二维超音速喷管型线设计仿真研究

采用计算软件FLUENT,对四种经典收缩段型线下的流场特性进行数值模拟,为选择超声速风洞收缩段的型线提供依据。基于特征线理论,利用解析法完成超音速喷管膨胀段型线设计,通过分析总压恢复系数及均匀度等流场参数,确定型线膨胀角角度及喷管长度。结果表明,收缩段型线选用双三次曲线,膨胀角度3.5°的情况下,超音速喷管出口达到了设计要求马赫数,并获得了较好的气流品质。     

The effect of the secondary annular stream on supersonic jet

The present study addresses an experimental investigation of the near field flow structures of supersonic, dual, coaxial, free, jet, which is discharged from the coaxial annular nozzle. The secondary stream is made from the annular nozzle of a design Mach number of 1.0 and the primary inner stream from a convergent-divergent nozzle. The objective of the present study is to investigate the interactions between the secondary stream and inner supersonic jets. The resulting flow fields are quantified by pitot impact and static pressure measurements and are visualized by using a shadowgraph optical method. The pressure ratios of the primary jet are varied to obtain over-expanded flows and moderately under-expanded flows at the exit of the coaxial nozzle. The pressure ratio of the secondary annular stream is varied between 1.0 and 4.0. The results show that the secondary annular stream significantly changes the Mach disc diameter and location, and the impact pressure distributions. The effects of the secondary annular stream on the primary supersonic jet flow are strongly dependent on whether the primary jet is underexpanded or over-expanded at the exit of the coaxial nozzle.     

超音速气流粉碎机喷嘴的结构设计研究

本文应用气体动力学理论，对超音速气流粉碎机喷嘴的流动及结构设计进行了分析，给出了喷嘴气动外形设计的要求和能获得最有效喷射速度的结构设计方法，并给出了喷嘴流态的计算和相应的流动多数计算公式。     

磨料喷射喷嘴的应力分析及FGM模型设计

分析表明磨料喷射加工中喷嘴承受的应力在喷嘴入口最大、出口次之、中间区域相对较小。试验显示喷嘴的冲蚀磨损在喷嘴入口最严重、出口次之、中间区域相对较小。试验结果与应力分析结果一致,得出磨料喷射加工中喷嘴入口、出口处承受的高拉应力是造成其冲蚀磨损严重的主要原因的结论。针对提高均质陶瓷材料喷嘴抗冲蚀磨损能力的力度有限,提出运用梯度功能材料(Function gradient material,FGM)理论于喷嘴材料的设计和制造中,研发新型非均质的陶瓷喷嘴材料,将梯度功能陶瓷喷嘴制备中产生的残余压应力引入喷嘴入口、出口,使其减缓磨料喷射加工中喷嘴承受的拉应力,以提高喷嘴抗冲蚀磨损能力。结合喷嘴的冲蚀磨损特点、结构特点及陶瓷喷嘴材料制备工艺性等,建立了梯度陶瓷喷嘴物理模型和成分分布模型。根据本模型设计结果,研制出梯度 SiC／(W,Ti)C陶瓷喷嘴。结果表明,相同条件下制各的梯度SiC／(W,Ti)C陶瓷喷嘴的抗冲蚀磨损能力高于非梯度 SiC／(W,Ti)C陶瓷喷嘴。     

Design and evaluation of high-pressure nozzle assembly for laser cutting of thick carbon steel

Laser cutting of carbon steel is extensively used across a range of industries, due to its advantage of high speed, low kerf and high quality. Currently, a 1-kW carbon dioxide (CO₂) laser with its subsonic nozzle assembly can be used only to cut steel plates up to around 10 mm. This paper aims to design and evaluate a high-pressure supersonic laser cutting nozzle assembly, which can enable a 1-kW CO₂ laser to cut steel of up to 50 mm thickness. Basic gas dynamic and compressible flow equations were used to design the supersonic nozzle assembly. The flow of the high-pressure gas jet inside the nozzle assembly was investigated using computational fluid dynamics (CFD), and the structural integrity of the high-pressure nozzle assembly was ensured using finite element analysis (FEA). The gas flow pattern at the exit of the nozzle assembly was computed and compared with the experimental observation made through a shadowgraph technique. Laser cutting experiments were performed with the developed supersonic nozzle assembly to demonstrate cutting of 50-mm-thick low carbon steel with 1-kW CO₂ laser.     

Effect of boundary layer swirl on supersonic jet instabilities and thrust

This paper reports the effects of nozzle exit boundary layer swirl on the instability modes of underexpanded supersonic jets emerging from plane rectangular nozzles. The effects of boundary layer swirl at the nozzle exit on thrust and mixing of supersonic rectangular jets are also considered. The previous study was performed with a 30° boundary layer swirl (S=0.41) in a plane rectangular nozzle exit. At this study, a 45° boundary layer swirl (S=1.0) is applied in a plane rectangular nozzle exit. A three-dimensional unsteady compressible Reynolds-Averaged Navier-Stokes code with Baldwin-Lomax and Chien’sk-ε two-equation turbulence models was used for numerical simulation. A shock adaptive grid system was applied to enhance shock resolution. The nozzle aspect ratio used in this study was 5.0, and the fully-expanded jet Mach number was 1.526. The “flapping” and “pumping” oscillations were observed in the jet’s small dimension at frequencies of about 3,900Hz and 7,800Hz, respectively. In the jefs large dimension, “spanwise” oscillations at the same frequency as the small dimension’s “flapping“ oscillations were captured. As reported before with a 30° nozzle exit boundary layer swirl, the induction of 45° swirl to the nozzle exit boundary layer also strongly enhances jet mixing with the reduction of thrust by 10%.     

An experimental study of supersonic dual coaxial free jet

A supersonic dual coaxial jet has been employed popularly for various industrial purposes, such as gasdynamic laser, supersonic ejector, noise control and enhancement of mixing. Detailed characteristics of supersonic dual coaxial jets issuing from an inner supersonic nozzle and outer sonic nozzles with various ejection angles are experimentally investigated. Three important parameters, such as pressure ratios of the inner and outer nozzles, and outer nozzle ejection angle, are chosen for a better understanding of jet structures in the present study. The results obtained from the present experimental study show that the Mach disk diameter becomes smaller, and the Mach disk moves toward the nozzle exit, and the length of the first shock cell decreases with the pressure ratio of the outer nozzle. It was also found that the highly underexpanded outer jet produces a new oblique shock wave, which makes jet structure much more complicated. On the other hand the outer jet ejection angle affects the structure of the inner jet structure less than the pressure ratio of the outer nozzle, relatively.     

A study on the flow with nonequilibrium condensation in a minimum length nozzle

As recognized previously, a minimum-length nozzle has the smallest possible throat-to-exit length that is still capable of maintaining uniform supersonic flow at the nozzle exit. In the present study, for the flow of moist air through a nearly minimum-length nozzle designed by the method of characteristics, the effects of nonequilibrium condensation on the uniformity of flow properties, the momentum efflux, and the flow distortion at the nozzle exit plane are discussed by experiment and numerical analysis of a third-order Total Variation Diminishing (TVD) finite difference scheme. The onset and zone of nonequilibrium condensation in a minimum-length nozzle are quite different from those of a general convergent-divergent supersonic nozzle. We know that the uniformity of flow properties at the nozzle exit with regard to the flow with nonequilibrium condensation in a minimum-length nozzle cannot be guaranteed. On the other hand, owing to the positions of the onset of condensation at the incident region of expansion waves from the sharp corner just downstream of the nozzle throat, the deceleration gradient and magnitude of heat released from the process of nonequilibrium condensation to the surrounding of ϕ₀=60% are greater than those of ϕ₀=70% in the case of T₀=290K. Furthermore, it has been determined that the decrease in efflux of momentum from the nozzle exit for the stagnation relative humidity of ϕ₀=70%(T₀=290K), which corresponds to the case with nonequilibrium condensation shock, is 6.8% smaller than that of isentropic expansion. This paper was recommended for publication in revised form by Associate Editor Do Hyung Lee Soon-Bum Kwon received his B.S. and M.S. degrees in Mechanical Engineering from Kyungpook National University in 1974 and 1980, respectively, and his Ph.D. degree from Kyushu University in 1987. He is a Professor at the School of Mechanical Engineering at Kyungpook National University. His research interests are compressible gas dynamics and nonequilibrium condensation.     

Flow characteristics of pyramidal shaped small sonic nozzles

Four types of pyramidal sonic nozzles made of silicon crystal were studied experimentally. The throat sizes varied from 38 to 140 μm for type A and D nozzles and from 75 to 188 μm for type B and C nozzles. For each of the nozzle types, the results show that the discharge coefficient is proportional to the throat size, and the critical back pressure ratio for choking is insensitive to Reynolds’ number. In parallel, the flow field of a type B nozzle was investigated by numerical simulation. The effect of heat flux coming from the nozzle body was examined and the flow patterns obtained from Spalart-Allmaras and standard k−ω turbulence models were compared. The simulation results indicate the heat flux does not noticeably change the velocity field and discharge coefficient. Also, the flow downstream of the nozzle throat develops into an under-expanded supersonic jet in which expansion and oblique shock waves appear alternately.     

Supersonic jet noise control via trailing edge modifications

Various experimental data, including mixing areas, cross correlation factors, surface flow patterns on nozzle walls, and far field noise spectra, was used to draw a noise control mechanism in a supersonic jet. In the underexpanded case, mixing of the jet air with ambient air was significantly enhanced as presented before, and mixing noise was also dramatically reduced. Screech tones, in the overexpanded case, were effectively suppressed by trailing edge modifications, although mixing enhancement was not noticeable. From mixing and noise performance of nozzles with modified trailing edges, enhancing mixing through streamwise vortices seems an effective way to reduce mixing noise in the underexpanded flow regime. However, screech tones in the overexpanded flow regime is well controlled or suppressed by making shock cells and/or spanwise large scale structures irregular and/or less organized by a proper selection of trailing edges. The noise field in the overexpanded flow regime was greatly affected by the symmetricity of the nozzle exit geometry. In the underexpanded flow regime, the effects of the symmetricity of the nozzle exit on mixing were negligible.     

An experimental study of the nozzle lip thickness effect on supersonic jet screech tones

It is well known that screech tones of supersonic jet are generated by a feedback loop driven by the instability waves. Near the nozzle lip where the supersonic jet mixing layer is receptive to external excitation, acoustic disturbances impinging on this area excite the instability waves. This fact implies that the nozzle lip thickness can influence the screech tones of supersonic jet. The objective of the present study is to experimentally investigate the effect of nozzle-lip thickness on screech tones of supersonic jets issuing from a convergent-divergent nozzle. A baffle plate was installed at the nozzle exit to change the nozzle-lip thickness. Detailed acoustic measurement and flow visualization were made to specify the screech tones. The results obtained obviously show that nozzle-lip thickness significantly affects the screech tones of supersonic jet, strongly depending on whether the jet at the nozzle exit is over-expanded or under-expanded.     

Development of weighing tank system employing rotating double wing diverter

A new design for a diverter and weighing tank system has been developed. Experiments have been carried out on a prototype. Diverter wings are set as partitions along the diameter of a half-cylinder tube. The rotation axis is at the center of the half cylinder and runs parallel to liquid jets flowing vertically downward from several independent nozzles. This design contributes to small diverter timing error, compact design, small wet area, a simple and robust mechanism, easy implementation of the ISO4185 test, and easy adjustment of diverter trigger timing. After adjustment of start and stop signal timings, the estimated standard uncertainty caused by diverter timing error is reduced within a negligible small value (0.0019% or less) over a wide range of flow rates at any nozzle position. The weighing tank system has been improved by some refined devices. A single movement of the weighing platform can prevent escaping vapor, enable connection of an air tube, provide protection against impact, and provide a fail-safe against overflow. It is confirmed that the liquid flow calibration facility using the present system achieves accurate measurement, easy operation, wide flow range, high throughput, low cost, small weighing tank, easy maintenance, and safety.     

缩扩型超音速喷管的设计与仿真

缩扩型的拉瓦尔喷管可以使通过其的气流获得超音速,出口速度的大小以及气流是否稳定是决定拉瓦尔喷管性能优劣的关键因素.通过Fluent软件对拉瓦尔喷管的相关流场进行了数值仿真,分析了入口压力、面积比、收缩段型面及扩张段的锥角对喷管出流速度的影响,可为喷嘴的设计提供理论指导.     

Optimization of a subsonic wind tunnel nozzle with low contraction ratio via ball-spine inverse design method

The goal of wind tunnel design is to generate a uniform air flow with minimum turbulence intensity and low flow angle. The nozzle is the main component of wind tunnels to create a uniform flow with minimal turbulence. Pressure distribution along nozzle walls directly affects the boundary layer thickness, pressure losses and non-uniformity of flow velocity through the test section. Although reduction of flow turbulences and non-uniformity through the test section can be carried out by nozzles with high contraction ratio, it increases the construction cost of the wind tunnel. For decreasing the construction cost of nozzle with constant test section size and mass flow rate, the contraction ratio and length of nozzle should be decreased; that causes the non-uniformity of outlet velocity to increase. In this study, first, three types of nozzle are numerically investigated to compare their performance. Then, Sargison nozzle with contraction ratio of 12.25 and length of 7 m is scaled down to decrease its weight and construction cost. Having scaled and changed to a nozzle with contraction ratio of 9 and length of 5 m, its numerical solution reveals that the non-uniformity of outlet velocity increases by 21%. By using the Ballspine inverse design method, the pressure distribution of the original Sargison nozzle is first scaled and set as the target pressure of the scaled down nozzle and geometry correction is done. Having reached the target nozzle, numerical solution of flow inside the optimized nozzle shows that the non-uniformity just increases by 5% in comparison with the original Sargison nozzle.     

Measurement of turbulent supersonic steam jet flow characteristics using TDLAS

Ejectors have no moving parts and are preferable to mechanical compressors in many applications, but ejectors typically have a relatively low efficiency. To aid in the ejector design process, thorough understanding of the turbulent mixing of multi-phase compressible jets is beneficial.This paper reports experimental results for Tunable Diode Laser Absorption Spectroscopy (TDLAS) measurements derived from an axisymmetric supersonic steam jet apparatus.In this experimental work, a supersonic steam jet nozzle exit of a diameter 13.6 mm was surrounded by a low-speed flow of dry nitrogen. The TDLAS system was traversed through the flow at three different planes downstream from the ejector nozzle exit: 15, 20, and 30 mm distance. At each of the three planes, line-of-sight measurements were made with the laser passing through locations between 0 and 15 mm from the jet centreline.Through the analysis of the TDLAS data and application of the Abel inversion method, the radial distribution of the pressure, temperature, and the concentration of the water-vapour were obtained. The key findings are that it is possible to determine key physical parameters using experimental TDLAS measurements when combined with a suitable numerical optimization approach.     

气溶性射流对钢板污垢清洗的应用研究

为了获得更好的清洗效果和节能环保,提出了一种新型的射流方式——气溶性射流。基于流体力学和气溶性射流的工作原理,根据气溶性射流的特性和喷嘴的几何特征,研究了气液相的流动特性方程,得到了气溶性射流的速度分布规律。利用自行设计的超音速喷嘴组装的带电气溶性射流实验装置对钢板进行清洗,经简易检测表明：气溶性射流喷嘴产生的超音速极大地提高了洗涤效率,与同流量的喷嘴相比,效率提高近10倍,具有广泛的应用前景。     

Transitional behavior of a supersonic flow in a two-dimensional diffuser

Two-dimensional blow-down type supersonic wind tunnel was designed and built to investigate the transient behavior of the startup of a supersonic flow from rest. The contour of the divergent part of the nozzle was determined by the MOC calculation. The converging part of the nozzle, upstream of the throat was contoured to make the flow profile uniform at the throat. The flow characteristics of the steady supersonic condition were visualized using the highspeed schlieren photography. The Mach number was evaluated from the oblique shock wave angle on a sharp wedge with half angle of 5 degree. The measured Mach number was 2.4 and was slightly less than the value predicted by the design calculation. The initial transient behavior of the nozzle was recorded by a high-speed digital video camera with schlieren technique. The measured transition time from standstill to a steady supersonic flow was estimated by analyzing the serial images. Typical transition time was approximately O.1sec.     

超音速射流噪声突变现象的实验和数值模拟研究

在利用“放气法”测量喷管的超音速射流噪声的实验过程中,发现部分结构喷管产生的射流噪声大小不是随着上游压强的减小而减小,而是在特定压强点出现突增现象。为了在喷管的设计阶段就能预测其喷流噪声是否发生突变,对两种结构喷管的射流流场进行了实验和数值模拟研究。结果表明,射流流场的轴线总压和噪声声压级存在强相关性,可将流场总压的变化作为噪声是否突变的判据。因此,在设计喷管时对其射流流场进行仿真研究,就可预测其噪声突变现象的产生。     

收缩喷嘴内部流道型线对射流流场的影响

为研究收缩喷嘴内部流道形成对射流流场的气体动特性参数的影响，根据可压缩流体轴对称N-S方程，采用非结构网格和二阶精度的有限体积法，对不同内部流道形线的喷嘴自由射流进行数值模拟。亚声速射流采用RNGk-ε湍流模型，超声速射流采用S-A湍流模型，计算结果与实验较吻合。在亚声速流动中，收缩喷嘴的收缩角大小会影响其对射流的阻滞效果，内部流道形线设计为维多辛斯基曲线可以获得更好的流场动特性参数，有利于提高喷嘴的工作效率。在超声速流动中，喷嘴流道型线对出口膨胀波的角度与强弱影响较大，要根据射流的有效作用区域选择合适的喷嘴，才能使能量的损失最小。若要获得较佳的外部流场参数，优化喷嘴内部流道设计十分重要。     

音速喷嘴气体流量标准装置流量点精确控制研究

音速喷嘴作为气体流量标准装置的标准表,具有精度高、性能稳定、维护容易等优点,已经成为传递标准。音速喷嘴法气体计量标准装置包括音速喷嘴、真空泵、滞止容器等,由于真空泵过高的频率不仅会造成浪费,还会产生严重的振动问题,而过低的频率又达不到要求,如何在这对矛盾之中寻找一个最优的方案是研究的重点。为了能够使测量数据更加准确,选择更优频率使整个装置配合运转达到最佳,通过实验找到最优频率点,通过变频真空泵的频率和音速喷嘴出口质量流量的优化设计,实现流量点精确控制。