets has been found to enhance ame speeds, decrease minimum ignition energy, and improve the ignitability of overall lean mixtures. In order to understand which factors are conducive to ignition of sprays, a spherically expanding ame is investigated, which is produced by a laser spark in a uniform dispersion of ethanol droplets in turbulent air. The ame is visualised by schlieren and OH*-chemiluminescence for overall equivalence ratios of 0.8 to 2, Sauter mean diameter of approximately 25 m, and u0=SL ranging from 0.9 to 1.3, where u0 and SL denote the rms axial velocity and laminar burning velocity, respectively. The timescales of the spark's effects on the ame are measured, as well as quenching timescales and initial kernel sizes conditional on ignition or failure. Small kernels quenched faster than approximately 0.6 ms, that is, the duration of the ame overdrive, and a minimum kernel radius for ignition of 1mm was observed. The short-mode of ignition failure was suppressed by increasing the laser energy and, consequently, the initial kernel size. Nevertheless, the ignitability of lean mixtures was only e ectively improved through high-energy sparks and partial prevaporisation of the fuel. Virtually all kernels ignited once prevaporisation was increased, and the gas-phase equivalence ratio was approximately 75% of the lower ammability limit, with ignition being limited only by laser breakdown.European Commission Clean Sky project AMEL (641453); Brazilian Space Agency and Brazil's National Council for Scientific and Technological Developmen

Mechanisms of flame propagation in jet fuel sprays as revealed by OH/fuel planar laser-induced fluorescence and OH* chemiluminescence

© 2019 The Combustion Institute Previous work on spray flames has shown that different propagation mechanisms may occur depending on the size and number density of droplets. In this work, the structure and propagation of flames in uniformly dispersed sprays of low-volatility fuels is experimentally examined. The effect of the Sauter mean diameter (SMD) of the spray (16–33 µm) on the propagation modes, flame speed, and flame curvature is assessed in weakly turbulent sprays, with the ratio of axial velocity rms to the gaseous laminar burning velocity uz′/SL,g ranging from 0.5–2.5, and overall equivalence ratio ϕ of 0.8, 1, and 1.4. The growth of the flame is evaluated from OH*-chemiluminescence and schlieren visualisation, which combined with OH/fuel planar laser-induced fluorescence visualisation reveal details of the propagation mechanisms. The aviation fuels investigated – Jet A and a renewable alternative, ATJ-8 – exhibited similar flame speed behaviour due to changes in SMD in each of the propagation modes identified: the droplet, inter-droplet, and gaseous-like modes. Concentrated reactions around large droplets found in lean conditions (ϕ = 0.8) allowed for a slowly propagating flame front which, in turn, ignited new droplets. Stoichiometric to rich conditions (ϕ = 1, 1.4) were marked by stronger evaporation ahead of the flame and, therefore, higher and more uniform heat release across the flame. Still, droplets penetrated the flame, locally inducing regions of negative flame curvature and continuing to evaporate in the burnt products. The droplet-induced effects disappeared at low SMD (16 µm, ϕ = 1.4), giving rise to a fully gaseous layer ahead of the flame and the highest flame speeds. At rich conditions and high SMD, Jet A had a lower flame speed than ATJ-8

Effect of spark location and laminar flame speed on the ignition transient of a premixed annular combustor

The flame expansion process (``light-round'') during the ignition transient in annular combustors depends on a number of parameters such as equivalence ratio (and hence laminar burning velocity, $S_L$, of the mixture), turbulent intensity, mean flow magnitude and direction, geometry, and spark location. Here, an experimental study on a fully premixed, swirled, bluff-body stabilised annular combustor is carried out to identify the sensitivity of the light-round to these parameters. A wide range of conditions were assessed: two inter-burner spacing distances, two fuels (methane and ethylene), bulk velocities from 10 to 30 m/s, and $\phi$ between 0.75 and 1 for methane and 0.58 and 0.9 for ethylene. The spark location was varied longitudinally ($x/D$ = 0.5 and $x/D$ = 5, where $D$ is the bluff body diameter, expected to lie inside and downstream of the inner recirculation zone of a single burner, respectively) and azimuthally. The propagation of the flame during the ignition transient was investigated via high speed (10 kHz) OH$^*$ chemiluminescence using two cameras to simultaneously image the annular chamber from axially downstream and from the side of the combustor. The pattern of flame propagation depended on the initial longitudinal spark location and comprised of burner-to-burner propagation close to the bluff bodies and upstream propagation of the flame front. The spark azimuthal position\textcolor{red}{, in this horizontal configuration,} had a negligible impact on the light-round time ($\tau_{LR}$), thus buoyancy plays a minor role in the process. In contrast, sparking at $x/D$ = 5 resulted in an increase in $\tau_{LR}$ by $\sim$30-40\% for all the conditions examined. The inter-burner spacing had a negligible effect on $\tau_{LR}$. When increasing bulk velocity, $\tau_{LR}$ decreased. For a constant bulk velocity, $\tau_{LR}$ depended strongly on $S_L$ and it was found that mixtures with the same $S_L$ from different fuels resulted in the same $\tau_{LR}$. Further, the observed propagation speed, corrected for dilatation, was approximately proportional to $S_L$ and was within 30\% of estimates of the turbulent flame speed at the same conditions. These findings suggest that $S_L$ is one of the controlling parameters of the light-round process; hence turbulent flame propagation has a major role in the light-round process, in addition to dilatation and flame advection by the mean flow. The results reported in the study help explain the mechanism of light-round and can assist the development of efficient ignition procedures in aviation gas turbines.EU project ANNULIGHT (765998

Polydispersity Effects in Low-order Ignition Modeling of Jet Fuel Sprays

Low-order ignition models are important tools in the design of aviation gas turbines. In this paper, a stochastic model that predicts the ignition probability in a combustor based on a timeaveraged cold- ow solution is extended to include local fuel concentration uctuations due to the polydisperse nature of the spray. For this, a stochastic approach to modelling such uctuations is considered, and the e ects of the ow and mixture parameters on the resulting equivalence ratio pdfs are investigated. The concentration of fuel in large droplets results in a high variation of the local equivalence ratio, hence a ecting the local ammability factor at the model's cell scale. The extinction criterion of the ignition model based on a critical Karlovitz number is calibrated based on ignition probability data from canonical experiments using jet fuel, suggesting critical Karlovitz values of spray ames between 0.2-0.6, which is to be contrasted with values of 1.5 for gaseous fuels.EU Clean Sky 2 project PROTEUS (785349

Ignition probability and lean ignition behaviour of a swirled premixed bluff body stabilised annular combustor

Abstract An experimental investigation was performed in a premixed annular combustor equipped with multiple swirl, bluff body burners to assess the ignition probability and to provide insights into the mechanisms of failure and of successful propagation. The experiments are done at conditions that are close to the lean blow-off limit (LBO) and hence the ignition is difficult and close to the limiting condition when ignition is not possible. Two configurations were employed, with 12 and 18 burners, the mixture velocity was varied between 10 and 30 m/s, and the equivalence ratio (ϕ) between 0.58 and 0.68. Ignition was initiated by a sequence of sparks (2 mm gap, 10 sparks of 10 ms each) and “ignition” is defined as successful ignition of the whole annular combustor. The mechanism of success and failure of the ignition process and the flame propagation patterns were investigated via high-speed imaging (10 kHz) of OH* chemiluminescence. The lean ignition limits were evaluated and compared to the lean blow-off limits, finding the 12-burner configuration is more stable than the 18-burner. It was found that failure is linked to the trapping of the initial flame kernel inside the inner recirculation zone (IRZ) of a single burner adjacent to the spark, followed by localised quenching on the bluff body probably due to heat losses. In contrast, for a successful ignition, it was necessary for the flame kernel to propagate to the adjacent burner or for a flame pocket to be convected downstream in the chamber to grow and start propagating upwards. Finally, the ignition probability (Pign) was obtained for different spark locations. It was found that sparking inside the recirculation zone resulted in Pign ∼ 0 for most conditions, while Pign increased moving the spark away from the bluff-body or placing it between two burners and peaked to Pign ∼ 1 when the spark was located downstream in the combustion chamber, where the velocities are lower and the turbulence less intense. The results provide information on the most favourable conditions for achieving ignition in a complex multi-burner geometry and could help the design and optimisation of realistic gas turbine combustors.EU Project ANNULIGHT (765998

Pre-chamber ignition mechanism: Experiments and simulations on turbulent jet flame structure

This work investigates the effects of premixed combustion kinematics in pre-chamber volumes on the development of emitted hot jets from the igniter. The effects of fuel type, orifice diameter, and ignition location are evaluated experimentally, with high-speed OH* and CH* chemiluminescence imaging, and computationally with Large-Eddy Simulations (LES). The imaging experiments allowed for simultaneous viewing of combustion processes within a quartz chamber and of the developing jet flow. Results from these experiments provided insight on the temporal evolution of the jet relative to the growth of an ignited kernel within the chamber, as well as information on the emission or lack of emission of radical species from the chamber. Computational results provided data on the temporal behavior of the pressure within the chamber and profiles of the high velocity flow through the orifice. These results, combined, have shown that dependent on the strain rate and effective orifice size, local quenching of radical species at the orifice occurs which fundamentally change whether hot products, reactive layers, or both are present in the turbulent jet emission. The dynamic structure and composition of the turbulent jet controls its relevance as an effective ignition source

Minimally processed fruit salad enriched with Lactobacillus acidophilus: Viability of anti-browning compounds in the preservation of color

Minimal processing promotes browning of some vegetal tissues due to cell membrane disruption, which results in the release of oxidative enzymes. This study evaluated the efficiency of citric acid, ascorbic acid, sodium metabisulfite and L-cysteine hydrochloride to retard enzymatic browning of minimally processed fruit salad and enriched this product with Lactobacillus acidophilus LA-5. Control treatment was fruit salad immersed in water. Polyphenol oxidase (PPO) and color (L*, a*, b*, index color - CI, browning index - BI, c*, and h°) were analyzed. The viability of L. acidophilus was also evaluated using Rogosa agar in fruit salads containing anti-browning compounds in higher concentrations. PPO presented a significant difference among control and fruit salad treated with ascorbic acid and L-cysteine hydrochloride, indicating the highest anti-browning activity of these compounds. The fruit color was affected by processing and storage time, with a reduction in the values of L* over time. Values of a*, c*, h° angle and CI indicated a predominance of red color in the fruit salad. Salads containing anti-browning compounds in higher concentrations presented viability of L. acidophilus above 7.43 log CFU/g up to the fifth day of storage, indicating that the product can be promised as probiotic. Thus, the fruit salad treated with anti-browning compounds has potential use as a probiotic carrier.Keywords: Fresh-cut fruits, color, ascorbic acid, vegetable matrix, probiotic culture

Soot particle size distribution measurements in a turbulent ethylene swirl flame

There is a need to better understand particle size distributions (PSDs) from turbulent flames from a theoretical, practical and even regulatory perspective. Experiments were conducted on a sooting turbulent non-premixed swirled ethylene flame with secondary (dilution) air injection to investigate exhaust and in-burner PSDs measured with a Scanning Mobility Particle Sizer (SMPS) and soot volume fractions (fv) using extinction measurements. The focus was to understand the effect of systematically changing the amount and location of dilution air injection on the PSDs and fv inside the burner and at the exhaust. The PSDs were also compared with planar Laser Induced Incandescence (LII) calibrated against the average fv. LII provides some supplemental information on the relative soot amounts and spatial distribution among the various flow conditions that helps interpret the results. For the flame with no air dilution, fv drops gradually along the centreline of the burner towards the exhaust and the PSD shows a shift from larger particles to smaller. However, with dilution air fv reduces sharply where the dilution jets meet the burner axis. Downstream of the dilution jets fv reduces gradually and the PSDs remain unchanged until the exhaust. At the exhaust, the flame with no air dilution shows significantly more particles with an fv one to two orders of magnitude greater compared to the Cases with dilution. This dataset provides insights into soot spatial and particle size distributions within turbulent flames of relevance to gas turbine combustion with differing dilution parameters and the effect dilution has on the particle size. Additionally, this work measures fv using both ex situ and in situ techniques, and highlights the difficulties associated with comparing results across the two. The results are useful for validating advanced models for turbulent combustion.European Union (EU), Horizon 2020 (H2020), Clean Sky 2 Joint Undertaking, Industrial Leadership (IL) (Project LEAFINNOX, grant number 831804

Evolution of spray and aerosol from respiratory releases: theoretical estimates for insight on viral transmission

By modelling the evaporation and settling of droplets emitted during respiratory releases and using previous measurements of droplet size distributions and SARS-CoV-2 viral load, estimates of the evolution of the liquid mass and the number of viral copies suspended were performed as a function of time from the release. The settling times of a droplet cloud and its suspended viral dose are significantly affected by the droplet composition. The aerosol (defined as droplets smaller than 5 μm) resulting from 30 s of continued speech has O(1 h) settling time and a viable viral dose an order-of-magnitude higher than in a short cough. The time-of-flight to reach 2 m is only a few seconds resulting in a viral dose above the minimum required for infection, implying that physical distancing in the absence of ventilation is not sufficient to provide safety for long exposure times. The suspended aerosol emitted by continuous speaking for 1 h in a poorly ventilated room gives 0.1–11% infection risk for initial viral loads of 108–1010 copies ml−ll, respectively, decreasing to 0.03–3% for 10 air changes per hour by ventilation. The present results provide quantitative estimates useful for the development of physical distancing and ventilation controls

Soot-free low-NOx aeronautical combustor concept: the lean azimuthal flame for kerosene sprays

An ultralow emission combustor concept based on “flameless oxidation” is demonstrated in this paper for aviation kerosene. Measurements of gas emissions, as well as of the size and number of nanoparticles via scanning mobility particle sizing, are carried out at the combustor outlet, revealing simultaneously soot-free and single-digit NOx levels for operation at atmospheric conditions. Such performance, achieved with direct spray injection of the fuel without any external preheating or prevaporization, is attributed to the unique mixing configuration of the combustor. The combustor consists of azimuthally arranged fuel sprays at the upstream boundary and reverse-flow air jets injected from downstream. This creates locally sequential combustion, good mixing with hot products, and a strong whirling motion that increases residence time and homogenizes the mixture. Under ideal conditions, a clean, bright-blue kerosene flame is observed, free of soot luminescence. Although soot is intermittently formed during operation around optimal conditions, high-speed imaging of the soot luminescence shows that particles are subjected to long residence times at O2-rich conditions and high temperatures, which likely promotes their oxidation. As a result, only nanoparticles in the 2–10 nm range are measured at the outlet under all tested conditions. The NOx emissions and completeness of the combustion are strongly affected by the splitting of the air flow. Numerical simulations confirm the trend observed in the experiment and provide more insight into the mixing and air dilution