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在开源的CFD工具包OpenFOAM环境下开发了基于低磁雷诺数的磁流体湍流数值模拟求解器,对2π×1×1的方管中无磁场湍流和磁流体湍流进行直接数值模拟研究,给出了截面瞬时速度、平均速度的分布,截面对称中心线上的脉动速度的均方根值、湍动能的分布。计算结果表明,外加磁场对磁流体湍流具有抑制作用和并且这种抑制作用具有各向异性。     

磁流体管流的直接数值模拟研究

在开源的CFD 工具包OpenFOAM 环境下开发了基于低磁雷诺数的磁流体湍流数值模拟求解器,对 2π ×1×1的方管中无磁场湍流和磁流体湍流进行直接数值模拟研究,给出了截面瞬时速度、平均速度的分布,截面对称中心线上的脉动速度的均方根值、湍动能的分布。计算结果表明,外加磁场对磁流体湍流具有抑制作用和并且这种抑制作用具有各向异性。     

Complex chemistry simulations of spark ignition in turbulent sprays

Three-dimensional DNS of two-phase flows with the point-source approximation and with complex chemistry for n-heptane has been used to extract physical information on the structure of igniting kernels following localised heat deposition in turbulent monodisperse sprays. Consistent with experiment, small sparks fail to ignite and sprays ignite later than premixed gaseous mixtures. Reaction rates are intense in spherical zones near droplets and much lower in the interdroplet spacing, resulting in a highly wrinkled flame surface. The propagation of these reaction zones was observed. The flame shows a locally non-premixed character, with reactions proceeding at a wide range of mixture fractions, which increases as evaporation progresses. The distribution of various chemical species is presented. The results constitute a database for model validation and physical analysis.     

An experimental study of hydrogen autoignition in a turbulent co-flow of heated air

The autoignition behaviour of hydrogen in a turbulent co-flow of heated air at atmospheric pressures was examined experimentally. Turbulent flows of air, with temperatures up to 1015 K and velocities up to 35 m/s, were set up in an optically accessible tube of circular cross-section. The fuel, pure or diluted with nitrogen, was continuously injected along the centreline of the tube, with velocities equal to or larger than those of the air, and temperatures that were lower. The fuel mixing patterns hence obtained were akin to diffusion from a point source or to an axisymmetric jet within a co-flow. For a relatively wide range of temperatures and velocities, a statistically steady condition of randomly occurring autoignition kernels was observed, whose axial location was measured by hydroxyl radical chemiluminescence. The probability density function of autoignition location was sharp enough to allow the accurate determination of a minimum autoignition length and smooth enough to allow the mean and variance to be calculated. It was found that both autoignition lengths increased with the air velocity and decreased with the air temperature, as expected. An estimate of the residence time up to autoignition showed that the autoignition delay times increased with the air velocity for the same temperature, suggesting a delaying effect of the turbulence on autoignition. The connection between these findings and previous experimental and direct numerical simulation studies is discussed.     

On the behavior of spray combustion in a turbulent spatially-evolving jet investigated by direct numerical simulation

Detailed investigations of turbulent spray combustion are very challenging due to the complexity of the underlying physicochemical processes. Experimentally, laboratory-scale burners are increasingly used to investigate these processes and support model development. One ultimate objective of these studies would be to deliver suitable benchmark data. In the present paper, the focus is similar but relying exclusively on direct numerical simulations. Conditions close that found in lab-scale burners are considered in the simulations, so that direct comparisons will ultimately become possible. The current analysis concentrates on the temporal evolution of temperature and concentrations of OH, CH₂O, and CH₄. The profiles of these variables show very complex features, therefore separate zones corresponding to characteristic physicochemical regimes have been tracked in time and space. It is found that, based on the temperature profile, four different zones coexist in the domain, associated to different degrees of competition between evaporation and reaction. It is observed that high concentrations of CH₂O and CH₄ can be used to delineate between three characteristic locations: 1) the evaporation zone; 2) close to the jet tip, at high temperatures; and 3) regions where evaporated droplets are entrained by mixing. This study demonstrates that direct numerical simulation of small spray burners can be used to deliver important information and to contribute useful benchmark data.     

Modelling of turbulent scalar flux in turbulent premixed flames based on DNS databases

A transport equation for scalar flux in turbulent premixed flames was modelled on the basis of DNS databases. Fully developed turbulent premixed flames were obtained for three different density ratios of flames with a single-step irreversible reaction, while the turbulent intensity was comparable to the laminar burning velocity. These DNS databases showed that the countergradient diffusion was dominant in the flame region. Analyses of the Favre-averaged transport equation for turbulent scalar flux proved that the pressure related terms and the velocity–reaction rate correlation term played important roles on the countergradient diffusion, while the mean velocity gradient term, the mean progress variable gradient term and dissipation terms suppressed it. Based on these analyses, modelling of the combustion-related terms was discussed. The mean pressure gradient term and the fluctuating pressure term were modelled by scaling, and these models were in good agreement with DNS databases. The dissipation terms and the velocity–reaction rate correlation term were also modelled, and these models mimicked DNS well.     

Direct numerical simulations of rich premixed turbulent n-dodecane/air flames at diesel engine conditions

Rich premixed turbulent n-dodecane/air flames at diesel engine conditions are analyzed using direct numerical simulations. The conditions correspond to a parametric variation of the Engine Combustion Network Spray A (pressure 60 atm; oxidizer oxygen level and temperature 21% and 900 K, respectively; fuel temperature 363 K). Three simulations with equivalence ratios of 3, 5, and 7 are performed with a Karlovitz number (Ka, based on flame time) of order 100 to match the estimated Ka of the rich premixed combustion region in Spray A. At these conditions, the reference laminar flames exhibit a complex structure which involves both low-temperature chemistry (LTC) and high-temperature chemistry over a wide range of length scales. In the presence of turbulence, the flame structure is strongly affected in physical space and the reaction zone exhibits a very complex structure in which broken, distributed, and thin regions co-exist, especially for the leanest case. However, the contribution of the LTC pathway is only weakly affected by turbulence. In progress variable space, the mean flame structure, including the chemical source terms, is found to match remarkably well that of the corresponding unity Lewis number laminar flame, particularly for the $?=$ 3 and 5 cases. This behavior is attributed to the strong turbulent mixing occurring throughout the flames/reaction zones, which suppresses differential diffusion effects. Nevertheless, large conditional fluctuations around the mean chemical source terms are identified. These are found to correlate very well with radical species mass fractions such as OH. In addition, a similar functional dependence is obtained from counterflow laminar flames. As such, it appears from these results that laminar flame models have a potential to be used to represent the thermochemical state of rich premixed turbulent flames under diesel engine conditions.     

Direct numerical simulations of turbulent flame expansion in fine sprays

Direct Numerical Simulations of expanding flame kernels following localized ignition in decaying turbulence with the fuel in the form of a fine mist have been performed to identify the effects of the spray parameters on the possibility of self-sustained combustion. Simulations show that the flame kernel may quench due to fuel starvation in the gaseous phase if the droplets are large or if their number is insufficient to result in significant heat release to allow for self-sustained flame propagation for the given turbulent environment. The reaction proceeds in a large range of equivalence ratios due to the random location of the droplets relative to the igniter location that causes a wide range of mixture fractions to develop through pre-evaporation in the unreacted gas and through evaporation in the preheat zone of the propagating flame. The resulting flame exhibits both premixed and non-premixed characteristics.     

Conditional statistics of nonreacting and reacting sprays in turbulent flows by direct numerical simulation

Conditional statistics concerning evaporation and combustion of a spray are investigated in homogeneous, isotropic, and decaying two-dimensional (2D) turbulence. Randomly distributed, polydisperse droplets of n-heptane go through single-step combustion chemistry. Attention is focused on parametric effects of initial Sauter mean radius (SMR), turbulence level and droplet velocity in both reacting and nonreacting cases. A simple linear model for the conditional evaporation rate is proposed and validated against DNS data. A conventional β-probability density function (pdf) is shown to be valid with no peak occurring on the fuel side. The amplitude mapping closure (AMC) model works well for the conditional scalar dissipation rate with evaporating and reacting sprays. Parametric study shows that initial SMR and droplet velocity are major factors affecting conditional flame structures, whereas the effect of reaction is not significant except during autoignition.     

Direct numerical simulation of spray flame/acoustic interactions

Interactions between conical spray flames and sinusoidal velocity modulations due to the propagation of acoustic waves have been studied thanks to direct numerical simulations (DNS). A 2D axi-symmetric configuration has been used to capture the evolution of the pulsating laminar flames. The DNS solver has been coupled with a Lagrangian model to account for the dispersion and evaporation of the liquid fuel in the computational domain. Four main configurations, with a unitary global equivalence ratio, have been studied. Apart from a gaseous reference case, one polydispersed and two monodispersed Bunsen-type injections with various droplets density and inertia have been simulated. DNS results are in good agreement with experimental data. For significant acoustic Stokes numbers, results showed a double effect of the modulations on the flame: a direct disturbance of the flame front and a secondary impact through the local variation of the mixture fraction due to droplets preferential segregation.     

Nonlinear wave-wave interactions in stratified flows: Direct numerical simulations

To investigate the formation mechanism of energy spectra of internal waves in the oceans, direct numerical simulations are performed. The simulations are based on the reduced dynamical equations of rotating stratified turbulence. In the reduced dynamical equations only wave modes are retained, and vortices and horizontally uniform vertical shears are excluded. Despite the simplifications, our simulations reproduce some key features of oceanic internal-wave spectra: accumulation of energy at near-inertial waves and realistic frequency and horizontal wavenumber dependencies. Furthermore, we provide evidence that formation of the energy spectra in the inertial subrange is dominated by scale-separated interactions with the near-inertial waves. These findings support observationally based intuition that spectral energy density of internal waves is the result of predominantly wave-wave interactions.     

Prediction of wall-pressure fluctuation in turbulent flows with an immersed boundary method

The objective of this paper is to assess the accuracy and efficiency of the immersed boundary (IB) method to predict the wall pressure fluctuations in turbulent flows, where the flow dynamics in the near-wall region is fundamental to correctly predict the overall flow. The present approach achieves sufficient accuracy at the immersed boundary and overcomes deficiencies in previous IB methods by introducing additional constraints – a compatibility for the interpolated velocity boundary condition related to mass conservation and the formal decoupling of the pressure on this surfaces. The immersed boundary-approximated domain method (IB-ADM) developed in the present study satisfies these conditions with an inexpensive computational overhead. The IB-ADM correctly predicts the near-wall velocity, pressure and scalar fields in several example problems, including flows around a very thin solid object for which incorrect results were obtained with previous IB methods. In order to have sufficient near-wall mesh resolution for LES and DNS computations, the present approach uses local mesh refinement. The present method has been also successfully applied to computation of the wall-pressure space–time correlation in DNS of turbulent channel flow on grids not aligned with the boundaries. When applied to a turbulent flow around an airfoil, the computed flow statistics – the mean/RMS flow field and power spectra of the wall pressure – are in good agreement with experiment.     

Using self-similar properties of turbulent premixed flames to downsize chemical tables in high-performance numerical simulations

Detailed chemical mechanisms have to be incorporated in turbulent combustion modelling to predict flame propagation, ignition, extinction or pollutant formation. Unfortunately, hundreds of species and thousands of elementary reactions are involved in hydrocarbon chemical schemes and cannot be handled in practical simulations, because of the related computational costs and the need to model the complexity of their interaction with turbulent motions. Detailed chemistry may be handled using look-up tables, where chemical parameters such as reaction rates and/or species mass fractions are determined from a reduced set of coordinates, progress variables or mixture fractions, as proposed in ILDM, FPI or FGM methods. Nevertheless, these tables may require large computer memory spaces and non-negligible access times. This issue becomes of crucial importance when running on massively parallel computers: to implement these databases in shared memories would induce a large number of data exchanges, reducing the overall code performance; on the other hand duplicating databases in every local processor memory may become impossible either for large databases or small local memories. This work proposes to take advantage of the self-similar behaviour of turbulent premixed flames to reduce the size of these chemical databases, specifically when running on massively parallel machines, under the FPI (Flame Prolongation of ILDM) framework. Several approaches to reduce the database are investigated and discussed both in terms of memory requirements and access times. A very good compromise is obtained for methane–air turbulent premixed flames, where the size of the database is decreased by a factor of 1000, while the access time is reduced by about 60%.     

Experimental and numerical characterization of a turbulent spray flame

A turbulent ethanol spray flame is characterized through quantitative experiments using laser-based imaging techniques. The data set is used to validate a numerical code for the simulation of spray combustion. The spray burner has been designed to generate a stable flame without the use of a bluff body or a pilot flame facilitating numerical simulations. The experiments include spatially-resolved measurements of droplet sizes (Mie/LIF-dropsizing and PDA), droplet velocity (PDA), liquid-phase temperature (2-color LIF temperature imaging with Rhodamine B) and gas-phase temperature (multi-line NO-LIF temperature imaging). The measurements close to the nozzle exit are used to determine the initial conditions for numerical simulations. An Eulerian–Lagrangian model including spray flamelet modeling is applied to calculate the development of the spray. Good agreement with the experimental data is found. The experimental data set and the numerical results will be published on a website to allow other groups to evaluate their experimental and/or numerical data.     

Experimental and computational investigation of autoignition of jet fuels and surrogates in nonpremixed flows at elevated pressures

Experimental and computational investigations are carried out to elucidate the fundamental mechanisms of autoignition of surrogates of jet-fuels at elevated pressures up to 6 bar. The jet-fuels tested are JP-8, Jet-A, and JP-5, and the surrogates tested are the Aachen Surrogate made up of 80 % n-decane and 20 % 1,3,5-trimethylbenzene by mass, Surrogate C made up of 60 % n-dodecane, 20 % methylcyclohexane and 20 % o-xylene by volume, and the 2nd generation Princeton Surrogate made up of 40.4 % n-dodecane, 29.5 % 2,2,4-trimethylpentane, 7.3 % 1,3,5-trimethylbenzene and 22.8 % n-propylbenzene by mole. Using the counterflow configuration, an axisymmetric flow of a gaseous oxidizer stream, made up of a mixture of oxygen and nitrogen, is directed over the surface of an evaporating pool of a liquid fuel. The experiments are conducted at a fixed value of mass fraction of oxygen in the oxidizer stream and at a fixed value of the strain rate. The temperature of the oxidizer stream at autoignition, T_ig, is measured as a function of pressure, p. Experimental results show that the critical conditions, of autoignition of the surrogates are close to that of the jet-fuels. Overall the critical conditions of autoignition of Surrogate C agree best with those of the jet-fuels. Computations were performed using skeletal mechanisms constructed from a detailed mechanism. Predictions of the critical conditions of autoignition of the surrogates are found to agree well with measurements. Computations show that low-temperature chemistry plays a significant role in promoting autoignition for all surrogates. The low-temperature chemistry, of the component of the surrogate with the greatest volatility, was found to have the most influence on the critical conditions of autoignition.     

Direct numerical simulation of the turbulent flows of power-law fluids in a circular pipe

Fully developed turbulent pipe flows of power-law fluids are studied by means of direct numerical simulation. Two series of calculations at generalised Reynolds numbers of approximately 10000 and 20000 were carried out. Five different power law indexes n from 0.4 to 1 were considered. The distributions of components of Reynolds stress tensor, averaged viscosity, viscosity fluctuations, and measures of turbulent anisotropy are presented. The friction coefficient predicted by the simulations is in a good agreement with the correlation obtained from experiment. Flows of power-law fluids exhibit stronger anisotropy of the Reynolds stress tensor compared with the flow of Newtonian fluid. The turbulence anisotropy becomes more significant with the decreasing flow index n. An increase in apparent viscosity away from the wall leads to the damping of the wall-normal velocity pulsations. The suppression of the turbulent energy redistribution between the Reynolds stress tensor components observed in the simulations leads to a strong domination of the axial velocity pulsations. The damping of wall-normal velocity pulsations leads to a reduction of the fluctuating transport of momentum from the core toward the wall, which explains the effect of drag reduction.     

Direct numerical simulations and analysis of three-dimensional n-heptane spray flames in a model swirl combustor

Three-dimensional n-heptane spray flames in a swirl combustor are investigated by means of direct numerical simulation (DNS) to provide insight into realistic spray evaporation and combustion as well as relevant modeling issues. The variable-density, low-Mach number Navier–Stokes equations are solved using a fully conservative and kinetic energy conserving finite difference scheme in cylindrical coordinates. Dispersed droplets are tracked in a Lagrangian framework. Droplet evaporation is described by an equilibrium model. Gas combustion is represented using an adaptive one-step irreversible reaction. Two different cases are studied: a lean case that resembles a lean direct injection combustion, and a rich case that represents the primary combustion region of a rich-burn/quick-quench/lean-burn combustor. The results suggest that premixed combustion contribute more than 70% to the total heat release rate, although diffusion flame have volumetrically a higher contribution. The conditional mean scalar dissipation rate is shown to be strongly influenced, especially in the rich case. The conditional mean evaporation rate increases almost linearly with mixture fraction in the lean case, but shows a more complex behavior in the rich case. The probability density functions (PDF) of mixture fraction in spray combustion are shown to be quite complex. To model this behavior, the formulation of the PDF in a transformed mixture fraction space is proposed and demonstrated to predict the DNS data reasonably well.     

Effects of temperature and equivalence ratio on the ignition of n-heptane fuel spray in turbulent flow

Direct numerical simulations were performed to study the autoignition process of n-heptane fuel spray in a turbulent field. For the solution of the carrier gas fluid, the Eulerian method is employed, while for the fuel droplets, the Lagrangian method is used. Droplets are initialized at random locations in a two-dimensional isotropic turbulent field. A chemistry mechanism for n-heptane with 44 species and 112 reactions was adopted to describe the chemical reactions. Three cases with the same initial global equivalence ratio (0.5) and different initial gas phase temperatures (1100, 1200, and 1300 K) were simulated. In addition, two cases with initial global equivalence ratios of 1.0 and 1.5 and initial temperature 1300 K were simulated to examine the effect of equivalence ratio. Evolution of temperature, species mass fraction, reaction rate, and the joint PDF of temperature and equivalence ratio are presented. Effects of the initial gas temperature and equivalence ratio on vaporization and ignition are discussed. A correlation was derived relating ignition delay times to temperature and equivalence ratio. It was confirmed that with the increase of initial temperature, the autoignition occurs earlier. With the increase of the initial equivalence ratio, however, autoignition occurs later due to a larger decrease in gas phase temperature caused by fuel droplet evaporation. The results obtained in this study are expected to be constructive in understanding fuel spray combustion, such as that in homogeneous charge compression ignition systems.