Combustion-formed nanoparticles

The processes by which carbonaceous nanoparticles are produced from combustion of liquid and gaseous fuels are reviewed. The focus of the paper is on the formation and properties of nanoparticles in laboratory laminar, premixed and diffusion flames and on the most popular methods of sampling and detection of these particles. Particle chemical nature is analyzed from data obtained by several measurement techniques. Measurements characterizing nanoparticles in the exhausts of practical combustion systems such as engines and commercial burners are also reported. Two classes of carbonaceous material are mainly formed in combustion: nanoparticles with sizes in the range 1-5 nm, and soot particles, with sizes from 10 to 100 nm. Nanoparticles show unique chemical composition and morphology; they maintain molecular characteristics in terms of chemical reactivity, but at the same time exhibit transport and surface related phenomena typical of particles. The emission of these particles contributes to atmospheric pollution and constitutes a serious health concern. A simplified modeling analysis is used to show how the growth of aromatics and the chemical nature of the particles depend on temperature and radical concentration distributions encountered in flames.     

Experimental and kinetic modeling study of sooting atmospheric-pressure cyclohexane flame

Experimental data and modelling results of the main products and intermediates from a fuel-rich sooting premixed cyclohexane flame were presented in this work. Model predictions well agree with experimental data both in sooting and non-sooting flames. Major and minor species are properly predicted, together with the soot yield. The initial benzene peak was demonstrated to be due to the fast dehydrogenation reactions of the cycloalkane, which gives rise to cyclohexene and cyclohexadiene both via molecular and radical pathways. Once formed cyclohexadiene quickly forms benzene whereas in the postflame zone, benzene comes from the recombination and addition reactions of small radicals, with C₃H₃ + C₃H₃ playing the most important role in these conditions. An earlier soot inception was detected in the cyclohexane flame with respect to a n-hexane flame and this feature is not reproduced by the model that foresees soot formation significant only in the second part of the flame. The model insensitivity of soot to the reactant hydrocarbon was also observed comparing the predictions of three flames of cyclohexane, 1-hexene and n-hexane with the same temperature profile. A sensitivity analysis revealed that soot primarily comes from the HACA mechanism for the three flames, acetylene being the key species in the nucleation. Experimental data on soot inception seem to indicate the importance of the early formation of benzene, that depends on the fuel structure. It is thus important to further investigate the role of benzene and aromatics in order to explain this discrepancy.     

Turbulent flame simulation taking advantage of tabulated chemistry self-similar properties

Predicting the flame shape, its stabilization process, and pollutant emissions in practical combustion devices requires to incorporate complex chemistry features. As detailed chemical schemes are too voluminous for practical numerical simulations, tabulated chemistry techniques have been proposed to account for the complexity of kinetics in turbulent flame simulations. Unfortunately, the size of these databases may become a crucial issue for efficient implementation on massively parallel computers. A reduction strategy that takes advantage of self-similar properties of tabulated chemistry is proposed for turbulent combustion modeling. A reduction of the database size by a factor of 1000 is achieved. This procedure is successfully applied to a RANS simulation of a turbulent jet flame.     

Modeling the combustion of JA2 and solid propellants of similar composition

A theoretical study on combustion of JA2, RPD-380, and RPD-351, which are modified double-base propellants composed primarily of three identical nitrate ester ingredients, is presented. A one-dimensional, two-phase model was used [M.S. Miller, W.R. Anderson, in: V. Yang, T.B. Brill, W.Z. Ren (Eds.), Solid Propellant Combustion Chemistry, Combustion and Motor Interior Ballistics, Progress in Astronautics and Aeronautics, vol. 185, AIAA, Reston, VA, 2000, pp. 501–531, (a) M.S. Miller, W.R. Anderson, J. Propul. Power 20 (3) (2004) 440–454. (b) M.S. Miller, W.R. Anderson, CYCLOPS, A Breakthrough Code to Predict Solid-Propellant Burning Rates, U.S. Army Research Laboratory Technical Report, 1987 ARL-TR-2910.]. This approach has been shown to give good agreement between predicted and experimental results for several nitrate ester propellants, including JA2 [(a) M.S. Miller, W.R. Anderson, J. Propul. Power 20 (3) (2004) 440–454. (b) M.S. Miller, W.R. Anderson, CYCLOPS, A Breakthrough Code to Predict Solid-Propellant Burning Rates, U.S. Army Research Laboratory Technical Report, 1987 ARL-TR-2910.]. Extension of the model to the two RPD variants yields results in good agreement with existing experimental data. Comparisons of the response of predicted burning rates to experimental formulation changes at gun pressures, and to the initial propellant temperature are particularly encouraging. Our results show the burning rate ordering of these propellants is JA2 < RPD-380 < RPD-351 at all pressures. Chemistry which appears to account for this ordering is discussed. Also, an upgraded mechanism was used, and the reasons for some slight changes in results vs. an older one are identified. Sensitivities of the computed temperatures near the propellant surface to the various reactions’ rate coefficients are discussed; these provide insights regarding which reactions are centrally important to the computed burning rates and solutions. The spatial structure of one propellant flame – temperature and species profiles – is given; variations vs. the formulations and pressure are discussed. The fidelity of burning rate response to mixture ratio and initial propellant temperature are encouraging that the model may find application in propellant formulation science and elsewhere.     

Physical meaning of the stretched exponential Kohlrausch function

In this paper the physical meaning of the empirical Kohlrausch-Williams-Watts (KWW) function is explained in terms of the linear oscillator theory. It is shown that the KWW function is a solution of the non-autonomous linear first order equation for an overdamped linear oscillator. From the linear oscillator model it follows that the KWW-type relaxation is the linear relaxation with a time (coordinate, stress, voltage, etc.) dependent dissipation of energy. The theoretical results are validated by measurements. A method for modeling KWW-type relaxation using simple electrical circuits is proposed.     

On the characterization of novel biologically active steroids: Selection of lipophilicity models of newly synthesized steroidal derivatives by classical and non-parametric ranking approaches

In this paper, the guidelines for the interpretation of the results of quantitative structure-retention relationship (QSRR) modeling, comparison and assessment of the established models, as well as the selection of the best and most consistent QSRR model were presented. Various linear and non-linear chemometric regression techniques were used to build QSRR models for chromatographic lipophilicity prediction of a series of triazole, tetrazole, toluenesulfonylhydrazide, nitrile, dinitrile and dione steroid derivatives. Linear regression (LR) and multiple linear regression (MLR) were used as linear techniques, while artificial neural networks (ANNs) were applied as non-linear modeling techniques. Generated models were statistically evaluated applying different approaches for model comparison and ranking. Two non-parametric methods (generalized pair correlation method – GPCM and sum of ranking differences – SRD) were used for model ranking and assessment of the best model for chromatographic lipophilicity prediction using experimentally obtained logk values and row average as a reference ranking. Both, GPCM and SRD, provided highly similar model choice regardless on a different background. These results are in agreement with the classical approach.     

A stress transport equation model for simulating turbulence at any mesh resolution

Prior work has demonstrated the effectiveness of using two-equation closures as the basis for universal, self-adapting turbulence models that are effective at any mesh resolution (Perot and Gadebusch in Phys. Fluids 19:115105, 2007). In order to demonstrate the broad applicability of the fundamental approach, the same behavior is now demonstrated for a second-moment closure (SMC). The SMC has the advantage over the earlier two-equation universal closure of being more accurate in the coarse mesh limit and of having a natural mechanism for backscattering energy from the modeled to the resolved turbulent fluctuations. The mathematical explanation for why Reynolds averaged (RANS) transport equation closures are applicable at any mesh resolution, including the large eddy simulation (LES) regime, is reviewed. It is demonstrated that for the problem of isotropic decaying turbulence, the SMC model produces good predictions at any mesh resolution and with arbitrary initial conditions. In addition, it is shown that the proposed model automatically adapts to the mesh resolution provided. The self-adaptive nature of the method is clearly observed when different initial conditions are used. It is shown that classic RANS models (often thought to produce steady and smooth solutions) can produce three-dimensional, unsteady, and chaotic solutions when generalized correctly and when provided with sufficient mesh resolution. The implications of these observations on the fundamental theories of RANS and LES turbulence modeling are discussed.      

基于OSC-PLS算法对大麦蛋白质含量进行定量分析的研究

用色散扫描型仪器采集大麦样品的近红外光谱,扫描出的光谱携带了大量样品化学值信息,采用正交信号校正(OSC)预处理方法对这些原始光谱进行处理,剔除噪声等不相关因子以后建立偏最小二乘(PLS)近红外光谱分析模型(OSC-PLS),预测大麦蛋白质的含量,并与传统PLS建模方法进行对比。基于OSC-PLS算法的蛋白质含量近红外光谱分析模型的测定系数R2为0.901,检验集的化学值与模型预测值的相关系数r达到0.971 7,分析模型的预测标准偏差SD为0.545 0,相对标准偏差RSD为4.2%。结果表明,OSC-PLS回归方法能在较大程度上消除无关因素的影响,在简化模型的同时提高了模型的可解释性,能够建立准确的大麦蛋白质含量近红外预测模型,可代替经典分析方法,满足农产品快速分析的需要。     

Experimental and numerical investigation on performance of a swirling jet reactor

In this work, a three-dimensional Computational Fluid Dynamic (CFD) analysis of a swirling jet reactor was implemented to gain a better understanding of fluid dynamics into the reactor. The effect of different geometries of the reactor, by considering different diameters of the injection slots of the reactor, on flow velocity and flow pressure distributions was investigated. Firstly, a one-phase model was implemented by considering only water into the reactor. Then, a two-phase model was defined including dissolved air into the water. The inlet flow pressure was set to 0.25 bar to consider non-cavitating conditions and, then, to get more accurate results on fluid dynamics into the reactor due to the absence of cavitating conditions. Data collected from experimental tests were used to calibrate and validate the model. Results of numerical simulations were in good agreement with experimental data, showing for all the geometries a rotating flow around the central axis of the reactor and at the exit of the double cone. The highest flow velocities and flow pressure drops were observed for the reactor geometry with the smallest injection slots diameters. Finally, noise measurements were performed during another set of experimental tests by considering different inlet flow pressures.     

A study on validity of using average fiber aspect ratio for mechanical properties of aligned short fiber composites with different fiber aspect ratios

A closed-form solution using the actual distribution of the fiber aspect ratio is proposed for predicting the stiffness of aligned short fiber composite. The present model is the simplified form of Takao and Taya’s model and the extended version of Taya and Chou’s model, where Eshelby’s equivalent inclusion method modified for finite fiber volume fraction is employed. The validity of using average fiber aspect ratio for predicting the composite stiffness is justified in terms of the scatter of fiber aspect ratio, fiber volume fraction, and constituents‘ Young’s modulus ratio, comparing with the results by the present model. The guideline for selection of either the actual distribution or the average fiber aspect ratio is presented for the better prediction of the composite stiffness.