Mixture Fraction Distribution Research Articles

FGM modeling considering preferential diffusion, flame stretch, and non-adiabatic effects for hydrogen-air premixed flame wall flashback

Preferential diffusion plays an important role especially in hydrogen flames. Flame stretch significantly affects the flame structure and induces preferential diffusion. A problematic phenomenon occurring in real combustion devices is flashback, which is influenced by non-adiabatic effects, such as wall heat loss. In this paper, an extended flamelet-generated manifold (FGM) method that explicitly considers the preferential diffusion, flame stretch, and non-adiabatic effects is proposed. In this method, the diffusion terms in the transport equations of scalars, viz. the progress variable, mixture fraction, and enthalpy, are formulated employing non-unity Lewis numbers that are variable in space and different for each chemical species. The applicability of the extended FGM method to hydrogen flames is investigated using two- and three-dimensional numerical simulations of hydrogen-air flame flashback in channel flows. The results of the extended FGM method are compared with those of detailed calculations and other FGM methods. The two-dimensional numerical simulations show that considering both preferential diffusion and flame stretch improves the prediction accuracy of the mixture fraction distribution and flashback speed. The three-dimensional numerical simulations show that the prediction accuracy of the flashback speed, backflow region, and distributions of physical quantities near the flame front is improved by employing the extended FGM method, compared with the FGM method that considers only the heat loss effect. In particular, the extended FGM method successfully reproduced the relationship between the reaction rate and curvature. These results demonstrate the effectiveness of the extended FGM method.Novelty and Significance Statement The novelty of this research is the development of a flamelet-generated mani-fold (FGM) method that explicitly considers preferential diffusion, flame stretch, and non-adiabatic effects. To the best of the authors’ knowledge, no studies have performed numerical simulations of pure hydrogen flames using such an FGM method. The developed FGM method was applied to numerical simula- tions of hydrogen-air premixed flame flashback at an equivalence ratio of 0.5 and reproduced the flashback speed of lean hydrogen-air premixed flame. The applicability of the FGM method to the numerical simulation of hydrogen-air flashback is reported first. This research is significant because the FGM method is one of the most widely used combustion models for premixed combustion, and the development of an accurate FGM method will contribute to the engineering field. The accurate prediction of the flame flashback attempted in this study is particularly important for the development of hydrogen-fueled combustion devices.

Numerical simulation and spray model development of liquid ammonia injection under diesel-engine conditions

Ammonia is regarded as a promising alternative fuel in internal combustion engine for its potential to reduce carbon emissions. Injection process of liquid ammonia will affect air-fuel mixture distribution and therefore combustion efficiency and NOx emissions. However, the experimental studies of liquid ammonia injection under engine conditions are limited, and the key impact factors of spray evolution are still unclear. This study aims to evaluate the applicability of numerical models for liquid ammonia spray, and numerically investigate spray characteristics under diesel-engine conditions (ambient temperature > 800K, ambient pressure > 2 MPa). Large eddy simulation coupled with Lagrangian particle tracking method were conducted, and the results show that the non equilibrium - Frössling phase change model coupled with Schiller-Naumann drag model can provide a good prediction of spray penetration. Liquid penetration increases with ambient pressure decreasing; vapor penetration increases with ambient density decreasing, but insensitive to ambient temperature. Temperature - mixture fraction distribution is sensitive to ambient temperature and superheat degree. Besides, a three-stage 0D model of spray tip penetration under diesel-engine conditions is developed firstly, which can reflect the key impact factors of ammonia spray evolution and provide theoretical support to optimize fuel injection strategy of liquid ammonia-fueled engines.

Numerical Study of Velocity and Mixture Fraction Fields in a Turbulent Non-Reacting Propane Jet Flow Issuing into Parallel Co-Flowing Air in Isothermal Condition through OpenFOAM

This research employs computational methods to analyze the velocity and mixture fraction distributions of a non-reacting Propane jet flow that is discharged into parallel co-flowing air under iso-thermal conditions. This study includes a comparison between the numerical results and experimental results obtained from the Sandia Laboratory (USA). The objective is to improve the understanding of flow structure and mixing mechanisms in situations where there is no involvement of chemical reactions or heat transfer. In this experiment, the Realizable k-ε eddy viscosity turbulence model with two equations was utilized to simulate turbulent flow on a nearly 2D plane (specifically, a 5-degree partition of the experimental cylinder domain). This was achieved using OpenFOAM open-source software and swak4Foam utility, with the reactingFoam solver being manipulated carefully. The selection of this turbulence model was based on its superior predictive capability for the spreading rate of both planar and round jets, as compared to other variants of the k-ε models. Numerical axial and radial profiles of different parameters were obtained for a mesh that is independent of the grid (mesh B). These profiles were then compared with experimental data to assess the accuracy of the numerical model. The parameters that are being referred to are mean velocities, turbulence kinetic energy, mean mixture fraction, mixture fraction half radius (Lf), and the mass flux diagram. The validity of the assumption that w߰ = v߰ for the determination of turbulence kinetic energy, k, seems to hold true in situations where experimental data is deficient in w߰. The simulations have successfully obtained the mean mixture fraction and its half radius, Lf, which is a measure of the jet’s width. These values were determined from radial profiles taken at specific locations along the X-axis, including x/D = 0, 4, 15, 30, and 50. The accuracy of the mean vertical velocity fields in the X-direction (Umean) is noticeable, despite being less well-captured. The resolution of mean vertical velocity fields in the Y-direction (Vmean) is comparatively lower. The accuracy of turbulence kinetic energy (k) is moderate when it is within the range of Umean and Vmean. The absence of empirical data for absolute pressure (p) is compensated by the provision of numerical pressure contours.

Numerical investigation of combustion characteristics for hydrogen mixed fuel in a can-type model of the gas turbine combustor

Numerical simulations are performed to analyze the combustion characteristics of propane fuel mixed with different amounts of hydrogen in a can-type combustor. The volume fraction of the hydrogen fuel varies from 0% to 100% in the fuel mixture. The results indicate that the hydrogen enrichment of the fuel significantly affects the flow structure, mixture fraction, and combustion characteristics. An increase in the volume fraction of hydrogen significantly affects the mean mixture fraction distribution, promotes combustion, and increases the flame temperature and the width of the flammable range within the combustor. Therefore, the degree of temperature uniformity at the outlet of the combustor increases with hydrogen enrichment, corresponding to an increase of 49.64% in the uniformity factor. The hydrogen enriched fuel can also reduce the emissions of CO and CO2, owing to the reduced amount of carbonaceous fuel.

Soot structure and flow characteristics in turbulent non-premixed methane flames stabilised on a bluff-body

The soot properties of methane in turbulent regimes are not well characterised but are highly desirable. Methane is the main constituent of natural gas that is broadly used in many industrial combustors. Investigation of turbulent methane flames under well-defined boundary conditions is therefore useful for interpreting soot formation in practical burners and can be used for further model development. This study presents a joint experimental and numerical study of a series of turbulent non-premixed bluff-body flames fuelled with pure methane for three values of the momentum flux ratio of fuel jet to co-flowing air. Soot volume fraction (SVF) and flowfield are measured simultaneously using planar laser-induced incandescence (P-LII) and 2D-polarised particle image velocimetry (P-PIV). Additionally, time-averaged temperature, mixture fraction, OH and C2H2 concentrations are estimated numerically using RANS models. The global flame structure for all three flames features a recirculation zone with a double-vortex structure, a jet-propagating zone, and a neck zone connecting the two regions. The soot distribution within the recirculation zone shows clear distinct features, which is attributed to the mean mixture fraction distribution in this zone. Increasing the momentum flux ratio shifts the location of the mean stoichiometric mixture fraction to the rich inner vortex core, leading to a distinct peak of the total integrated soot in the inner vortex of the recirculation zone that is not observed in other cases. Also, it is deduced that the soot inception starts earlier in the recirculation zone for the flame with the highest momentum flux ratio and in the jet zone for the other two flames. Much higher soot concentration and lower intermittency are found with ethylene-based flames stabilised on the same burner and with the same operating conditions. In addition, the study has generated a database of soot and flowfield results, which can be helpful for future model validations.

The influence of the evaporation model on ethanol turbulent jet flame simulation

High-precision numerical simulation of aero-engine combustors requires a suitable evaporation model. At present, quantitative research on the effects of the evaporation model on two-phase turbulence combustion simulation is insufficient. Ethanol has a higher latent heat of evaporation than other fuels like acetone and aero kerosene, thus it is especially sensitive to the evaporation model. This paper uses LES to simulate turbulence in an aero-engine combustor, and TPDF to describe the interaction between turbulence and chemical reactions. Numerical simulation results of the Abramzon-Sirignano (A-S), Spalding, and thick exchange layer (NC-TEL) evaporation models for high-Reynolds number ethanol combustion in two-phase turbulent jet flames are analyzed and compared with experimental data. Results suggest that per unit time, the A-S evaporation model has the largest evaporation mass, followed by the NC-TEL evaporation model, while the Spalding evaporation model has the smallest evaporation mass. Through quantitative analysis of the different evaporation models’ influence on temperature, mixture fraction, and liquid phase distribution, it is found that the A-S evaporation model fits the experimental data better in the central region of jet flame but has worse performance in the other flame regions. The simulation result of the Spalding model is good in the X/D = 10 section, and that of the NC-TEL model shows the best result at X/D > 10 sections and in the outer regions of the jet flame. The maximum difference between the NC-TEL model results and the experimental data was 21.856 % in liquid phase velocity. Hence, the NC-TEL model has the best simulation performance.

Modeling and LES of high-pressure liquid injection under evaporating and non-evaporating conditions by a real fluid model and surface density approach

Numerical modeling of high-pressure liquid fuel injection remains a challenge in various applications. Indeed, experimental observations have shown that injected liquid fuel jet undergoes a continuous change of state from classical two-phase atomization and spray droplets evaporation to a dense-fluid mixing phenomenon depending on the ambient pressure, temperature, and fuel properties. Accordingly, a predictive and efficient computational fluid dynamics (CFD) model that can represent the possible coexistence of subcritical and supercritical regimes during the fuel injection event is required. The widely used Lagrangian Discrete Droplet Method (DDM) requires parameter tuning of model constants and cannot model the dense near-nozzle region. Meanwhile, the high computational cost of Interface Capturing Methods (ICM) has prohibited their application to industrial cases. Thus, another alternative is an Eulerian Diffuse Interface Model (DIM), where the unresolved interface features are modeled instead of being tracked. Accordingly, the current work proposes a fully compressible multi-component two-phase real-fluid model (RFM) with a diffused interface and closed by a thermodynamic equilibrium tabulation method based on a real-fluid equation of state. The RFM model is complemented with a postulated surface density equation for fuel atomization modeling within the Large Eddy Simulation (LES) framework. The Engine Combustion Network (ECN) Spray A injector non-evaporating and nominal evaporating conditions are used as a reference for the proposed model validation. Simulations are performed using the proposed RFM model that has been implemented in the CONVERGE CFD solver. Under the non-evaporating condition, the RFM model can capture well the fuel mass distribution in the near-nozzle field, but also the interfacial surface area. Besides, the predicted drop size from simulations falls within the experimental data range. On the other hand, under the evaporating condition, spray liquid and vapor penetrations and fuel mixture fraction distribution are also accurately predicted. The vaporization effect on the surface area density is revealed to enhance surface generation in the dense spray region while reducing the surface density in the dilute spray region. The mean droplet size is also relatively reduced under the evaporating condition in the diluted spray region. Overall, the accuracy and computationally efficiency of the proposed RFM model coupled with the surface density equation for high-pressure fuel injection modeling are confirmed, allowing its use for high pressure industrial configurations in future studies.

Large eddy simulation of multi-regime combustion with a two-progress variable approach for carbon monoxide

Simulations of two cases in a novel multi-regime burner configuration are undertaken using a presumed joint probability density function (PDF) approach with tabulated chemistry. The flame conditions are varied by changing the central jet equivalence ratio, which produces different multi-regime combustion modes in the non-premixed inner flame. An outer premixed flame and recirculation zone behind a bluff body are present to supply heat and combustion products to stabilise the inner flame. A two-progress variable approach is tested to improve predictions of carbon monoxide (CO) in the post-flame regions, where CO oxidation occurs. The large eddy simulation set-up and sub-grid combustion model are assessed through comparisons with time-averaged measurements for radial profiles at different streamwise locations. The jet break-up length, the shear layers and the mixture fraction distribution are well captured in both cases. The temperature distribution is well captured for the inner flame in each case but the temperature and mixture fraction are over predicted in the downstream regions of the outer premixed flame, which is due to increased dilatation that suppresses air entrainment. Improved predictions of the CO mass fraction are obtained for the outer premixed flames with the two-progress variable approach. Over predictions are seen in the upstream regions of the inner flame when the CO mass fraction is obtained from a look-up table, suggesting that the CO mass fraction should be transported to include the convection/diffusion balance in regions where there is no flame. Furthermore, transporting the CO mass fraction with a one-progress variable approach produces over predictions in the burnt regions, suggesting a two-progress variable model is needed to capture the consumption region of CO. The multi-regime combustion characteristics are observed to be stronger in flame MRB26b, where non-premixed and rich premixed combustion is present. For flame MRB18b, the non-premixed contribution is smaller and weak stratified combustion is observed.

Spatial characteristics and modelling of mixture fraction variance and scalar dissipation rate in steady turbulent round jets

The spatial characteristics of scalar dissipation rate (SDR) are investigated in a steady-state turbulent round jet using Direct Numerical Simulation (DNS). Radial distribution of mixture fraction is found to agree well with the experimental data. Radial distributions of the turbulent diffusivities and the turbulent Schmidt numbers show different behaviours, based on how these quantities are defined. Point-wise statistics confirm that the log-normal distribution fits the probability distribution functions (PDFs) of the SDR well, except for a small positive skewness attributed to flow anisotropy. Joint PDFs of mixture fraction and SDR are symmetric with respect to mixture fraction. Self-similarity of the mean SDR, as well as its axial, radial and azimuthal components appears after a certain downstream distance. In the self-similar region, centreline SDR scales with downstream distance, x−4. Normalised SDR radial profiles show an increase near the centreline, followed by a steep decrease, after an off-centreline peak. Magnitudes of the axial, radial and azimuthal components of the mean SDR are approximately the same in the self-similar region. The budget analysis of the mean SDR transport equation reveals that the production term by scalar field stretch and the destruction term due to local curvature effects are the two dominant terms. A modification of an existing model for the self-similar mean SDR radial profiles is proposed. This is found to improve the prediction of mean SDR distributions.

Large eddy simulation of n-dodecane spray flame: Effects of injection pressure on spray combustion characteristics at low ambient temperature

The present study uses large-eddy simulations (LES) to identify the underlying mechanism that governs the ignition and stabilization mechanisms of ECN Spray A flame for different injection pressures (Pinj) and ambient temperatures (Tam). Two Pinj of 50 MPa and 150 MPa, as well as two Tam of 750 K and 900 K are considered. The numerical model is validated against the experimental fuel penetration, radial mixture fraction distribution, ignition delay time, and lift-off length. The combustion characteristics of all four spray flames are well predicted, with a maximum relative difference of 15% to the measurements. At 900 K, high-temperature ignition (HTI) occurs in the fuel-rich mixture at the spray head of the high Pinj spray flame, but at the spray periphery of the low Pinj spray flame. This is due to the low Pinj case having fuel-richer mixture in the inner spray region. Nonetheless, the spray flames at both Pinj exhibit double-flame structure. At 750 K, HTI occurs at the fuel-rich and fuel-lean regions for spray flames with Pinj=50 MPa and 150 MPa, respectively. Reducing the Pinj leads to a lower injection velocity, less turbulent fluctuation, slower mixing, and hence the occurrence of HTI at the fuel-rich mixtures. The spray flame in the low Pinj case at 750 K exhibits a triple-flame structure at the lift-off position, while the high Pinj case exhibits a lean premixed reaction zone. This difference is attributed to the distribution of fuel-rich mixtures. Despite differences in the flame structures, auto-ignition process plays a key role to stabilize the lift-off position for all four spray flames. The auto-ignition process is also found to be dependent on the cool-flame products upstream of the lift-off position. In particular for the low Tam cases, the heat transfer effect from the main flame to the fuel-rich regions is suggested to also contribute to the flame stabilization mechanisms.

Scalar Forcing Methodology for Direct Numerical Simulations of Turbulent Stratified Mixture Combustion

Scalar forcing in the context of turbulent stratified flame simulations aims to maintain the fuel-air inhomogeneity in the unburned gas. With scalar forcing, stratified flame simulations have the potential to reach a statistically stationary state with a prescribed mixture fraction distribution and root-mean-square value in the unburned gas, irrespective of the turbulence intensity. The applicability of scalar forcing for Direct Numerical Simulations of stratified mixture combustion is assessed by considering a recently developed scalar forcing scheme, known as the reaction analogy method, applied to both passive scalar mixing and the imperfectly mixed unburned reactants of statistically planar stratified flames under low Mach number conditions. The newly developed method enables statistically symmetric scalar distributions between bell-shaped and bimodal to be maintained without any significant departure from the specified bounds of the scalar. Moreover, the performance of the newly proposed scalar forcing methodology has been assessed for a range of different velocity forcing schemes (Lundgren forcing and modified bandwidth forcing) and also without any velocity forcing. It has been found that the scalar forcing scheme has no adverse impact on flame-turbulence interaction and it only maintains the prescribed root-mean-square value of the scalar fluctuation, and its distribution. The scalar integral length scale evolution is shown to be unaffected by the scalar forcing scheme studied in this paper. Thus, the scalar forcing scheme has a high potential to provide a valuable computational tool to enable analysis of the effects of unburned mixture stratification on turbulent flame dynamics.

Improved delayed detached eddy simulation of supersonic combustion fueled by liquid kerosene

The purpose of this study is to quantitatively investigate the influence of diffusion characteristics and equivalence ratios (ERs) of gaseous/liquid kerosene on transient combustions in a three-dimensional cavity-based scramjet combustor using Improved Delayed Detached Eddy Simulation (IDDES) with a 19 species and 54 reactions kerosene/air mechanism. Additionally, the similarities and differences between gaseous and liquid kerosene supersonic combustion are identified based on the pressure, mixture fraction, temperature, and heat release rate distributions. The findings indicated that the injection velocity of liquid kerosene is an order of magnitude lower than that of gaseous kerosene; however, the residence time of liquid kerosene in the cavity was amplified by two orders of magnitude. The results also highlighted the substantial differences in the reaction heat release position between gaseous and liquid kerosene combustion. For a combustion process of liquid kerosene at an ER of 0.215, there is no obvious boundary layer separation in the isolator. The combustion process is controlled by the mixing efficiency of the shear layer, and the mode of combustion is cavity shear-layer stabilized combustion. When the ERs are 0.27 and 0.43, the flame propagates upstream of the cavity and forms boundary layer separation and oblique shock waves. Then, the combustion process is controlled by the fuel transportation in the cavity recirculation zone, and the mode of combustion is the cavity recirculation-zone stabilized combustion.

Examination of probability distribution of mixture fraction in LES/FDF modelling of a turbulent partially premixed jet flame

An accurate prediction of the probability density function (PDF) of the mixture fraction is crucial to the prediction of combustion since mixing plays an important role in turbulent non-premixed and partially premixed flames. This work provides an assessment of the large-eddy simulation (LES)/filtered density function (FDF) method for the prediction of the PDF of the mixture fraction. The advantage of the LES/FDF method is that it provides the full predictions of the statistical distribution of scalars including the mixture fraction. The predictive accuracy of the method for the PDF is yet to be fully validated. Assessing the prediction of the PDF of the mixture fraction, a conserved scalar, is an important starting point. The Sydney/Sandia inhomogeneous inlet jet flame is used as a test case. A quick comparison shows that the LES/FDF predicted PDF shapes of the mixture fraction deviate significantly from the commonly presumed Beta-PDF as well as from the experimental data in the flame. To examine the source of the discrepancy, we clarify the different PDF definitions used in the comparison among the predictions, measurements, and presumed shape PDFs. The discrepancy observed from the comparison is largely reconciled by clarifying the difference between the PDFs that are examined. The PDF of the resolved mixture fraction is shown to be close to the Beta-PDF in both the measurements and predictions, while the PDF directly deduced from the LES/FDF particles deviates significantly from the Beta-PDF. A multimodal PDF analysis and a pseudo convergence analysis are conducted to provide plausible evidence to support the predicted multimodal PDF shapes. The sub-filter scale FDF is shown to be close to the Beta-PDF too through the construction of a synthesised PDF, which supports the common presumed Beta-PDF assumption used in the presumed PDF methods when combined with LES.

Large-eddy simulation of n-dodecane spray flame: Effects of nozzle diameters on autoignition at varying ambient temperatures

In the present study, large-eddy simulations (LES) are used to identify the underlying mechanism that governs the ignition phenomena of spray flames from different nozzle diameters when the ambient temperature (Tam) varies. Two nozzle sizes of 90µm and 186µm are chosen. They correspond to the nozzle sizes used by Spray A and Spray D, respectively, in the Engine Combustion Network. LES studies of both nozzles are performed at three Tam of 800K, 900K, and 1000K. The numerical models are validated using the experimental liquid and vapor penetration, mixture fraction (Z) distribution, as well as ignition delay time (IDT). The ignition characteristics of both Spray A and Spray D are well predicted, with a maximum relative difference of 14% as compared to the experiments. The simulations also predict the annular ignition sites for Spray D at Tam ⩾ 900K, which is consistent with the experimental observation. It is found that the mixture with Z ⩽ 0.2 at the spray periphery is more favorable for ignition to occur than the overly fuel-rich mixture of Z > 0.2 formed in the core of spray. This leads to the annular ignition sites at higher Tam. Significantly longer IDT for Spray D is obtained at Tam of 800K due to higher scalar dissipation rates (χ) during high temperature (HT) ignition. The maximum χ during HT ignition for Spray D is larger than that in Spray A by approximately a factor of 5. In contrast, at Tam=1000K, the χ values are similar between Spray A and Spray D. This elucidates the increase in the difference of IDT between Spray D and Spray A as Tam decreases. This may explain the contradicting findings on the effects of nozzle diameters on IDT from literature.

Interpreting diffusion flame structure by simultaneous mixture fraction and temperature measurements using optical and acoustic signals from laser-induced plasmas

The measurement of mixture fraction and temperature is of great importance in non-premixed flame structure studies and combustion optimization. However, obtaining profiles of mixture fraction and temperature simultaneously for a wide range of fuels and compositions has proven to be challenging, especially when suitable spatial and temporal resolutions are desired. In this study, the acoustic and optical signals from laser induced plasmas were simultaneously used for the first time to obtain acoustic-based laser-induced breakdown thermometry (LIBT) combined with the laser induced breakdown spectroscopy (LIBS). The system was first calibrated in an ethylene-air premixed flame. Then the atomic-ratio and temperature distributions along the centerline of an ethylene counterflow diffusion flame were measured. The strong compositional and temperature gradients in diffusion flames represents a potential challenge, but simultaneous measurements were successfully performed within a 1.5 mm wide physical region, where the equivalence ratio ranged from 0.5 to 12 and temperature ranged from 1100 K to 2000 K. The elemental mass fractions and mixture fraction distribution were inferred based on measured atomic ratio distributions. The preferential diffusion of H relative to C was directly observed by the measurement of C/H ratio. Lastly, the physics behind the LIBT technique is discussed and analyzed. This work demonstrates that the combination of LIBT and LIBS holds promise as a simple tool to measure the mixture fraction and temperature in a broader range of combustion conditions; and facilitates a better understanding of flame structure.

Comparison of Finite Rate Chemistry and Flamelet/Progress-Variable Models: Sandia Flames and the Effect of Differential Diffusion

ABSTRACT In this study, large eddy simulations (LES) are conducted using both a finite-rate chemistry (FRC) model and a flamelet/progress-variable (FPV) model for a series of piloted partially premixed methane/air flames of increasing turbulence intensity (Sandia Flames D, E, and F). From Flame D to E to F, as flow velocity and strain rate increase, the flame is either pushed downstream and extended radially or weakened by enhanced local extinction. The two combustion models produce different spatial distributions of both time-averaged quantities and instantaneous flame field. The FPV model provides an overall better prediction of the time-averaged axial and radial profiles of Flame D, but a significantly worse prediction of Flame F, primarily because the FPV model significantly over predicts local extinction. In terms of the conditional statistics, in which the effects of spatial distribution of mixture fraction and subgrid-scale (SGS) modeling are largely “removed,” the FRC model provides better predictions than the FPV model for all quantities at most locations and mixture fractions in all three flames. The effect of differential diffusion on the prediction of a species depends on the molecular diffusivity of that species; the effect is typically smaller than the difference between the FRC and FPV models.

Effects of the Bluff-Body Diameter on the Flow-Field Characteristics of Non-Premixed Turbulent Highly-Sooting Flames

ABSTRACT This paper presents a joint experimental and computational study of the effect of the bluff-body diameter on the flow field and residence time distribution (RTD) in a set of turbulent non-premixed ethylene/nitrogen flames with a high soot load. A novel optical design has been developed to undertake the PIV measurements successfully in highly-sooting turbulent flames, using polarizing filters. The mean velocity components and turbulent intensity are reported for three bluff-body burners with different bluff-body diameters (38, 50, and 64 mm), but which are otherwise identical in all other dimensions. The central jet diameter of 4.6 mm was supplied with a mixture of ethylene and nitrogen (4:1, by volume) to achieve a bulk Reynolds number of 15,000 for the reacting cases. Isothermal cases were also investigated to isolate the effect of heat release on the flow fiel. Pure nitrogen was utilized in the isothermal cases where the Reynolds number was kept the same as the reacting cases. The annular bulk velocity of the co-flowing air was kept constant at 20 m/s for all experiments. Computationally, a 2-D RANS model was developed, validated against the experimental data and was mainly used to investigate the effect of the bluff body diameter on the residence time in the recirculation zone. The flow structure for both isothermal and reacting cases was found to be consistent with the literature, exhibiting similar vortical structures of the recirculating zone and mixture fraction distribution, for similar momentum flux ratios. The flame length and volume were found to decrease by 20% and 9%, respectively, as the bluff diameter was increased from 38mm to 64mm. The length of the recirculation zone for the isothermal cases was found to be ~1.2DBB, while for the reacting cases it was ~1.5–1.75DBB. A stochastic tracking model was employed to estimate the pseudo-particles’ residence time distribution in the recirculation region. The model revealed that an increase in the bluff body diameter from 38mm to 64 mm leads to tripling of the mean residence time within the recirculation zone. Thermal radiation measurements from the recirculation zone show a 35% increase as the bluff body diameter is increased from 38 mm to 64 mm, whilst the total radiation from the whole flame drops by 15%, which is deduced to be due mostly to the decrease in flame volume. The effect of these differences on soot propensity and transport are described briefly and will be the subject of future investigations.

Flame–spray interaction and combustion features in split-injection spray flames under diesel engine-like conditions

In compression ignition engines, split-injection strategy has shown great benefits in reducing pollutant emissions and improving combustion efficiency. Spray–flame interaction involving in split injections is significantly complex, which affects the ignition process and even pollutant emissions. Therefore, the objective of this study is to investigate how the flame–spray interaction affects the subsequent ignition process and combustion features in split injections under diesel engine-like conditions. In this work, large eddy simulation coupled with a 54-species mechanism for split injections of n-dodecane is performed to study the effect of injection duration and dwell times (DTs) on spray–flame interactions and the ignition mechanism. The numerical model gives a reasonable agreement with the experiments in terms of the vapor penetration length, ignition delay times, mixture fraction distributions and the flame structures. The present study revealed that combustion for split injections is a multi-stage process and the ignition processes for the first and second injections are controlled by different mechanisms, namely autoignition for the first injection, and the accelerating ignition for the second injection due to the intermediate species and heating effect formed in the first injection. Moreover, the increase in dwell time between individual injections reduces the subsequently promoting ignition effect for the second injection and thus weakens the interacting process between the two injections. Consumption of the fuel in the first injection leads to a temperature increase and production of different species, which in turn accelerates the ignition of the second injection. Finally, the competition between the local flow timescale and chemical timescale is investigated based on the chemical explosive mode analysis (CEMA) methods. A balance between reaction and mixing processes dominates the combustion of the quasi-steady spray in the second injection with a short DT. However, the flame is controlled by autoignition when a longer DT is used.

A new perspective on modelling passive scalar conditional mixing statistics in turbulent spray flames

Carrier-phase direct numerical simulations (CP-DNS) are performed for turbulent spray combustion. The CP-DNS database is used to evaluate scaling laws for modelling characteristic mixing quantities, such as the mixture fraction conditional scalar dissipation, Nf, and its probability density function (PDF or Pf) in inter-droplet space. Three different methods to process the input parameters for the scaling laws are evaluated. If conditionally averaged quantities are used as input, the models can better capture (1) the scalar dissipation’s exponential increase for larger mixture fraction values and (2) the overall shape of the mixture fraction distribution featuring a distinct secondary peak at stoichiometry that results from increased droplet evaporation in regions where the flame is located. It is also found that the scaling laws provide acceptable estimates independent of droplet equivalence ratio, droplet pre-evaporation rate, spray combustion regime, droplet Stokes number, droplet size and large turbulence scale. This study provides a more physical perspective to the understanding of the mixing in spray combustion. The scaling laws for turbulent micro-mixing may have better predictive capability than the existing sub-models for single phase combustion used in flamelet and conditional moment closure (CMC) approaches that provide sub-grid closures for turbulence-chemistry interactions in LES or RANS.

Modelling Sub-Grid Passive Scalar Statistics in Moderately Dense Evaporating Sprays

Spray evaporation in spatially decaying turbulence is simulated using carrier-phase direct numerical simulations (CP-DNS). The CP-DNS cover a much wider parameter range than earlier fully resolved DNS of regular droplet arrays that were used to calibrate scaling laws for sub-grid closures of the distribution of mixture fraction and its conditionally averaged scalar dissipation. The scaling laws include the effects of sub-grid interactions between droplet evaporation and turbulence, and here they are assessed by direct comparison with the statistics from the CP-DNS. Two issues can be observed: Firstly, care must be taken when interpreting the CP-DNS statistics as the lack of resolution of the point particle surface could impact on the values in DNS cells that are located within the (unresolved) quasi-laminar wake. Secondly, the scaling laws present similar agreement with CP-DNS data as had been observed before for the fully resolved DNS. The scaling law for the scalar dissipation approximates the DNS statistics well, independent of the droplet number density, Stokes number and turbulence intensity. The estimated mixture fraction distribution (the PDF) is good for the mixture fraction values larger than a suitable average value but deteriorates for smaller mixture fractions due to inherent model limitations. The data corroborate that the scaling laws for turbulent micro-mixing can potentially serve as sub-grid closures for mixture fraction based combustion models such as flamelet and conditional moment closure approaches in large eddy simulations and may provide better approximations than existing expressions derived from single-phase non-premixed combustion.