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• Virtual (screen-based) simulations assist nursing students' clinical learning. • A collection of open access and fee-based virtual technologies is presented. • Results meet a need for educators to access virtual simulation applications. Virtual (screen-based) simulations have been utilized to help progress pre-licensure nursing students' remote clinical learning during the recent pandemic. This mapping review, reports an anthology of virtual simulation technology sources from simulation education web sites and library sources. Two authors verified available sources and categorized these based on cost; either open access, or subscription-based. A list of 40 virtual simulation sources including virtual simulations, virtual reality simulations and virtual games, is presented. These provide faculty with a choice of virtual simulation modalities for various levels of nurse learners. Numerous virtual simulation technology applications are available for educators to utilize in teaching nursing students. Results from this review meet a need for educators to access virtual simulation applications to include in their education curricula. [ABSTRACT FROM AUTHOR]

The purpose of this study was to examine the accuracy of methods to estimate ball spin rate and spin axis direction using multi-camera tracking technology. The method implemented by Hawk-Eye and a theoretical ball trajectory model were assessed for their accuracy against high speed-vision. A theoretical ball trajectory model was found to estimate ball spin rate and the spin axis direction more accurately than the method used by Hawk-Eye. The superior performance of the ball trajectory model was indicated by a lower mean bias and standard deviation (2.92 ± 222.76) than Hawk-Eye (−100.01 ± 542.44), as well as narrower limits of agreement and a 327.63 RPM lower RMSE. Spin rates less than 4500 RPM were estimated with the highest accuracy, but neither method was able to consistently and accurately estimate spin rates >4500 RPM. The application of a trajectory model to multi-camera ball tracking data provides a practical and non-invasive method to accurately estimate the spin imparted when hitting, allowing for large-scale collection of spin rates during matches. [ABSTRACT FROM AUTHOR]

The turbulent burning velocity of premixed flames is sensitive to the turbulence intensity of the unburned mixture. Premixed flame propagation models that incorporate these effects of turbulence rest on either of the two hypotheses proposed by Damköhler. The first hypothesis applies to lowintensity turbulence that acts mainly to increase the turbulent burning velocity by increasing the flame surface area. The second hypothesis states that, at sufficiently high intensities of turbulence, the turbulent burning velocity is governed mainly by enhanced diffusivity. Most studies to date have examined the validity of the first hypothesis under increasingly high intensities of turbulence. In the present study, the validity of Damköhler's second hypothesis is investigated. A range of turbulence intensities is addressed by means of direct numerical simulations spanning the "flamelet" and "broken reaction zones" regimes. The validity of Damköhler's second hypothesis is found to be strongly linked to the behaviour of turbulent transport within the flame. [ABSTRACT FROM AUTHOR]

This paper extends a straightforward technique for the calculation of shell thicknesses in core-shell nanoparticles to the case of core-shell-shell nanoparticles using X-ray photoelectron spectroscopy (XPS) data. This method can be applied by XPS analysts and does not require any numerical simulation or advanced knowledge, although iteration is required in the case where both shell thicknesses are unknown. The standard deviation in the calculated thicknesses versus simulated values is typically less than 10%, which is the uncertainty of the electron attenuation lengths used in XPS analysis.© 2016 Crown copyright. Surface and Interface Analysis © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Autoignition of turbulent methane/air mixing layers, in which n-heptane droplets have been added, was investigated by DNS. This configuration is relevant to dual-fuel, pilot-ignited natural gas engines under direct injection conditions. Two passive scalars were introduced in order to describe the dual fuel combustion. It was shown that the pre-ignition phase is dominated by n-heptane oxidation while methane oxidation is less intense. During the pre-ignition phase, the methane/air mixing layer is distorted due to turbulence creating regions around the n-heptane droplets allowing the transport of intermediate species to the methane reaction zone. According to the passive scalars introduced, it was shown that ignition occurs at mixtures rich in n-heptane vapor. Subsequently, consumption of both n-heptane and methane is rapidly increased and promoted by the high temperatures achieved. The competition of the two fuels makes autoignition retarded relative to the pure n-heptane case, but accelerated relative to the pure methane case. [ABSTRACT FROM PUBLISHER]

Bibliometrics is an emerging science in nursing. Quantitative methods were used to conduct a bibliometric analysis of highly cited virtual simulation nursing education articles to describe rank order, breadth of topics and authorship patterns. A desktop analysis of publication performance was conducted using the Scopus database as the source of article citation data. The top 100 cited articles clustered over 14-years, 2008 to 2021. Citations per paper ranged from 88 to a low of 3 (median 18, mean 22.1) and in the top 10 studies, from 88 to 41. The citation trajectory was moderately correlated with article maturity (r = −.384, p ≤0.001). Article citations in subsequent publications commenced the first year after publication and three-year-old papers reached the mean citation rate of 22. Nurse Education Today was the most cited journal. There was no significant impact of article type (72% primary research, 17% literature reviews, 11% descriptive papers) (p = 0.755). International representation was strong, as first authors in 21 countries were cited – many (43%) from USA. One highly cited author (M. Verkuyl) from Canada led six papers, with six other authors each leading two studies. Virtual simulation modalities included virtual simulation, virtual reality simulation, 3D virtual reality simulation, virtual games and virtual worlds. The top 10 articles offer a diverse resource for faculty and educators who wish to consider using virtual simulation. Virtual simulation studies in nursing education cover an emerging field of research that has relatively low citation rates. Nursing researchers and faculty need to understand the usefulness and limitations of bibliometric analysis as this methodology can make a unique contribution to research, policy, and funding decisions, and enable productivity assessments of faculty staff and departments. • Bibliometrics is a novel quantitative evaluation method. • Bibliometrics analysis identifies leaders in specific disciplinary fields. • Our results may benefit uptake of virtual simulation in nursing. [ABSTRACT FROM AUTHOR]

The influence of turbulent Reynolds number (Ret) on the statistical behavior and modeling of turbulent scalar flux (TSF) has been analyzed using a direct numerical simulation database of freely propagating turbulent premixed flames. A range of different values of Retis considered in which the Damköhler and Karlovitz numbers are modified independent of each other to bring about the variation of Ret. It has been found that the qualitative behavior of the various terms of the TSF transport equation does not change by the variation of Ret, but their relative contributions to the transport of TSF are affected to some extent. The effects of Reton the modeling of the TSF using both algebraic and transport equation-based closures are addressed in detail. It is demonstrated that model parameters for an existing algebraic model, and the models for turbulent transport, pressure gradient and the reaction rate terms in the TSF transport equation, all exhibit Retdependence for small values of Retbut all assume asymptotic values for Ret ≥ 50. By contrast, the model parameters for the combined molecular diffusion term are found to be insensitive to the variation of turbulent Ret. Existing models for algebraic and transport equation-based closures of TSF have been modified to account for the observed Retdependence. [ABSTRACT FROM AUTHOR]

Three-dimensional direct numerical simulation (DNS) of turbulent combustion under moderate and intense low-oxygen dilution (MILD) conditions has been carried out inside a cuboid with inflow and outflow boundaries on the upstream and downstream faces, respectively. The initial and inflowing mixture and turbulence fields are constructed carefully to be representative of MILD conditions involving partially mixed pockets of unburned and burned gases. The combustion kinetics are modeled using a skeletal mechanism for methane-air combustion, including non-unity Lewis numbers for species and temperature-dependent transport properties. The DNS data is analyzed to study the MILD reaction zone structure and its behavior. The results show that the instantaneous reaction zones are convoluted and the degree of convolution increases with dilution and turbulence levels. Interactions of reaction zones occur frequently and are spread out in a large portion of the computational domain due to the mixture non-uniformity and high turbulence level. These interactions lead to local thickening of reaction zones yielding an appearance of distributed combustion despite the presence of local thin reaction zones. A canonical MILD flame element, called MIFE, is proposed, which represents the averaged mass fraction variation for major species reasonably well, although a fully representative canonical element needs to include the effect of reaction zone interactions and associated thickening effects on the mean reaction rate. [ABSTRACT FROM AUTHOR]

Abstract: The influence of turbulent Reynolds number on the statistical behaviour and modelling of flame surface density (FSD) transport has been analysed using a direct numerical simulation (DNS) database of freely propagating turbulent premixed flames with values of ranging from 20 to 100. The variation of is brought about by modifying the Damköhler and Karlovitz numbers independently of each other. The findings indicate that the qualitative behaviour of the various terms of the FSD transport equation remains unaffected by the changes in turbulent Reynolds number, but their relative contributions to the transport of FSD are affected to some extent. The effects of turbulent Reynolds number on the modelling of the turbulent transport term, the tangential strain rate term and the combined propagation and curvature terms are addressed in detail. It is demonstrated that model parameters for the turbulent transport and tangential strain rate term exhibit turbulent Reynolds number dependence for small values of and assume asymptotic values for . By contrast, the model parameters for the combined propagation and curvature term are found to be insensitive to the variation of turbulent Reynolds number. Existing models for the turbulent transport term and the tangential strain rate term are modified to account for the observed turbulent Reynolds number dependence. [Copyright &y& Elsevier]

We derive a measure that describes the complexity and consistency of a 3D virtual scene, so as to be able to compare scenes used for training purposes, and to allow changes in a scene to generate appropriate responses within games. The measure is calculated by constructing an entropy tree that links all the objects within the scene together. The total entropy is thus the sum of all the individual 'object to object' entropies within the tree. The object to object entropy is derived from the nature of the objects (their semantic relevance to each other, termed selection entropy) and to their geometric arrangement. We calculate the selection entropy for a number of examples to show how it quantifies the aesthetic consistency and complexity of the scene. [ABSTRACT FROM AUTHOR]

The performance of algebraic flame surface density (FSD) models has been assessed for flames with nonunity Lewis number (Le) in the thin reaction zones regime, using a direct numerical simulation (DNS) database of freely propagating turbulent premixed flames with Le ranging from 0.34 to 1.2. The focus is on algebraic FSD models based on a power-law approach, and the effects of Lewis number on the fractal dimension D and inner cut-off scale ηi have been studied in detail. It has been found that D is strongly affected by Lewis number and increases significantly with decreasing Le. By contrast, ηi remains close to the laminar flame thermal thickness for all values of Le considered here. A parameterisation of D is proposed such that the effects of Lewis number are explicitly accounted for. The new parameterisation is used to propose a new algebraic model for FSD. The performance of the new model is assessed with respect to results for the generalised FSD obtained from explicitly LES-filtered DNS data. It has been found that the performance of the most existing models deteriorates with decreasing Lewis number, while the newly proposed model is found to perform as well or better than the most existing algebraic models for FSD. [ABSTRACT FROM AUTHOR]

Background: Assessment using case based practical exams that require demonstration of clinical practice skills and articulation of clinical reasoning is commonly used in undergraduate allied health and nursing professions. However, the student often struggles in understanding and evaluating their performance of practical skills in preparation for summative practical assessment tasks. The fear of attempting and completing practical tasks does not contribute to learning. Any means of encouraging self evaluation, reflection and facilitating learning that is driven by the learner and not the assessor will contribute to the performance of the student in the short term and the practitioner in the future. Aims: This project used a mixed-methodological approach to explore the use of audio-visual technology to enhance 61 undergraduate physiotherapy students' practice of self evaluation and then sought to explore and understand the student experience of the activity and the teacher feedback provided. Findings: Themes relating to the students concerns, strengths and levels of self-confidence were identified. Reflection on the student feedback provided and the implications for teaching are also discussed. [ABSTRACT FROM AUTHOR]

Abstract: The transport of flame surface density (FSD) in turbulent premixed flames has been studied using a database obtained from Direct Numerical Simulation (DNS). Three-dimensional freely propagating developing statistically planar turbulent premixed flames have been examined over a range of global Lewis numbers from 0.6 to 1.2. Simplified chemistry has been used and the emphasis is on the effects of Lewis number on FSD transport in the context of Reynolds-averaged closure modelling. Under the same initial conditions of turbulence, flames with low Lewis numbers are found to exhibit counter-gradient transport of FSD, whereas flames with higher Lewis numbers tend to exhibit gradient transport of FSD. Stronger heat release effects for lower Lewis number flames are found to lead to an increase in the positive (negative) value of the dilatation rate (normal strain rate) term in the FSD transport equation with decreasing Lewis number. The contribution of flame curvature to FSD transport is found to be influenced significantly by the effects of Lewis number on the curvature dependence of the magnitude of the reaction progress variable gradient, and on the combined reaction and normal diffusion components of displacement speed. The modelling of the various terms of the FSD transport equation has been analysed in detail and the performance of existing models is assessed with respect to the terms assembled from corresponding quantities extracted from DNS data. Based on this assessment, suitable models are identified which are able to address the effects of non-unity Lewis number on FSD transport, and new or modified models are suggested wherever necessary. [Copyright &y& Elsevier]

The effects of global Lewis number on the statistical behaviour of turbulent kinetic energy transport in turbulent premixed flames are analysed using three-dimensional direct numerical simulation (DNS) data for freely propagating statistically planar flames with Lewis number ranging from 0.34 to 1.2. For flames with Lewis number significantly smaller than unity, it is observed that the turbulent kinetic energy is significantly augmented within the flame brush due to flame-generated turbulence. In these flames, it is demonstrated that effects of the mean pressure gradient and pressure dilatation are sufficient to overcome the effects of viscous dissipation. By contrast, for flames with Lewis number close to unity, it is found that the turbulent kinetic energy decays monotonically through the flame brush. In these flames, the effects of the mean pressure gradient and pressure dilatation terms are relatively much weaker than those of viscous dissipation. The modelling of the various unclosed terms of the turbulent kinetic energy transport equation has been analysed in detail. The predictions of existing models are compared with corresponding quantities extracted from DNS data. Based on this a-priori DNS assessment, either appropriate models are identified or new models are proposed where necessary. It is shown that the turbulent flux of turbulent kinetic energy exhibits counter-gradient transport for the low Lewis number flames where the turbulent scalar flux is also counter-gradient in nature. However, for flames with Lewis number close to unity, the turbulent flux of turbulent kinetic energy exhibits predominantly gradient type transport. A new model has been proposed for the flux of turbulent kinetic energy in premixed flames and is found to capture the qualitative and quantitative behaviour obtained from DNS data for all the different Lewis numbers considered. [ABSTRACT FROM AUTHOR]

Abstract: The wavelet-based multiresolution analysis (MRA) technique is used to develop a modelling approach to large-eddy simulation (LES) and its associated subgrid closure problem. The LES equations are derived by projecting the Navier–Stokes (N–S) equations onto a hierarchy of wavelet spaces. A numerical framework is then developed for the solution of the large and the small-scale equations. This is done in one dimension, for the Burgers equation, and in three dimensions, for the N–S problem. The proposed methodology is assessed in a priori tests on an atmospheric turbulent time series and on data from direct numerical simulation. A posteriori (dynamic) tests are also carried out for decaying and force-driven Burgers turbulence. [ABSTRACT FROM AUTHOR]

Fully compressible direct numerical simulation (DNS) is used to study the behaviour of a stratified turbulent premixed flame in the wrinkled flamelet regime (ReΛ = 38, u'/SL = 0.7, Da = 2.3) with reduced multi-step chemistry. A laminar premixed flame in an initially quiescent domain propagates into a turbulent flow so that the scalar and velocity fields are correctly coupled. Comparison of a stratified scenario (0.4 < φ < 1.2) with an otherwise identical homogeneous case indicates that in this regime the turbulent flame speed is increased by enhanced flame front wrinkling as a result of differential propagation speeds. The preferential propagation into more combustible mixture results in a correlation between the mixture fraction and progress variable, particularly at the leading edge of the flame brush. In front of the flame, the composition gradient preferentially aligns with the most compressive strain. Within the flame front, the progress variable gradient preferentially aligns with the most extensive turbulent strain, and in the latter portion of the flame the alignment between the mixture fraction gradient and the most compressive principal strain deteriorates. Despite this, the convenient approximation of the joint pdf by assuming statistical independence of the mixture fraction and progress variable does not introduce a significant error in the approximation of the reaction rates. [ABSTRACT FROM AUTHOR]

Abstract: The behaviour of the turbulent scalar flux in premixed flames has been studied using Direct Numerical Simulation (DNS) with emphasis on the effects of Lewis number in the context of Reynolds-averaged closure modelling. A database was obtained from DNS of three-dimensional freely propagating statistically planar turbulent premixed flames with simplified chemistry and a range of global Lewis numbers from 0.34 to 1.2. Under the same initial conditions of turbulence, flames with low Lewis numbers are found to exhibit counter-gradient transport, whereas flames with higher Lewis numbers tend to exhibit gradient transport. The Reynolds-averaged transport equation for the turbulent scalar flux is analysed in detail and the performance of existing models for the unclosed terms is assessed with respect to corresponding quantities extracted from DNS data. Based on this assessment, existing models which are able to address the effects of non-unity Lewis number on turbulent scalar flux transport are identified, and new or modified models are suggested wherever necessary. In this way, a complete set of closure models for the scalar flux transport equation is prescribed for use in Reynolds-Averaged Navier–Stokes simulations. [Copyright &y& Elsevier]

Abstract: Three-dimensional direct numerical simulations (DNS) were carried out to investigate the impact of evaporation of droplets on the autoignition process under decaying turbulence. The droplets were taken as point sources and were tracked in a Lagrangian manner. Three cases with the same initial equivalence ratio but different initial droplet size were simulated and the focus was to examine the influence of the droplet evaporation process on the location of autoignition. It was found that an increase in the initial droplet size results in an increase in the autoignition time, that highest reaction rates always occur at a specific mixture fraction ξ MR, as in purely gaseous flows, and that changes in the initial droplet size did not affect the value of ξ MR. The conditional correlation coefficient between scalar dissipation rate and reaction rates was only mildly negative, contrary to the strongly negative values for purely gaseous autoigniting flows, possibly due to the continuous generation of mixture fraction by the droplet evaporation process that randomizes both the mixture fraction and the scalar dissipation fields. [Copyright &y& Elsevier]

The effects of global Lewis number on scalar transport in turbulent premixed flames are studied using direct numerical simulation. Under the same initial conditions of turbulent flow field, it is observed that flames with global Lewis numbers significantly smaller than unity tend to exhibit countergradient transport, whereas the extent of gradient transport is shown to increase with increasing global Lewis numbers. The velocity difference between reactants and products in the flame normal direction is shown to be significantly affected by the global Lewis number. The flame normal acceleration is shown to increase with decreasing Lewis number, leading to an increase in the magnitude of the mean pressure gradient in the mean direction of flame propagation. This effect is shown to be is responsible for promoting countergradient transport in low Lewis number flames. It is also shown that turbulent transport of flame surface density tends to exhibit countergradient behavior for the flames with Lewis number significantly smaller than unity. [ABSTRACT FROM AUTHOR]

Abstract: Turbulent reaction rate closure modelling using the concept of Flame Surface Density (FSD) is now well-established in the context of Reynolds Averaged Navier Stokes (RANS) simulations. In the present study, three-dimensional DNS data is explicitly filtered in order to evaluate different terms of the FSD transport equation in the context of Large Eddy Simulation (LES). Existing sub-models for these unclosed terms are assessed with respect to the filtered DNS data, resulting in a recommended modelled form for each term. Previously assessed modelling of the propagation and curvature terms is combined with new model formulations for the turbulent transport and strain rate terms to produce a complete modelled FSD transport equation for use in LES. Displacement speed is shown to play a pivotal role in the modelling of several terms and its curvature dependence is explicitly accounted for in order to ensure that the FSD model will be valid in both the corrugated flamelets and the thin reaction zones regimes. [Copyright &y& Elsevier]

This paper reviews the current state of the art in accidental explosion modeling using methods based on computational fluid dynamics (CFD) in the petrochemical process industries. We discuss the problem in terms of its industrial importance and its technical difficulty, which stems mainly from the large range of length and timescales that must be represented. Explicit representation of all scales is not feasible due to limitations of computational cost, and modeling of unresolved physical features is required. We also discuss geometry modeling using the porosity/distributed resistance (PDR) method and review relevant combustion modeling. We describe an advanced CFD approach using unstructured adaptive gridding and discuss its usefulness in the context of results obtained for both two dimensional and three dimensional simulations of gas explosion phenomena in complex geometries. [ABSTRACT FROM AUTHOR]

Direct numerical simulation (DNS) results of turbulent MILD premixed and conventional (undiluted) premixed combustion have been investigated to shed light on the physical aspects of reaction zones and their morphology in MILD combustion. Results of a premixed case are used for comparative analyses. The analyses show that the regions with strong chemical activity in MILD combustion are distributed over a substantial portion of the computational domain unlike in the premixed case where these regions are confined to a small portion of the domain. Also, interactions of reaction zones are observed in MILD combustion with their spatial extent increasing with dilution level. These interactions give an appearance of distributed combustion for MILD conditions. The morphology of these reaction zones is investigated using the Minkowski functionals and shapefinders commonly employed in cosmology. Predominant sheet-like structures are observed for the premixed combustion case whereas a pancake-like structure is observed as the most probable shape for the MILD cases. Spatial and statistical analyses of various fluxes involved in a progress variable transport equation are conducted to study autoignitive or propagative characteristics of MILD reaction zones. The results suggest that there are local regions with autoignition, propagating-flames, and their coexistence for the conditions considered in this study. Typically, reaction dominated or ignition front and propagating-flame dominated regions are entangled for high dilution cases. Scalar gradient plays a strong role on whether reaction or propagating-flame dominated activities are favoured locally. [ABSTRACT FROM AUTHOR]

Abstract: Direct Numerical Simulations (DNS) of turbulent n-heptane sprays autoigniting at high pressure (P =24bar) and intermediate air temperature (T air =1000K) have been performed to investigate the physical mechanisms present under conditions where low-temperature chemistry is expected to be important. The initial turbulence in the carrier gas, the global equivalence ratio in the spray region, and the initial droplet size distribution of the spray were varied. Results show that spray ignition exhibits a spotty nature, with several kernels developing independently in those regions where the mixture fraction is close to its most reactive value ξ MR (as determined from homogeneous reactor calculations) and the scalar dissipation rate is low. Turbulence reduces the ignition delay time as it promotes mixing between air and the fuel vapor, eventually resulting in lower values of scalar dissipation. High values of the global equivalence ratio are responsible for a larger number of ignition kernels, due to the higher probability of finding regions where ξ = ξ MR. Spray polydispersity results in the occurrence of ignition over a wider range of mixture fraction values. This is a consequence of the inhomogeneities in the mixing field that characterize these sprays, where poorly mixed rich spots are seen to alternate with leaner ones which are well-mixed. The DNS simulations presented in this work have also been used to assess the applicability of the Conditional Moment Closure (CMC) method to the simulation of spray combustion. CMC is found to be a valid method for capturing spray autoignition, although care should be taken in the modelling of the unclosed terms appearing in the CMC equations. [Copyright &y& Elsevier]

Abstract: This paper analyses reaction and thermal front development in porous reservoirs with reacting flows, such as those encountered in oil shale upgrading. A set of dimensionless groups and a 1D code are developed in order to investigate the important physical and chemical variables of such reservoirs when heated by in situ methods. Theory necessary for this study is presented, namely shale decomposition chemical mechanisms, governing equations for multiphase flow in porous media and necessary closure models. Plotting the ratio of the thermal front speed to the fluid speed allows one to infer that the reaction front ends where this ratio is at a minimum. The reaction front follows the thermal front closely, thus allowing assumptions to be made about the extent of decomposition solely by looking at thermal front progression. Furthermore, this sensitivity analysis showed that a certain minimum permeability is required in order to ensure the formation of a traveling thermal front. Compared to varying deposit porosities and kerogen activations energies, varying temperature, pressure and permeability are more important. [Copyright &y& Elsevier]

Abstract: A parametric study of spark ignition in a uniform monodisperse turbulent spray is performed with complex chemistry three-dimensional Direct Numerical Simulations in order to improve the understanding of the structure of the ignition kernel. The heat produced by the kernel increases with the amount of fuel evaporated inside the spark volume. Moreover, the heat sink by evaporation is initially higher than the heat release and can have a negative effect on ignition. With the sprays investigated, heat release occurs over a large range of mixture fractions, being high within the nominal flammability limits and finite but low below the lean flammability limit. The burning of very lean regions is attributed to the diffusion of heat and species from regions of high heat release, and from the spark, to lean regions. Two modes of spray ignition are reported. With a relatively dilute spray, nominally flammable material exists only near the droplets. Reaction zones are created locally near the droplets and have a non-premixed character. They spread from droplet to droplet through a very lean interdroplet spacing. With a dense spray, the hot spark region is rich due to substantial evaporation but the cold region remains lean. In between, a large surface of flammable material is generated by evaporation. Ignition occurs there and a large reaction zone propagates from the rich burned region to the cold lean region. This flame is wrinkled due to the stratified mixture fraction field and evaporative cooling. In the dilute spray, the reaction front curvature pdf contains high values associated with single droplet combustion, while in the dense spray, the curvature is lower and closer to the curvature associated with gaseous fuel ignition kernels. [Copyright &y& Elsevier]

Autoignition of n-heptane sprays in a methane/air mixture and the subsequent methane premixed flame ignition, a constant volume configuration relevant to pilot-ignited dual fuel engines, was investigated by DNS. It was found that reducing the pilot fuel quantity, increases its autoignition time. This is attributed to the faster disappearance of the most reactive mixture fraction (predicted from homogeneous reactor calculations) which is quite rich. Consequently, ignition of the n-heptane occurs at leaner mixtures. The premixed methane flame is eventually ignited due to heating gained by the pressure rise caused by the n-heptane oxidation, and heat and mass transfer of intermediates from the n-heptane autoignition kernels. For large amounts of the pilot fuel, the combustion of the n-heptane results in significant adiabatic compression of the methane–air mixture. Hence the slow methane oxidation is accelerated and is further promoted by the presence of species in the oxidizer stream originating from the already ignited regions. For small amounts of the pilot fuel intermediates reach the oxidizer stream faster due to the very lean mixtures surrounding the n-heptane ignition kernels. Therefore, the premixed methane oxidation is initiated at intermediate temperatures. Depending on the amount of n-heptane, different statistical behaviour of the methane oxidation is observed when this is investigated in a reaction progress variable space. In particular for large amounts of n-heptane the methane oxidation follows roughly an autoignition regime, whereas for small amounts of n-heptane methane oxidation is similar to a canonical premixed flame. The data can be used for validation of various turbulent combustion models for dual-fuel combustion. [ABSTRACT FROM AUTHOR]

Three-dimensional carrier-phase Direct Numerical Simulations (DNS), combined with a Lagrangian representation of individual droplets, have been employed in this parametric study to examine the physical effects of liquid water mist interacting with laminar and turbulent premixed stoichiometric n-heptane/air flames. Significant reductions of flame temperature and burning velocity have been observed in the presence of water droplets. In agreement with the laws governing evaporation, a strongly non-linear influence of the droplet size on the overall burning rate has been noted, whereas the influence of water loading is fairly linear. Different regimes of droplet-flame interaction are known to exist and this has been investigated by numerical experiments focusing on the influence of the latent heat of vaporization. When using realistic fluid properties of water, the cooling effect associated to evaporating droplets outweighs the dilution effect due to the local decrease of fuel and oxidizer concentrations. Under turbulent conditions, the effectiveness of the droplets in reducing the overall burning rate changes owing to the transient nature of the droplet-flame interaction. Furthermore, it was found that the evaporating droplets significantly diminish the flame-generated turbulence and this leads to weaker turbulent wrinkling of the flame surface as compared to gaseous premixed reference simulations without droplets. Based on a comparison of the time scales representing droplet evaporation and the droplet residence time within the flame, a reduced-order model is proposed to account for both the cooling and dilution effects with respect to flame temperature and laminar burning velocity. [ABSTRACT FROM AUTHOR]