Publications
68 results found
Giusti A, Fredrich D, 2024, Fuel effects on the electrostatic control of charged droplet trajectories, Fuel, Vol: 366, ISSN: 0016-2361
The use of electrostatic fields to control the trajectory of charged droplets is investigated as a new technology to enhance mixing in liquid-fuelled combustors. The canonical configuration of a spray in crossflow is numerically studied with a focus on the effects of the fuel type on the onset of charge-induced breakup and droplet trajectories for a range of bulk flow velocities and electrostatic field strengths at conditions relevant to gas turbine applications. The investigated fuels are ethanol, n-heptane, and n-decane. Operational maps for each fuel are provided to assist the selection of the external electrostatic field required to achieve a balance between the drag and electrostatic forces, and enable a system design that considers fuel flexibility. The results demonstrate that the fuel type has an important impact on the diameter at which the charge-induced breakup is achieved, which mainly depends on the droplet equilibrium temperature. It is also shown that, for cases where the droplet net charge is fixed to a given fraction of the maximum possible charge (based on the Rayleigh limit), the temperature of the droplet at injection could be used as a parameter to control the onset of secondary breakup. Analysis of the strength of the electrostatic field necessary to achieve droplet stabilisation in a bulk flow shows that a balance between the electrostatic and drag forces can only be achieved for relatively low values of the bulk flow velocity, if the strength of the electrostatic field is kept below the breakdown limit of the carrier phase. This balance mainly depends on the droplet net charge and flow conditions, whereas the effect of the type of fuel on the drag force is less important. When the charge is imposed as a fraction of the maximum possible value at injection, for low values of the bulk flow velocity, the strength of the electrostatic field that balances the drag tends to become independent of the droplet diameter for a wide range of droplet sizes
Thijs LC, Kritikos EM, Giusti A, et al., 2023, Effect of Fe–O ReaxFF on liquid iron oxide properties derived from reactive molecular dynamics, The Journal of Physical Chemistry A: Isolated Molecules, Clusters, Radicals, and Ions; Environmental Chemistry, Geochemistry, and Astrochemistry; Theory, Vol: 127, Pages: 10339-10355, ISSN: 1089-5639
As iron powder nowadays attracts research attention as a carbon-free, circular energy carrier, molecular dynamics (MD) simulations can be used to better understand the mechanisms of liquid iron oxidation at elevated temperatures. However, prudence must be practiced in the selection of a reactive force field. This work investigates the influence of currently available reactive force fields (ReaxFFs) on a number of properties of the liquid iron–oxygen (Fe–O) system derived (or resulting) from MD simulations. Liquid Fe–O systems are considered over a range of oxidation degrees ZO, which represents the molar ratio of O/(O + Fe), with 0 < ZO < 0.6 and at a constant temperature of 2000 K, which is representative of the combustion temperature of micrometric iron particles burning in air. The investigated properties include the minimum energy path, system structure, (im)miscibility, transport properties, and the mass and thermal accommodation coefficients. The properties are compared to experimental values and thermodynamic calculation results if available. Results show that there are significant differences in the properties obtained with MD using the various ReaxFF parameter sets. Based on the available experimental data and equilibrium calculation results, an improved ReaxFF is required to better capture the properties of a liquid Fe–O system.
Miniero L, Pandey K, Fredrich D, et al., 2023, Air-blast atomization and ignition of a kerosene spray in hot vitiated crossflow, Combustion and Flame, Vol: 256, Pages: 1-13, ISSN: 0010-2180
Increasingly stringent regulations of pollutant emissions from aviation require rapid implementation ofnovel combustion technologies. Promising concepts based on moderate or intense low-oxygen dilution(MILD) combustion have been investigated in academia and industry. This MILD regime can be obtainedfrom the recirculation of the hot vitiated combustion products to raise the temperature of the reactants,resulting in distributed reaction regions and lower flame temperatures. In the present work, we considerthe air-blast atomization of a kerosene spray in crossflow, which enables efficient mixing between fueland oxidizer. We investigate experimentally and numerically the effect of the spray air-to-liquid massflow ratio (ALR) variation on the reaction front and flame topology of a kerosene spray flame. The sprayis injected transversely into a turbulent vitiated crossflow composed of the products of a lean CH4-H2flame. The spray flame thermal power is varied between 2.5 and 5 kW, along with the atomizer ALRbetween 2 and 6. The experimental characterization of the reaction zone is performed using OH∗ chemiluminescence and OH and fuel planar laser-induced fluorescence (PLIF). The Large Eddy Simulations (LES)of the multiphase reactive flow provide good agreement with the experimental observations. Experimentsand simulations show that the ALR governs mixing, resulting in different flame stabilization mechanismsand combustion regimes. Low ALR results in a relatively small jet-to-crossflow momentum ratio and alarge spray Sauter mean diameter (SMD). A thick windward reaction region is formed due to inefficientshear layer mixing between the fuel spray and the crossflow. Meanwhile, the correspondingly large spraySMD leads to isolated penetration and localized combustion of fuel clusters. At high ALR, the higher penetration and the faster droplet evaporation due to the lower spray SMD result in an efficient entrainmentinduced mixing between the two streams, forming more dist
Petersen JE, Kapur S, Gkantonas S, et al., 2023, Modelling and optimisation of extinction actions for wildfire suppression, Combustion Science and Technology, Vol: 195, Pages: 3584-3595, ISSN: 0010-2202
A physics-based model for the prediction of wildfire propagation, which combines the cellular automata concept with virtual Lagrangian fire particles, is further developed to include fire extinction actions. Deposition of water and firebreaks are included in the formulation. The fire propagation model is then coupled with a Monte Carlo Tree Search (MCTS) algorithm to optimize the allocation of fire extinction actions. Starting from an ignited fire, and fixing the amount of resources available for firefighting, the model suggests which series of actions minimizes the loss of wildland. The model has been assessed and validated with model fires and then applied to a realistic scenario. MCTS optimization is found to autonomously outperform human intuition for medium-scale fires and to successfully enhance human decision-making capabilities for large-scale fires with the use of convolution-based terrain re-sampling. This study opens up new possibilities for the development of decision-making tools to assist the real-time allocation of firefighting resources as well as to support the design of preventive measures to preserve the environment and reduce the potential impact of wildfires.
Lalli NS, Giusti A, 2023, Coherence effects on the interference colors of soap films, Journal of Applied Physics, Vol: 134, Pages: 1-18, ISSN: 0021-8979
Acquiring the thickness field of a soap film from interference colors requires an accurate relationship between color and film thickness. Throughout the literature, an interference relation derived using monochromatic waves is widely used to calculate the colors of soap films illuminated by light sources with significant frequency bandwidths by applying the relation at a number of discrete wavelengths in the source, which assumes that the interfering waves are perfectly coherent. However, since the coherence between waves is expected to decrease with increasing film thickness, it is poorly understood when interference relations derived using monochromatic waves can be applied. In this study, an interference relation incorporating the coherence between interfering waves is derived. The effects of coherence on the interference colors of soap films are then studied by comparing the colors computed using each of these two interference relations for light sources with different frequency bandwidths. As the frequency bandwidth of the light source increases, the difference in the colors computed using each interference relation increases, which implies that the accuracy of the method involving the monochromatic relation decreases with increasing frequency bandwidth of the source. The findings of this study will allow for more accurate measurements of the thickness of soap films from their interference colors.
Sayed Ahmad M, Kritikos EM, Giusti A, 2023, A reactive molecular dynamics investigation of nanoparticle interactions in hydrocarbon combustion, Combustion Science and Technology, Vol: 195, Pages: 3281-3295, ISSN: 0010-2202
The use of energetic nanoparticles to tailor the properties of a base liquid fuel has attracted attention due to the possibility of decreasing fuel consumption and increasing control over the combustion process. In this study, the role of nanomaterials in the consumption of hydrocarbon fuel vapor is investigated using reactive molecular dynamics. Simulations are performed with aluminum and iron nanoparticles inside an n-heptane and oxygen gas mixture. The role of atomic charges on the dynamics of nanoparticle-hydrocarbon interactions is also investigated using different charge equilibration methods. Results show that both nanomaterials act as catalysts and enhance fuel decomposition. The decomposition of fuel molecules is initiated by dehydrogenation at the particle’s surface. This reaction path occurs significantly faster than the oxidation and pyrolysis paths observed for n-heptane in absence of nanoparticles. The oxidation in the presence of aluminum is characterized by more rapid particle heating and fragmentation compared to iron. Metal fragments further enhance the reactivity of the system due to a higher surface area available for reactions. The atomic charge distribution was found to affect the kinetics and reactivity of the system, showing that the non-bonded interactions influence the oxidation process. This study confirms that the use of nanomaterials is beneficial to accelerate the decomposition of fuel and that the combustion behavior of the selected hydrocarbon is strongly dependent on the type of nanomaterial used in combination with the base fuel.
Thijs LC, Kritikos EM, Giusti A, et al., 2023, On the surface chemisorption of oxidizing fine iron particles: insights gained from molecular dynamics simulations, Combustion and Flame, Vol: 254, Pages: 1-14, ISSN: 0010-2180
Molecular dynamics (MD) simulations are performed to investigate the thermal and mass accommodationcoefficients (TAC and MAC, respectively) for the combination of iron(-oxide) and air. The obtained valuesof TAC and MAC are then used in a point-particle Knudsen model to investigate the effect of chemisorption and the Knudsen transition regime on the combustion behavior of (fine) iron particles. The thermalaccommodation for the interactions of Fe with N2 and FexOy with O2 is investigated for different surfacetemperatures, while the mass accommodation coefficient for iron(-oxide) with oxygen is investigated fordifferent initial oxidation stages ZO, which represents the molar ratio of O/(O + Fe), and different surfacetemperatures. The MAC decreases fast from unity to 0.03 as ZO increases from 0 to 0.5 and then diminishes as ZO further increases to 0.57. By incorporating the MD-informed accommodation coefficientsinto the single iron particle combustion model, the oxidation beyond ZO = 0.5 (from stoichiometric FeOto Fe3O4) is modeled. A new temperature evolution for single iron particles is observed compared toresults obtained with previously developed continuum models. Specifically, results of the present simulations show that the oxidation process continues after the particle reaching the peak temperature, whileprevious models predicting that the maximum temperature was attained when the particle is oxidizedto ZO = 0.5. Since the rate of oxidation slows down as the MAC decreases with an increasing oxidationstage, the rate of heat loss exceeds the rate of heat release upon reaching the maximum temperature,while the particle is not yet oxidized to ZO = 0.5. Finally, the effect of transition-regime heat and masstransfer on the combustion behavior of fine iron particles is investigated and discussed.
Efstathiou G, Gkantonas S, Giusti A, et al., 2023, Simulation of the December 2021 Marshall fire with a hybrid stochastic Lagrangian–cellular automata model, Fire Safety Journal, Vol: 138, Pages: 1-16, ISSN: 0379-7112
A stochastic model based on a combination of the cellular automata approach for forest fires and a random walk for firebrands and hot gases has been further developed and used to simulate the Marshall fire, Colorado, December 2021. Typical heat release profiles for burning wooden houses from the literature were used to distill information on the burning duration and ignition delay time needed to model the hours-long firebrand emission from wooden buildings in the Marshall area during this fire. In addition to information on vegetation and housing structures from in-person inspection, satellite images were used to estimate various model parameters. The results give reasonable predictions for the extent of the fire and its time evolution. A parametric analysis further highlighted the sensitivity of predictions to the parameters used in the model and suggested areas for improvement. The very low computational cost of the model, ease of operation, and acceptable accuracy suggest that the proposed framework can be used for operational decision-making and damage assessment.
Jain A, Giusti A, Magri L, 2023, Indirect noise from weakly reacting inhomogeneities, Journal of Fluid Mechanics, Vol: 965, Pages: 1-26, ISSN: 0022-1120
Indirect noise is a significant contributor to aircraft engine noise, which needs to be minimized in the design of aircraft engines. Indirect noise is caused by the acceleration of flow inhomogeneities through a nozzle. High-fidelity simulations showed that some flow inhomogeneities can be chemically reacting when they leave the combustor and enter the nozzle (Giusti et al., Trans. ASME J. Engng Gas Turbines Power, vol. 141, issue 1, 2019). The state-of-the-art models, however, are limited to chemically non-reacting (frozen) flows. In this work, first, we propose a low-order model to predict indirect noise in nozzle flows with reacting inhomogeneities. Second, we identify the physical sources of sound, which generate indirect noise via two physical mechanisms: (i) chemical reaction generates compositional perturbations, thereby adding to compositional noise; and (ii) exothermic reaction generates entropy perturbations. Third, we numerically compute the nozzle transfer functions for different frequency ranges (Helmholtz numbers) and reaction rates (Damköhler numbers) in subsonic flows with hydrogen and methane inhomogeneities. Fourth, we extend the model to supersonic flows. We find that hydrogen inhomogeneities have a larger impact on indirect noise than methane inhomogeneities. Both the Damköhler number and the Helmholtz number markedly influence the phase and magnitude of the transmitted and reflected waves, which affect sound generation and thermoacoustic stability. This work provides a physics-based low-order model which can open new opportunities for predicting noise emissions and instabilities in aeronautical gas turbines with multi-physics flows.
Kritikos E, Giusti A, 2023, Investigation of the effect of iron nanoparticles on n-dodecane combustion under external electrostatic fields, Proceedings of the Combustion Institute, Vol: 39, Pages: 5667-5676, ISSN: 1540-7489
Reactive molecular dynamics simulations are performed to investigate the combined effects of iron nanoparticles and external electrostatic fields on the combustion of n-dodecane. Results suggest that iron nanoparticle additives significantly accelerate fuel and oxidizer consumption. In particular, the decomposition ofn-dodecane is initiated at the nanoparticle’s surface by hydrogen abstraction and subsequent absorption ofthe hydrogen and carbon atoms. Products, such as H2 and H2O, are formed in the nanoparticle’s shell andreleased back into the gas phase, demonstrating a catalytic behaviour of the nanoparticle. Additionally, theapplication of an external electrostatic field further increases the n-dodecane consumption rate. A rise in thevariety of product species is also observed when an external electrostatic field is applied due to the overallaccelerated kinetics of the system. Analysis of the system’s kinetic energy suggests that the presence of anexternal electrostatic field leads to an increase in the translational energy of the molecules. The chemical composition of the nanoparticle is also affected. The absorbed species diffuse along the surface of the nanoparticle to counteract the externally applied electric field. This species rearrangement leads to the formation of ananisotropic shell with varying chemical composition. This study suggests that the use of electrostatic fieldswith nanomaterial-based catalysis can offer new possibilities for the control of the reaction process as wellas for the synthesis of tailored nanoparticles.
Mastorakos E, Gkantonas S, Efstathiou G, et al., 2023, A hybrid stochastic Lagrangian – cellular automata framework for modelling fire propagation in inhomogeneous terrains, Proceedings of the Combustion Institute, Vol: 39, Pages: 3853-3862, ISSN: 0082-0784
A stochastic model motivated by the Lagrangian transported probability density function method for turbulent reacting flows and the cellular automata approach for forest fires was put together to simulate propagation of fires in terrains with inhomogeneous composition. In contrast to the usual cellular automata models for fires where the probability of ignition is prescribed, here the ignition of cells is determined by a random walk that mimics turbulent convection and diffusion of the hot gases and firebrands from upwind and neighbouring fire fronts. Radiation is also included. The model is aimed at speed of computation while approximating the key physics through only a few terrain-related inputs and tunable parameters representing fire intensity, hot gas and ember decay timescales, cell ignition delay and local turbulence. These parameters were calibrated against controlled fire experiments and the model was then used to give reasonable predictions for fires of increasing complexity. The presented framework allows improvements for more accurate representation of the flammable material characteristics, fire-induced flow modifications, and most other phenomena present in fires, hence providing an extendable and simple yet physically-realistic novel modelling approach.
Lalli NS, Shen L, Dini D, et al., 2023, The stability of magnetic soap films, Physics of Fluids, Vol: 35, Pages: 1-16, ISSN: 1070-6631
Although previous studies have shown that a magnetic field can drastically alter drainage in soap films containing particles responsive to a magnetic field, which we refer to as magnetic soap films, it is yet to be understood whether a magnetic field may be able to control the rate of drainage and film stability. Furthermore, the effect of a magnetic field on key drainage mechanisms, such as marginal regeneration, is unknown. An experimental investigation involving interferometry is conducted here to develop further understanding of the behavior of horizontal soap films containing magnetite nanoparticles. Three scenarios are considered: soap films, magnetic soap films, and magnetic soap films in an inhomogeneous magnetic field. In each of the three scenarios, high-resolution images capturing the time evolution of each film are acquired, and the lifetime of each film is measured. In addition, a measure of the rate of drainage and the velocities of thin patches of fluid arising from marginal regeneration are evaluated. The results suggest that a magnetic field may be able to have either a stabilizing or destabilizing effect on magnetic soap films, depending on their composition. Furthermore, applying a magnetic field to magnetic soap films alters the trajectory of thin patches of fluid arising from marginal regeneration. This study reveals how a magnetic field can be used in conjunction with magnetic particles to control the stability of soap films, which opens up the possibility for new technologies that require a fine control of film stability.
Fredrich D, Weiand E, Giusti A, 2023, Electrostatic fields for the control of evaporating charged fuel sprays, International Journal of Multiphase Flow, Vol: 160, Pages: 1-8, ISSN: 0301-9322
The current socio-economic shift towards electrification of the transport sector and development of hybrid thermal–electric propulsion systems provides new opportunities for the development of ‘clean’ aviation technologies. In this work, the use of electrostatic fields to control the location of electrically charged fuel droplets is proposed as a novel technology to enhance pre-evaporation of liquid sprays in confined spaces. An electrospray in cross-flow is numerically investigated using large-eddy simulations for a range of flow and droplet conditions in order to study the feasibility of the approach. A deterministic model is further introduced to compute the trajectory of single droplets in a steady cross-flow. This enables a separation of the effects of turbulence, droplet repulsion and evaporation through comparison with data obtained from the large-eddy simulations, and at the same time provides a cheap computational tool to explore a wider range of operating conditions. It is shown that external electrostatic fields below the breakdown threshold of air can significantly change the trajectory of charged droplets at moderate flow velocities. Moreover, electrostatic forces acting in the opposite direction of the mean cross-flow can potentially be used to stabilise the spray position within a confined region, hence allowing for an increase of the residence time available for full evaporation. The application and modulation of such electrostatic forces is envisioned as a new paradigm to achieve ‘targeted evaporation’ in next-generation hybrid thermal–electric aero-engines and to improve the fuel-oxidiser mixing quality. The electrical power associated with the external electrostatic field to achieve droplet stabilisation is negligible compared to the thermal power released by complete combustion of the injected fuel. In addition, it is shown that stabilisation of the droplets enhances the evaporation rate (by more than 30%) and
Kritikos E, Lele A, van Duin ACT, et al., 2023, Atomistic insight into the effects of electrostatic fields on hydrocarbon reaction kinetics, The Journal of Chemical Physics, Vol: 158, Pages: 1-18, ISSN: 0021-9606
Reactive Molecular Dynamics (MD) and Density Functional Theory (DFT) computations are performed to provide insight into the effects of external electrostatic fields on hydrocarbon reaction kinetics. By comparing the results from MD and DFT, the suitability of the MD method in modeling electrodynamics is first assessed. Results show that the electric field-induced polarization predicted by the MD charge equilibration method is in good agreement with various DFT charge partitioning schemes. Then, the effects of oriented external electric fields on the transition pathways of non-redox reactions are investigated. Results on the minimum energy path suggest that electric fields can cause catalysis or inhibition of oxidation reactions, whereas pyrolysis reactions are not affected due to the weaker electronegativity of the hydrogen and carbon atoms. MD simulations of isolated reactions show that the reaction kinetics is also affected by applied external Lorentz forces and interatomic Coulomb forces since they can increase or decrease the energy of collision depending on the molecular conformation. In addition, electric fields can affect the kinetics of polar species and force them to align in the direction of field lines. These effects are attributed to energy transfer via intermolecular collisions and stabilization under the external Lorentz force. The kinetics of apolar species is not significantly affected mainly due to the weak induced dipole moment even under strong electric fields. The dynamics and reaction rates of species are studied by means of large-scale combustion simulations of n-dodecane and oxygen mixtures. Results show that under strong electric fields, the fuel, oxidizer, and most product molecules experience translational and rotational acceleration mainly due to close charge transfer along with a reduction in their vibrational energy due to stabilization. This study will serve as a basis to improve the current methods used in MD and to develop novel metho
Fredrich D, Akbar AM, bin Mohd Fadzil MF, et al., 2022, Modelling of human exhaled sprays and aerosols to enable real-time estimation of spatially-resolved infection risk in indoor environments, Publisher: arXiv
A numerical framework for the ‘real-time’ estimation of the infection risk from airborne diseases (e.g., SARS-CoV-2) in indoor spaces such as hospitals, restaurants, cinemas or teaching rooms is proposed. The developed model is based on the use of computational fluid dynamics as a pre-processor to obtain the time-averaged ventilation pattern inside a room, and a post-processing tool for the computation of the dispersion of sprays and aerosols emitted by its occupants in ‘real time’. The model can predict the dispersion and concentration of droplets carrying viable viral copies in the air, the contamination of surfaces, and the related spatially-resolved infection risk. It may therefore provide useful information for the management of indoor environments in terms of, e.g., maximum occupancy, air changes per hour and cleaning of surfaces. This work describes the fundamentals of the model and its main characteristics. The model was developed using open-source software and is conceived to be simple, user-friendly and highly automated to enable any potential user to perform estimations of the local infection risk.
Kritikos E, Lele A, van Duin ACT, et al., 2022, A reactive molecular dynamics study of the effects of an electric field on n-dodecane combustion, Combustion and Flame, Vol: 244, Pages: 1-16, ISSN: 0010-2180
A reactive Molecular Dynamics (MD) study of n-dodecane combustion at high temperatures under externally applied electrostatic fields is performed to investigate their effect on chemical kinetics. A local charge equilibration method is used to enable charge transfers up to the overlap of the atomic orbitals and introduce molecular polarization induced by an electric field. The atomic charges of an isolated n-dodecane molecule with and without external electrostatic fields are first compared with Density Functional Theory (DFT) computations, to assess the accuracy of the charge equilibration method and its ability to capture polarization. Then, the impact of external electrostatic fields on the reaction kinetics of fuel, oxidizer and products is studied for a range of ambient temperatures and densities. The activation energy and pre-exponential factor of Arrhenius-type reactions under various electrostatic fields are also investigated by performing Nudged Elastic Band (NEB) computations on selected reactions’ Minimum Energy Path (MEP) and by analysing the collision frequency, respectively. Results show that the atomic charge transfers due to close interactions and molecular polarisation are relatively weak in all investigated conditions, leading to the necessity of strong external electric fields to induce changes to chemical kinetics. The consumption rate of n-dodecane decreases for strong electrostatic fields, whereas for low values of the electrostatic field strength no clear trend is observed. In addition, at high temperature and density conditions, oxygen consumption increases under strong electrostatic fields, whereas the opposite trend is observed as the temperature and density decrease. NEB analysis shows alterations of the activation energy up to 2.3 kcal/mol for oxygen compound reactions with varying strength of the external electrostatic field. Furthermore, analysis of the translational, rotational and vibrational kinetic energy modes and collision fr
Lopez L, Giusti A, Gutheil E, et al., 2022, On the effects of the fuel injection phase on heat release and soot formation in counterflow flames, Energy, Vol: 254, ISSN: 0360-5442
Total rates of heat release, , and soot emission, , are studied in ethanol-N2/air and n-butanol-N2/air counterflow flames. A gas flame is first established as a base case and fractions of the gaseous fuel are then replaced by droplets and nitrogen, keeping the total fuel mass flux constant. Several values of the liquid to total fuel mass ratio, φl, are employed, covering the entire range from a gas flame to a spray flame (0 ≤ φl ≤ 1). Different initial droplet radii, R0, are considered, as well as both low and close to extinction strain rates. Results show qualitative similarities for both fuels, even though quantitative differences are observed. In general, ethanol flames release more heat and less soot than n-butanol flames. For low strain rates, φl = 1 leads to lower soot emissions than for the reference gas flame, independently of R0. Also, is higher for R0 = 25 and 40 μm. Close to extinction, increasing φl notoriously improved without considerably raising for R0 = 25 μm. Also, for R0 = 40 μm, there are some particular values of φl for which similar situations occur. These results show that the fuel injection phase plays an important role in optimizing combustion processes.
Giusti A, Zhang H, Kypraiou A-M, et al., 2022, Numerical investigation of the response of turbulent swirl non-premixed flames to air flow oscillations, International Journal of Spray and Combustion Dynamics, Vol: 14, Pages: 229-237, ISSN: 1756-8277
The response of swirl non-premixed flames to air flow oscillations is studied using Large-Eddy Simulation (LES) and the Conditional Moment Closure (CMC) combustion model, focusing on the physical mechanisms leading to the heat release rate oscillations observed in a parallel experimental study. Cases relatively close to blow-off and characterized by different amplitude of the flow oscillations are considered. Numerical results are in good agreement with the experiment in terms of both mean flame shape and heat release rate response. Simulations show that the oscillation of the air flow leads to an axial movement and fragmentation of the flame that are more pronounced with increasing amplitude of the forcing. The flame response is characterized by fluctuations of the flame area, time-varying local extinction and lift-off from the fuel injection point. LES-CMC, due to the inherent capability to capture burning state transitions, predicts properly the flame transfer function as a function of the amplitude of the air flow oscillations. This suggests that the response mechanism for this flame is not only due to time-varying flame area, but also local extinction and re-ignition. This study demonstrates that LES-CMC is a useful tool for the analysis of the response of flames of technical interest to large velocity oscillations and for the prediction of the flame transfer function in conditions close to blow-off.
Fredrich D, Miniero L, Pandey K, et al., 2022, Large Eddy simulation of a reacting kerosene spray in hot vitiated cross-flow, Flow, Turbulence and Combustion, Vol: 109, Pages: 991-1010, ISSN: 1386-6184
The evaporation and combustion characteristics of a kerosene spray injected perpendicularly into a cross-flow of high-temperature vitiated air is investigated. This fundamental flow configuration has wider implications for the future development of ultra-low emission aeronautical combustors, particularly with respect to technologies involving MILD combustion. Large eddy simulations with a Eulerian–Lagrangian framework are performed to investigate the spray evolution and the characteristics of the reaction zone for a range of conditions. For the closure of turbulence-chemistry interactions at the sub-grid scales, a transported probability density function approach solved by the Eulerian stochastic fields method is applied. A configuration based on the use of airblast atomisation is assessed first and compared with experimental observations. The effect of the atomiser air-to-liquid mass flow ratio is studied in greater detail, both in terms of the resulting gas-phase properties and the droplet evaporation process. Then, the effect of ambient pressure on the global spray flame behaviour is examined. For this part of the study, no atomising air is included in the simulation to separate the effects of ambient pressure on the spray from the interaction with the air jet. Analysis of the flame and spray properties at cross-flow operating pressures of 1 atm, 2 bar and 4 bar highlights the strong coupling between the reacting flow and droplet evaporation characteristics, which are highly affected by the penetration of the spray into a flow field characterised by relatively large gradients of temperature. The results reported in this work provide fundamental understanding for the development of novel low-emission combustion technologies and demonstrate the feasibility of applying large eddy simulation with detailed chemistry for the investigation of reacting aviation fuel sprays in hot vitiated cross-flow.
Fredrich D, Giusti A, 2022, Numerical investigation of multi-component droplet evaporation and autoignition for aero-engine applications, Combustion and Flame, Vol: 241, Pages: 1-16, ISSN: 0010-2180
The droplet evaporation and autoignition behaviour of a three-component kerosene surrogate is numerically investigated for a wide range of ambient pressures and temperatures representing realistic aero-engine conditions. Vitiated air with different levels of dilution is considered to represent mixing of air with combustion products. Particular attention is given to the analysis of multi-component fuel effects at the various operating conditions. The investigation also considers the impact of fuel preheating and multiple initial droplet diameters, and extends to the quantification of NOx emissions at the droplet scale level. Considering pure evaporation, results show that preferential evaporation and variation of the droplet composition are mainly affected by the gas temperature. An increase of the pressure generally increases the duration of the droplet heat-up period and reduces the effects of preferential evaporation, especially when high temperatures are considered. Autoignition in vitiated air is strongly influenced by both the level of dilution and ambient pressure, with the latter playing an important role in determining the value of the initial droplet diameter below which no autoignition occurs. Lower pressures generally make the kerosene droplet more resistant to autoignition for the same level of dilution or gas temperature. Droplet preheating mainly affects the heat-up period and can be used as a design parameter to control the autoignition delay time. NOx levels are substantially related to the gas-phase temperature and the existenceof a flame at the droplet scale. Potential implications for the design of future low-emission combustor technologies are discussed with an emphasis on the fuel preparation.
Leung HY, Karlis E, Hardalupas I, et al., 2021, EVALUATION OF BLOW-OFF DYNAMICS IN AERO-ENGINE COMBUSTORS USING RECURRENCE QUANTIFICATION ANALYSIS, ASME Turbo Expo Turbomachinery Technical Conference and Exposition
Fredrich D, Jones WP, Giusti A, 2021, Numerical investigation of a reacting kerosene spray in hot vitiated cross-flow, 13th International ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements, Publisher: ERCOFTAC, Pages: 1-6
The behaviour of a liquid kerosene spray injectedinto a cross-flow of high temperature vitiated air is in-vestigated. This fundamental flow configuration haswider implications for the design of future aeronauti-cal fuel injectors, particularly with respect to MILDcombustion concepts. Large eddy simulation is ap-plied to numerically study the global flame character-istics, with the main objectives of giving further in-sight into the reacting behaviour of sprays in cross-flow and comparing two different approaches to modelturbulence-chemistry interaction: the conditional mo-ment closure model and the Eulerian stochastic fieldsmethod. Results show that the two approaches givesimilar predictions of the location of the peak meantemperature. Some differences appear in the vicinityof the spray injection location, possibly highlightingthe important role of the modelling of the interactionbetween evaporation and sub-grid mixing. Analysisof the flame and spray behaviour at different pressuresalso demonstrates the strong coupling between the re-acting field and evaporation characteristics, which arehighly affected by the penetration of the spray into aflow field characterised by relatively large gradients oftemperature. Results obtained in this work provide abenchmark for the application of sprays in hot cross-flow for the development of novel combustion tech-nologies.
de Oliveira PM, Sitte MP, Zedda M, et al., 2021, Low-order modeling of high-altitude relight of jet engine combustors, International Journal of Spray and Combustion Dynamics, Vol: 13, Pages: 20-34, ISSN: 1756-8277
A physics-based, low-order ignition model is used to assess the ignition performance of a kerosene-fueled gas-turbine combustor under high-altitude relight conditions. The ignition model used in this study is based on the motion of virtual flame particles and their extinction according to a Karlovitz number criterion, and a stochastic procedure is used to account for the effects of spray polydispersity on the flame’s extinction behavior. The effects of large droplets arising from poor fuel atomization at sub-idle conditions are then investigated in the context of the model parameters and the combustor’s ignition behavior. For that, a Reynolds-averaged Navier-Stokes simulation of the cold flow in the combustor was performed and used as an input for the ignition model. Ignition was possible with a Sauter mean diameter (SMD) of 50 μm, and was enhanced by increasing the spark volume. Although doubling the spark volume at larger SMDs (75 and 100 μm) resulted in the suppression of short-mode failure events, ignition was not achieved due to a reduction of the effective flammable volume in the combustor. Overall, a lower ignition probability is obtained when using the stochastic procedure for the spray, which is to be expected due to the additional detrimental effects associated with poor spray atomisation and high polydispersity.
Sitte MP, Turquand dAuzay C, Giusti A, et al., 2021, A-priori validation of scalar dissipation rate models for turbulent non-premixed flames, Flow, Turbulence and Combustion, Vol: 107, Pages: 201-218, ISSN: 1386-6184
The modelling of scalar dissipation rate in conditional methods for large-eddy simulations is investigated based on a priori direct numerical simulation analysis using a dataset representing an igniting non-premixed planar jet flame. The main objective is to provide a comprehensive assessment of models typically used for large-eddy simulations of non-premixed turbulent flames with the Conditional Moment Closure combustion model. The linear relaxation model gives a good estimate of the Favre-filtered scalar dissipation rate throughout the ignition with a value of the related constant close to the one deduced from theoretical arguments. Such value of the constant is one order of magnitude higher than typical values used in Reynolds-averaged approaches. The amplitude mapping closure model provides a satisfactory estimate of the conditionally filtered scalar dissipation rate even in flows characterised by shear driven turbulence and strong density variation.
de Oliveira PM, Fredrich D, De Falco G, et al., 2021, Soot-free low-NOx aeronautical combustor concept: the lean azimuthal flame for kerosene sprays, Energy and Fuels, Vol: 35, Pages: 7092-7106, ISSN: 0887-0624
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.
de Oliveira PM, Mesquita LCC, Gkantonas S, et al., 2021, Evolution of spray and aerosol from respiratory releases: theoretical estimates for insight on viral transmission, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 477, ISSN: 1364-5021
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.
Foale JM, Giusti A, Mastorakos E, 2021, Simulating the blowoff transient of a swirling, bluff body-stabilized kerosene spray flame using detailed chemistry, Pages: 1-12
The lean blowoff transient of a Jet-A spray flame in a lab-scale swirl burner is simulated using Large Eddy Simulation (LES) and the Conditional Moment Closure (CMC) combustion model. The objectives are: (i) verify the capability of LES-CMC to predict the blowoff of heavy hydrocarbon spray flames, (ii) to investigate local flame behavior such as local extinction and flame lift-off, and (iii) to analyze species behavior during the blowoff transient. The kerosene in this study is a conventional Jet-A reference fuel standardized under the USA National Jet Fuels Combustion Program. A detailed hybrid chemistry (HyChem) mechanism based on the lumped pyrolysis assumption for high temperatures was used. The flame shape changes as it experiences the transient, shrinking down and then retreating from the edges of the bluff body until mixture fraction is present only in regions along the spray cone. During the simulation of the blowoff transient, local extinctions are identified both visually along the flame stoichiometric isosurface and quantitatively in mixture fraction space by regions of low OH and temperature and high fuel and formaldehyde (CH2 O) mass fractions. Pyrolysis species benzene (C6 H6 ) and ethylene (C2 H4 ) are analyzed and shown to reduce during blowoff, due to reduced temperatures preventing vaporized kerosene from undergoing pyrolysis. Without sufficient pyrolysis products available, the flame experiences fuel starvation. The combination of fuel starvation and increased number of local extinctions causes the global lean blowoff event. CH2 O notably builds up around the edges of the combustion chamber toward the end of blowoff, as less OH mass fraction is present to consume the formaldehyde. LES-CMC simulates blowoff to occur at a bulk air velocity within 5% of the experimental value, affirming that the LES-CMC approach coupled with detailed chemistry is able to accurately predict the blow off of heavy hydrocarbon flames.
Kritikos E, Giusti A, 2020, Reactive molecular dynamics investigation of Toluene Oxidation under electrostatic fields: effect of the modeling of local charge distribution., The Journal of Physical Chemistry A: Isolated Molecules, Clusters, Radicals, and Ions; Environmental Chemistry, Geochemistry, and Astrochemistry; Theory, Vol: 124, Pages: 10705-10716, ISSN: 1089-5639
A reactive Molecular Dynamics (MD) study of toluene oxidation at high temperatures under externally applied electrostatic fields has been performed. The impact of the modeling of local charge distribution has been investigated by comparing the widely used Charge Equilibration (QEq) method with the Charge Transfer with Polarization Current Equalization (QTPIE) method, which shields charge transfers up to atomic orbitals and introduces molecular polarization. Using the latter method, it is possible to improve the computation of the atomic charges, which are a critical aspect for the numerical study of electric fields, and to capture important effects of the electric field on rotational and vibrational energies of the toluene molecule. Results show that a more comprehensive treatment of inter- and intramolecular charge distribution achieved through the QTPIE method leads to substantially different applied forces and oxidation rates of toluene compared to the QEq method. Using the QTPIE method, no significant effects of the electrostatic field on the toluene oxidation rate were observed for the range of temperatures and pressures studied here, which is in disagreement with the results obtained with the QEq method where a clear impact of the electrostatic field on the average oxidation rate was found. Therefore, when studying electric field effects with MD simulations, the choice of the method used for the charge equilibration is a key modeling assumption whose impact should be carefully evaluated.
de Oliveira PM, Mesquita LCC, Gkantonas S, et al., 2020, Evolution of spray and aerosol from respiratory releases: theoretical estimates for insight on viral transmission, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, ISSN: 1364-5021
<jats:p>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 seconds 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 hour in a poorly ventilated room gives 0.1–11% infection risk for initial viral loads of 10<jats:sup>8</jats:sup>-10<jats:sup>10</jats:sup> copies/ml<jats:sub>l</jats:sub>, 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.</jats:p>
Gkantonas S, Foale JM, Giusti A, et al., 2020, Soot emission simulations of a single sector model combustor using incompletely stirred reactor network modeling, Journal of Engineering for Gas Turbines and Power, Vol: 142, Pages: 1-11, ISSN: 0742-4795
The simulation of soot evolution is a problem of relevance for the development of low-emission aero-engine combustors. Apart from detailed CFD approaches, it is important to also develop models with modest computational cost so that a large number of geometries can be explored, especially in view of the need to predict engine-out soot particle size distributions (PSDs) to meet future regulations. This paper presents an approach based on Incompletely Stirred Reactor Network (ISRN) modeling that simplifies calculations, allowing for the use of very complex chemistry and soot models. The method relies on a network of incompletely stirred reactors (ISRs), which are inhomogeneous in terms of mixture fraction but characterized by homogeneous conditional averages, with the conditioning performed on the mixture fraction. The ISRN approach is demonstrated here for a single sector lean-burn model combustor operating on Jet-A1 fuel in pilot-only mode, for which detailed CFD and experimental data are available. Results show that reasonable accuracy is obtained at a significantly reduced computational cost. Real fuel chemistry and a detailed physicochemical sectional soot model are consequently employed to investigate the sensitivity of ISRN predictions to the chosen chemical mechanism and provide an estimate of the soot PSD at the combustor exit.
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