Particle image velocimetry investigation on mixing enhancement of non-circular sharp edge nozzles

An experimental study using Particle Image Velocimetry (PIV) on free jets issuing from different orifice plate (OP) nozzles is reported. Mean velocity, turbulence intensity and higher order profiles relevant for large and small scale mixing are considered in the near field and interaction zone (0 < X/D < 20). This is done to determine mixing enhancement due to rectangular, squared, elliptic and triangular nozzles in comparison to circular nozzle results in two orthogonal planes. The effect of Reynolds number on the differences among the nozzle shapes is also considered by performing measurements just after laminar–turbulent transition (Re = 8000) and in the fully turbulent regime (Re = 35,000). The results at low Reynolds number show two classes of jets, i.e. at one side, those closer to axial-symmetric conditions, as circular, square and triangular jets, whereas on the other side those with elongated nozzles as rectangular and elliptic. The reason for the different behavior of the latter is connected to the phenomenon of axis-switching which allows a rearrangement of turbulence over the different velocity components and directions. However, for the highest Reynolds number investigated, all nozzles show similar behavior especially in the jet far field (X/D > 10), thus suggesting a significant Reynolds number dependence of the results.     

Oscillations of the fluid flow and the free surface in a cavity with a submerged bifurcated nozzle

The free surface dynamics and sub-surface flow behavior in a thin (height and width much larger than thickness), liquid filled, rectangular cavity with a submerged bifurcated nozzle were investigated using free surface visualization and particle image velocimetry (PIV). Three regimes in the free surface behavior were identified, depending on nozzle depth and inlet velocity. For small nozzle depths, an irregular free surface is observed without clear periodicities. For intermediate nozzle depths and sufficiently high inlet velocities, natural mode oscillations consistent with gravity waves are present, while at large nozzle depths long term self-sustained asymmetric oscillations occur.For the latter case, time-resolved PIV measurements of the flow below the free surface indicated a strong oscillation of the direction with which each of the two jets issue from the nozzle. The frequency of the jet oscillation is identical to the free surface oscillation frequency. The two jets oscillate in anti-phase, causing the asymmetric free surface oscillation. The jets interact through a cross-flow in the gaps between the inlet channel and the front and back walls of the cavity.     

Jet impingement onto a cylindrical cavity: consideration of annular nozzle cone angles,and cavity diameter

In the present study, gas jet emerging from an annular nozzle and impinging onto a cylindrical cavity is considered. The geometric configuration of the nozzle is varied in the simulations. Air is used as impinging gas while stainless steel is considered as workpiece material. Reynolds turbulence model is accommodated to account for the turbulence. A numerical scheme employing a control volume approach is used to simulate the flow field. Heat transfer characteristic and shear stress distribution around the cavity are computed. It is found that outer cone angle of the annular nozzle influences the heat transfer rates from the cavity wall. The flow structure around the cavity changes significantly with increasing cavity diameter. Moreover, increasing cavity depth results in stagnation zone moving into the cavity.     

锯齿喷口参数对喷流噪声特性的影响

喷流噪声是主要的飞机噪声源之一,影响飞机的适航噪声水平和舱内乘坐舒适性。针对单涵冷喷流噪声,通过试验研究V形喷口几何参数,锯齿个数、锯齿长度、锯齿的嵌入角度等对噪声抑制效果的影响。研究发现:压比1.52和1.69时,增加锯齿的个数,可以有效地提高降噪水平;压比1.89时,增加锯齿个数增大了高频噪声的水平;增加锯齿的长度,并不能改善降噪效果。增加锯齿的嵌入角度,可以有效地降低中低频噪声,但高频噪声显著增加;在压比1.69和1.89时,锯齿喷口的噪声谱线中有幅值较高的驼峰,可能是锯齿诱导产生了高频激波造成的。     

氧气转炉的氧枪喷头优化及操作改进

整体性是马克思主义理论的重要特性,整体性问题是研究马克思主义的一个基本问题,也是当前我国学者研究马克思主义的一个热点问题。我国理论界围绕此问题进行了深入而系统的研究,取得了丰硕成果,但也存在一些问题,对这些问题的反思,有益于进一步实现马克思主义整体发展和创新的前瞻性使命。     

斯特林发动机压力涡流喷嘴的喷雾特性试验

采用高速摄影技术,研究了斯特林发动机压力涡流喷嘴的喷雾发展过程及喷雾特性,并考察了喷射压力和气体引射对喷雾特性的影响.结果表明,压力涡流喷嘴初始喷雾阶段雾化效果较差,持续时间较短,主喷雾发展过程有明显的空气卷吸涡流现象;柴油主喷雾锥角随喷油时间先迅速增大后逐渐减小至稳定,而贯穿距离则逐渐增大至稳定,喷雾锥角随喷油压力的提高而增大;引射空气对稳态喷雾形态有明显影响,使得喷雾扩散范围明显扩大,油气混合更加充分.     

分区分级双P型辐射管喷口结构位置特性

为了对分区分级双P型燃气辐射管喷口结构、位置进行优化,提高燃烧效率.首先对分区分级双P型燃气辐射管进行了实验和数值研究,结果发现,除NOx体积分数的误差为11.6%外,其他参数的偏差都在1%以内,证明该模型具有可靠性.在此基础上,通过研究主管和支管的喷口位置及喷口结构等参数,进行了气体温度和壁面温度的研究分析.结果显示:随着主管喷口位置向外移动,分区分级燃气辐射管表面温度的最高值逐渐减小,壁面温度的最低值逐渐增大.支管喷口位于三通管与支管交线处时,可以减少高温气体对辐射管管壁的冲击作用,提高支管径向的温度均匀性,延长辐射管使用寿命;主管喷口的形式为完全预混式喷口时,壁面温差最小;支管喷口的形式为不对称式时,分区分级燃气辐射管壁面温差最小,燃烧热效率最高.     

含钛高强钢中夹杂物析出行为研究

以不同钛含量(0.032%~0.153%)的高强钢为研究对象,利用SEM、EDS等对钢中氧化物及氮化物夹杂的形貌、组成、数量及尺寸分布进行表征,结合热力学与动力学计算,对TiN的析出行为进行分析。结果表明,当钢中Ti含量较低(0.032%)时,冶炼过程会形成富含MgAl_2O_4的多边形高熔点固态夹杂,易造成水口堵塞;当Ti含量大于0.106%时,钢中形成了富含TiAl_2O_5的液态球形夹杂,可通过上浮去除,不易堵塞水口;Ca处理能将低钛钢中的夹杂改性为低熔点的铝酸钙,不仅降低了水口堵塞风险,还能还原夹杂物中的Ti,提高Ti的收得率。含钛高强钢中夹杂物平均尺寸为1~2μm,氮化物平均尺寸为1~3μm,其均随着Ti含量的增加而稍有增加。当Ti含量小于0.04%时,将钢中N含量控制在0.0018%以下,能有效减小TiN的析出尺寸;当Ti含量大于0.1%时,实际工业生产中很难阻止TiN的液析,但可通过提高钢中细小氧化物的数量,来细化TiN的尺寸。     

Single-mode low threshold current multi-hole vertical-cavity surface-emitting lasers

A multi-hole vertical-cavity surface-emitting laser(VCSEL) operating in stable single mode with a low threshold current was produced by introducing multi-leaf scallop holes on the top distributed Bragg-reflector of an oxidationconfined 850 nm VCSEL.The single-mode output power of 2.6 mW,threshold current of 0.6 mA,full width of half maximum lasing spectrum of less than 0.1 nm,side mode suppression ratio of 28.4 dB,and far-field divergence angle of about 10 are obtained.The effects of different hole depths on the optical characteristics are simulated and analysed,including far-field divergence,spectrum and lateral cavity mode.The single-mode performance of this multi-hole device is attributed to the large radiation loss from the inter-hole spacing and the scattering loss at the bottom of the holes,particularly for higher order modes.