Coaxial turbulent jet flames: Scaling relations for measured stoichiometric mixing lengths

Previous studies have quantified the fuel–air mixing processes within jet flames having a central fuel jet surrounded by an oxidizer coflow, where the mixing primarily occurs in the far field. The present work instead quantifies mixing within coaxial jet flames having a central jet of oxygen and a surrounding finite thickness coflow of hydrogen. These flames are relatively short and the primary mixing occurs in the near field. The stoichiometric mixing length (L_S) was measured, which is the distance along the centerline at which the stoichiometric condition occurs. Values of L_S were measured for a wide range of velocity ratios for both reacting and nonreacting cases using Planar Laser Induced Fluorescence (PLIF). Acetone PLIF was utilized for nonreacting cases, while OH PLIF was used for reacting. In nonreacting cases the use of a nondimensional momentum flux ratio collapses the nonreacting coaxial jet values of L_S to a single curve, which confirms previous theory. It also was found that the reacting and nonreacting coaxial jet data collapses approximately to a single curve, if one uses both the nondimensional momentum flux ratio and an effective outer flow gas density which is predicted by the analysis of Tacina and Dahm.     

Impact of swirl and bluff-body on the transfer function of premixed flames

The frequency response of three lean methane/air flames submitted to flowrate perturbations is analyzed for flames featuring the same equivalence ratio and thermal power, but a different stabilization mechanism. The first flame is stabilized by a central bluff body without swirl, the second one by the same bluff body with the addition of swirl and the last one only by swirl without central insert. In the two last cases, the swirl level is roughly the same. These three flames feature different shapes and heat release distributions, but their Flame Transfer Function (FTF) feature about the same phase lag at low frequencies. The gain of the FTF also shows the same behavior for the flame stabilized by the central insert without swirl and the one fully aerodynamically stabilized by swirl. Shedding of vortical structures from the injector nozzle that grow and rollup the flame tip controls the FTF of these flames. The flame stabilized by the swirler-plus-bluff-body system features a peculiar response with a large drop of the FTF gain around a frequency at which large swirl number oscillations are observed. Velocity measurements in cold flow conditions reveal a strong reduction of the size of the vortical structures shed from the injector lip at this forcing condition. The flame stabilized aerodynamically only by swirl and the one stabilized by the bluff body without swirl do not exhibit any FTF gain drop at low frequencies. In the former case, large swirl number oscillations are still identified, but large vortical structures shed from the nozzle also persist at the same forcing frequency in the cold flow response. These different flame responses are found to be intimately related to the dynamics of the internal recirculation region, which response strongly differs depending upon the injector used to stabilize the flame.     

Experimental investigation of flame length of buoyancy-controlled jet flames from inclined rectangular nozzles

In this paper, the buoyancy-controlled jet flames from the inclined rectangular nozzles with different aspect ratios have been investigated. The length-to-width aspect ratios of nozzles are 1, 2.25, 4, 9, and 16 with the same cross-sectional area. The inclined angles of the nozzle to the horizontal direction are 0°, 30°, 60°, and 90°. The value of flame length changes at the same inclined angle for different aspect ratios. A correlation has been developed to calculate the flame length of jet flame for different inclined nozzle and aspect ratios with an arbitrary angle.     

Effect of stoichiometric mixture fraction on soot fraction and emission spectra with application to oxy-combustion

Many proposed oxy-combustion concepts for carbon capture incorporate the recycling of flue gas which is used as a dilution gas to aid in the control of temperature and heat flux. Improvements in efficiency may be realized by significantly reducing the recycle flue gas (RFG), however, in application, care must be taken to avoid excessive radiant heat flux and gas temperature. One of the features oxy-combustion, unlike air-fired combustion, is that the oxygen and dilution gases are initially separated. RFG can, for example, be strategically blended with either the fuel stream, or oxidizer stream, or both, which affects the stoichiometric mixture fraction, Z_st. In this work, the effects of the amount of dilution, or RFG, and Z_st on soot fraction are experimentally investigated in a laminar coflow flame. Carbon dioxide is employed as the dilution gas to simulate the recycling of dry flue gas. Soot fraction and temperature are quantitatively determined by a flame image processing technique. In addition, the visible and near-IR emission spectra are given. When dilution, or RFG, is reduced, while holding Z_st constant, soot formation and thermal radiation increase due to higher temperature. However, high temperature flames with reduced or zero soot are achieved by increasing Z_st via the combination of fuel dilution and oxygen enrichment. This study highlights the inherent flexibility of oxy-fuel combustion, which offers the opportunity to control flame temperature and gas volume while independently controlling soot formation and radiant heat transfer.     

The burning rate’s effect on the flame length of weak fire whirls

This paper discusses why the visibly-determined flame length of a weak fire whirl increases as compared with the corresponding pool fire without spin. Here, a fire whirl is called weak when the pure aerodynamic effect of flow circulation has a negligible influence on the flame length. Split cylinders were used to apply a flow circulation to a 3-cm-diameter methane burner flame and a 3-cm-diameter ethanol pool fire. After applying the flow circulation, the flame length of the ethanol pool fire increased about three times, while little change was observed in the flame length of the methane burner flame. The difference is explained by the fact that the burning rate of the methane burner flame was fixed constant, whereas that of the ethanol pool fire increased due to the increased heat input to the fuel surface caused by a change in flame shape pushed toward the fuel surface. The experimental observations thus demonstrate that the burning-rate effect can significantly increase the flame length even under a weak circulation condition. Computational fluid dynamics (CFD) simulations were conducted to understand the detailed flow structure of a fire whirl. An analytical model was then developed based on the experimental observations and CFD calculations; the predicted relationship between the flame height and the burning rate agreed with experimental data.     

μs级充电时水介质同轴电容击穿实验研究

 研制了水介质同轴电容的实验装置，在此装置上进行了大量的实验。测得μs级充电时，水介质同轴电容分别在正脉冲和负脉冲充电时的击穿电压，负击穿电压的理论值与测量值相差较大，正击穿电压的理论值大于测量值的18%左右，对此实验结果进行了分析。此外，当去离子水的电阻率从13MΩ·cm降到5 MΩ·cm时，水介质同轴电容器的击穿电压基本不变。     

半径渐变同轴波导模式耦合系数研究

 为了用耦合波理论研究一些同轴结构的高频特性，推导了半径渐变同轴波导内模式耦合系数的计算公式，发现半径渐变圆波导模式耦合系数只是同轴波导模式耦合系数的特例。对同轴波导中的模式及其本征方程根的特点进行了讨论，通过建立同轴波导模式和矩形波导模式之间的对应关系，给出了同轴波导模式本征方程根的快速计算方法。对所给模式耦合系数公式进行了验证，耦合波理论计算结果与有限元方法计算结果吻合良好。     

利用湍流廓线获得斜程大气相干长度的实验研究

 用两种方法获得有限高度斜程大气相干长度。一种方法用安置在测量铁塔上的温度脉动仪以及多普勒声雷达测量了大气边界层下部折射率结构常数廓线，用以计算有限高度球面波斜程大气相干长度；另一种方法用系留气球搭载一个点光源，在地面用差分光学系统接收球面波到达角起伏法测量了实时斜程大气相干长度。对实验结果进行了比较和分析，二者都反映了实际大气斜程相干长度的时间变化特点。两种方法结果的相关性比较好，因此两种方法都可以在大气光学传输研究中应用。最后在实验的基础上对湍流廓线计算球面波斜程大气相干长度作了讨论。     

大耦合孔同轴输出腔的3维解析分析

 在高功率微波器件中，通常采用大耦合孔的同轴微波输出腔，该腔为3维结构。采用场等效原理将腔体、耦合孔及其输出负载(行波边界)进行分区,每个区域中的场由界面上的磁流密度决定。采用格林函数积分法可得每个区域中的3维场,再由各个区域的场匹配方程求得腔体的谐振频率、特性阻抗、有载品质因数、模式分布等参数。为腔体的计算和设计提供一种计算模型。     

On the beam transport of a helical coaxial wiggler for a free-electron laser

Dynamics of an electron beam in a helical coaxial wiggler is studied. It is observed that the electrons perform betatron oscillation in a potential well that reflects the effective radiation pressure of the wiggler field and a centrifugal force due to an average azimuthal drift.     

LES of the Sydney swirl flame series: A study of vortex breakdown in isothermal and reacting flows

This work examines the flow and mixing in selected non-premixed cases of the Sydney Swirl Flame series by Large Eddy Simulation. A mixture fraction approach with a steady flamelet model, based on a detailed chemical mechanism, is applied to determine the chemical state in the flame. The isothermal case N29S054 is simulated to provide insights into the flow field, the resolution requirements for the simulation, and to allow for various measures of validation and verification. For the reactive case, the high-speed hydrogen/methane flame SMH1 is chosen for its similarity to the non-swirling Sydney flames and its good stability. In experiments carried out previously at Sydney University, vortex breakdown has been observed and in the isothermal case, this is clearly predicted by the LES. However, no vortex breakdown is observed in the simulations of flame SMH1, which necessitates further studies on this and similar flames to investigate this phenomenon. Studies of the low-velocity flames SM1 and SM2 show that reactive vortex breakdown can be predicted successfully. This difficulty in the prediction of vortex breakdown is another indication that the Sydney Swirl Flame series, especially at high velocities of the central jet, is an interesting and challenging test-case for the development of combustion LES.     

高功率微波同轴阻抗变换器优化设计及功率计算

 在高功率微波应用中,同轴线的阻抗匹配十分重要。通过同轴线内导体的渐变,可以实现阻抗变换与阻抗匹配。内导体渐变可以采用多项式、余弦、指数等函数形式。用时域有限差分方法计算同轴线的反射系数,以同轴线内导体渐变段的长度和反射系数达到最小为目标,采用遗传算法优化渐变段的结构参数,得到了反射系数为0.015、渐变长度为148 mm的同轴线阻抗变换结构。在一套具有16节点的Beowulf型并行计算机系统上采用主从式并行计算技术完成了计算,缩短了遗传算法搜索时间。最后计算和分析了该同轴线阻抗变换结构的带宽和微波功率容量,该结构峰值功率达8.734 MW。     

同轴膜片加载圆波导相对论行波管注-波互作用线性理论研究

 从改善圆盘加载波导作为相对论行波管慢波结构的特性入手，加入同轴内导体，着重研究其小信号理论。利用场匹配法导出了“热”色散方程并计算求解，具体分析了系统的电子注参量及慢波结构几何尺寸对小信号增益及慢波相速的影响,为同轴圆盘加载相对论行波管的设计提供了理论基础。     

同轴慢波结构相对论高功率微波产生器理论分析

 推导了同轴波导的空间电荷限制流,其值大于圆波导的空间电荷限制流。因此在阴极电势和束流相等的情况下,同轴波导中的束流具有更高的动能,同轴器件有可能获得更高的微波转换效率。理论推导出同轴慢波结构中考虑束流空间电荷影响的色散方程,利用Matlab进行了编程求解。不考虑束流空间电荷影响时,编程计算结果与Superfish模拟结果一致。由考虑束流空间电荷影响的色散方程数值计算结果,可知文献中提出的同轴慢波结构相对论高功率微波产生器工作在准TEM模的π模,频率为7.67 GHz,峰值时间增长率较高,电子束损失的能量与其初始能量之比为34%。这些结果均与文献中的数值模拟结果一致。同时理论分析说明该种器件无论在能量转换效率,还是在产生微波脉冲的上升时间上均具有优势。     

大气压氩气冷等离子体射流处理对芥菜种子发芽率及根长的影响

研究大气压条件下空气环境中氩气放电冷等离子体射流处理对芥菜种子发芽率及幼苗根长的影响。比较了不同的放电电压和不同的处理时间下芥菜种子的发芽率及根长。结果表明,与对照组比较,在放电电压1.6kV、处理时间120s 和30s 的条件下,等离子体射流处理对种子的生长起到了促进作用;在放电电压2.2kV、处理时间120s 及放电电压2.8kV 的条件下,等离子体射流处理对种子的生长发育起到了抑制作用。     

纳秒脉冲电压同轴电场下有机玻璃和尼龙的闪络特性

 利用基于SOS的SPG200高压脉冲功率装置研究了有机玻璃(PMMA)和尼龙1010在变压器油中的闪络特性。研究结果表明：闪络电压随内电极直径增加近似指数增加，随闪络距离延长近似线性增加，相应的击穿时延也近似线性增加。受实验条件的影响，有机玻璃和尼龙1010的闪络性能略有差异。同轴径向电场降落集中在内电极表面附近一定径向距离内，同轴电场下的闪络电压主要由内电极表面电场与径向电场分布的均匀程度决定。增加内电极直径和延长闪络距离都可以提高闪络电压，而增加内电极直径的效果更显著。     

气液同轴式喷嘴自激振荡的试验现象和声学模型及对火箭发动机不稳定燃烧的影响

首先简述了气液同轴式喷嘴的试验现象。在一定的工作条件下,同轴式喷嘴会产生啸叫,随气、液喷注压降的连续变化,啸叫会周期地产生和消失。通过对试验现象的分析,又提出了喷嘴啸叫的声学模型:喷嘴的自激振荡是环形通道中气流与液体喷嘴气涡共振的结果。声学模型能很好地解释冷态试验现象。最后,将喷嘴自激振荡的现象与模型和液氢液氧火箭发动机的不稳定燃烧的热试结果进行了比较。声学模型能较好地解释热态试车的主要结论。以同轴式喷嘴为喷注器的液氢液氧火箭发动机的高频不稳定燃烧现象有可能是喷嘴自激振荡引起燃烧室中压力振荡的结果。     

用传输线和时域有限差分法计算电缆X射线响应

 介绍了时域传输线和时域有限差分两种计算电缆X射线瞬态响应的方法,并利用电缆芯线总电荷以及介质层的电荷分布,结合实验结果,分别给出了时域传输线模型中的电流源和时域有限差分模型中电流密度的计算方法。作为例子,在60 keV单能X射线垂直于电缆轴向辐照的条件下,采用两种方法计算了电缆两端接匹配负载时负载上的电流响应;采用传输线方法计算了不同能量X射线辐照电缆时的芯线负载电流响应。计算结果表明,两种方法计算的电缆X射线瞬态响应是一致的;电缆介质层中存在静电场;X射线能量不同时,电缆芯线负载电流存在最大值。计算中,电缆与X射线的作用采用蒙特卡罗方法计算。     

Modified nodal point method for the measurement of the large focal length of lenses

Some modifications have been suggested to the nodal slide and microscope method to suit the focal length measurement of large focal length lenses. In this procedure, the experimental set-up has to be accommodated on the two usual optical benches in line, and the position of the nodal point of the lens under test is easily located by introducing an extra lens of comparatively shorter focal length between the test lens and microscope. The results of the measurement of ‘f’ for a few lenses, and the experimental parameters, are presented.     

On the length of stabilograms: A study performed with detrended fluctuation analysis

The effects associated to the length of stabilograms, a measure of the time dependence of the center of pressure of an individual standing up, are analyzed. The fractal characteristics of 27 signals with a length of 2¹⁴$2^{14}$ points, each one corresponding to a different individual, are studied by using the Detrended Fluctuation Analysis technique. The properties of the complete signals are compared to those of various subsignals extracted from them. No differences have been found between the characteristic exponents found for x$x$ and y$y$ signals. The relation between the exponents of the position and velocity signals is accomplished by the 2¹⁴$2^{14}$ point signals, while subsignals with up to 2¹²$2^{12}$ points do not verify it. Using artificial signals with 2¹⁴$2^{14}$ points, generated for α$\alpha$ values given, it has been demonstrated that the exponents obtained from these signals take values larger than expected for α<0.3$\alpha <0.3$, while the exponents of the accumulated series are smaller than expected for 0.7<α$0.7<\alpha$. For CoP trajectories this indicates that DFA-1 provides feasible exponents for the short τ$\tau$-end region of the velocity signal and the large τ$\tau$-end region of the accumulated (position) one. It has been found that the characteristic exponents vary along the series. A slightly larger persistence is found in the last part of the signal for large frequencies in the x$x$ direction.