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

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

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

Dynamics of acoustically forced non-premixed flames close to blow-off

The effect of forced oscillations on the behaviour of non-premixed swirling methane flames close to the lean blow out limits was investigated using experiments in a lab-scale burner. Two different fuel injection geometries, non-premixed with radial -NPR- and non-premixed with axial -NPA- fuel injection, are considered. The flame behaviour is studied using 5 kHz OH* chemiluminescence and OH Planar Laser Induced Fluorescence (OH PLIF) imaging. In both systems, acoustic forcing reduces the stability of the flame, and in particular, the stability was found to decrease with the increase in forcing amplitude. Flame lift-off was observed in both configurations, with the magnitude of the effect of forcing depending on the fuel injection configuration. The results provide insight on the effect of superimposed flow field fluctuations in systems operating close to the lean blow out limits and offer useful data for the development and validation of numerical models for the prediction of the dynamic behaviour of flames of industrial interest

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

Response of flames with different degrees of premixedness to acoustic oscillations

The response of three flames with different degrees of premixedness (fully premixed, non-premixed with radial, and non-premixed with axial fuel injection) to acoustic oscillations is studied experimentally. The flames were imaged using OH* chemiluminescence and OH Planar Laser Induced Fluorescence (OH PLIF) at 5 kHz. In addition to a flame kinematics analysis, the amplitude dependence of the transfer function was calculated. The dominant spatial structures of the heat release and their periodicity were examined using the Proper Orthogonal Decomposition (POD) method. The non-premixed system with radial fuel injection showed the highest response to acoustic forcing, followed by the fully premixed and the non-premixed system with axial fuel injection. In addition, the response of the non-premixed system with radial fuel injection was greater than that of the fully premixed system for various bulk velocities U, global φ, forcing amplitudes, A and forcing frequencies, f. About 70% of the energy of the total fluctuations in the NPR case is contained in the first four POD modes, a percentage that decreases with overall equivalence ratio, but only this drops to about 40% for the NPA flame. The frequency spectra of the coefficients of the POD modes have peaks at the forcing frequency, with increasing broadband contributions in higher modes and for the NPA flame

The moderating role of overcommitment in the relationship between psychological contract breach and employee mental health

Reimann M. The moderating role of overcommitment in the relationship between psychological contract breach and employee mental health. JOURNAL OF OCCUPATIONAL HEALTH. 2016;58(4-5):425-433.Objectives: This study investigated whether the association between perceived psychological contract breach (PCB) and employee mental health is moderated by the cognitive-motivational pattern of overcommitment (OC). Linking the psychological contract approach to the effort-reward imbalance model, this study examines PCB as an imbalance in employment relationships that acts as a psychosocial stressor in the work environment and is associated with stress reactions that in turn negatively affect mental health. Methods: The analyses were based on a sample of 3,667 employees who participated in a longitudinal linked employer employee survey representative of large organizations (with at least 500 employees who are subject so social security contributions) in Germany. Fixed-effects regression models, including PCB and OC, were estimated for employee mental health, and interaction effects between PCB and OC were assessed. Results: The multivariate fixed-effects regression analyses showed a significant negative association between PCB and employee mental health. The results also confirmed that OC does indeed significantly increase the negative effect of PCB on mental health and that OC itself has a significant and negative effect on mental health. Conclusions: The results suggest that employees characterized by the cognitive-motivational pattern of OC are at an increased risk of developing poor mental health if they experience PCB compared with employees who are not overly committed to their work. The results of this study support the assumption that psychosocial work stressors play an important role in employee mental health

Clinical prevalence of Lewy body dementia.

BACKGROUND: The prevalence of dementia with Lewy bodies (DLB) and dementia in Parkinson's disease (PDD) in routine clinical practice is unclear. Prevalence rates observed in clinical and population-based cohorts and neuropathological studies vary greatly. Small sample sizes and methodological factors in these studies limit generalisability to clinical practice. METHODS: We investigated prevalence in a case series across nine secondary care services over an 18-month period, to determine how commonly DLB and PDD cases are diagnosed and reviewed within two regions of the UK. RESULTS: Patients with DLB comprised 4.6% (95% CI 4.0-5.2%) of all dementia cases. DLB was represented in a significantly higher proportion of dementia cases in services in the North East (5.6%) than those in East Anglia (3.3%; χ2 = 13.6, p < 0.01). DLB prevalence in individual services ranged from 2.4 to 5.9%. PDD comprised 9.7% (95% CI 8.3-11.1%) of Parkinson's disease cases. No significant variation in PDD prevalence was observed between regions or between services. CONCLUSIONS: We found that the frequency of clinical diagnosis of DLB varied between geographical regions in the UK, and that the prevalence of both DLB and PDD was much lower than would be expected in this case series, suggesting considerable under-diagnosis of both disorders. The significant variation in DLB diagnostic rates between these two regions may reflect true differences in disease prevalence, but more likely differences in diagnostic practice. The systematic introduction of more standardised diagnostic practice could improve the rates of diagnosis of both conditions

Functional diversity of marine ecosystems after the Late Permian mass extinction event

Article can be accessed from http://www.nature.com/ngeo/journal/v7/n3/full/ngeo2079.htmlThe Late Permian mass extinction event was the most severe such crisis of the past 500 million years and occurred during an episode of global warming. It is assumed to have had significant ecological impact, but its effects on marine ecosystem functioning are unknown and the patterns of marine recovery are debated. We analysed the fossil occurrences of all known Permian-Triassic benthic marine genera and assigned each to a functional group based on their inferred life habit. We show that despite the selective extinction of 62-74% of marine genera there was no significant loss of functional diversity at the global scale, and only one novel mode of life originated in the extinction aftermath. Early Triassic marine ecosystems were not as ecologically depauperate as widely assumed, which explains the absence of a Cambrian-style Triassic radiation in higher taxa. Functional diversity was, however, significantly reduced in particular regions and habitats, such as tropical reefs, and at these scales recovery varied spatially and temporally, probably driven by migration of surviving groups. Marine ecosystems did not return to their pre-extinction state, however, and radiation of previously subordinate groups such as motile, epifaunal grazers led to greater functional evenness by the Middle Triassic

Plant Trait Diversity Buffers Variability in Denitrification Potential over Changes in Season and Soil Conditions

BACKGROUND: Denitrification is an important ecosystem service that removes nitrogen (N) from N-polluted watersheds, buffering soil, stream, and river water quality from excess N by returning N to the atmosphere before it reaches lakes or oceans and leads to eutrophication. The denitrification enzyme activity (DEA) assay is widely used for measuring denitrification potential. Because DEA is a function of enzyme levels in soils, most ecologists studying denitrification have assumed that DEA is less sensitive to ambient levels of nitrate (NO(3)(-)) and soil carbon and thus, less variable over time than field measurements. In addition, plant diversity has been shown to have strong effects on microbial communities and belowground processes and could potentially alter the functional capacity of denitrifiers. Here, we examined three questions: (1) Does DEA vary through the growing season? (2) If so, can we predict DEA variability with environmental variables? (3) Does plant functional diversity affect DEA variability? METHODOLOGY/PRINCIPAL FINDINGS: The study site is a restored wetland in North Carolina, US with native wetland herbs planted in monocultures or mixes of four or eight species. We found that denitrification potentials for soils collected in July 2006 were significantly greater than for soils collected in May and late August 2006 (p<0.0001). Similarly, microbial biomass standardized DEA rates were significantly greater in July than May and August (p<0.0001). Of the soil variables measured--soil moisture, organic matter, total inorganic nitrogen, and microbial biomass--none consistently explained the pattern observed in DEA through time. There was no significant relationship between DEA and plant species richness or functional diversity. However, the seasonal variance in microbial biomass standardized DEA rates was significantly inversely related to plant species functional diversity (p<0.01). CONCLUSIONS/SIGNIFICANCE: These findings suggest that higher plant functional diversity may support a more constant level of DEA through time, buffering the ecosystem from changes in season and soil conditions