STUDY OF THE IMPULSE WAVE DISCHARGED FROM THE EXIT OF A RIGHT-ANGLE PIPE BEND

The impulse wave, which is usually generated by a shock wave discharge from the exit of a pipe, almost always leads to undesirable noise and vibration problems. The present study addresses experimental and computational work of the impulse wave discharged from the exit of two kinds of right-angle pipe bends, which are attached to the open end of a simple shock tube. The weak normal shock wave with its magnitude of Mach number from 1·02 to 1·20 is employed to obtain the impulse wave propagating outside the exit of the pipe bends. A Schlieren optical system is employed to visualize the impulse wave discharged from the exit of the pipe bends at an instant. The experimental data of the magnitude of the impulse wave and its propagation directivity are analyzed to characterize the impulse waves discharged from the exit of the pipe bends and compared with those discharged from a straight pipe. Computational analysis using the unsteady, inviscid, compressible equations is complemented to represent the major features of the impulse wave obtained from the shock tube experiments. Computational results well predict the experimented dynamic behaviors of the impulse wave. The results obtained show that a right-angle miter bend considerably reduces the magnitude of the impulse wave and its directivity toward to the pipe axis, compared with the straight pipe. It is believed that the right-angle miter bend pipe can play one role of a passive control against the impulse wave.     

Visualization and force measurement of high-temperature,supersonic impulse jet impinging on baffle plate

The flow visualization and force measurement of a supersonic impinging jet on a center-holed vertical baffle plate were investigated. Center-holed baffle plates of 2d to 5d in diameter, with a 1d center hole were tested, where d is the bore of the launch tube. The standoff distance of the baffle plates from the open end of the launch tube were varied to be from 2d to 5d. The supersonic impulse jet, with an incident shock wave of Mach 2.89 was produced by a high-enthalpy blast-wave simulator. The direction-indicating color schlieren method produced a two-dimensional density gradient of the flow field around the baffle plate. In addition, the flow fields were numerically analyzed, using two-dimensional asymmetric Euler equations. The results of the numerically-analyzed and the experimentally-visualized flow field agreed well. The visualized flow field indicates that the baffle plate should be at least 3.5d in diameter to deflect the supersonic impinging jet at an angle greater than a right angle. We have concluded that the representative method of designing muzzle brakes for military purpose accurately predicts the force yielded by the supersonic impinging impulse jet on the vertical baffle plate only when there is a large ratio of the baffle-plate diameter to the bore of the launch tube.     

层流与湍流等离子体冲击射流特性比较

本文采用数值模拟方法,对层流与湍流氩等离子体射流在空气环境中冲击平板时的流动与传热特性进行了对比研究.结果表明,在平板和射流进口间的距离较大时,平板的存在只对其附近的射流参数分布有较大影响,层流等离子体冲击射流的温度与轴向速度的轴向梯度明显小于湍流等离子体冲击射流情形;由于在平板表面形成的径向壁面射流对引射的附加贡献,层流和湍流等离子体冲击射流对环境空气的引射量明显增加.     

The connection between sound production and jet structure of the supersonic impinging jet

An experimental investigation into the sound-producing characteristics of moderately and highly underexpanded supersonic impinging jets exhausting from a round convergent nozzle is presented. The production of large plate tones by impingement on a square plate with a side dimension equal to 12 nozzle exit diameters is studied using random and phase-locked shadowgraph photography. Discrete frequency sound is produced in the near-wall region of the jet when a Mach disk occurs upstream of the standoff shock wave. Tones cease when the plate distance is approximately 2.2 free-jet cell lengths and the first and second shock waves are located in the free-jet positions. The production of impulsive sound appears to be associated with the collapse of the standoff shock wave during a portion of the oscillation cycle. Results from unsteady plate-pressure measurements indicate that plane-wave motion occurs in the impingement region and a secondary pressure maximum is observed on the plate adjacent to the flow region where sound appears to originate.     

Influence of Burner Geometry on Heat Transfer Characteristics of Methane/Air Flame Impinging on Flat Surface

An experimental study has been conducted to find the heat transfer characteristics of methane/air flames impinging normally to a flat surface using different burner geometries. The burners used were of nozzle, tube, and orifice type each with a diameter of 10 mm. Due to different exit velocity profiles, the flame structures were different in each case. Because of nearly flat velocity profile, the flame spread was more in case of orifice and nozzle burners as compared to tube burner. Effects of varying the value of Reynolds number (600–2500), equivalence ratio (0.8–1.5) and dimensionless separation distance (0.7–8) on heat transfer characteristics on the flat plate have been investigated for the tube burner. Different flame shapes were observed for different impingement conditions. It has been observed that the heat transfer characteristics were intimately related to flame shapes. Heat transfer characteristics were discussed for the cases when the flame inner reaction cone was far away, just touched, and was intercepted by the plate. Negative heat fluxes at the stagnation point were observed when the inner reaction cone was intercepted by the plate due to impingement of cool un-burnt mixture directly on the surface. Different heat transfer characteristics were observed for different burner geometries with similar operating conditions. In case of tube burner, the maximum heat flux is around the stagnation point and decay is faster in the radial direction. In case of nozzle and orifice burner, the heat transfer distribution is more uniform over the surface.     

Numerical analysis of supersonic gas-dynamic characteristic in laser cutting

The influence of the processing parameters on the dynamic characteristic of supersonic impinging jet in laser cutting is studied numerically. The numerical modeling of a supersonic jet impinging on a plate with a hole is presented to analyze the gas jet–workpiece interaction. The model is able to make quantitative predictions of the effect of the standoff distance and exit Mach number on the mass flow rate and the axial thrust. The numerical results show that the suitable cutting range is slightly different for different exit Mach number, but the optimal cutting parameter for certain exit total pressure is nearly changeless. So the better cut quality and capacity can be obtained mainly by setting the suitable standoff distance for a certain nozzle pressure.     

HEAT TRANSFER CHARACTERISTICS OF AN IMPINGING BUTANE/AIR FLAME JET OF LOW REYNOLDS NUMBER

An experimental study was performed to determine the heat transfer characteristics of a premixed butane/air round flame jet, of low Reynolds number, impinging upwards normally on a flat rectangular plate. The effects of the exit Reynolds number and equivalence ratio of the flame jet, and the distance between the nozzle and the impingement plate, on the thermal performance of the jet were examined. The range of Reynolds numbers was selected to cover the laminar to the transitional flow conditions. The investigations were conducted with equivalence ratios corresponding to the fuel-rich, stoichiometric, and fuel-lean conditions. The nozzle-plate distance was varied from 1d to 8d. Within the range of Reynolds numbers investigated, the highest Nusselt numbers were obtained at the equivalence ratio of φ = 0.85 when the nozzle-plate distance was maintained at 5d. At the stoichiometric condition, the highest Nusselt number was obtained at the nozzle- plate distance of 6d. Nondimensional correlations were obtained from the experimental results and presented to predict the maximum Nusselt number and average Nusselt number for laminar flame jets as a function of the nozzle-plate distance, Reynolds number, and equivalence ratio.     

Normal interaction between a supersonic axisymmetric jet and a surface containing a hole coaxial with the jet

The influence of the position of a supersonic jet source relative to a flat plate and of the size of a hole on it on the amplitude and frequency of shock wave oscillations is numerically investigated by integrating 2D Navier-Stokes equations using the predictor-corrector scheme of the second-order accuracy in time and space. Depending on the source-plate distance, an increase in the hole size raises or lowers the oscillation frequency. The oscillation amplitude decreases with increasing hole size.     

冲击射流激波振荡与抑制

欠膨胀冲击射流具有复杂的激波结构,并伴随产生高幅值的离散频率单音.通过高速摄像获取的纹影图像并结合噪声测量,对欠膨胀冲击射流激波振荡过程、剪切层不稳定波的模态和离散频率单音的产生进行了系列研究.给出了冲击距离为5倍喷嘴出口直径的复杂流动实验结果分析,射流剪切层不稳定波有对称和非对称两种模态,发现不同模态下的离散频率单音...     

Numerical Study of Air Flow Induced by Shock Impact on an Array of Perforated Plates

This study is focused on the propagation behavior and attenuation characteristics of a planar incident shock wave when propagating through an array of perforated plates. Based on a density-based coupled explicit algorithm, combined with a third-order MUSCL scheme and the Roe averaged flux difference splitting method, the Navier–Stokes equations and the realizable k-ε turbulence model equations describing the air flow are numerically solved. The evolution of the dynamic wave and ring vortex systems is effectively captured and analyzed. The influence of incident shock Mach number, perforated-plate porosity, and plate number on the propagation and attenuation of the shock wave was studied by using pressure- and entropy-based attenuation rates. The results indicate that the reflection, diffraction, transmission, and interference behaviors of the leading shock wave and the superimposed effects due to the trailing secondary shock wave are the main reasons that cause the intensity of the leading shock wave to experience a complex process consisting of attenuation, local enhancement, attenuation, enhancement, and attenuation. The reflected shock interactions with transmitted shock induced ring vortices and jets lead to the deformation and local intensification of the shock wave. The formation of nearly steady jets following the array of perforated plates is attributed to the generation of an oscillation chamber for the inside dynamic wave system between two perforated plates. The vorticity diffusion, merging and splitting of vortex cores dissipate the wave energy. Furthermore, the leading transmitted shock wave attenuates more significantly whereas the reflected shock wave from the first plate of the array attenuates less significantly as the shock Mach number increases. The increase in the porosity weakens the suppression effects on the leading shock wave while increases the attenuation rate of the reflected shock wave. The first perforated plate in the array plays a major role in the attenuation of the shock wave.     

Numerical analysis on auto-ignition of a high pressure hydrogen jet spouting from a tube

In this study, a direct numerical simulation based on compressible flow dynamics has been applied to the autoignition and extinction of a high-pressure hydrogen jet spouting from a tube. The diameter of the tube is 4.8 mm. The length of the tube is 71 mm. At the inlet, pressure is set at 3.6, 5.3 and 21.1 MPa, and temperature is set at 300 K for all cases. To explore the autoignition of hydrogen jet, two-dimensional axisymmetric Navier–Stokes equations with a detailed chemical kinetics and rigorous transport properties have been employed. The hydrogen jet through the tube is choked. The numerical results show that the high-pressure hydrogen jet produces a semi-spherical shock wave in the ambient air at the early time of jetting. The shock wave heats up the air to a high temperature and causes the autoignition of the hydrogen and air mixture in the tube as well as at the tube exit.     

平板约束下激光诱导等离子体的动态仿真

为深入认识空间约束增强的物理机理,采用二维可压缩流体模型,建立平板约束下激光诱导等离子体动力学行为的数值模拟模型,计算了平板约束下等离子体的演化过程,得到的一系列时间分辨的温度分布结果与实验结果基本一致.揭示了平板约束下,反射激波对等离子体的压缩作用导致等离子体温度升高的机理.对不同激光能量和不同约束板间距对等离子体温度增强效果和增强时刻的影响进行了研究,两板间距增加,增强时刻明显延迟,等离子体温度的增强效果削弱.     

保护气对切割弧特性影响的模拟研究

通过比较两种不同结构切割炬所产生的等离子体流场,发现保护气对等离子体的温度和速度分布影响很小.垂直保护气在切割炬喷口形成阻碍作用,造成切割炬内的压强有所升高,但是增加不大.两种结构保护气对切割弧的影响只是在炬喷口外的激波附近.加入保护气后激波的强度会减弱.相对于没有保护气的情况,保护气增加冷却作用,弧电压会略有升高.当改变保护气的成分时,发现弧柱区的氧气含量不受影响,所以保护气成分的改变不会影响到弧电压.计算发现轴线处氧气和周围气体的混合很少,在喷口下游10mm处,氧气的摩尔分数仍在90%以上. 关键词： 等离子体切割弧 保护气 数值模拟     

MEASUREMENTS OF LOCAL HEAT TRANSFER COEFFICIENTS FROM A FLAT PLATE TO A PAIR OF CIRCULAR AIR IMPINGING JETS

Local heat transfer coefficients from a flat plate to a pair of circular air impinging jets are investigated experimentally, A pair of well-controlled, fully developed circular air impinging jets at room temperature are used in the experiments. The experimental method in this investigation is the transient liquid-crystal technique. During the experiments, the surface liquid-crystal color distribution of the test plate is recorded using a video imaging acquisition system, and the color information is translated into a surface temperature distribution through a digital color image processing unit. Local heat transfer coefficients art obtained using a surface transient heat conduction analysis. The flow Reynolds number of the jet is kept at 23,000. The jet-to-plate distance and the jet-to-jet spacing are varied in the experiment. Detailed radial heat transfer distributions at different radial directions are obtained and analyzed for L/D = 2, 4, 6, 8, and 10. The effect of jet spacing distance (S/D =1.75, 3.5,5.25, 7.0) is analyzed by comparing to data obtained from a single jet with similar flow configurations.     

矩形喷口欠膨胀超声速射流对撞的实验研究

在不同喷口间距和射流压力下开展了矩形喷口欠膨胀超声速射流对撞实验并与自由射流进行了对比. 实验表明:超声速射流对撞的辐射噪声中存在四种不同的啸音模式, 且随喷口距离和射流压力的变化在不同模式间切换. 在射流压力大于0.5 MPa且喷口间距小于50 mm时, 射流对撞面在两个喷口外形成两道正激波之间, 啸音基频维持在3 kHz左右. 随喷口间距的增大或射流压力的降低, 射流对撞面在一侧喷口外的弓形激波与另一侧喷口外的正激波之间. 对撞面也有可能出现在两个弓形激波之间, 对应的啸音基频约为9 kHz, 但容易受扰动而回到喷口一侧或是在喷口之间大幅度振荡. 当射流压力小于0.36 MPa且喷口间距大于70 mm后, 对撞面在两个喷口之间大幅度振荡, 产生基频在1 kHz左右并随射流压力的降低和喷口间距的增大而降低的啸音. 关键词： 超声速射流 啸音 射流对撞 激波     

An experimental investigation of the noise produced by air jet impingement on flat plates

The sound power level produced by an air jet impinging upon a flat solid boundary is investigated. Measurements are recorded for nozzle diameters and exit velocities in the ranges $\text{0·75 in < D < 1·5}$ in and $\text{270 ft/s <}\text{U}\text{?}\text{< 700 ft/s}$. The effect of varying the nozzle to plate spacing is investigated for plates normal to and inclined to the jet axis.At low velocities and large nozzle diameters the sound power level was found to increase continuously as the nozzle to plate spacing decreased, the maximum increase being 7 dB above that of the free jet. Under certain conditions, usually small nozzle diameter and high velocities, a maximum in sound power level was observed in the nozzle to plate spacing range, increases of 27 dB above that of the free jet levels being recorded and a distinct tonal character being detected.The reasons for this phenomenon cannot be fully explained, but it is likely that this can be avoided by inclining the plate to the jet axis.     

High strain rate deformation and fracture of the magnesium alloy Ma2-1 under shock wave loading

This paper presents the results of measurements of the dynamic elastic limit and spall strength under shock wave loading of specimens of the magnesium alloy Ma2-1 with a thickness ranging from 0.25 to 10 mm at normal and elevated (to 550°C) temperatures. From the results of measurements of the decay of the elastic precursor of a shock compression wave, it has been found that the plastic strain rate behind the front of the elastic precursor decreases from 2 × 10⁵ s^?1 at a distance of 0.25 mm to 10³ s^?1 at a distance of 10 mm. The plastic strain rate in a shock wave is one order of magnitude higher than that in the elastic precursor at the same value of the shear stress. The spall strength of the alloy decreases as the solidus temperature is approached.     

Visualization of impinging supersonic free jet on a tilt plate by LIF and PSP

The flow field structures of low density supersonic free jets impinging on a tilt plate are studied by hybrid use of LIF (Laser Induced Fluorescence) and PSP (Pressure Sensitive Paint). The jet through an orifice flows into a low pressure chamber and impinges on the tilt plate with angle from jet axis 45, 60 or 90 degrees. A plane including the jet axis and the normal of the plate is visualized by LIF of seeded iodine molecules, scanning a laser beam along the jet axis. On the other hand, the pressure distribution on the tilt plate is visualized by PSP. In comparing the results of the two methods, the complicated shock wave system is analyzed. Deformations of the Mach disk and the barrel shock are also confirmed.     

Frequency modulation at a moving material interface and a conservation law for wave number

An integral conservation law for wave numbers is considered. In order to test the validity of the proposed conservation law, a complete solution for the reflection and transmission of an acoustic wave impinging normally on a material interface moving at a constant speed is derived. The agreement between the frequency condition thus deduced from the dynamic equations of motion and the frequency condition derived from the jump condition associated with the integral equation supports the proposed law as a true conservation law. Additional comparisons such as amplitude discontinuities and Snells' law in a moving media further confirm the stated proposition. Results are stated concerning frequency and wave number relations across a shock front as predicted by the proposed conservation law.     

Influence of Streamwise Pitch on Local Heat Transfer Distribution for In-Line Arrays of Circular Jets with Spent Air Flow in Two Opposite Directions

Abstract

The effect of streamwise jet-to-jet spacing on local heat transfer distribution due to an in-line rectangular array of confined multiple circular air jets impinging on a surface parallel to the jet plate are experimentally studied. The length-to-diameter ratio of nozzles of the jet plate is 1.0. The flow, after impingement, is constrained to exit in two opposite directions from the confined passage formed between the jet plate and target plate. Mean jet Reynolds numbers based on the nozzle exit diameter (d) covered are 3,000, 5,000, 7,500, and 10,000; jet-to-plate distances studied are d, 2d, and 3d. Streamwise jet-to-jet distances of 3d, 4d, and 5d, and a constant spanwise pitch of 4d, are considered. The jet plates have ten spanwise rows in the streamwise direction and six jets in each spanwise row. The flat heat transfer surface is made of thin stainless-steel metal foil. Local temperature distribution on a target plate is measured using a thermal infrared camera. Wall static pressures in the streamwise direction are measured midway between the spanwise jets to estimate cross-flow velocities and individual jet velocities. The streamwise distribution of the jet flow and the cross flow is found to be least influenced by the streamwise pitch variation for the range of parameters considered during the present study. Heat transfer characteristics are explained partially on the basis of flow distribution. The cooling performance, based on the strip-averaged Nusselt number per unit mass flow rate of coolant per unit area of cooled surface, indicates deterioration for lower streamwise pitch and higher jet-to-plate distance.