针板型大气压氦气冷等离子体射流的二维模拟

大气压冷等离子体射流的传播机理一直是人们研究的一个热点. 本文采用自洽的二维等离子体流体模型, 研究了大气压氦气冷等离子体射流在自身环境气体中以及在介质管中的传播问题. 得到了电子密度、电离速率、空间电场以及电子温度等参量的时空分布规律, 分析了介质管大小以及介质管介电常数对射流放电性质的影响, 得到了一种提高电子密度和射流尺寸的新方法. 关键词： 大气压冷等离子体射流 等离子体子弹 数值模拟 流体模型     

水下超声速气体射流气液两相复杂流动研究

本文实验研究了水下超声速气体射流气液两相复杂流动。利用高速摄影仪和电子相机分别实时记录了过膨胀超声速工况水下气体射流的喷射状态和整体形貌,显示了不同工况水下高速气体垂直射流的演化过程和动态不稳定性形貌。研究结果表明:在射流的初始段存在与射流内部复杂波系相关的激波反馈特性,激波反馈特性发生之前存在能量积聚的高频低幅的胀鼓过程,二者均随机发生;在射流的主体段,在气水掺混和卷吸大规模能量交换作用下,射流呈现随机的偏摆效应,并且偏摆受环境流场影响明显。     

Mixing by turbulent buoyant jets in slender containers

A turbulent buoyant jet injected vertically into a slender cylinder containing a stratified fluid is investigated experimentally. The working fluid is water, and salt is used to change its density to obtain either a positively or negatively buoyant jet. The interest is the vertical density distribution in container and its dependence on time and other parameters. For each case (lighter or heavier jet) the experimental data could be collapsed into a ‘universal’ time dependent behavior, when properly non-dimensionalized. A theoretical model is advanced to explain the results. Possible applications include refilling of crude oil into U.S. strategic petroleum reserves caverns.     

An experimental study of circular sand–water wall jets

Laboratory experiments were carried out to study the effects of sand particles on circular sand–water wall jets. Mean and turbulence characteristics of sand particles in the sand–water wall jets were measured for different sand concentrations c_o ranging from 0.5% to 2.5%. Effects of sand particle size on the centerline sand velocity of the jets were evaluated for sand size ranging from 0.21 mm to 0.54 mm. Interesting results with the range of measurements are presented in this paper. It was found that the centerline sand velocity of the wall jets with larger particle size were 15% higher than the jets with smaller particle size. Concentration profiles in the vertical direction showed a peak value at x/d = 5 (where x is the longitudinal distance from the nozzle and d is the nozzle diameter) and the sand concentration decreased linearly for x/d > 5. Experimental results showed that the turbulence level enhanced from the nozzle to x/d = 10. For sand–water wall jets with a higher concentration (c_o = 1.5–2.5%), the turbulence intensity became smaller than the corresponding single-phase wall jets by 34% due to turbulent modulation. A modified logarithmic formulation was introduced to model the longitudinal turbulent intensity at the centerline and along the axis of the jet.     

Wavepacket models for subsonic twin jets using 3D parabolized stability equations

An extension of the classical parabolized stability equations to flows strongly dependent on the two cross-stream spatial directions and weakly dependent on the streamwise one is applied to model the large-scale structures present in twin-jet configurations. The existence of these unsteady flow structures, usually referred to as wavepackets, has been demonstrated in the literature for both subsonic and supersonic round jets, along with their relation to the generation of highly directional noise emitted in the aft direction. The present study considers twin-jet configurations with different separations at high Reynolds number and subsonic conditions. The existing instability modes for the twin-jet mean flow, their dependence on the separation of the two jets, and the interaction between the wavepackets originating from the two jets is investigated here. Arising from the axisymmetric mode for single round jets, two dominant modes are found for twin jets: a varicose one, relatively insensitive to jets' proximity, but likely to be efficient in radiating noise; a sinuous one, whose amplification is strongly dependent on the jets' distance, and which can be expected to produce weaker acoustic signatures.     

Transient and steady-state behavior of auto-igniting propane and dimethyl ether fuel jets in high-temperature vitiated coflows

In the current study, the auto-ignition dynamics of cold fuel jets issuing into a high-temperature, vitiated environments is investigated. Due to the short time scale of these events, high-speed measurements are used to resolve the coupled spatio-temporal behavior. The present study uses high-speed (20-kHz) OH* chemiluminescence imaging to identify the location and timing of the formation of the initial ignition kernels, providing visualization of the ignition dynamics and a detailed statistical evaluation of ignition heights and ignition delay times across a broad parameter space which includes variations in fuel type, dilution levels, coflow temperature, and coflow oxidizer content. The auto-ignition location and ignition delay times show a strong sensitivity to coflow temperature with increased sensitivities at lower coflow temperatures. Comparisons between kernel formation location for the transient jet and the fluctuating flame base of the subsequent, steady-state flame is presented, highlighting the role of flame propagation on flame stabilization. Results indicate that at lower temperatures the flame stabilization mechanism is dominated by auto-ignition, but at higher coflow temperatures, flame propagation plays a key role. The effects of variations in the hot, coflow oxidizer content on ignition properties were found to be noticeable, but still significantly less than variations in the temperature.     

The impact of dilatation,scrambling, and pressure transport in turbulent premixed flames

Premixed turbulent flames feature strong interactions between chemical reactions and turbulence that affect scalar and turbulence statistics. The focus of the present work is on clarifying the impact of pressure dilatation/flamelet scrambling effects with a comprehensive second-moment closure used for evaluation purposes. Model extensions that take into account flamelet orientation and molecular diffusion are derived. Isothermal pressure transport is included with an additional variable density contribution derived for the flamelet regime of combustion. Full closure is assessed by comparisons with Direct Numerical Simulations (DNSs) of statistically ‘steady’ fully developed premixed turbulent planar flames at different expansion ratios. Subsequently, the prediction of lean premixed turbulent methane–air flames featuring fractal grid generated turbulence in an opposed jet geometry is considered. The overall agreement shows that ‘dilatation’ effects contribute to counter-gradient transport and can also increase the turbulent kinetic energy significantly. Levels of anisotropy are broadly consistent with the DNS data and key aspects of opposed jet flames are well predicted. However, it is also shown that complications arise due to interactions between the imposed pressure gradient and combustion and that redistribution is affected along with the scalar flux at the leading edge. The latter is strongly affected by the reaction rate closure and, potentially, by pressure transport. Overall, the derived models offer significant improvements and can readily be applied to the modelling of premixed turbulent flames at practical rates of heat release.