Lean Regions Research Articles

Plant-based marbled salami analogs: Emulsion-loaded microgels embedded within protein-polysaccharide hydrogel matrices

The inherent molecular differences between plant and animal proteins make it challenging to accurately simulate the optical and mechanical properties of real meat products using plant-derived ingredients. Soft matter physics approaches are therefore being employed to direct the assembly of plant proteins, polysaccharides, and lipids into meat-like structures. The main focus of the current study was to use soft matter physics approaches to create plant-based analogs of marbled meat products, such as salami, chorizo, or pepperoni. These products have whitish fatty regions embedded within reddish/brown lean regions. In this study, we mimicked the fatty regions in salami using emulsion-loaded alginate beads and the lean regions using composite hydrogels assembled from potato protein and gellan gum. Initially, the textural attributes of the plant-based lean regions were optimized by varying potato protein type (low or high isoelectric point) and concentration. As expected, the mechanical strength of the lean region analogs increased with increasing protein content and depended on protein type. Introducing alginate beads into the salami analogs had little impact on their textural attributes when measured under compression conditions but decreased their stiffness and fracture stress when measured under tensile conditions. In particular, introducing either small or large beads reduced the stiffness and breaking stress of the salami analogs, which was attributed to the introduction of weak points in the composite hydrogel. Finally, we showed that the appearance and textural attributes of real salami could be mimicked in plant-based salami by optimizing its composition and structure. This research provides valuable insights into methods of better simulating the properties of marbled meat products, like sausages and cold-cut meat products, thereby creating a more sustainable food supply.

Investigating the potential of a higher reactivity fuel to achieve faster heat-up of aftertreatment systems

Shortening the aftertreatment system heat-up period will be crucial to meet upcoming emissions regulations, which mandate significant reductions in tail-pipe emissions for on-road heavy-duty vehicles. This work studies the impact of increasing fuel reactivity on the combustion of post injections during the expansion stroke. The work demonstrates an operating strategy using a blend of di-butyl ether (a potential low-carbon biofuel component) and #2 diesel that achieves higher exhaust enthalpy, while maintaining similar engine-out emissions (NOx and combustion efficiency) in comparison to the baseline #2 diesel fuel (cetane number 41). On average, a 12% increase in exhaust enthalpy was observed for the blend (cetane number 60) relative to the baseline fuel. The ability to operate the higher reactivity fuel at a more retarded post-injection timing, where combustion with #2 diesel was found to be more unstable, and less complete was key to achieving higher exhaust enthalpy. However, a small fuel penalty (∼ 3%–5%) was also associated with the higher reactivity fuel with this operating strategy. The formation of fuel lean regions, due to the long ignition delays at late injection conditions, was predicted to be the leading contributor to the increased emissions of products of partial combustion. Chemical kinetic simulations using surrogates for the fuels used in the experiments, demonstrated the ability of a higher cetane fuel to outperform the baseline fuel under fuel lean conditions with equivalence ratios less than 0.7.

3D simulation to evaluate MTU4000 heavy-duty diesel engine power enhancement considering performance limitations and combustion improvement by reducing lean regions

3D simulation to evaluate MTU4000 heavy-duty diesel engine power enhancement considering performance limitations and combustion improvement by reducing lean regions

3D simulation to evaluate MTU4000 heavy-duty diesel engine power enhancement considering performance limitations and combustion improvement by reducing lean regions

3D simulation to evaluate MTU4000 heavy-duty diesel engine power enhancement considering performance limitations and combustion improvement by reducing lean regions

Combustion Regimes in Turbulent Non-Premixed Flames for Space Propulsion

Direct numerical simulations of non-premixed fuel-rich methane–oxygen flames at 20 bar are conducted to investigate the turbulent mixing burning of gaseous propellants in rocket engines. The reacting flow is simulated by using an EBI-DNS solver within an OpenFOAM frame. The transport of species is resolved with finite-rate chemistry by using a complex skeletal mechanism that entails 21 species. Two different flames at low and high Reynolds numbers are considered to study the sensitivity of the flame dynamics to turbulence. Regime markers are used to measure the probability of the flow to burn in premixed and non-premixed conditions at different regions. The local heat release statistics are studied in order to understand the drivers in the development of the turbulent diffusion flame. Despite the eminent non-premixed configuration, a significant amount of combustion takes place in premixed conditions. Premixed combustion is viable in both lean and fuel-rich regions, relatively far from the stoichiometric line. It has been found that a growing turbulent kinetic energy is detrimental to combustion in fuel-rich premixed conditions. This is motivated by the disruption of the local premixed flame front, which promotes fuel transport into the diffusion flame. In addition, at downstream positions, higher turbulence enables the advection of methane into the lean core of the flame, enhancing the burning rates in these regions. Therefore, the primary effect of turbulence is to increase the fraction of propellants burnt in oxygen-rich and near-stoichiometric conditions. Consequently, the mixture fraction of the products shifts towards lean conditions, influencing combustion completion at downstream positions.

Time-resolved study of mixing and reaction in an aero-engine model combustor at increased pressure

In this study, we experimentally investigate large-scale vortex structures, fuel-air mixing and reaction processes, which occur in a partially premixed swirl-stabilized ethylene/air flame in a laboratory-scale gas turbine model combustor. Time-resolved stereo-PIV, OH-PLIF, and acetone/PAH-PLIF systems with a repetition rate of 10 kHz are used to investigate the flame behavior at 3 bar pressure and an equivalence ratio of Φ=1.2. For isothermal conditions, the results strongly indicate that vortex-induced roll-up of fuel is a main driver of the mixing process. For the reacting case, a precessing vortex core (PVC) and a double helical vortex (DHV), both co-rotating with the swirl direction and occurring simultaneously in the inner and outer shear layer, respectively, are identified by using proper orthogonal decomposition (POD) and phase-averaging techniques. Acetone-PLIF measurements show that the fuel exhibits a motion that resembles a strong axial flapping at the frequency of the DHV at reacting conditions when considering the measurement plane. The measurements show that the PVC and DHV cause a regular sequence of flame roll-up, mixture of burned and unburned gases, and the subsequent ignition of this mixture. The pockets of rich burned gas formed by mixing and reaction can eventually be oxidized by OH-rich lean burned gas regions that have the potential to prevent soot formation; however, this process strongly depends on intermittent motion within the inner recirculation zone. Based on the additional visualization of PAH-PLIF distributions, the present study further reveals for the first time the complete sequence of formation of rich burned gas, PAH and soot in a GT combustor. It is shown that soot forms only in certain regions of the high-PAH rich burned gas pockets. This indicates that other factors, such as the detailed composition of PAHs or local temperatures, which cannot be resolved here, likely also play an important role in soot formation.

Simulation of the Self-Ignition of a Cold Premixed Ethylene-Air Jet in Hot Vitiated Crossflow

The aim of this paper is to analyze the self-ignition of a jet flame in hot vitiated cross flow using Large Eddy Simulation with analytically reduced chemistry. A rich premixed ethylene-air mixture (phi = 1.2) at 300 K is injected into a hot vitiated crossflow at 1500 K. The simulated reacting flow steady-state was validated against experiments in previous publications and the focus of the present work is the transient self-ignition of the jet. It is shown that spontaneous ignition occurs at very lean mixture fractions in the form of reacting patches in the windward jet mixing layer. These patches grow, laterally wrap the jet and extend into the recirculation region. Chemical explosive mode analysis is performed to identify the chemically active regions that are precursors of the patches undergoing spontaneous ignition. It is shown that the self-ignition occurs at very lean fuel concentrations regions, which are leaner and hotter than the most reactive mixture fraction of the jet and crossflow. This is explained by the fact that the scalar dissipation is significantly lower in these very lean regions. Ultimately, the peak heat release moves toward the richer regions and an autoignition cascade governs the steady state flame anchoring.

The role of alcohol biofuels in advanced combustion: An analysis

The production of alcohol fuels from bioderived feedstocks and the performance of next generation stratified low temperature combustion (LTC) modes for internal combustion engines are two research areas that have recently undergone rapid growth independently. Now, there is a need to bridge these two fields and identify the optimal combustion strategy for these biosynthesized alcohol fuels. This work addresses this need by using a metric called the normalized φ-sensitivity to analyze how the local ignition delays of regions in the combustion chamber respond to stratification induced by the compression stroke injection of fuel, thereby identifying the optimal next generation stratified LTC strategy for a number of biosynthesized alcohol fuels.The large set of next generation stratified LTC modes are generalized into two groups based on how the heat release process proceeds in the compositionally stratified combustion chamber: lean-to-rich burn stratified combustion, where the lean regions ignite first followed sequentially by progressively richer regions, or rich-to-lean burn stratified combustion, the inverse of lean-to-rich. The C1-C4 alcohols are prime candidates to enable lean-to-rich burn stratified combustion based on their high cooling potentials and lack of negative temperature coefficient (NTC) behavior. Specifically, methanol or water-alcohol blends were found to be the best candidates. The C5 + alcohols were found to be prime candidates to enable rich-to-lean burn combustion based on their emergent NTC-behavior and low cooling potentials that approach their alkane counterparts. Specifically, n-pentanol, n-hexanol, and hexanol isomers were found to be the best candidates based on balancing their normalized φ-sensitivity and their autoignition resistance.

Role of intrinsic and extrinsic defects in H implanted hydrothermally grown ZnO

The impact of hydrogen in ZnO is revealed by combining reaction dynamics calculations with temperature dependent Hall (TDH), photoluminescence, and secondary ion mass spectrometry measurements performed on H, 2H, and He implanted ZnO. H and 2H box profiles with a concentration ranging from ∼3×1017 cm−3 to ∼1019 cm−3 and He to produce as much as damage as in the [H] ∼3×1017 cm−3 case were implanted in the samples. The formation of Li lean regions has been observed for [2H] <1019 cm−3 after annealing at 400 °C. This is attributed to Lii presence consequent to the diffusion of Zni created during the H/2H implantation process. Results extracted from the TDH measurements performed prior to the annealing at 400 °C evidence that Lii contributes to an increase in carrier concentration up to ∼1017 cm−3 by providing a donor level with an activation energy of ∼40 meV and thus is very close to the value of ∼47 meV expected for H in the oxygen site. The reaction dynamics analysis evidences that the amount of Lii introduced is decreasing at higher H implantation doses as a result of increasing VZn and H-VZn retrapping, reactions in which Lii is competing with H. Overall, due to Lii formation as well as the presence of Al the maximum percentage of the implanted H or 2H acting as a donor in the investigated range is found to be ≲ 2%, which is considerably lower than previously reported.

Effect of varying fuel types on oxy‐combustion performance

In search for clean energy solutions in a global warming era, oxy-fuel combustion systems are promising. In the study, combustion products are calculated, and exergy analysis is done using the proposed multifeature equilibrium combustion model. And the results obtained for oxy-combustion of different fuels at various oxygen fractions are given in comparison with conventional combustion. For validation, the model results are compared with popular combustion calculation tools, GASEQ and CEA. Effect of oxygen content on oxy-combustion exergy analysis is calculated, also considering changes in equivalence ratio and combustion chamber inlet temperature. Moreover, indicating parameters for combustion performance, temperature ratio, chemical exergy, physical exergy, total specific exergy, and exergy destruction are utilized in the calculations elaborately. Changes in combustion product mole fractions are explained for rich and lean combustion regions. And also, specific exergy results are presented. In terms of exergy destruction, oxy-combustion is more advantageous than conventional combustion. It has been shown that exergy destruction in combustion process with conventional air is approximately 1.5 times higher compared with 21% oxy-combustion, both at different equivalence ratios and at different combustion chamber inlet temperatures. Nowadays, environment-friendly, clean energy production systems are growing in numbers. In this concept, exergetic analyses of combustion for different fuels and greener natural gas, compared with diesel, gasoline, and methanol, are given in comparison. Considering four fuel types, advantageous and disadvantageous cases are presented for oxy-combustion at different oxygen fractions and conventional combustion. As a result, diesel fuel is more advantageous than the other three fuel types, in terms of temperature ratio and exergy. Natural gas combustion appears to be disadvantageous in terms of specific exergy and temperature ratio, but it is the most advantageous in terms of exergy destruction. Consequently, distinctive comparison is done for oxy-combustion and conventional combustion, determining positive and negative effects for different fuels.

Experimental investigation of pilot-fuel combustion in dual-fuel engines, Part 2: Understanding the underlying mechanisms by means of optical diagnostics

The pilot-fuel auto-ignition and combustion under engine-like conditions in compressed methane/air mixtures are investigated in a RCEM using a single-hole coaxial injector. In Part 1, the phenomenology of the pilot-fuel combustion was studied based on the thermodynamic analysis. With the addition of methane, a prolonged pilot-fuel combustion duration was observed, especially at increased EGR rates. The aim of Part 2 is to improve the understanding of the underlying processes governing the pilot-fuel burning and premixed flame initiation in dual-fuel engines. The thermodynamic analysis is supplemented by optical diagnostics including the high-speed CH2O-PLIF, Schlieren and OH*, and corroborated with homogeneous reactor and laminar flame speed calculations. The investigations focus on determining the role of (a) ignition location and number of ignition kernels, (b) stratification of the autoignition time due to the methane chemistry effects, and (c) the role of flame propagation during the pilot-fuel burning. In the initial phase, combustion is found to propagate through an auto-igniting front. When combustion reaches the lean zones with a high spatial stratification of the autoignition time, premixed flame propagation becomes the dominant mechanism, owing to its higher spreading rate. Both processes influence the pilot-fuel combustion duration. At higher methane concentration, the simulations predict an increasing stratification of the ignition delay in lean regions, while the laminar flame speed in the pilot-fuel lean regions moderately increases. Overall, this explains the observed trend of longer pilot-fuel combustion duration in the dual-fuel cases and indicates an increasing role of flame-propagation in the dual-fuel combustion pilot-fuel burning.

Large-scale dislocation dynamics simulations of strain hardening of Ni microcrystals under tensile loading

The strain hardening in FCC Ni was studied along low index directions using 3-dimensional discrete dislocation dynamics. Large (20 × 20 × 50 μm) Ni microcrystals were simulated using rectangular parallelepiped-cells loaded in tension along four low-index directions ([111], [001], [110] and [112]) to shear strains of ∼0.01–0.02. Loading was at a constant strain rate of 10/sec, and all surfaces of the cell are treated as free surfaces. The FCC dislocation mobility routines were modified to include thermally activated cross-slip processes, as a function of three different stress components, using the results of previous atomistic simulations. These include bulk cross slip (cross-slip at atomic jogs), intersection cross slip (attractive and repulsive) as well as surface cross slip. One of these, repulsive intersection cross-slip, has zero activation energy and is present at all simulated deformation temperatures. The simulations were performed for three different temperatures, 5, 150 and 300 K. The strain-hardening rate is independent of temperature and of the order of μ/200 – μ/400, in agreement with experimental data for the <111> and <001> orientations of deformation. For the [001] orientation, at 5 and 150 K, the strain hardening rate decreases considerably when repulsive intersection cross-slip is removed from the simulations. The [110] and [112] orientations exhibit single-slip glide and a very low strain-hardening rate (∼μ/3000). Heterogeneity of dislocation microstructure develops spontaneously at the higher temperatures as a result of increased cross slip. Even though the strain-hardening rate is independent of temperature, the increase in dislocation density with shear strain is larger at higher temperatures. It is proposed that higher temperatures deformation produces larger dislocation-microstructure heterogeneities, providing for higher average dislocation densities and regions of low density where deformation can proceed. Also, the strain hardening rate at 300 K is controlled by the rate of increase of forest dislocation density in these lean regions.

Influence of piston shape and injector geometry on combustion and emission characteristics of syngas in direct-injection spark-ignition engine

This paper presents a numerical study of the influence of piston shape and injector geometry on combustion and emissions characteristics of a direct-injection spark-ignition engine fueled by syngas (50% by volume of hydrogen, 50% by volume of carbon monoxide) under low/medium load conditions. Three different piston cup geometries namely: High-clearance Combustion Cup (HCC), Low-clearance Combustion Cup (LCC) and Omega Combustion Cup (OCC) have been considered with a compression ratio of 14. An axial full-cone gas jet injector has been considered together with a hollow-cone gas jet injector with several included half-angles, i.e. 30°, 45°, 52.5° and 60°. Computational fluid dynamics modelling has been performed to simulate the combustion process. The results indicate that, in terms of performance, OCC shape is favorable, even if OCC generates relatively higher NOx than the other two configurations. A further analysis has been performed by simulating an engine with OCC piston shape and an included half-angle of injection of 30°, by varying the Start Of Injection (SOI). The results show that the flame propagation velocity reduces as the SOI advances, since the fuel distribution becomes more homogeneous approaching to a premixed case. However, the flame speed reduction is partially balanced by the disappearance of very lean regions thanks to fuel convection and diffusion.

Importance of turbulence-chemistry interactions at low temperature engine conditions

The role of turbulence-chemistry interaction in autoignition and flame stabilization is investigated for spray flames at low temperature combustion (LTC) conditions by performing high-fidelity three-dimensional computational fluid dynamics (CFD) simulations. A recently developed Tabulated Flamelet Model (TFM) is coupled with a large eddy simulation (LES) framework and validated across a range of Engine Combustion Network (ECN) ambient temperature conditions for n-dodecane fuel. High resolution grids with 0.0625 mm minimum cell size and 25 million total cell count are implemented using adaptive mesh refinement over the spray and combustion regions. Simulations with these grids and multiple LES realizations, with a 103 species n-dodecane mechanism show good agreement with experimental data for all the ambient conditions investigated. This modeling approach with the computational cost advantage of tabulated chemistry is then extended towards understanding the auto-ignition and flame stabilization at an ambient temperature of 750 K. These low temperature conditions lead to substantially higher ignition delays and flame liftoff lengths, and significantly leaner combustion compared to conventional high temperature diesel combustion. These conditions also require the simulations to span significantly larger temporal and spatial dimensions thereby increasing the computational cost. The TFM approach is able to capture autoignition and flame liftoff length at the low temperature conditions. Significant differences with respect to mixing, species formation and flame stabilization are observed under low temperature compared to conventional diesel combustion. At higher ambient temperatures, formation of formaldehyde is observed in the rich region (phi > 1) followed by the formation of OH in the stoichiometric regions. Under low temperature conditions, formaldehyde is observed to form at leaner regions followed by the onset of OH formation in significantly lean regions of the flame. Qualitative differences between species formation and transient flame development for the high and low temperature conditions are presented. The two stage ignition process is further investigated by studying the species formation in mixture fraction space by solving 1D flamelet equations for different scalar dissipation rates and homogeneous reactor assumption. Results show that scalar dissipation causes these radicals to diffuse within the mixture fraction space. This significantly enhances ignition and plays a dominant role at such low temperature conditions which cannot be captured by the homogeneous reaction assumption based model.

Production, analysis and combustion characterization of biomass fast pyrolysis oil – Biodiesel blends for use in diesel engines

In this study, blends of biodiesel and softwood derived fast pyrolysis oil (FPO) were produced, characterized, and evaluated for thermodynamic and emission performance in a single cylinder research engine. FPO was supplied from an in-house fluidized bed reactor as well as a commercial source. Separate FPO-biodiesel blends from both FPO sources were prepared using initial volumetric ratios of 80:20 and 60:40 (biodiesel:FPO, by volume). After blending, mixing, and a 24h settling period, two layers formed and the top, biodiesel-rich layers containing about 5 and 10vol% FPO were decanted and characterized on the basis of a thermogravimetric analysis, viscosity, acid number, water content, elemental analysis, and heating value. Significant decreases in viscosity, acidity, and water content from the original FPO validated blending as means of extracting compounds suitable for use as fuels from pyrolytic liquids in biodiesel. A single cylinder, common rail, direct injection diesel engine was used to analyze the combustion performance of the FPO fuel blends against neat diesel and biodiesel. Fuel performance was characterized on the basis of a thermodynamic combustion analysis, as well as corresponding exhaust emission measurements for CO2, CO, unburned hydrocarbons, particulate matter, and NOx.Engine testing revealed similar indicated efficiencies for biodiesel and diesel at all considered engine operating modes, while blend fuels showed indicated efficiencies between 75 and 95% of diesel values. In general, FPO fuels exhibited increased ignition delays and shorter combustion durations with greater FPO blend concentrations, though this could be partially compensated for using a pilot injection strategy. The longer ignition delays of the blend fuels resulted in overly lean regions of the cylinder, which produced largely premixed combustion events contributing to brake specific CO and uHC emissions up to 1.5 and 3.5 greater than diesel, respectively. Specific PM emissions were 41–62% lower for blend fuels than diesel. Both blends of in-house FPO showed similar PM emission performance, however at higher concentration than low blend commercial fuel.

Nonintrusive Uncertainty Quantification of Computational Fluid Dynamics Simulations of a Bench-Scale Fluidized-Bed Gasifier

Uncertainty quantification (UQ) analysis is increasingly becoming one of the major requirements of simulation-based engineering to assess the confidence in the results and make better-informed decisions based on the insight derived from the simulations. In an earlier study, Bayesian UQ analysis was applied to existing bench-scale fluidized-bed gasifier experiment results. In the current study, a series of simulations were carried over with the open-source computational fluid dynamics software MFiX to reproduce the experimental conditions, where three operating factors, i.e., coal flow rate, coal particle diameter, and steam-to-oxygen ratio, were systematically varied to understand their effect on the syngas composition. Bayesian UQ analysis was this time performed on the numerical results for comparison purposes. This is part of ongoing research efforts to explore the applicability of advanced UQ methods and processes such as Bayesian methods for large-scale complex multiphase flow simulations. As part of Bayesian UQ analysis, a global sensitivity analysis was performed based on the simulation results, which shows that the predicted syngas composition is strongly affected not only by the steam-to-oxygen ratio (which was observed in experiments as well) but also by variation in the coal flow rate and particle diameter (which was not observed in experiments). The carbon monoxide mole fraction is underpredicted at lower steam-to-oxygen ratios and overpredicted at higher steam-to-oxygen ratios. The opposite trend is observed for the carbon dioxide mole fraction. These discrepancies are attributed to either excessive segregation of the phases that leads to the fuel-rich or -lean regions or alternatively the selection of reaction models, where different reaction models and kinetics can lead to different syngas compositions throughout the gasifier.

A direct numerical simulation of cool-flame affected autoignition in diesel engine-relevant conditions

In diesel engines, combustion is initiated by a two-staged autoignition that includes both low- and high-temperature chemistry. The location and timing of both stages of autoignition are important parameters that influence the development and stabilisation of the flame. In this study, a two-dimensional direct numerical simulation (DNS) is conducted to provide a fully resolved description of ignition at diesel engine-relevant conditions. The DNS is performed at a pressure of 40 atmospheres and at an ambient temperature of 900 K using dimethyl ether (DME) as the fuel, with a 30 species reduced chemical mechanism. At these conditions, similar to diesel fuel, DME exhibits two-stage ignition. The focus of this study is on the behaviour of the low-temperature chemistry (LTC) and the way in which it influences the high-temperature ignition. The results show that the LTC develops as a “spotty” first-stage autoignition in lean regions which transitions to a diffusively supported cool-flame and then propagates up the local mixture fraction gradient towards richer regions. The cool-flame speed is much faster than can be attributed to spatial gradients in first-stage ignition delay time in homogeneous reactors. The cool-flame causes a shortening of the second-stage ignition delay times compared to a homogeneous reactor and the shortening becomes more pronounced at richer mixtures. Multiple high-temperature ignition kernels are observed over a range of rich mixtures that are much richer than the homogeneous most reactive mixture and most kernels form much earlier than suggested by the homogeneous ignition delay time of the corresponding local mixture. Overall, the results suggest that LTC can strongly influence both the timing and location in composition space of the high-temperature ignition.

An analysis of the structure of an n-dodecane spray flame using TPDF modelling

With a view to understanding ignition and combustion behaviours in diesel engines, this study investigates several aspects of ignition and combustion of an n-dodecane spray in a high pressure, high temperature chamber, known as Spray A, using data resulting from modelling using the transported probability density function (TPDF) method. The model has been validated comprehensively with good to excellent agreement in our previous work against all available experimental data including for mixture-fraction and velocity fields in non-reacting cases, and flame lift-off length and ignition delay in reacting cases. This good agreement encourages further investigation of the numerical model results to help understand the structure of this flame, which serves to complement the experimental information that is available, which is very limited due to the difficult experimental conditions in which this flame exists. For example, quantitative experimental measurements of local mixture-fraction, temperature, velocity gradients, etc. are not yet possible in reacting cases. Analysis of the model results shows that two-stage ignition is found to occur across the ambient temperature conditions considered: the first stage is rapidly initiated on the lean side where temperatures are high and sequentially moves to richer, cooler conditions. The first stage is extremely resilient to turbulence, occurring in a region of very low Damköhler number. The second stage of ignition occurs first in rich mixtures in a region behind the head of the fuel jet where mixture gradients are low, and appears to be influenced strongly by turbulence. Relative to a homogeneous reactor, it is delayed on the lean side but advanced on the rich side, suggesting entrainment and mixing from the early igniting lean regions into richer mixtures is an important moderator of the ignition process. The second-stage ignition front propagates at very high velocities initially, suggesting it is a sequential ignition moving according to gradients of ignition delay and/or residence time. The flame stabilises however on the lean side in a region of much lower velocity, where turbulent velocity fluctuations are sufficiently high such that turbulent transport influences the propagation. It stabilises in a region of low Damköhler number which implies that a competition of chemistry versus micro-mixing might also be involved in stabilisation. The stabilisation mechanism is investigated by an analysis of the transport budgets, showing the flame is stabilised by autoignition but moderated by turbulent diffusion. Further analysis of the flame index supports this stabilisation mechanism, and demonstrates the simultaneous existence of non-premixed and premixed combustion modes in the same flame. Analysis of the flow fields also reveals that local entrainment and dilatation are important flow features near the flame base.

Modeling the homogenization kinetics of as-cast U-10wt% Mo alloys

Low-enriched U-22at% Mo (U–10Mo) alloy has been considered as an alternative material to replace the highly enriched fuels in research reactors. For the U–10Mo to work effectively and replace the existing fuel material, a thorough understanding of the microstructure development from as-cast to the final formed structure is required. The as-cast microstructure typically resembles an inhomogeneous microstructure with regions containing molybdenum-rich and -lean regions, which may affect the processing and possibly the in-reactor performance. This as-cast structure must be homogenized by thermal treatment to produce a uniform Mo distribution. The development of a modeling capability will improve the understanding of the effect of initial microstructures on the Mo homogenization kinetics. In the current work, we investigated the effect of as-cast microstructure on the homogenization kinetics. The kinetics of the homogenization was modeled based on a rigorous algorithm that relates the line scan data of Mo concentration to the gray scale in energy dispersive spectroscopy images, which was used to generate a reconstructed Mo concentration map. The map was then used as realistic microstructure input for physics-based homogenization models, where the entire homogenization kinetics can be simulated and validated against the available experiment data at different homogenization times and temperatures.

Effects of stratification on locally lean, near-stoichiometric, and rich iso-octane/air turbulent V-flames

The effects of partial premixing on locally rich, near-stoichiometric, and lean flame regions were investigated in stratified, iso-octane/air turbulent V-flames by varying the mean equivalence ratio gradient along the exit plane of a rectangular slot burner. Instantaneous heat release rate (HRR) images were obtained from the product of spatially registered, near-simultaneously acquired OH and CH2O planar laser induced fluorescence (PLIF) images. HRR data were analyzed within a region of interest (ROI) that was determined from separate 3-pentanone tracer PLIF measurements. The ROI was unique to each gradient flame setting, and was configured to ensure the mean range of equivalence ratios being analyzed was constant among gradient conditions. This allowed distinction of the effects of mean equivalence ratio gradient at the flame front from effects associated with having different ranges of equivalence ratios within the flame zone.Individual flame realizations were studied for differences in the local peak HRR and instantaneous flame thickness δt as they varied with curvature among gradient conditions. While general trends for the fully-premixed cases were consistent with Lewis number theory, subtle changes in the normalized distribution of local peak HRR vs. curvature were observed for locally rich and locally lean flames propagating in different mean ϕ gradients. Negligible changes were observed for near-stoichiometric flames, suggesting that gradient effects may influence the local thermodiffusive stability of off-stoichiometric mixtures more significantly.Ensemble averages of individual peak HRR and δt values within each ROI were separately evaluated, and differences among gradient conditions were greater than those observed for the normalized distributions with curvature. For all flame settings considered, an increase in either of the peak HRR or δt led to a decrease in the other. Gradient effects were observed when comparing back- and front-supported locally rich flames, which experienced opposite changes in peak HRR of +10.1% and −5.2% for gradient settings ∂ϕ/∂y = −0.014 mm−1 and ∂ϕ/∂y = 0.012 mm−1 respectively, coupled with a thinning and thickening of δt of −7.2% and +2.4%. Similar but weaker trends were observed for near-stoichiometric flame regions, with a decrease in peak HRR of up to −3.5% and a thickening up to +2.8% for the steepest gradient ∂ϕ/∂y = 0.029 mm−1. Locally lean flames showed small increases in peak HRR of up to +3.8%, and decreases in δt of up to −2.1% for back-supported gradient case ∂ϕ/∂y = 0.024 mm−1, however, variations were not as significant as those observed for back-supported rich flame regions of equivalent gradients. The presented results show that mean gradients of equivalence ratio can alter the local characteristics of partially premixed flames. Through subtle differences in the local distribution of peak HRR with curvature, more pronounced variations in the magnitude of the mean local peak HRR and δt, and opposing effects in both peak HRR and δt for back- and front-supported rich flames, the data reveal the specific influence of equivalence ratio gradients in experiments where the mean range of equivalence ratios in the analysis region is fixed.