激光诱导荧光技术在高焓空气氮原子测量中的应用

高超声速飞行器再入大气层时,受到激波的压缩和激波层内粘性阻滞作用,周围绕流流场的空气温度在4 000～15 000 K之间,使空气中的氧、氮分子发生离解,从而出现高温气体效应,形成高焓化学非平衡流。飞行器表面防热材料基本都含有大量的碳元素,通常情况下主要是氧原子与碳发生反应,但在焓值大于18 MJ·kg-1的情况下,氮原子与飞行器表面碳反应的无量纲烧蚀因子BC_N＞0.172 5,此时碳在高焓空气介质中的无量纲质量烧蚀因子BC_air＞0.345,因此,碳的氮化烧蚀变得非常剧烈,和氧化烧蚀相当;同时离解的氮原子也会在飞行器表面发生催化复合反应放出大量的热,使飞行器表面承受更多的热载荷。因此,分析高焓化学非平衡流流场中的氮原子具有现实意义。在地面模拟设备中建立高焓化学非平衡流场,对氮原子进行测量技术,可以很好的对其进行研究。双光子吸收激光诱导荧光（TALIF）技术作为一种非接触测量,在不干扰流场环境的情况下,可以直接获得氮原子的浓度分布。对流场氮原子激发荧光,通过布置在风洞试验段外与流场和激光形成的平面呈垂直方向的ICCD获取到二维氮原子的荧光信号。为确保荧光图像的清晰及视场合适,选择了Nikon f=50 mm F/1.4镜头作为前级收光设备。实验成像采用50次曝光的累计结果,以消除气流扰动及激光能量抖动造成的不确定性。实验中,在理论激发波长附件进行测试,优化选择出206.717 nm作为正式实验中脉冲激光的最佳激发波长。在最佳激光波长条件下,从小到大调整激发能量,获得了该环境下的氮原子激发的非饱和线性区为1.8 mJ以下。正式实验过程中激光能量为1.6 mJ,处于线性区。对所获取的荧光图像进行处理,提取激光中心线上的荧光强度进行分析,发现无论是亚声速还是超声速,荧光强度沿径向都呈驼峰状分布,与之前氧原子测试结果比较,发现流场中氮分子还未完全离解,这符合实验风洞流场特性。     

Spray flame dynamics with oscillating flow and droplet grouping

The co-flow laminar spray diffusion flame in an oscillating flow field is investigated. Mild slip is permitted between the droplets and their host surroundings and droplet grouping resulting from the host flow oscillations is accounted for. The spray is modelled using the sectional approach and a perturbation analysis using a small sectional Stokes number is utilised for solving the liquid phase governing equations. The effect of droplet grouping is described through a specially constructed model for the vaporisation Damkohler number. The large chemical Damkohler number assumption is adopted and a formal analytical solution is developed for Schwab-Zeldovitch parameters through which the dynamics of the spray flame front shapes and thermal fields are deduced. Computed results based on the solutions demonstrate how the phenomenon of droplet grouping can lead to the existence of multiple flame sheets as a result of the dynamic change in the type of the main homogeneous flame from under- to over-ventilated as the flow field oscillates. Concomitant fluctuating thermal fields are also shown to be present indicating a potential impact on undesirable pollutants production.     

Infrared thermography-based visualization of droplet transport in liquid sprays

An infrared thermography-based technique for the characterization and visualization of liquid sprays was developed. The technique was tested on two atomizers: a high-speed rotary bell atomizer and a high volume low pressure air-assisted atomizer. The technique uses an infrared thermography-based measurement in which a uniformly heated background acts as a thermal radiation source, and an infrared camera as the receiver. The infrared energy emitted by the radiation source in traveling through the spray is attenuated by the presence of the droplets inside the spray. The infrared intensity is captured by the receiver showing the attenuation in the image as a result of the presence of the spray. The captured thermal image is used to study detailed macroscopic features of the spray flow field and the evolution of the paint droplets as they are transferred from the applicator to the target surface.     

Numerical study of the effect of turbulence on rate of reactions in the MILD combustion regime

In this paper, the importance of fluctuations in flow field parameters is studied under MILD combustion conditions. In this way, a turbulent non-premixed CH₄+H₂ jet flame issuing into a hot and deficient co-flow air is modeled using the RANS Axisymmetric equations. The modeling is carried out using the EDC model to describe the turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI2.11 full mechanism are used to represent the chemical reactions of H₂/methane jet flame. Results illustrate that although the fluctuations in temperature field are small and the reaction zone volume are large in the MILD regime, the fluctuations in temperature and species concentrations are still effective on the flow field. Also, inappropriate dealing with the turbulence effect on chemistry leads to errors in prediction of temperature up to 15% in the present flame. By decreasing of O₂ concentration of hot co-flow air, the effect of fluctuations in flow field parameters on flame characteristics are still significant and its effect on species reaction rates does not decrease. On the other hand, although decreasing of jet inlet Reynolds number at constant inlet turbulence intensity addresses to smaller fluctuations in flow filed, it does not lead to lower the effect of turbulence on species distribution and temperature field under MILD combustion conditions.     

Thermal analysis of Polymer Dispersed Liquid Crystal under electric field using photopyroelectric technique

In this paper the first measurement of effective thermal parameters (thermal diffusivity, effusivity, conductivity and heat capacity) of Polymer Dispersed Liquid Crystal (PDLC) composites using the photopyroelectric (PPE) calorimetry is reported. The PPE technique is used in the standard “back” configuration and the cell has been designed for allowing the application of an electric field to the sample. Results show a dependence of the thermal parameters on the applied electric field which is explained by the reorientation of the liquid crystal molecules inside the droplets.     

Numerical study of methanol flames in laminar forced convective environment using short chemical kinetics mechanism

Due to recent interest in methanol economy, it is seen that a numerical study of combustion of methanol in a comprehensive manner is necessary. Motivated from this interest and based on the studies from literature, a numerical study on prediction of structures of non-premixed methanol-air flames in laminar forced convective environment is reported. Two-dimensional, planar and axisymmetric, computational domains are considered. Corresponding governing equations for conservation of mass, momentum, species and energy have been solved using Ansys FLUENT. The numerical model incorporates multi-component diffusion, variable thermal and physical properties, a short chemical kinetics mechanism with 18 species and 38 elementary reactions, and a non-luminous thermal radiation model. Homogeneous flames in opposed flow and heterogeneous flames in cross-flow and co-flow configurations are studied. For heterogeneous flames, interface conditions at the liquid methanol surface are defined systematically using a user-defined function. Numerical results are validated against the experimental results available in literature. Results in terms of mass burning rates, flow, species and temperature fields have been presented to describe the flame characteristics.     

Thermal modes of a plug flow reactor with a liquid–liquid heterogeneous system

The thermal modes of a flow plug reactor with an exothermic chemical reaction are numerically simulated. A heterogeneous reaction system consisting of two immiscible liquids is studied: one of the liquids (dispersed phase) in the form of droplets is distributed in the other (dispersion phase). The characteristics of the thermal modes of the reactor at various values of two governing parameters, the Damköhler number and the rate of extraction of the dissolved substance from the dispersed phase into the dispersion phase is examined. Two modes of chemical reaction in the reactor are demonstrated to be possible: low-temperature and high-temperature. Critical criteria of thermal ignition are formulated. The dependence of the structure of the thermal wave on the governing parameters is investigated.     

Investigation of the transition from lightly sooting towards heavily sooting co-flow ethylene diffusion flames

Laminar, sooting, ethylene-fuelled, co-flow diffusion flames at atmospheric pressure have been studied experimentally and theoretically as a function of fuel dilution by inert nitrogen. The flames have been investigated experimentally using a combination of laser diagnostics and thermocouple-gas sampling probe measurements. Numerical simulations have been based on a fully coupled solution of the flow conservation equations, gas-phase species conservation equations with complex chemistry and the dynamical equations for soot spheroid growth. Predicted flame heights, temperatures and the important soot growth species, acetylene, are in good agreement with experiment. Benzene simulations are less satisfactory and are significantly under-predicted at low dilution levels of ethylene. As ethylene dilution is decreased and soot levels increase, the experimental maximum in soot moves from the flame centreline toward the wings of the flame. Simulations of the soot field show similar trends with decreasing dilution of the fuel and predicted peak soot levels are in reasonable agreement with the data. Computations are also presented for modifications to the model that include: (i) use of a more comprehensive chemical kinetics model; (ii) a revised inception model; (iii) a maximum size limit to the primary particle size; and (iv) estimates of radiative optical thickness corrections to computed flame temperatures.     

Polydisperse spray diffusion flames in oscillating flow

The phenomenon of droplet clustering or grouping found when a spray of droplets is moving in an oscillating host flow field is investigated for the case of a polydisperse spray that fuels a laminar co-flow diffusion flame. A mathematical solution is developed for the liquid phase based on use of small Stokes numbers for size sections into which the polydisperse spray size distribution is divided. Droplet clustering in the oscillatory flow field is accounted for by constructing a special model for the sectional vaporization Damkohler numbers in accordance with droplet size. Combining this with a formal solution for a gas phase Schvab-Zel'dovich variable yields the means whereby flame dynamics can be described. Results calculated from this solution demonstrate that preferential droplet size behaviour (with smaller droplets tending to cluster to a greater extent and reduce the vaporization Damkohler number more than larger ones) can have a major impact on the flame dynamics through local droplet enrichment with attendant consequences on the production of fuel vapour. The dynamics of the sort of flame (over- or under-ventilated) and the occurrence of flame pinching leading to multiple flame sheets are altered under these circumstances. However, potential control of the actual initial spray polydispersity may reduce the intensity of such effects.     

Synthesis,growth and optical properties of semi organic non linear optical single crystal: l-Arginine acetate

In the recent past, nonlinear optical materials are getting attention in the field fibre optic communication and optical signal processing. In the present study, the title compound was successfully synthesized by conventional chemical reaction and then the single crystal was grown by slow evaporation solution growth technique. l-Arginine acetate (LAA) is semi organic non linear optical single crystal and crystallizes in noncentrosymmetric space group. The grown good quality single crystals have been analyzed by different characterization analyses such as powder X-ray diffraction, high resolution X-ray diffraction (HRXRD), Raman spectroscopy, photoluminescence, SEM, micro-hardness, thermal and birefringence analyses.     

一种新型FBG热式液体流量传感器

光纤布拉格光栅热式流量传感器目前只适用于气体流量,为扩大其应用领域,设计了一种可用于液体流量测量的新型光纤布拉格光栅热式流量传感器。该光纤布拉格光栅热式流量传感器使用陶瓷加热片以恒定功率提供热量,不同流量的液体经过传感器时带走的热量不同,通过检测光纤布拉格光栅中心波长的变化就可以测得传感器的温度变化,进而推导出液体流量大小。通过温度传感测试实验和流量传感测试实验,验证了所设计的传感器可用于液体流量测量。实验结果表明,该传感器的流量测量范围为40.575~550.664 L/h。     

Simulation of front motion in a reacting condensed two phase mixture

The computational technique is developed in order to provide the scale capturing for numerical simulation of the thermal processes. The thermal front motion and gas flow dynamics as well as the rate of particle growth during the Carbon Combustion Synthesis of Oxides (CCSO) were predicted using the numerical simulation. In CCSO the exothermic oxidation of carbon nanoparticles generates a self-sustained thermal reaction front that propagates through the solid reactant mixture converting it to the desired complex oxides. The combusted carbon is emitted from the sample as carbon dioxide and its high rate of release increases the product porosity and friability. It was shown that the complicated finger front instability can be developed during the carbon combustion synthesis. This phenomenon results from a vortex gas flow in the reaction zone fed by the carbon dioxide co-flow and oxygen counter-flow filtration.     

电场对协流式微流控装置中乳液液滴生成行为的调控机理

建立了直流电场作用下协流式微流控装置中单乳液液滴乳化生成过程的非稳态理论模型,并开展了数值模拟研究,揭示了电场对液滴乳化生成动力学行为的调控机理,阐明了流场/电场参数对液滴乳化生成特性的影响规律.研究结果表明:沿流体流动方向施加静电场可在电物性参数不同的两相流体界面法线方向上产生指向内相流体的电场力,进而强化了内相流体界面的颈缩和断裂,提升了液滴生成速率和形变程度,减小了液滴生成尺寸;在同一毛细数下,随着电毛细数的增大,乳液乳化流型由每周期仅有单一液滴生成的滴式流型转变为每周期有一个主液滴并伴随有卫星液滴生成的滴式流型;随着毛细数和电毛细数的增大,黏性拖曳力以及电场力作用增强,使内相流体颈缩过程后期更容易形成细长型液线,从而有助于诱发液线上产生Rayleigh-Plateau不稳定现象,继而促进卫星液滴的形成.     

A rapid,spatially dispersive frequency comb spectrograph aimed at gas phase chemical reaction kinetics

This New Views article will highlight some recent advances in high sensitivity gas detection using direct infrared absorption frequency comb laser spectroscopy, with a focus on frequency comb use in chemical reaction kinetics and our own contribution to this field. Our recently implemented detection technique uses a combination of a 12.9?GHz free spectral range virtually imaged phased array and diffraction grating to spatially disperse the mid-infrared frequency comb onto a camera. Individual frequencies or ‘comb teeth’ of a 250?MHz repetition-rate frequency comb are able to be resolved. High molecular sensitivity is achieved by increasing the interaction path length using a Herriott multipass cell. High spectral resolution, broadband spectral coverage, and high molecular sensitivity are all achieved on an adjustable 1–50 µs timescale, making this frequency comb apparatus ideal for measuring chemical reaction kinetics where multiple absorbing species can be monitored simultaneously. This New Views article will also discuss some of the challenges and decisions that chemists might face in implementing this advanced physics technology in their own laboratory.

Spatially dispersed 250 MHz mid-infrared frequency comb laser, with absorption of some frequencies by a dilute sample of methane.     

Infrared thermography as a tool for thermal surface flow visualization

Infrared (IR) thermography is a two-dimensional, non-contact technique of temperature measurement which can be usefully exploited in a vast variety of heat transfer industrial applications as well as research fields. The present work focuses attention on thermal surface flow visualizations of several types of fluid flow studied by means of the IR imaging system and in particular: the flow over a delta wing at angle of attack; the flow generated by a disk rotating in still air; air jets impinging on a flat wall; the flow inside a 180deg turn in astatic channel with, or without, turbulence promoters; the flow inside a 180deg turn in arotating square channel. Each flow visualization is illustrated through thermographic images and/or Nusselt number maps. The emphasis is on the capability of the infrared system to study: laminar-to-turbulent transition and location of primary and secondary vortices over the delta wing at angle of attack; the spiral vortical structure developing at transition over the disk; azimuthal structures arising for certain jet conditions; the influence of the channel aspect ratio (width to height ratio) on the heat transfer coefficient distribution along the 180deg turn, as well as the influence of ribs, in the case ofstatic channel; the influence of rotation for the rotating channel.     

Simultaneous Measurement of Droplet Size and Three-Components of Velocity in Spray

The combination of the optical diagnostics and image processing technique has produced various methods of practical measurement in engineering. The measurement of size and velocity of droplets in fuel spray is one typical example. In general, the spray is a complicated and three-dimensional two-phase flow; therefore multi-dimensional field measurement of the spray field is indispensable. In the present paper, a technique of simultaneous measurement of droplet size and three-components of velocity is proposed and applied to spray issuing from the swirl nozzle as a practical application. The focused image of glare points is captured by a stereoscopic arrangement, and droplet size and the three-components of velocity are evaluated from the doublet image. Fundamental optical arrangements to capture the image of the glare points with a stereo view are established, and the feasibility and the accuracy of the technique are confirmed. © 2004 The Optical Society of Japan     

Frequency modulated infrared imaging for thermal characterization of nanomaterials

The paper presents frequency modulated thermal wave imaging (FMTWI) as a fast and efficient non-contact technique for in-plane thermal characterization of thin plate nanomaterials. A novel excitation signal in the form of an up-chirp is applied and the thermal response is monitored using an infrared (IR) thermography based temperature sensing system. The in-plane thermal diffusivity of any sample can be measured using the multiple phase information extracted from a single run of the experiment. This feature provides a time efficient approach for thermal measurements using infrared thermography techniques. The theoretical background and experimental details of the technique are discussed, with practical measurement of thermal diffusivity of an empty anodic alumina (AAO) template in direction perpendicular to the nanochannel axis, in support.     

光镊技术在气溶胶物理化学表征中的应用

有机气溶胶的热/动力学研究是多学科交叉的前沿研究领域,其核心问题主要是非理想混合包括挥发性、液-液相分离、非平衡传质动力学等,精确测量这些过程相关理化参数是目前研究的瓶颈。光镊系统可以悬浮气溶胶单颗粒,获得高信噪比受激拉曼光谱,在研究气溶胶物理化学性质与其大气效应中具有独到优势。被广泛用于有机及其与无机混合体系气溶胶的吸湿性、挥发性、水传质动力学、液-液相分离过程研究中。本文综述了激光悬浮气溶胶单颗粒技术的研究进展,主要包括光镊技术的原理和技术手段,以及在气溶胶关键物理化学参数测量中的应用。通过光镊系统,一方面可以获得重要理化参数的精确结果,另一方面可以对实际环境中悬浮液滴的状态进行模拟测量,从而为大气科学的研究与污染治理提供重要支撑。     

Melt Crystallization Behavior of Injection-Molded High-Density Polyethylene Based Upon a Solidification Kinetic Analysis

Rheological characterization and an in-situ temperature measurement were carried out to investigate the solidification behavior and crystallization kinetics of injection-molded (IM) high-density polyethylene (HDPE) samples. The temperature profiles obtained via the enthalpy transformation method (ETM) were also used to disclose the effect of cooling rate on the solidification kinetics during the IM process. A four parameter model (FPM) was developed in the present work, based on a three-parameter model (TPM) we proposed previously, with the FPM shown to have an obviously better fitting effect. It was seen that heat transfer at locations χ?≤?0.5 was primarily dominated by a thermal conduction mechanism, which was unlike the situation when χ?>?0.5, with χ being the fractional distance from the mold surface to the center. The results of the present study are suggested to be of significance to further research on the correlation between microstructures and properties of IM products.     

甲醇喷雾湍流反应流的JPDF数值模拟

喷雾湍流燃烧过程中,液滴、湍流和化学反应之间强烈耦合,物理化学机理非常复杂。本文将速度-标量-频率联合概率密度函数JPDF输运方程方法应用于两相喷雾湍流反应流问题,利用火焰面模型解耦流动和化学反应动力学的耦合关系,建立起相应的数值计算模型。采用Monte-Carlo数值计算方法,针对澳大利亚悉尼大学Masri等人以甲醇为燃料所进行的湍流喷雾燃烧值班火焰这一试验进行了数值模拟,通过与Fluent下的计算结果及试验结果的对比分析,验证了本文所建模型的准确性。