Mass Fraction Distribution Research Articles

Effects of inflow velocity on transverse jet injection in a supersonic cavity combustor

The flow, mixing, and combustion mechanisms in the wide range scramjet engine are complex and far from clear. In the present work, the mixing flow of a sonic transverse jet injection in a supersonic cavity combustor is numerically investigated at two typical inflow velocities. The basic flow structures, unsteady flow dynamics, average flow structures, and several significant mixing performance parameters are well captured and compared based on high-resolution large eddy simulation. The simulation results show that separation shock induced by the jet is gradually merged with the bow shock at low Mach inflow so that the curved shock flow patter is produced. In addition, smaller large-scale coherent structures at the windward side and slower large-scale vortex transport are observed at low Mach inflow. At low Mach inflow, moreover, much narrower range of jet species mass fraction distributions and more upstream large-scale vortices breakdown and dissipation can be observed. The low Mach inflow generates weaker pair of counter-rotating vortices and some trailing counter-rotating vortices, which primarily leads to the weaker jet/cavity interaction. The baroclinic term effects are considerably weaker at low Mach inflow in the near field. In view of mixing efficiency and flammability efficiency, the effect of the cavity in enhancing mixing is more evident at low Mach inflow.

A numerical investigation of the thermal–hydraulic performance during subcooled flow boiling in MMCs with different manifolds

Thermal management of high-heat-flux electronics has been a critical bottleneck for integrated circuit miniaturization and increased power density, and manifold microchannel (MMC) heat sinks are expected to be an efficient near-junction thermal solution. A numerical investigation based a self-developed solver has been conducted to explore the effects of manifold geometries on the thermal–hydraulic performance of MMC heat sinks during subcooled flow boiling, and phase, pressure, and mass distribution among microchannels are analyzed combined with temperature distribution. Heat flux makes little difference on the mass maldistribution among microchannels of MMCs regardless of single-phase and flow boiling, and flow patterns in the outlet manifolds of Z-type MMCs changes from bubbly flow, slug flow to vapor columns with segmented bubbles with increasing heat fluxes. As for the effect of manifold numbers, both the maximum temperature and the pressure drop decrease with increasing manifold numbers, and a relatively uniform distribution of mass and void fraction has been found for all cases of HU-type MMCs regardless of heat fluxes with the best thermal–hydraulic performance at N = 10. However, the thermal performance of Z-type MMCs would improve firstly and then flatten out with increasing manifold number, while the pressure drop presents an opposite trend. The differences in thermal–hydraulic performance are attributed to the longer flow path in Z-type manifolds, and the increasing manifold number presents a higher manifold pressure drop and thus would worsen the mass maldistribution. As for tapered manifolds, a tapered ratio of 0.33 could provide a slightly lower base temperature than the original case with a pressure drop decreased by over 25 %, and the pressure drop fluctuation at high heat fluxes could be suppressed by 70 % with significantly improved flow boiling stability. This work can serve as a basis for the further optimization of subcooled flow boiling in MMC heat sinks, and the results indicate that both an even mass distribution among microchannels and a desirable inlet/outlet port ratio for improved flow field are of great importance for the manifold design applied in MMC heat sinks for two-phase heat dissipation.

In-line measurement of fiber mass fraction using Terahertz spectroscopy for a function-oriented quality assurance of glass fiber sheet molding compound

Glass fiber sheet molding compounds are a popular fiber-reinforced polymer because of their good processability. Both fiber mass fraction (FMF) and fiber orientation distribution significantly determine the mechanical properties of manufactured components. Local random variations in both properties occur during prepreg production and molding. This contribution expands a non-destructive measurement model for local FMF using Terahertz spectroscopy towards multiple specimen thicknesses. A thorough measurement uncertainty analysis according to the Guide to the Expression of Uncertainty in Measurement (GUM) shows an uncertainty reduction by 70 % compared to the uncertainty resulting from the manufacturing process. Measurement times of 0.5 s qualify the methodology for an in-line application. Integrating the measurement results into a parametrized, component-specific finite element model reduces the root mean squared error between physical experiments and simulations by 33 and 54 % compared to simulations with nominal fiber mass fractions.

Flame Surface Density and Artificially Thickened Flame Combustion Models Applied to a Turbulent Partially-Premixed Flame

The Sydney Flame provides an excellent framework for systematic analysis of premixed, partially premixed and non-premixed methane/air combustion. It represents (depending on the axial position from the fuel inlet) non-premixed, partially premixed and fully premixed combustion. We simulate the test case FJ200-5GP-Lr75-57, which represents the partially premixed mode. Large-eddy simulations (LES) are conducted using a flame surface density combustion model (FSD) using one-step chemistry and an artificially thickened flame model (ATF) with a global two-step and an analytically reduced multi-step chemistry reaction mechanism. Unity Lewis numbers are assumed (Le=1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Le=1$$\\end{document}) at first. The FSD and ATF models are compared using the same computational mesh, numerical scheme and boundary conditions. Both models will be analysed regarding their accuracy and computational efficiency in the regime of partially premixed combustion. A comparison of the results points out the strengths and weaknesses of the FSD models. The FSD model with inclusion of flame stretch effects yields good agreement with mean experimental temperature, CO2 and H2O mass fraction distributions in contrast to the ATF model using the global two-step mechanism, which overestimates downstream temperature, CO2 and H2O mass fractions. The FSD model performs well in all regions, which are dominated first by premixed, then partially premixed and finally non-premixed combustion along the flame. The ATF multi-step chemistry shows good results only in the premixed mode region, while mean temperature, CO2 and mass fractions are overestimated in the non-premixed mode regions at higher mixture fraction values. Including differential diffusion into the transport equations improved the ATF model results in comparison with experiments. A mesh study revealed, that the ATF model performs only after mesh refinement. In particular results are improved in the non-premixed combustion region. In contrast, the FSD model is less resolution sensitive and performs very well for both meshes. An evaluation of the Wasserstein metric provides a quantitative assessment of different ATF model setups on simulation accuracy. Finally, computational times for all simulation setups are compared.

A pressure-coupled Representative Interactive Linear Eddy Model (RILEM) for engine simulations

The Representative Linear Eddy Model (RILEM) was introduced by Lackmann et al. (2018) as an alternative modeling approach to simulate turbulent non-premixed combustion in engines. The model utilizes a RANS approach for turbulence and the Linear Eddy Model (LEM) with a presumed probability density function (PDF) approach for combustion closure. A distinct feature of RILEM is its potential to handle arbitrary combustion regimes and the consideration of complex physical phenomena such as differential diffusion effects. The original version of RILEM implemented a volume-based coupling between LEM and the flow solver. This work presents a new variant of RILEM, i.e., Multiple Representative Interactive Linear Eddy Model (MRILEM) based on a pressure-based coupling, to overcome some deficiencies of the original RILEM, namely statistical fidelity. Due to the introduced pressure coupling, the effects of heat losses (wall heat fluxes, latent heat of evaporation) on combustion are intrinsically included via the pressure trace. Furthermore, we introduce a new step function PDF for the progress variable defined by its mean value only. Issues with an incomplete solution space for mixture fraction and progress variable due to the stochastic nature of LEM are remedied with a PDF scaling technique, aided by a novel parameterization of the progress-variable PDF The new variant of RILEM is evaluated using part- and full-load cases of a heavy-duty metal engine. The impact of utilizing multiple LEM lines on the completeness of the solution space and its influence on the distribution of scalar values in the CFD domain was demonstrated. Results for pressure trace, flame structure, and CO emissions are analyzed and compared with simulations using the Multi-Zone Well-Mixed Model (MZWM) model and experiments. While pressure traces agree well among the different models and experiments, noteworthy differences are observed between the models regarding CO emissions and temperature. Effects of turbulence chemistry interaction were noticed when comparing MRILEM to the results of the MZWM simulation, namely flame brush and species mass fraction distribution.

ExoMDN: Rapid characterization of exoplanet interior structures with mixture density networks

Aims. Characterizing the interior structure of exoplanets is essential for understanding their diversity, formation, and evolution. As the interior of exoplanets is inaccessible to observations, an inverse problem must be solved, where numerical structure models need to conform to observable parameters such as mass and radius. This is a highly degenerate problem whose solution often relies on computationally expensive and time-consuming inference methods such as Markov chain Monte Carlo. Methods. We present ExoMDN, a machine-learning model for the interior characterization of exoplanets based on mixture density networks (MDN). The model is trained on a large dataset of more than 5.6 million synthetic planets below 25 Earth masses consisting of an iron core, a silicate mantle, a water and high-pressure ice layer, and a H/He atmosphere. We employ log-ratio transformations to convert the interior structure data into a form that the MDN can easily handle. Results. Given mass, radius, and equilibrium temperature, we show that ExoMDN can deliver a full posterior distribution of mass fractions and thicknesses of each planetary layer in under a second on a standard Intel i5 CPU. Observational uncertainties can be easily accounted for through repeated predictions from within the uncertainties. We used ExoMDN to characterize the interiors of 22 confirmed exoplanets with mass and radius uncertainties below 10 and 5%, respectively, including the well studied GJ 1214 b, GJ 486 b, and the TRAPPIST-1 planets. We discuss the inclusion of the fluid Love number k2 as an additional (potential) observable, showing how it can significantly reduce the degeneracy of interior structures. Utilizing the fast predictions of ExoMDN, we show that measuring k2 with an accuracy of 10% can constrain the thickness of core and mantle of an Earth analog to ≈13% of the true values.

Importance of nasal air conditioning in pre-septoplasty planning: A heat and mass transfer perspective

Nasal air conditioning (heat and mass transfer) is one of the essential functions of the nasal cavity, which can get significantly altered due to deviated nasal septum (DNS). Its importance in pre-septoplasty planning remains poorly investigated, with existing literature focusing primarily on the importance of airflow and pressure drop in determining the efficacy of septoplasty. Nevertheless, this parameter should be considered as ignorance can lead to an erroneous selection of septoplasty patients. In this regard, the current study uses virtual septoplasty and image-based CFD modeling based on the finite volume method (FVM) to investigate the alteration in nasal air conditioning in seventeen DNS patients. Changes in relative humidity (RH) at the nasopharynx region are highlighted. In addition, a new parameter known as the septoplasty score is introduced, which considers the combined effect of pressure drop and RH to quantify the effectiveness of the septoplasty. The insilico results show that DNS leads to asymmetric distribution of water mass fraction and temperature, with higher values on the narrower side of the nasal cavity leading to local drying of the mucosal layer. This asymmetric effect becomes less potent after virtual correction with a maximum decrease in the mass transfer of up to 85 gm per week. Further, the inspired air's RH is significantly altered at the nasopharynx region before and after the virtual septoplasty with a maximum difference of up to 10%. The septoplasty score always increases after correction with a maximum increase of up to 90%; thereby, it can be used by ENT surgeons to quantify septoplasty's benefits.

Mass transfer of the multicomponent free jet and one numerical implementation on a graphic processing unit platform

Jet flows are ubiquitous in nature and laboratories. For example, helium and argon jets into ambient air are commonly used to produce atmospheric plasma jets. The jets before and after discharge both are typical multicomponent flows. This study reports on the implementation of a numerical solver based on the graphic processing units to model multicomponent mixture gas flows. To this end, two- and three-dimensional Jacobian matrices for the multicomponent Euler equations were obtained to reconstruct numerical fluxes and build a total variation diminishing scheme. The two-dimensional early-stage mass transfer of gas jets was studied for three mixture gases: He/air, Ar/air, and N2/air to numerically verify the steady functioning of the numerical solver despite the high mass fraction gradients. The mass fraction distribution of helium in ambient air exhibited more complex substructures than those of argon and nitrogen in ambient air. Using the watershed method, self-generated bullets were extracted from the mass fraction distributions of the helium jet. The proposed numerical solver implemented in this study is expected to provide a better understanding of gas jet dynamics, and the Jacobian matrices can provide a mathematical foundation for constructing potential higher schemes.

The profiles of bars in spiral galaxies

ABSTRACT We present an analysis of major-axis surface-brightness profiles of bars in a volume-limited sample of 182 barred spiral galaxies, using Spitzer 3.6 $\mu$m images. Unlike most previous studies, we use the entire bar profile, and we classify profiles into four categories. These are ‘Peak+Shoulders’ (P+Sh) – updating the classic ‘flat bar’ profile – and three subtypes of the classic ‘exponential’ profile: (true) Exponential, ‘Two-Slope’ (shallow inner slope + steeper outer slope), and ‘Flat-Top’ (constant inner region, steep outer slope). P+Sh profiles are preferentially found in galaxies with high stellar masses, early Hubble types, red colours, and low gas fractions; the most significant factor is stellar mass, and previous correlations with Hubble type can be explained by the tendency of higher-mass galaxies to have earlier Hubble types. The most common type of non-P+Sh profile is Exponential, followed by Flat-Top profiles; all non-P+Sh profiles appear to have similar distributions of stellar mass, Hubble type, colour, and gas fraction. We also morphologically classify the bars of an inclined subsample into those with and without boxy/peanut-shaped (B/P) bulges; as previously reported, the presence of a B/P bulge is very strong function of stellar mass. Essentially all bars with B/P bulges have P+Sh profiles; we associate the profile shoulders with the outer, vertically thin part of the bar. We find a small number of P+Sh profiles in bars without clear B/P bulges, which may indicate that P+Sh formation precedes the formation of B/P bulges.

CO2 diffusive characteristics and influencing factors in the porous medium with saturated polyacrylamide solutions

Polyacrylamide has been widely utilized to store CO2 in reservoirs in order to alleviate the greenhouse impact. However, no studies have been conducted on the motion of CO2 in a polyacrylamide solution within porous media. This study illustrates the effect of polyacrylamide molecular weight, concentration, pressure, temperature, and pore size on the dynamic diffusion of CO2 in the core with saturated polyacrylamide solutions. The diffusion motion characteristics of CO2 were obtained by diffusion experiments. The microscopic analysis of core slices following testing showed the presence of polyacrylamide on rock particles. The pressure and CO2 mass fraction distribution inside the core were calculated by constructing a mathematical model. The results show that the increase in molecular weight and concentration of polyacrylamide decreased the diffusion ability of CO2, while temperature and pressure accelerated the diffusion of CO2. The polyacrylamide moved into the core to form an effective inhibiting effect, which was affected by CO2 and pressure. While the increase in pore size accelerates the flow rate of the polyacrylamide, it also enhances the sealing capacity of the polyacrylamide solution. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) showed that the core had a high content of polyacrylamide solution inside it, and these molecules were attached to the surface of the core particles in an irregular lamellar pattern. The calculation model was consistent with the diffusion experimental results, where the CO2 diffusion coefficient in the polyacrylamide solution was in the order of 10-12 m2/s under the conditions of 5–30 MPa, 323–343 K, and a core permeability of 60 × 10-3 μm2. The results of this paper provide a new calculation for evaluating the performance of polymer plugging.

Online monitoring of spatial-temporal distribution of harmful gases during advanced oxidation of NO by convolutional networks and gated recurrent units

Online monitoring of the spatial-temporal distribution of harmful gases has always been a complex problem in the environmental field. This paper proposes a novel mathematical method for online monitoring of the spatial-temporal distribution of reactants by machine learning, which can help to remove harmful gases efficiently. In this model, we take the advanced oxidation of NO as an example to evaluate the model performance. The spatial features were extracted by CNN, and GRU extracted the temporal features in the sequence of spatial features. Five physical field variables (mass fraction of ozone, velocity, temperature, the wind direction of the horizontal plane, and the wind direction of the vertical plane) were put into the network to predict NO's spatial-temporal mass fraction distribution. Furthermore, the impact of sampling time interval on monitoring performance was also evaluated. The results show that both the instantaneous and continuous CFD (Computational Fluid Mechanics) and predicted values show high consistency, which indicates that the model can online monitor the spatial-temporal distribution of reactants successfully. In addition, the most suitable sampling time interval is 0.5 s, with low training error (RMSE=0.06 and nRMSE=0.3) and high relation coefficient (r=0.99), which shows the model has great perceived and predicted performance under this condition.

Numerical studies of flow patterns and fuel distribution of tandem-arranged multi jets in a hypersonic crossflow

In this study, the numerical analysis of the three-dimensional flow fields of a series of tandem-arranged equal-strength hydrogen jets injected into a Mach 8 hypersonic crossflow is presented. The study comparatively analyzes the flow field similarities and differences between different configurations, namely, single, double, and quadruple jets. The results indicate that downstream jets do not have a significant effect on the first Mach cell and only slightly lift the bow shock at downstream compared with the single jet case. Additionally, high-temperature regions in all cases are in close proximity to the hydrogen stoichiometric equivalence ratio interface, making the fuel more prone to pre-ignition. Moreover, flow patterns in inter-gap regions and vortex structures are compared casewise. In addition, hydrogen injected from each orifice is traced, and the mass fraction distribution contributed by each jet on the symmetry plane and the inter-gap region is analyzed to derive the mixing characteristics.

Numerical analysis of combustion noise in an atmospheric swirl-stabilized LDI burner through modal decomposition techniques

Combustion noise in gas turbine engines has recently become a relevant source of noise in the aircraft due to the appearance of new burner architectures that are intrinsically more unstable, and the optimization of other conventional noise sources in this mean of transport (e.g., jet, fan, airframe). In this work, a simulation setup for reactive conditions was prepared in the CONVERGE finite-volume package using the detailed chemistry SAGE solver to model the combustion of a benchmark case, which was solved using a LES approach with three different cell base sizes: 8,10,12 mm. A confined liquid-fueled swirl-stabilized burner located at the CORIA Laboratory, France, was used to validate the numerical results with the experimental measurements obtained at this facility. OH-PLIF measurements and PDA results for both phases were used to guarantee the accuracy of the numerical OH contours and the velocity profiles of both phases. These experimental measurements were collected at CORIA. After ensuring the stabilization of the numerical flame, the reactive simulations were extended with some adjustments in the time step to capture the acoustic motion. Several techniques like Fast Fourier Transform (FFT), Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) were used to analyze these results and confirm the presence of a Precessing Vortex Core (PVC) and a Vortex Breakdown Bubble (VBB) during the coupling of pressure, axial velocity and fuel mass fraction in reactive conditions. Furthermore, the acoustic analysis performed with a Helmholtz solver proved that the second longitudinal mode of the chamber (310 Hz) was present in the pressure signal (300 Hz in the LES calculations) and resonated with the Vortex Breakdown Bubble (VBB). However, this dominant frequency did not appear in the frequency distribution of the OH mass fraction and no feedback interaction between the acoustic and the combustion happened. Thus, only combustion noise was obtained.

Computational fluid dynamics study of Y3+-doped Ba(Ce,Zr)O3 based single channel proton ceramic fuel cell

Theoretical studies using Computational Fluid Dynamics (CFD) modeling have been established in the field of polymer electrolyte membrane fuel cells (PEMFCs) and oxygen ion solid oxide fuel cells (O2-SOFCs). However, its implementation in the proton ceramic fuel cell (PCFC) development is still in progress and very limited literature can be found. Thus, in this simulation study, ANSYS 2022 CFD software has been employed to predict hydrogen mass fraction distribution and power density of a single-channel PCFC operating in 100 % hydrogen fuel. This simulation utilized input data based on previously published experimental works. The mass fraction of H2 was 0.0 at the cathode area indicating that the electrolyte layer is fully dense and no leakage of H2 from the anode area into the cathode area. The maximum power density in 100 % H2 was 0.34 W/cm2 at 800° C. This is in agreement with the power density produced by the in-house fabricated button cell with the configuration of NiO-BCZY|BCZY|LSCF (BCZY=BaCe0.54Zr0.36Y0.1O2.95, LSCF=La0.6Sr0.4Co0.2Fe0.8O3-δ.) that showed a maximum power density of 0.33W/cm2 in 100 % H2. This analysis will contribute to insight information on the relationship between fuel mass fraction distribution and fuel cell performance for future improvements in the field of PCFC.

Effect of improved atmospheric opacities in modelling sub-Neptunes

Aims. We investigate the impact of updated atmospheric mean opacity input values on modelled transit radius and the distribution of interior layer mass fractions. Methods. We developed and applied a coupled interior-atmosphere model. Our straightforward semi-grey calculation of atmospheric temperature enables us to perform thousands of model realisations in a Monte Carlo approach to address potential degeneracies in interior and atmospheric mass fraction. Our main constraints are planetary mass and radius from which our model infers distributions of the internal structure of exoplanetary classes ranging from Super-Earth to Mini-Neptune. We varied the relative masses of gas, envelope, mantle, and core layers subject to constraints on the bulk density from observations, and investigated the effect of updating atmospheric mean opacities. Results. First, we validate our model output with observed temperature profiles for modern Neptune. We can reproduce the basic features in the middle atmosphere but not the temperature inversion in the upper layers, which is likely because our model lacks aerosol heating. Calculated interiors are generally consistent with modern Neptune. Second, we compare with the well-studied object GJ 1214 b and obtain results that are broadly consistent with previous findings; they suggest correlations between modelled gas, water, and core mass fractions, although these are generally weak. Updating the opacities leads to a change on the order of a few percent in the modelled transit radius. This is comparable in magnitude to the planned accuracy of the PLATO data for planetary radius, suggesting that the opacity update in the model is important to implement.

Optimal and modeling study of air-cooled proton exchange membrane fuel cell with various length-width ratio and numbers

Air-cooled proton exchange membrane fuel cell is a positive and effective energy strategy to improve the efficiency of applications in robots and unmanned aerial vehicles. In this paper, novel air-cooled proton exchange membrane fuel cells based on different length-width ratio (RL/W) of the cell and the number of anode serpentine are designed to explore their effects on oxygen mass fraction distribution, temperature distribution, pressure drop, as well as the cell performance. The results show that RL/W has outstanding impact on the performance, and the uniformity of current density, oxygen mass fraction, temperature and water distribution can be improved by increasing RL/W, so as to obtain better performance. Increasing the amount of anode serpentine channels also has positive effect. While when RL/W is small, the number of anode flow channels has a greater impact on the performance and parameter distribution than the RL/W of the bipolar plate. When RL/W is greater than 2.94, the performance mainly depends on RL/W. This research provides basic data that is significant for the system optimization and application of air-cooled proton exchange membrane fuel cells in the future.

Electrical/flow heterogeneity of gas diffusion layer and inlet humidity induced performance variation in polymer electrolyte fuel cells

A three-dimensional single-flow channel computational model is used to investigate the performance characteristics of polymer electrolyte fuel cells (PEFC). The combined influence of non-uniform interfacial contact resistance (ICR) and inlet relative humidity (RH), along with the heterogeneous flow properties of the gas diffusion layer (GDL) on the PEFC performance is evaluated. The study considers combinations of full and partial humidification of anode and cathode reactants. Results reveal heterogeneous GDL with non-uniform ICR distribution results in a slight ∼4.4% reduction in current density at 0.3V compared to the homogeneous case. However, under same electrical/flow heterogeneities, the current density is observed to increase by ∼19% to ∼1.3A/cm2 under fully humidified anode and partially humidified cathode (i.e., RHa|RHc = 100%|60%) as compared to ∼1.1A/cm2 under symmetric RHa|RHc = 100%|100%. Interesting observations are made on the temperature distribution and cathodic water fractions. The variation in anodic inlet humidity is observed to have no impact on temperature distribution in the membrane, whereas variation in cathodic inlet humidity is effective in reducing the temperature in the channel regime with a 4K (kelvin) difference among all the cases. It is noted here that the overpotential map is not an indicator for performance loss, at least at full inlet humidity. This parameter is observed to depend on water concentration in the cathode. The study provides a detailed analysis of the distribution of reactant mass fraction, water concentration, current density, temperature, cathodic overpotential, and cell performance for all the simulated cases.

LES flamelet modeling of hydrogen combustion considering preferential diffusion effect

A flamelet-generated manifold (FGM) method that explicitly considers the preferential diffusion effect, referred to as FGM-PD method, is employed for large-eddy simulations (LESs) of a lean-premixed H2/air low-swirl lifted flame, and the validity is examined by comparing with the experiment. First, the applicability of the FGM-PD method is investigated by one-dimensional numerical simulations of planar laminar premixed H2/air flames. Next, LESs of a lean-premixed H2/air low-swirl lifted flame are performed employing the FGM-PD and conventional FGM methods. Results of the one-dimensional numerical simulations show the importance of considering preferential diffusion to accurately predict species concentrations near the flame front. The FGM-PD method accurately predicts this, and therefore, reproduces the laminar burning velocity and spatial distributions of temperature and mixture fraction. Three-dimensional LES results confirm that the prediction accuracy of the velocities near the flame front is improved by employing this FGM-PD method. Additionally, the OH mass fraction distribution predicted by the FGM-PD method exhibits the inhomogeneous finger-like structure, which has been observed in previous experiments. This inhomogeneity of OH mass fraction distribution, which corresponds to that of the reaction rate, predicted by the FGM-PD method, strongly affects the flame front structure.

A Comparison between Statistical Behaviours of Scalar Dissipation Rate between Homogeneous MILD Combustion and Premixed Turbulent Flames

Three-dimensional Direct Numerical Simulations (DNS) data has been utilised to analyse statistical behaviours of the scalar dissipation rate (SDR) and its transport for homogeneous methane-air mixture turbulent Moderate or Intense Low oxygen Dilution (MILD) combustion for different O2 dilution levels and turbulence intensities for different reaction progress variable definitions. Additional DNS has been conducted for turbulent premixed flames and passive scalar mixing for the purpose of comparison with the SDR statistics of the homogeneous mixture MILD combustion with that in conventional premixed combustion and passive scalar mixing. It has been found that the peak mean value of the scalar dissipation rate decreases with decreasing O2 concentration for MILD combustion cases. Moreover, SDR magnitudes increase with increasing turbulence intensity for both MILD and conventional premixed combustion cases. The profiles and mean values of the scalar dissipation rate conditioned upon the reaction progress variable are found to be sensitive to the choice of the reaction progress variable definition. This behaviour arises due to the differences in the distributions of the species mass fractions within the flame. The strain rate contribution and the molecular dissipation term are found to be the leading order contributors in the scalar dissipation rate transport for MILD combustion; whereas, in conventional premixed flames, the terms rising from density variation and reaction rate gradient also play leading roles in addition to the strain rate and molecular dissipation contributions. By contrast, the terms due to density gradient and reaction rate gradient remain negligible in comparison to the leading order contributors in MILD combustion cases due to small density variation because of moderate temperature rise and small reaction rate gradient magnitudes. Furthermore, the qualitative behaviour of the strain rate contribution to the SDR transport in premixed flames is significantly different to that in the case of MILD combustion and passive scalar mixing. The findings of the current analysis indicate that the scalar dissipation rate statistics in MILD combustion show several qualitative similarities to the passive scalar mixing despite major differences with the SDR transport in conventional turbulent premixed flames. This further suggests that the scalar dissipation rate models, which were originally proposed in the context of passive scalar mixing, have the potential to be applicable for MILD combustion but the models for the premixed turbulent combustion may not be applicable for MILD combustion of homogeneous mixtures.

Surface profile evolution model for titanium alloy machined using abrasive waterjet

The surface profile evolution model, which was initially developed for glass and polymers, can accurately predict a channel profile cross-section produced by abrasive jet (AJ) machining. In this study, the model is modified and applied for estimating the profiles of a Ti-6Al-4V alloy eroded by an abrasive waterjet (AWJ). First, the velocity and mass fraction distributions of the gas–liquid–solid phases in the AWJ at the nozzle exit were derived and compared, and several improvements were proposed, such as considering the divergence angle of the jet and particles, as well as the length of the jet core area, to precisely construct a theoretical connection of the erosion efficiency distribution before impacting the workpiece. Computational fluid dynamics (CFD) simulations were then performed to investigate the behaviour of the erosion jets during surface evolution. The results revealed that the jet diffusion provoked by the stagnation zone effect became more pronounced as the surface profile depth deepened, which led to jet directional deflection and suppressed the erosion capacities of the AWJ. Therefore, a central erosion depth function was introduced to correct this detrimental effect with the intention of obtaining an accurate channel profile. In addition, a second-order single-step fitting function was suggested to eliminate the fluctuations caused by uneven abrasive particles and the problem of reduced erosion efficiency due to channel depth variation. Finally, based on the determination of the parameters affecting the channel profile, a normalised centre erosion rate function, which only depends on the channel depth and is isolated from the material properties and the standoff distance, was recommended to simplify the calculation. The erosion function conforming to a Gaussian surface was fitted using MATLAB (R2019b, MathWorks, USA). The results demonstrated that the channel profiles predicted by the surface evolution model were consistent with the measured profiles, with an average error of 11.4%.