Pressure fluctuations within a turbulent gas flow and their interaction with a shock wave

The interaction of a shock wave with a turbulent air flow is investigated experimentally. The turbulence was created with the aid of a grid. On its reflection from a perforated disc the wave propagated through a turbulent flow. The Mach number of the incident shock was equal to 1.9–4, the Mach number of the reflected wave was equal to 1.6–2.5. We found the autocorrelation functions of pressure fluctuations and their phase diagrams. The turbulent length scale of pressure fluctuations behind the incident shock was determined. The appropriate quantity behind the reflected wave is less of an order as compared with the previous case. It is established that the pressure behind the reflected wave in the turbulent flow is 7–8% higher as compared with the pressure in the laminar flow, if other conditions are the same.     

Radiation Characteristics of Air in the Ultraviolet and Vacuum Ultraviolet Regions of the Spectrum behind the Front of Strong Shock Waves

Experiments on the measurement of air emission intensity behind the front of incident shock wave were carried out in a shock tube at an initial pressure of 0.25 Torr and shock wave velocities of 6.3–8.4 km/s. The emission intensity was measured in absolute units both in the form of an integral spectral distribution in a wavelength range of 120?400 nm (panoramic spectra) and as the time evolution of emission at the individual atomic lines of nitrogen and oxygen atoms. The results of the measurements demonstrated that the emission in air behind a shock wave in the vacuum ultraviolet region of 120–200 nm had a much higher radiation flux level than the emission in a range of 200–900 nm.     

Application of an aerosol shock tube to the measurement of diesel ignition delay times

Shock tube ignition delay times were measured for DF-2 diesel/21% O₂/argon mixtures at pressures from 2.3 to 8.0 atm, equivalence ratios from 0.3 to 1.35, and temperatures from 900 to 1300 K using a new experimental flow facility, an aerosol shock tube. The aerosol shock tube combines conventional shock tube methodology with aerosol loading of fuel-oxidizer mixtures. Significant efforts have been made to ensure that the aerosol mixtures were spatially uniform, that the incident shock wave was well-behaved, and that the post-shock conditions and mixture fractions were accurately determined. The nebulizer-generated, narrow, micron-sized aerosol size distribution permitted rapid evaporation of the fuel mixture and enabled separation of the diesel fuel evaporation and diffusion processes that occurred behind the incident shock wave from the chemical ignition processes that occurred behind the higher temperature and pressure reflected shock wave. This rapid evaporation technique enables the study of a wide range of low-vapor-pressure practical fuels and fuel surrogates without the complication of fuel cracking that can occur with heated experimental facilities. These diesel ignition delay measurements extend the temperature and pressure range of earlier flow reactor studies, provide evidence for NTC behavior in diesel fuel ignition delay times at lower temperatures, and provide an accurate data base for the development and comparison of kinetic mechanisms for diesel fuel and surrogate mixtures. Representative comparisons with several single-component diesel surrogate models are also given.     

A shock tube study of line broadening in a temperature range of 6100 to 8300 K

Line widths of the Ca(II) 3968 Å and Na(I) 5890 Å resonance lines broadened by electric microfields in a plasma were measured. A pressure-driven shock tube was used as the light source. Radiation from the equilibrium region behind the reflected shock wave was studied using a rapid scanning Fabry-Perot interferometer. Electron and argon atom densities of about 8 x 10¹⁶ and 1 x 10¹⁹ per cm³, respectively, were achieved at the relatively low temperature of around 7500 K by vaporizing a cesium powder that had been added to the shock tube as a solid aerosol in argon gas. The measured line widths were predominantly Stark broadened by the electrons and ions in the plasma, although Doppler, van der Waals, instrument function and optical depth effects had to be taken into account. The electron broadened line width of Na(I) 5890 Å was lower by about 16% than the predicted value from a semiclassical calculation, in agreement with a recently reported measurement, while for Ca(II) 3968 Å the results agree with previous reliable experiments and are lower than two calculated values by 20 and 36% respectively. The van der Waals broadening by argon was also determined and showed qualitative agreement with other results.     

Stark broadening of singly ionized strontium and calcium lines

Profiles of five calcium II and six strontium II lines have been measured in an argon plasma behind the reflected shock wave. The plasma was produced in an electromagnetically drivenT tube and calcium and strontium were present as an impurities. Electron densities were in the range 1.91–3.40·10¹⁷ cm^?3 and the electron temperatures between 10300 and 14200 °K. The half halfwidths of the measured profiles of some Ca II and Sr II multiplets show large discrepancies with theoretical predictions.     

Ignition delay times of methyl oleate and methyl linoleate behind reflected shock waves

Ignition delay times for methyl oleate (C₁₉H₃₆O₂, CAS: 112-62-9) and methyl linoleate (C₁₉H₃₄O₂, CAS: 112-63-0) were measured for the first time behind reflected shock waves, using an aerosol shock tube. The aerosol shock tube enabled study of these very-low-vapor-pressure fuels by introducing a spatially-uniform fuel aerosol/4% oxygen/argon mixture into the shock tube and employing the incident shock wave to produce complete fuel evaporation, diffusion, and mixing. Reflected shock conditions covered temperatures from 1100 to 1400 K, pressures of 3.5 and 7.0 atm, and equivalence ratios from 0.6 to 2.4. Ignition delay times for both fuels were found to be similar over a wide range of conditions. The most notable trend in the observed ignition delay times was that the pressure and equivalence ratio scaling were a strong function of temperature, and exhibited cross-over temperatures at which there was no sensitivity to either parameter. Data were also compared to the biodiesel kinetic mechanism of Westbrook et al. (2011) [10], which underpredicts ignition delay times by about 50%. Differences between experimental and computed ignition delay times were strongly related to existing errors and uncertainties in the thermochemistry of the large methyl ester species, and when these were corrected, the kinetic simulations agreed significantly better with the experimental measurements.     

Measurement of the Stark broadening parameters of some singly ionized argon lines

Half-widths of fifteen Stark broadened argon II lines have been measured in argon plasma behind the reflected shock wave produced in an electromagnetically driven “T” tube. The plasma electron density was determined by the laser interferometry at three different wavelengths, while the plasma temperature was measured from relative intensities of A II lines. Temperatures were in the range 8,500–16,500 °K; electron densities varied from 1.82 to 3.94 · 10¹⁷ cm^?3. The measured A II linewidths are compared with theoretical and other experimental results. It is shown that a) the broadening of A II lines is in good agreement with the theory, b) line broadening increases linearly with electron density, and c) the Stark broadened lines follow the dispersion profile to the distance of at least three halfwidths from the line center.     

甲基环己烷燃烧反应特性的光谱研究

利用激波管实验装置由反射激波点火,在点火温度1 164～1 566 K,点火压力1.03～1.99 atm,燃料浓度为1.0%,当量比为1.0的条件下,用光谱单色仪、光电倍增管、压力传感器和示波器等组成测试系统,测量了甲基环己烷燃烧过程中主要中间产物OH,CH和C2自由基特征光辐射随时间的连续变化,并测得了甲基环己烷/氧气/氩气的点火延迟时间。通过对测量结果的分析,初步认识了甲基环己烷燃烧反应中几个主要中间产物的光辐射特性及其反映出的甲基环己烷燃烧反应特性。实验所测点火延迟时间与已报道的实验结果和燃烧反应机理预测结果符合较好。本文实验结果为构建和验证甲基环己烷燃烧反应机理提供了实验依据。     

Cesium halide photodissociation cross sections

The cross sections for the photodissociation of CsCl, CsBr and CsI have been measured using a cesium halide seeded shock tube. These measurements were made by using the ratio of two absorption signals: one in the incident and one in the reflected shock region. The cesium halide was the absorption species in the incident shock region and a halogen negative ion species absorbed in the reflected shock region. Previously measured and calculated halogen negative ion photodetachment cross sections were used in calculating the cesium halide photodissociation cross section. Measurements were made between 4000 and 2000 Å at 1900°K.     

Effect of pressure on soot formation in the pyrolysis of <Emphasis Type="Italic">n</Emphasis>-hexane and the oxidation of fuel-rich mixtures of <Emphasis Type="Italic">n</Emphasis>-heptane behind reflected shock waves

The results of detailed kinetic simulations of the formation of soot particles in the pyrolysis of n-hexane–argon mixtures and in the oxidation of fuel-rich (φ = 5) n-heptane–oxygen–argon mixtures behind reflected shock waves at pressures of 20–100 bar and a constant concentration of carbon atoms or a constant fraction of argon in the initial mixture within the framework of a modified reaction mechanism are reported. The choice of n-hexane and n-heptane for examining the effect of pressure on the process of soot formation was motivated by the availability for these hydrocarbons of experimental measurements in reflected shock waves at high pressures (up to ~100 bar). The temperature dependences of the yield of soot particles formed in the pyrolysis of n-hexane are found to be very weakly dependent on pressure and slightly shifting to lower temperatures with increasing pressure. In general, pressure produces a very weak effect on the soot formation in the pyrolysis of n-hexane. The effect of pressure and concentration of carbon atoms in the initial mixture on the process of soot formation during the oxidation of fuel-rich n-heptane mixtures behind reflected shock waves is studied. The results of our kinetic simulations show that, for both the pyrolysis of n-hexane and the oxidation of fuel-rich n-heptane–oxygen mixtures, the influence of pressure on the process of soot formation is negligible. By contrast, the concentration of carbon atoms in the initial reaction mixture produces a much more pronounced effect.     

一种准二维柱面爆炸波加载装置的设计与验证

基于竖直爆轰管和径向Hele-Shaw Cell,设计并搭建了一套准二维柱面爆炸波加载装置,可以实现对Hele-Shaw Cell内部材料界面的径向冲击加载.竖直爆轰管内部的预混气体在底部点燃后,形成向上传播的冲击波,冲击波冲破爆轰管开口与Hele-Shaw Cell底板开孔之间的隔膜后,被Hele-Shaw Cell...     

Experimental observations of flame acceleration and transition to detonation following shock-flame interaction

Observations are presented from experiments where laminar flame bubbles were perturbed successively by incident and reflected shock waves. Significant flame acceleration was observed in many instances, with the flame closely coupled to the reflected shock wave. The coupled waves are interpreted using a generalized Hugoniot analysis. As the incident shock velocity increased, detonation emerged near the highly convolved reaction zone. Prior to detonation the external visual attributes of the combustion fronts appear identical to turbulent combustion. However, they cannot be due to classical isotropic turbulence. The overall conclusion is that the observed enhancement of combustion is driven by chemi-acoustic interactions and related gas-dynamic effects. An analysis of the prevailing thermodynamic states suggests that thermal auto-ignition chemistry could also play a significant role prior to the onset of detonation.     

Ignition of cyclopropane in shock waves

The values of the ignition delay time of cyclopropane–oxygen–argon (cyclo-C₃H₆–O₂–Ar) mixtures of different compositions (φ = 0.333, 1, and 3) behind reflected shock waves at temperatures of 1200–1640 K and a pressure of (0.55 ± 0.05) MPa are measured. A kinetic mechanism of cyclopropane ignition using the known rate constants for the most important elementary reactions is developed. The mechanism closely describes both our own and published experimental data on the delay time of ignition of cyclopropane in shock waves over wide ranges of temperature (1200–2100 K), pressure (0.1–0.55 MPa), cyclopropane concentrations (0.05–11 vol %), and oxygen concentrations (0.25–21 vol %). It is shown that, with increasing fraction of diluent gas in the mixture, the dependence of the ignition delay time on the fuel-to-oxidizer equivalence ratio changes.     

基于波传播算法的火焰不稳定性

基于波传播算法构造了多组分反应流的数值格式,利用CH4空气基元反应动力学模型,并采用分离算法,对CH4空气混合物中,入射激波与火焰的相互作用,以及反射激波与火焰的二次作用过程进行了数值模拟.根据计算结果,讨论了激波诱导火焰失稳的发展过程及其特点.结果表明,Helmholtz不稳定、RichtmyerMeshkov不稳定以及反应放热速率对火焰失稳过程有重要影响.计算结果与实验结果进行了比较,对数值方法的有效性进行了验证.     

Analysis of Plasmadynamic Nanosecond-Range Processes in the Formation of Shock Waves from Pulsed Discharges

We have studied the dynamics of the plasma glow of pulsed discharges (sliding surface discharge and combined volume discharge with plasma electrodes) in the nanosecond range (100–12 000 ns) in stationary air and in the flow behind the front of a plane shock wave with Mach numbers 1.7–5.0 in the shock tube channel. The temporal characteristics of the flow, the radiation spectra, and the discharge currents in air are compared in the pressure range 5–150 Torr, a pulsed voltage of 20–30 kV, and a current of about 1 kA. It is shown that the time of current under various conditions does not exceed 400 ns, and the duration of the glow can reach a few microseconds. It is shown that as a result of energy supply near the planar shock wave front, the decay of discontinuities occurs with the formation of shock waves and contact surfaces. The positions of the plasma glow regions are compared with the positions of discontinuity surfaces of numerically calculated gasdynamic parameters in the flow.     

激光引致激波演化数值研究

在高重复频率激光推进的研究中,激波的合并是发生在激波演化后期的,同时由于脉冲间隔短,脉冲宽度对流场演化的影响也需要详细研究。考虑了激光辐照过程对流场演化的影响,通过数值计算对激波演化特性进行了研究。结果表明,初期波阵面的椭球形状逐渐转化为一个球形,球心与击穿点的距离随时间逐渐减小并最终趋于稳定。基于激波合并的应用,当激波马赫数在1~2之间时,给出了激波波阵面半径随时间的变化规律,以及激波高压区长度和波峰压强随激波波阵面半径变化规律的经验公式。     

Autoignition of gasoline surrogate mixtures at intermediate temperatures and high pressures: Experimental and numerical approaches

Ignition-delay times were measured in shock-heated gases for a surrogate gasoline fuel comprised of ethanol/iso-octane/n-heptane/toluene at a composition of 40%/37.8%/10.2%/12% by liquid volume with a calculated octane number of 98.8. The experiments were carried out in stoichiometric mixtures in air behind reflected shock waves in a heated high-pressure shock tube. Initial reflected shock conditions were as follows: Temperatures of 690-1200 K, and pressures of 10, 30 and 50 bar, respectively. Ignition delay times were determined from CH^∗ chemiluminescence at 431.5 nm measured at a sidewall location. The experimental results are compared to simulated ignition delay times based on detailed chemical kinetic mechanisms. The main mechanism is based on the primary reference fuels (PRF) model, and sub-mechanisms were incorporated to account for the effect of ethanol and/or toluene. The simulations are also compared to experimental ignition-delay data from the literature for ethanol/iso-octane/n-heptane (20%/62%/18% by liquid volume) and iso-octane/n-heptane/toluene (69%/17%/14% by liquid volume) surrogate fuels. The relative behavior of the ignition delay times of the different surrogates was well predicted, but the simulations overestimate the ignition delay, mostly at low temperatures.     

Flow visualization and density measurement of compressible flows by a laser speckle method

Laser speckle method is a well known technique that is useful for both visualization and quantitative measurement. This technique was applied to the density measurement of Mach reflection of shock waves in the present experiment. The Object of the measurement is the density field of simple Mach reflection in relatively low shock Mach number. The non-uniform flow field is divided into three regions by incident, Mach and reflected shock waves. A shock tube was employed in the present experiment. Wedges of 20 degrees and 45 degrees were placed in the test section. YAG laser was employed as a light source. Speckle photograph was taken by a digital still camera. Simple subtraction between the reference and flow images shows a shock pattern and a degree of the correlation of speckle pattern in the flow field. Thus, we can obtain a visualized flow image showing a configuration of Mach reflection from speckle photograph. Speckle photographs which was obtained in the experiments were processed with cross-correlation method. A reconstructed density gradient vector map of Mach reflection was obtained. Comparing the experimental result with numerical one, the measured density gradient shows a good agreement with theoretical prediction.     

An experimental determination of the cross section of the swings band system of C3

The spectral absorption cross section of the Swings band of C₃ was determined from measurements behind incident shock waves that heated a gas mixture of argon and acetylene. These measurements spanned the spectral region between 300 and 540 nm, and were obtained at temperatures between 3200 and 4000°K. An electronic oscillator strength of 0.033 was deduced from the measurements.     

Attenuation of shock parameters in air and water

This paper describes the results of shock pressure measurements in the range of 1–25 MPa in water and in the range of 60–500 kPa in air. Pressure pulses were generated by exploding wire technique and measured with a quartz piezo-electric transducer. The attenuation with distance of shock overpressure, impulse and energy in shock front has been studied. Experimental data on shock attenuation in air is scarce and the results presented here confirm the attenuation behaviour derived from theoretical considerations.