考虑两相流音速时气固两相流激波研究

1引言超音速冷喷涂(CSC),又称为冷空气动力学喷涂(CGDS),是一种新型的喷涂表面沉积的方法[1~3]。在超音速冷喷涂中,喷涂粉末与气体混合后,在缩放流道内形成超音速流动,与基板碰撞后喷涂粒子在基板上沉积。由于两相介质是超音速流动,此时气固两相流激波起着至关重要的作用。气固     

在绝热流动情况下非球形水滴的运动特性

采用非球形颗粒的阻力系数关联式，建立了水滴在绝热逆流空气中的动力学方程，得出了水滴所受阻力及其最终沉降速度的计算方法。在此基础上，分析了水滴变形程度和内部流动对水滴运动特性的影响，结果表明水滴变形程度增加使得水滴受到的空气阻力增大，而最终沉降速度减小，水滴的内部流动导致其受到的空气阻力减小，计算条件下最大偏差可达15％。     

Comprehensive study of the evolution of an annular edge flame during extinction and reignition of a counterflow diffusion flame perturbed by vortices

The structure of a time-dependent methane/enriched-air flame established in an axisymmetric, laminar counterflow configuration is investigated, as the flame interacts with two counterpropagating toroidal vortices. Computationally, the time-dependent equations are written using a modified vorticity–velocity formulation, with detailed chemistry and transport, and are solved implicitly on a nonstaggered, nonuniform grid. Boundary conditions are chosen to create local extinction and reignition in the vicinity of the axis of symmetry. Experimentally, CO planar laser-induced fluorescence (PLIF), OH PLIF, and an observable proportional to the forward reaction rate (RR) of the reaction CO+OH→CO₂+H are measured. Particle image velocimetry (PIV) is used to characterize the velocity field of the vortical structures and to provide detailed boundary conditions for the simulations. Excellent agreement is found between model and experiments to the minutest morphological details throughout the interaction. The validated model is then used to probe the dynamics of the two-dimensional extinction process with high temporal resolution. During the initial phase of the interaction, the flame is locally extinguished by the two vortices. The resulting edge flame propagates outward as an extinction front, with a structure that does not depart significantly from that of a diffusion flame. The front recedes from the axis of symmetry with a negative propagation speed that reaches a value as large as six times that of the freely propagating laminar flame with the same reactant concentrations found at the stoichiometric surface. As the front propagates outward, it transitions to an ignition front, and it reaches a positive propagation speed comparable to that of the freely propagating laminar flame. During this transition, it develops a characteristic premixed “hook,” with a lean premixed branch, a stoichiometric segment that evolves into the remnant of the original primary diffusion flame, and a much weaker secondary diffusion flame resulting from a secondary peak in heat release in the original unperturbed diffusion flame. No evidence of a distinct rich premixed flame is found. The edge flame stabilizes at a radial location where the local gaseous speed equals the propagation speed of the front. When the local perturbation has decayed below the flame propagation speed, the flame edge starts reigniting the mixing layer as an ignition wave that propagates with an essentially frozen structure along the stoichiometric surface until the original diffusion flame structure is fully recovered. Implications for flamelet modeling of turbulent flames with local extinction are discussed.     

Comparison of iso-octane burning rates between single-phase and two-phase combustion for small droplets

Two-phase combustion is a widespread mechanism of energy conversion that is of practical importance in gas turbines, diesel and spark ignition engines, furnaces, and hazardous environments. However, the exploration of important parameters in combustion systems of practical application is difficult, due to the multiplicity of dependent variables. In the present work, combustion rates of well-defined droplet suspensions of iso-octane have been measured using techniques employed for gaseous combustion. This required a full characterization of the aerosols produced in the combustion apparatus, which determined that the maximum droplet size produced was around 30 μm. Comparisons of two-phase with single-phase laminar mixtures suggest that there were negligible differences in the burning velocity of an aerosol and a gaseous mixture at the same overall equivalence ratio and similar conditions for iso-octane. At high stretch rates, flames remained smooth and droplet enhancement was negligible. However, at lower rates of stretch, both gaseous and aerosol flames became unstable and cellular, and this cellularity, in some cases, increased the burning rate. The values of Markstein length measured for aerosol flames had trends similar to those for gaseous-phase mixtures (Markstein length decreased with equivalence ratio), but were lower than in gaseous combustion. The values of Markstein length in aerosol flames also decreased with liquid equivalence ratio and/or Sauter mean diameter. All this indicates a higher tendency to instabilities in aerosol flames compared to gaseous combustion. A qualitative explanation for the lower values of Markstein length in aerosol combustion is given. It is suggested in the present work that aerosol flames became unstable, and hence had faster burning rates, under conditions that would not result in unstable gaseous flames. Comparisons, qualitative and in terms of dimensionless groups, of two-phase with single-phase turbulent combustion also suggest no enhancement.     

Steam reforming of acetic acid for hydrogenproduction: Catalytic performances of Ni and Co supported on Ce0·75Zr0·25O2 catalysts

The catalytic steam reforming of acetic acid over both Ni/ and Co/Ce_0·75Zr_0·25O₂ (CZO) catalysts in the temperature range of 450–650 °C and steam-to-carbon molar ratios of 3–9 was studied. It was found that the complete acetic acid conversion was achieved for all the conditions investigated. Nevertheless, the C–C bond cleavage conversion was attained less than the acetic acid conversion at a given condition due to carbon deposition on the catalyst. However, hydrogen yield was obtained in the same trend as C–C bond cleavage conversion as well. The results revealed that the CZO as an active support prefers to promote the ketonization reaction to the C-C bond cleavage reaction at a lower temperature, and vice versa at a higher temperature. The Ni/CZO catalyst exhibits higher C–C bond cleavage conversion than the Co/CZO catalyst particularly at 650 °C whereas the Co/CZO catalyst is more active for ketonization reaction at low temperatures. However, as an increase in reaction temperature, the Co/CZO catalyst promotes ketonization reaction more pronouncedly toward aldol-condensation reaction thus giving rise to the carbon deposition. The results deduced from the effect of space velocity on the activity and product distribution suggested that the steam reforming of acetic acid over Ni/CZO catalyst is dominated by decomposition of acetic acid, while that of Co/CZO catalyst by ketonization reaction.     

柴油机燃油喷射雾化的PIV测量试验研究

介绍了先进的粒子图像测速技术(PIV)应用于燃油喷雾过程的试验研究结果，燃油通过多种孔径的单孔和多孔喷油嘴喷入一个定容压力室内，采用高分辨率的CCD摄像机记录整个喷射过程，可以观察到喷雾场内部结构．结果分析显示，高速射流表面存在不稳定扰动波，在气动力作用下，液滴发生分裂破碎，呈现枝状分离表面．瞬态速度矢量图显示，气液界面处存在大尺度的卷吸涡流运动，有助于油气扩散混合。     

锅炉引风机改用调速型液力耦合器调节的经济分析

本文介绍热电厂引风机的几种调节方式，以员工锅炉引风机的改造为例，通过分析比较，得出结论，现阶段应用调速型液力偶合器调节是解决热电厂锅炉引风机经济运行的最有效方法。     

环状出口气泡雾化喷嘴液膜破碎过程与喷雾特性

主要研究了环状出口气泡雾化喷嘴出口下游液膜破碎过程与喷雾特性．当气液质量流量比为零时，出口下游形成空心封闭膜壳，表面波明显存在于液膜表面．随着气液质量流量比的增加，膜壳逐渐膨胀并在最薄弱部位被撕裂．利用DualPDA测量得到液雾颗粒的速度分布、直径分布与流通量分布特性；在主流区域存在负向运动的粒子同时颗粒平均速度明显降低．出口下游的速度分布曲线呈现双峰趋势，实验数据显示中心回流区域结束于距离出口30mm左右．索特平均直径的有关数据显示气泡的“爆炸”发生于出口下游5～15mm区域．流通量分布曲线也是双峰的，径向逐渐扩张，轴向逐渐降低，并且液雾主流区域流通量明显高于边缘区域的流通量．     

Premixed flamelet modelling: Factors influencing the turbulent heat release rate source term and the turbulent burning velocity

A flamelet approach is adopted in a study of the factors affecting the volumetric heat release source term in turbulent combustion. This term is expressed as the product of an instability enhanced burning rate factor, Pbi, and the mean volumetric heat release rate in an unstretched laminar flamelet of the mixture. Included in the expression for Pbi are a pdf of the flame stretch rate and a flame stretch factor. Fractal considerations link the turbulent burning velocity normalised by the effective rms turbulent velocity to Pbi. Evaluation of this last parameter focuses on problems of (i) the pdfs of the flame stretch rate, (ii) the effects of flame stretch rate on the burning rate, (iii) the effects of any flamelet instability on the burning rate, (iv) flamelet extinctions under positive and negative flame stretch rates, and (v) the effects of the unsteadiness of flame stretch rates. The Markstein number influences both the rate of burning and the possibility of flamelet instabilities developing which, through their ensuing wrinkling, increase the burning rate. The flame stretch factor is extended to embrace potential Darrieus-Landau thermo-diffusive flamelet instabilities. A major limitation is the insufficient understanding of the effects of negative stretch rates that might cause flame extinction. The influences of positive and negative Markstein numbers are considered separately. For the former, a computed theoretical relationship for turbulent burning velocity, normalised by the effective rms velocity, is developed which, although close to that measured experimentally, tends to be somewhat lower at the higher values of the Karlovitz stretch factor. This might be attributed to reduced flame extinction and reduced effective Markstein numbers when the increasingly nonsteady conditions reduce the ability of the flame to respond to changes in flame stretch rates. As the pressure increases, Markstein numbers decrease. For negative Markstein numbers the predicted values of Pbi and turbulent burning velocity are significantly increased above the values for positive Markstein numbers. This is confirmed experimentally and these values are close to those predicted theoretically. The increased values are due to the greater stretch rate required for flame extinction, the increased burning rate at positive values of flame stretch rate, and, in some instances, the development of flame instabilities. At lower values of turbulence than those covered by these computations, burning velocities can be enhanced by flame instabilities, as they are with laminar flames, particularly at negative Markstein numbers.     

基于惯性传感的人员行进动作识别方法

人员行进动作的识别对于行人航位推算(PDR)的计算精度有至关重要的影响。将连续的行进过程依据零速率修正点(ZUPT)划分为单步动作后,准确的动作识别是PDR系统的难题之一。将惯性测量单元IMU固连在胫骨中间外侧位置,采集了大量人员行进过程胫骨运动的加速度和角速度,建立了用于行进动作识别的惯性传感数据库。通过对人员的平地行走、上楼梯和下楼梯的惯性传感数据的分析,建议了一种结合运动特性寻找行进过程中零速率修正ZUPT点的方法。在完成单步步态划分的基础上,提出了一种基于支持向量机(SVM)理论,并采用惯性传感数据作为特征的SVM行进动作分类器。经大量人员行进实验验证,所建议的动作识别方法对于区分行走、上楼梯和行走、下楼梯,精度分别达到96%和85%。