Lagrangian conditional moment closure model with flame group interaction for lifted turbulent spray jet flames

A new Lagrangian conditional moment closure (CMC) model is developed for multiple Lagrangian groups of sequentially evaporating fuel in turbulent spray combustion. Flame group interaction is taken into account as premixed combustion by the eddy breakup (EBU) model in terms of the probability of finding flame groups in the burned and the unburned state. Evaporation source terms are included in the two phase conditional transport equations, although they turn out to have negligible influence on the mean temperature field during combustion. The Lagrangian CMC model is implemented in OpenFOAM [1 H.G. Weller, G. Tabor, H. Jasak, and C. Fureby, A tensorial approach to computational continuum mechanics using object-oriented techniques, Comput. Phys. 12 (1998), pp. 620–631.[Crossref] [Google Scholar]] and validated for test cases in the Engine Combustion Network (ECN) [2 Engine Combustion Network. (2011). Available at http://www.sandia.gov/ecn. [Google Scholar],3 L.M. Pickett, C.L. Genzale, G. Bruneaux, L. Malbec, L. Hermant, C. Christian, and J. Schramm, Comparison of diesel spray combustion in different high-temperature, high-pressure facilities, SAE technical paper 2010-01-2106 (2010). [Google Scholar]]. Similar ignition delays and lift-off lengths are predicted by the incompletely stirred reactor (ISR) and the Eulerian CMC models due to relatively uniform conditional flame structure in the domain. The improved Lagrangian CMC model shows no abrupt reaction or oscillatory behaviour with an appropriate model constant K and gives results in better agreement with measurements lying between the predictions by ISR and Lagrangian CMC without flame group interaction.     

Prediction of NO in turbulent diffusion flames using Eulerian particle flamelet model

The combustion characteristics for the turbulent diffusion flames using the unsteady flamelet concept have been numerically investigated. The Favre-averaged Navier–Stokes equations are solved by a finite volume method of SIMPLE type that incorporates the laminar flamelet concept with a modified k ? ε turbulence model. The NO formation is estimated by solving the Eulerian particle transport equations in a postprocessing mode. Two test problems are considered: CH₄/H₂/N₂ jet flame and CH₄/H₂ stabilised bluff body flame. The temperature and species profiles are well captured by the flamelet model. Two different chemical mechanisms (GRI 2.11 and 3.0) give nearly identical results for temperature and species except NO. The GRI 3.0 gives significantly higher NO levels compared to the GRI 2.11. This is mainly attributed to the difference in NO formation by the prompt mechanism. The NO formation is sensitive to the number of flamelet particles. The NO levels for two test flames do not change when the flamelet particle number exceeds six.     

Comparison of RANS/CMC modeling of Flame D with conventional and with Presumed Mapping Function statistics

Numerical simulations of Sandia Flame D are presented using Reynolds-averaged formulations plus a two equation turbulence model for the flow and mixing fields and a first order Conditional Moment Closure for the flame model. The distributions of probability and Conditional Scalar Dissipation (CSD) in mixture fraction space are modeled first using a Beta PDF (Probability Distribution Function) plus a theoretical model for CSD and then using the consistent, Presumed Mapping Function-based formulation of Mortensen for both PDF and CSD, in both two and three stream mixing modes. It is shown that there is an improvement in predictions compared with experiment when using the consistent models.     

Computations of turbulent lean premixed combustion using conditional moment closure

Conditional Moment Closure (CMC) is a suitable method for predicting scalars such as carbon monoxide with slow chemical time scales in turbulent combustion. Although this method has been successfully applied to non-premixed combustion, its application to lean premixed combustion is rare. In this study the CMC method is used to compute piloted lean premixed combustion in a distributed combustion regime. The conditional scalar dissipation rate of the conditioning scalar, the progress variable, is closed using an algebraic model and turbulence is modelled using the standard k–? model. The conditional mean reaction rate is closed using a first order CMC closure with the GRI-3.0 chemical mechanism to represent the chemical kinetics of methane oxidation. The PDF of the progress variable is obtained using a presumed shape with the Beta function. The computed results are compared with the experimental measurements and earlier computations using the transported PDF approach. The results show reasonable agreement with the experimental measurements and are consistent with the transported PDF computations. When the compounded effects of shear-turbulence and flame are strong, second order closures may be required for the CMC.     

Simulation of spray development and turbulent combustion processes in low and high speed diesel engines by the CMC-ISR model

Simulation is performed to analyse the characteristics of turbulent spray combustion in conventional low and high speed diesel engine conditions. Turbulence–chemistry interaction is resolved by the Conditional Moment Closure (CMC) model in the spatially integrated form of an Incompletely Stirred Reactor (ISR). After validation against measured pressure traces, characteristic length and time scales and dimensionless numbers are estimated at the locations of sequentially injected fuel groups. Conditional flame structures are calculated for sequentially evaporated fuel groups to consider different available periods for ignition chemistry. Injection overlaps the combustion period in the high rpm engine, while most combustion occurs after injection and evaporation are complete in the low rpm engine. Ignition occurs in rich premixture with the initial peak temperature at the equivalence ratio around 2–4 as observed in Dec [2]. It corresponds to the most reactive mixture fraction of the minimum ignition delay for the given mixture states. Combustion proceeds to lean and rich sides in the mixture fraction space as a diffusion process by turbulence. The mean scalar dissipation rates (SDRs) are lower than the extinction limit to show stability of diffusion flames throughout the combustion period.     

Pulverized coal swirl flames in oxy-fuel conditions: Effects of oxidizer O2 concentration on flow field and local gas composition

In an experimental study the effects of varied oxygen concentrations in the oxidizer gas on resulting flow fields, combustion products and general behavior of pulverized coal swirl flames under oxy-fuel conditions have been investigated. Experiments were carried out in a small scale down-fired cylindrical combustion chamber equipped with an annular swirl burner. Studied flames had a constant power output of 40 kW_th and O₂/CO₂ oxidizer gas mixtures with O₂ concentrations ranging from 23 to 33 vol%. Detailed two-dimensional flow field measurements are obtained from laser Doppler anemometry (LDA). Velocity profiles (Mean and RMS) have been obtained for all conditions investigated and serve as basis for identification of flow field characteristics. Velocity RMS values are provided as supplementary material. To complement flow field measurements, in-flame gas composition measurements were also conducted using a sampling probe combined with infrared gas absorption analysis via Fourier-transform infrared (FTIR) spectrometry. The results obtained show increased velocities, particularly along the main vortex for flames with increased oxygen contents, while lower velocities are found to occur inside the recirculation regions. The opposite occurs with lower O₂ concentrations, showing significantly reduced velocities in the main vortex, but stronger recirculation than the high oxygen counterparts. This effect is attributed to a modification of the swirl level introduced by the expansion of product gases. Measured NO and CO in-flame concentrations showed significant variations under different O₂ concentrations in the oxidizer.     

Comparisons of Lagrangian and Eulerian PDF methods in simulations of non-premixed turbulent jet flames with moderate-to-strong turbulence-chemistry interactions

Transported probability density function (PDF) methods have been applied widely and effectively for modelling turbulent reacting flows. In most applications of PDF methods to date, Lagrangian particle Monte Carlo algorithms have been used to solve a modelled PDF transport equation. However, Lagrangian particle PDF methods are computationally intensive and are not readily integrated into conventional Eulerian computational fluid dynamics (CFD) codes. Eulerian field PDF methods have been proposed as an alternative. Here a systematic comparison is performed among three methods for solving the same underlying modelled composition PDF transport equation: a consistent hybrid Lagrangian particle/Eulerian mesh (LPEM) method, a stochastic Eulerian field (SEF) method and a deterministic Eulerian field method with a direct-quadrature-method-of-moments closure (a multi-environment PDF-MEPDF method). The comparisons have been made in simulations of a series of three non-premixed, piloted methane–air turbulent jet flames that exhibit progressively increasing levels of local extinction and turbulence-chemistry interactions: Sandia/TUD flames D, E and F. The three PDF methods have been implemented using the same underlying CFD solver, and results obtained using the three methods have been compared using (to the extent possible) equivalent physical models and numerical parameters. Reasonably converged mean and rms scalar profiles are obtained using 40 particles per cell for the LPEM method or 40 Eulerian fields for the SEF method. Results from these stochastic methods are compared with results obtained using two- and three-environment MEPDF methods. The relative advantages and disadvantages of each method in terms of accuracy and computational requirements are explored and identified. In general, the results obtained from the two stochastic methods (LPEM and SEF) are very similar, and are in closer agreement with experimental measurements than those obtained using the MEPDF method, while MEPDF is the most computationally efficient of the three methods. These and other findings are discussed in detail.     

Modeling and measurements of size distributions in premixed ethylene and benzene flames

This study demonstrates the major differences in the evolution of the particle size distributions (PSDs), both measured and modeled, of soot in premixed benzene and ethylene flat flames. In the experiments, soot concentration and PSDs were measured by using a scanning mobility particle sizer (SMPS, over the size range of 3-80 nm). The model employed calculations of gas phase species coupled with a discrete sectional approach for the gas-to-particle conversion. The model includes reaction pathways leading to the formation of nano-sized particles and their coagulation to larger soot particles. The particle size distribution, both experimental and modeled, evolved from a single particle mode (the nucleation mode) to a bimodal size distribution. An important distinction between the results for the ethylene and benzene flames is the behavior of the nucleation mode which persists at all heights above the burner (HAB) for ethylene whereas it was greatly suppressed at greater HAB for the benzene flames. The explanation for the decreased nucleation mode at higher elevations in the benzene flame is that the aromatics are consumed in the oxidation zone of the flame. Fair predictions of particle-phase concentrations and particle sizes in the two flames were obtained with no adjustments to the kinetic scheme. In agreement with experimental data, the model predicts a higher formation of particulate in the benzene flame as compared with the ethylene flame.     

Modelling of the correlation between velocity and reactive scalar gradients in turbulent premixed flames based on DNS data

The interaction between turbulence and reactive scalar fields is discussed for the wrinkled flamelets regime of turbulent premixed combustion. Emphasis is placed on the effects associated with the turbulent straining term. In the regime of turbulent combustion under consideration, which corresponds to Karlovitz and Damköhler numbers such that Ka < 1 and Da > 1, a clear and simple formulation is proposed to explain and to model the influence of the correlation between velocity and reactive scalar gradients. This formulation is based on the conservative variables budget across one-dimensional premixed laminar flamelets. The analysis firmly confirms the dependence on both the Damköhler number and the expansion factor, a feature already foreseen in recent studies. Nevertheless, in contrast with previous work, (i) the scaling arguments used in the present contribution are different from those used in other recent proposals, and (ii) the proposed closures are not only deduced from dimensional arguments but also from the consideration of conservative variable budgets across laminar flamelets. The resulting functional dependence on the expansion factor is found to be influenced by the underlying one-dimensional flamelet representation and two possible closures are put forward to take this dependence into account. (iii) The two closures do not exhibit a proportionality to the mean scalar dissipation rate as suggested in previous studies but to the square of this quantity. This results in the presence of a second contribution proportional to in the modelled transport equation for the mean scalar dissipation rate, in addition to the modelled molecular dissipation term. (iv) Since previous Direct Numerical Simulation (DNS) studies have been essentially devoted to the influence of the Damköhler number, the present DNS validation step is focused on the effects of the expansion rate. To this purpose, the proposed models are validated against three available DNS databases obtained for turbulent premixed flames with different values of the density ratio between unburned and fully burned gases.     

时域有限差分法在建筑声学中的应用及前景

介绍了声波方程的基本差分格式及稳定条件、数值色散、吸收边界条件等数值计算理论，例举了前人用时域有限差分法对噪声传播过程的模拟和室内声学中座椅吸声低谷效应模拟的模拟结果。本文指出，由于时域有限差分法的特点使其具有在模拟脉冲响应方而的特别优势，因而，应用这一技术研究厅堂的声学特性，尤其是低频特性，将有重要意义。     

细菌觅食优化算法在光散射法颗粒粒度反演中的应用

针对函数约束算法中传统的智能算法反演时存在鲁棒性差和易陷入局部最优的缺点,提出了将正则化理论与细菌觅食优化算法相结合应用在颗粒粒度的测量中。引入Tikhonov平滑泛函来构建算法的目标函数,采用L曲线法确定正则化参数;再利用细菌觅食优化算法通过趋向、聚群、复制和迁徙等四种智能行为,迭代计算来搜寻函数的最优解。实验仿真结果表明:利用细菌觅食优化算法实现了在不同程度的随机噪声下的服从J-SB分布的单峰分布的均匀球形颗粒粒度分布反演,其反演结果更稳定,反演精度高,对于实现稳定、快速、准确的颗粒粒度在线测量具有重要的意义。     

Analysis on the efficiency of parallel FDTD method and its application in two-dimensional photonic crystal

A new kind of parallel finite-difference time-domain (FDTD) method is used to calculate and analyze the transmission properties of photonic crystal. The effectiveness and the accuracy of parallel FDTD method were proved by the typical two-dimensional sine wave. The result shows that parallel FDTD method, which can save time effectively in electromagnetic problems with large size and long time, performs as accurate as the series one. This method, whose results shows that the photonic band gap normalized frequency moves to the low-frequency direction with the enlargement of the section area of the square cylinder and that the band gap gets thinner, is applied to simulate and analyze the two-dimensional photonic crystal and gives calculating method and analyzing accordance to calculate and analyze large-sized photonic crystal structure.     

表面增强拉曼光谱对免疫球蛋白IgG分子与银基底作用的研究

采用高灵敏度的表面增强拉曼光谱(SERS)技术,以具有强SERS信号的金纳米粒子标记抗体,以此SERS标记免疫金溶胶为探针,结合扫描电镜技术,研究免疫球蛋白羊抗小鼠IgG分子与银基底的相互作用。我们发现,羊抗小鼠IgG分子可直接与银基底通过疏水作用或形成Ag-S键而牢固结合。为消除这种非特性吸附,本文以小牛血清白蛋白(BSA)封闭银基底,取得了较好的效果。     

非分离状态下FI-KR-FAAS测定铁的价态

建立了流动注射(FI)编结反应器(KR)在线预富集与火焰原子吸收光谱(FAAS)联用非分离状态下测定铁的价态的新方法。进样流速6.0 mL.min-1;进样时间60s,测定0.04 mg.L-1的Fe(Ⅲ)和Fe(Ⅱ),Fe(Ⅲ)和Fe(Ⅱ)的浓集系数(EF)分别为41和9;检出限分别为2.5和14.3μg.L-1;相对标准偏差(RSD,n=11)分别为2.3%和3.1%。以0.1‰φ的三乙醇胺为掩蔽剂,Fe(Ⅲ)和Fe(Ⅱ)在水样中的回收率分别为97%～101%和96%～100%。     

光谱分析在碲化镉晶体制备和晶体评价中的应用

籍助于原子吸收法及X射线光电子能谱(XPS)的分析,比较了CdTe晶体制备的不同工艺条件。根据红外光谱和光致发光光谱的测量结果,对不同方法制备的CdTe晶体质量作了评价。     

时域有限差分法在光波导分析中的应用及改进

对时域有限差分算法在圆柱坐标下弱导光纤中应用时吸收边界条件的设置及其对计算精度的影响进行了讨论,提出了改善的方法。并且将这种方法应用于一般光纤和光电子晶体光纤传输特性的计算,得到了有用结果。     

Locally rigorous electromagnetic analysis of non-periodic gratings with large size and small features in the non-scalar validity domain by the field stitching method

Scalar theory is a simple and rapid method to analyse and conceive diffractive optical element and gratings, in some limited cases. When we want to deflect wave fronts with large angles (for telecommunication devices, for instance) or when the features of the grating are around the order of the wavelength, scalar optics fail to be accurate, then adapted rigorous electromagnetic tools must be used. To correct the increasing cost in memory saving and time consuming with the size of the grating, we present here an original `field stitching' method adapted for aperiodic gratings (FSA). This method, initially developed for periodic gratings is extended here to analyse locally and rigorously aperiodic gratings with large sizes and small features without having some of the above-mentioned inconvenients.     

Influence of fibre extraction method,alkali and silane treatment on the interface of Agave americana waste HDPE composites as possible roof ceilings in Lesotho

Surface treatment is often necessary for strong composites. But the challenge for developing countries is to find chemicals and treatment procedures that are cheap and simple but maintain good composite properties. Mercerization followed by silane treatment of natural fibres is among the simplest and cheapest methods used to improve composite interfaces. This study investigates the effectiveness of this method to improve the bond between Agave americana fibres and post consumer HDPE. The influence of fibre extraction method, mercerization and mercerization followed by silane treatment on interfacial shear strength (ISS) and fibre properties is determined. The results indicate that ISS values are generally low but mercerization doubles the ISS values between Agave americana fibres extracted by traditional boiling of leaves and post consumer HDPE. Mercerization also improves fibre tensile and thermal properties. While triethoxyvinylsilane treatment of fibres after mercerization does not improve the ISS, it does not reduce it either, nor does it reduce tensile and thermal strengths of mercerized fibres. Fibres from non-boiled leaves resulted in poor fibre tensile strengths but improved ISS. There is a potential to use mercerization as cheap, simple technique to make Agave americana HDPE composites to provide cheap roof ceilings in Lesotho.     

Thermal Ionization and Inductively Coupled Plasma Mass Spectrometry: Potential for Application to Studies on the Biokinetics of Molybdenum in Humans

Abstract

The use of stable isotopes as tracers in biokinetic investigations provides a means to obtain important metabolic data directly in humans without exposing the subjects to undue risks. In this work, three types of mass spectrometers are compared with regard to the determination of the abundances of stable isotopes of molybdenum in natural and enriched aqueous samples. The data show a good response of thermal ionization mass spectrometry (TIMS) and of high-resolution inductively coupled plasma mass spectrometry (ICPMS) to the isotopic enrichment of the samples, whereas conventional quadrupole ICPMS shows an unsatisfactory reproducibility of the results. Moreover, only TIMS can achieve an accuracy of better than 1% for the obtained isotopic ratios. Although a tedious procedure for the preparation of the biological samples is required and less sensitivity is achieved as compared to ICPMS, TIMS still seems to be method of choice for the accurate assessment of isotope ratios as required in multitracer studies on human biokinetics of trace metals.     

A method for quantitative interpretation of fluorescence detection of poly(ethylene glycol)-mediated 1-palmitoyl-2-[[[2-[4-(phenyl-trans-1,3,5-hexatrienyl) phenyl]ethyl]oxyl]carbonyl]3-sn-phosphatidylcholine (DPHpPC) transfer and fusion between phospholipid vesicles in the dehydrated state

A method has been developed for calculating the expected fluorescence lifetime of the DPH _p PC probe distributed between different membrane environments. We show how this method can be used to distinguish between lipid transfer and fusion between large unilamellar vesicles occurring in the presence of poly(ethylene glycol) (PEG). This application of the calculation took into consideration the heterogeneity of microenvironments experienced by the probe in a sample containing vesicle aggregates of different sizes. Assuming that the aggregate size distribution was a delta function of the aggregate size, comparison of the calculated and observed lifetimes yielded an estimate of the vesicle aggregate size. For vesicles of varying compositions in the presence of dehydrating concentrations of PEG, this method suggested that only small aggreggates formed. For vesicles that could be demonstrated by other means not to have fused, the data were consistent with lipid transfer occurring only between the outer leaflets of two to four vesicles, even at high PEG concentrations. For vesicles that could be demonstrated to fuse by contents mixing and size changes, the fluorescence lifetime data were consistent with lipid transfer between both the inner and the outer leaflets of two to four fused vesicles. At very high PEG concentrations, where extensive rupture and large, multilamellar products were previously observed, the lifetime data were consistent with much more extensive lipid transfer within larger aggregates. The agreement of predictions made on the basis of lifetime measurements with other observations attests to the validity of the fluorescence lifetime method. In addition, the model and data presented here provide evidence that fusion occurs between small numbers of PEG-aggregated vesicles before the removal of PEG.