The effect of reflector with sound-absorbing material on supersonic jet noise

This paper describes an experimental work to investigate the effect of a reflector on supersonic jet noise radiated from a convergent-divergent nozzle with a design Mach number 2.0. In the present study, a metal reflector and reflectors made of three different sound-absorbing materials (grass wool and polyurethane foam) were employed, and the reflector size was varied. Acoustic measurement is carried out to obtain the acoustic characteristics such as frequency, amplitude of screech tone and overall sound pressure level (OASPL). A high-quality schlieren optical system is used to visualize the detailed structure of supersonic jet. The results obtained show that the acoustic characteristics of supersonic jet noise are strongly dependent upon the jet pressure ratio and the reflector size. It is also found that the reflector with sound-absorbing material reduces the screech tone amplitude by about 5-13dB and the overall sound pressure levels by about 2-5dB, compared with those of the metal reflector.     

The Effect of Spherical Surface on Noise Suppression of a Supersonic Jet

Experiments were carried out to eliminate the screech tone generated from a supersonic jet. Compressed air was passed through a circular convergent nozzle preceded by a straight tube of same diameter. In order to reduce the jet screech a spherical reflector was used and placed at the nozzle exit. The placement of the spherical reflector at the nozzle exit controlled the location of the image source as well as minimized the sound pressure at the nozzle exit. The weak sound pressure did not excite the unstable disturbance at the exit. Thus the loop of the feedback mechanism could not be accomplished and the jet screech was eliminated. The technique of screech reduction with a flat plate was also examined and compared with the present method. A good and effective performance in canceling the screech component by the new method was found by the investigation. Experimental results indicate that the new system suppresses not only the screech tones but also the broadband noise components and reduces the overal     

Three-dimensional rainbow schlieren measurements in underexpanded sonic jets from axisymmetric convergent nozzles

The rainbow schlieren deflectometry has been combined with the computed tomography to obtain three-dimensional density fields of shock containing free jets and we call the method the schlieren CT. Experiments on the schlieren CT have been performed at a nozzle pressure ratio of 4.0 by using an axisymmetric convergent nozzle with an inner diameter of 10 mm at the exit where the nozzle was operated at an underexpanded condition. Multidirectional rainbow schlieren pictures of an underexpanded sonic jet can be acquired by rotating the nozzle about its longitudinal axis in equal angular intervals and the three-dimensional density fields are reconstructed by the schlieren CT. The validity of the schlieren CT is verified by a comparison with the density fields reconstructed by the Abel inversion method. As a result, it is found that excellent quantitative agreement is reached between the three-dimensional jet density fields reconstructed from both methods.     

Jet screech reduction with perforated flat reflector

In the present experimental study, investigations have been carded out to evaluate the performance of the new control technique of jet screech with different perforated flat reflectors. Mainly two types of porous flat reflectors had been used in the experiment. One reflector （reflector-V） designed for placing the reflector surface vertical to the jet axis, when, another type of reflector （reflector-H） designed for placing the reflecting surface horizontal to the jet axis. In both cases the reflectors had been placed at the nozzle （base tube with uniform cross-sectional area） exit. The diameter of the reflector-V was 15D when the diameter of the reflector-H was 10D. The porous area of the reflector-V was 6D and 4.5D for reflector-H where D indicated the diameter of the nozzle exit. The placement of the reflector at the exit of the nozzle reduces the sound pressure at the nozzle exit. Thus the muted sound can not excite the unstable disturbance at the nozzle exit and the loop of the feedback mechanism disappeared, finally, the generation of jet screech be cancelled. The suction space located at the back side of the porous surface of the reflector-V improves the efficiency of the screech control technique. However, in the case of reflector-H, the receptivity process of feedback loop had been controlled by reducing the disturbances at the effective shock fronts as well as at the nozzle exit. The performance of the proposed method was verified with a flat reflector concept and good performance in jet screech suppression has been confirmed in the case of porous reflector.     

Configuration of Shock Waves in Two-Dimensional Overexpanded Jets

An experimental and analytical study has been carried out to obtain the clear understanding of a shock wave transition associated with a steady two-dimensional overexpanded flow. Two-dimensional inviscid theory with respect to a shock wave reflection is used in the present study on the characteristic of shock waves. The results obtained from the flow analysis are compared with those obtained from flow visualizations. It is shown that in the region of regular reflection, the angle of an incident shock wave becomes lower than that calculated by two shock theory with an increment in the ratio pe/pb of the nozzle exit pressure pe to the back pressure pb. It is indicated that the configuration of shock waves in overexpanded jets is influenced by the divergent angle at the nozzle exit. Also it is shown from the flow visualization that a series of shock waves move into the nozzle inside with a decrease in pressure ratio pe/pb, even if the pe/pb is under overexpanded conditions.     

Effect of boundary layer trip on reduction of jet noise in over-expanded nozzle flow

A flow resonance accompanied by the emission of acoustic tones occurs in an over-expanded convergent-divergent (C-D) nozzle when operated at comparatively low pressure ratios.This phenomenon is distinguished from conventional screech tones and is referred to as “transonic tones”.In contrast to screech tones,the peak resonant frequency for transonic tones increases with pressure ratio;the peak sound pressure level exceeds 110 dB.In this study,we investigated the basic characteristics of transonic resonance and tones using a circular C-D nozzle in an anechoic room.The effects of the boundary layer trip were also evaluated using a tripping wire for the suppressing transonic resonance and tones.The results of acoustic measurements show that several predominant peaks correspond to transonic tones.However,the boundary layer trip inside the nozzle effectively eliminated these tones and suppressed the unsteadiness of the flow inside the nozzle.     

Measurement of Supersonic Jet Noise with Optical Wave Microphone System

An optical wave microphone system is a new technique of sound measurement. This technique has been developed as a new plasma diagnostic technique to measure electron density fluctuations in the nuclear fusion research. Because the sound wave is a pressure or a density fluctuation, it is possible for this technique to measure the sound wave, too. The acoustical characteristics of the optical wave microphone system were examined by using a speaker as a first step. Next, feasibility of this device to measure jet noise was examined. It was found that the optical wave microphone system could measure the jet noise as well as a sound from speaker. Hence the optical wave microphone system can be considered one of the devices equivalent to condenser microphone. Because of these reason, this device is very convenient to scan the acoustic filed through jet flow from the inside to the out side and more preferable for not disturbing the observation field.     

三喷头蒸汽水下喷注噪声试验研究

蒸汽在过冷水下喷注过程中,由于汽泡溃灭及喷注射流扰动会产生强烈的噪声.通过试验研究了不同结构的三喷头蒸汽水下喷注在不同的水温及蒸汽流量下的噪声规律.试验结果表明:三喷头喷注A声压级噪声主要受水温、蒸汽流量及喷头结构的影响,其中水温对喷注噪声的影响最大;喷注噪声随着蒸汽流量的增加而增大;在试验范围内,减小喷头的孔径或相对孔间距,可以减小喷注噪声.     

Forced convective heat transfer by swirling impinging jets issuing from nozzles equipped with twisted tapes

Forced convective heat transfer on the impinged plate associated with swirling impinging jets (SIJ) issuing from nozzles inserted by twisted tapes has been investigated. Swirling impingement jets with several swirl rates were generated by twisted tapes at different twist ratios (y/W = 3, 4, 5, and 6). The experiments were performed by locating nozzle at 4 different jet-to-plate spacings of L/D = 2, 4, 6 and 8. A jet Reynolds number varied between 4000 and 16000. Attributing to a high momentum transfer rate, an efficient heat transfer was obtained by using the jet with a small jet-to-plate spacing and the twist tape with a large twist ratio at high Reynolds number. At small jet-to-plate spacings (L/D = 2 and 4), swirling impinging jets gave higher heat transfer rate than conventional impinging jets while at large jet-to-plate spacings (L/D = 6 and 8), the opposed result was obtained. Over the range examined, only SIJ induced by the twisted tape at a twist ratio of 6 consistently provided higher average Nusselt numbers than CIJ.     

Experimental investigation on starting process of supersonic single-stage air ejector

This paper deals with experimental study of flow field of starting process in two-dimensional, single-stage supersonic ejector on different air total pressure. Schlieren pictures of flow field were taken, static pressure distribu-tions on side wall were measured. The obtained results show that, on critical pressure, the starting main shock waves in ejector oscillated back and forth between the second throat and the middle section of the mixing chamber, it causes the pressure in the second half of the mixing chamber acutely fluctuated .When the working pressure of the active flow is higher than the critical starting pressure, ejector starts normally and the inner flow-field of the mixing chamber keeps stable and the shock waves in the second throat have a certain degree of oscillation . After ejector starts, the operating pressure of the active flow may be lower than the starting pressure .     

Experimental investigation on the microscopic characteristics of underexpanded transient hydrogen jets

The microscopic characteristics of hydrogen jet affect the concentration distribution and gas-air mixing of high-pressure gas jets for a hydrogen engine. In this study, the schlieren method was used to record the hydrogen jet for an outward-opening injector. The results showed that the jet asymmetricity was large in the initial stage of the jet development, then decreased rapidly as the jet developed, and finally entered a stable range of 13%–38% when the valve fully opened. The increase of the ambient pressure was beneficial to reduce the fluctuation range of the asymmetricity. The steady S of an outward-opening injector (S = 0.7 ± 0.03) is larger than that of a single-hole injector (S = 0.25 ± 0.05). The increase in injection pressure and the decrease in ambient pressure caused S to decrease to 1 faster. The steady Γ of an outward-opening injector was experimentally determined, and the value (Γ = 0.98 ± 0.1) was smaller than that of a single-hole injector. According to the transient Γ, the injection process can be divided into the developing phase and the self-similar phase. Γ increased rapidly with time and then stabilized after the injector valve reached its maximum lift position. The gas concentration distribution was uneven along the axial direction. The mole fraction decreased with the increase in axial penetration, and entered a stable range of 0.27–0.37 when axial penetration was greater than 20 mm. The average equivalent ratio was large at the beginning of injection under different pressure ratios, and then decreased and finally stabilized in the range of 1.4–5.     

Temperature distribution on inclined plate caused by interaction with supersonic jet

IntroductionThe phenomena of the impingement of a supersonicjet on a solid obstacle is very interesting and importantin relation to the aeronautical and other industrialengineerings[']-l']. A large number of papers hadconcerned with the impingement on a perpendicularplate and evidenced the flow phenomena and thepressure distribuhon on a plate and so on. However, theinvestigation of the impingement on an inclined platecan only be found in merely several papers['-']. Ih thesestudies, it is menti…     

化工废液喷嘴冷态雾化性能实验研究

用焚烧法处理有机废液不仅有良好的经济效益，而且有重大的环境效益。但在实际使用中，经常出现废液喷嘴烧损的现象，为此以油和压缩空气为介质，对内混式化工废液喷嘴进行了实验研究，获得了这类喷嘴的流量特性曲线及实验雾化角。实验发现并获得了液流量为零的流量阻塞压力比。在实际运行中，液气压力比必须大于流量阻塞压力比，才能确保安全可靠运行，否则废液喷嘴极有可能烧损。     

Numerical simulation on supersonic flow in High-Velocity Oxy-Fuel thermal spray gun

This paper analyzes the behaviour of coating particles as well as the gas flow both inside and outside of the High-Velocity Oxy-Fuel (HVOF) thermal spray gun by using a quasi-one-dimensional analysis and a numerical simulation. The HVOF gun in the present analysis is an axially symmetric convergent-divergent nozzle with the design Mach number of 2.0. From the present analysis, the distributions of velocity and temperature of the coating particles flying inside and outside of the HVOF gun are predicted. The velocity and temperature of the coating particles at the exit of the gun calculated by the present method agree well with the previous experimental results. Therefore, the present method of calculation is considered to be useful for predicting the HVOF gas and particle flows.     

含NaCl添加剂细水雾对不同燃料池火灭火性能的实验研究

为了提高普通细水雾的灭火有效性，克服不足之处，研究了含NaCl灭火添加剂的细水雾与油池火相互作用的过程。重点研究了不同质量分数、不同工作压力、不同燃料条件下，细水雾的灭火有效性。研究结果表明，向细水雾中加入NaCl，显影响其灭火性能。细水雾的灭火对间与加入的NaCl质量分数间呈现出“W”形的关系曲线；细水雾喷头工作压力和燃料性质也影响细水雾的灭火性能：工作压力越高，平均熄灭时间越短；在相同的实验条件下，含NaCl添加剂的细水雾熄灭煤油火的时间小于酒精火的时间。含添加剂细水雾的灭火有效性，存在着细水雾与灭火添加剂的最优质量分数配比关系。     

密闭容器内蒸汽水下喷注噪声试验研究

蒸汽水下喷注加热在工业及民用领域有着广泛的应用。蒸汽水下喷注加热过程中,由于汽泡体积的急剧变化以及喷注过程中产生的扰流作用,如设计或运行不当,在水空间中会产生强烈的噪声并引起振动,严重时会危及设备运行的可靠性。试验研究了密闭容器内喷头结构及布置参数和设备运行参数对三喷头蒸汽水下喷注噪声的影响规律,并根据试验结果得到了蒸汽水下喷注噪声计算公式,结果表明:常压下蒸汽水下喷注噪声随着水温的升高先逐渐增大而后迅速减小;随着水温的进一步升高,筒体内压力逐渐升高,蒸汽水下喷注噪声缓慢减小,且明显低于水温较低时;蒸汽水下喷注噪声随着蒸汽流量的增加而增大,不过增大幅度逐渐减小,且喷注噪声峰值在较高水温时出现;喷头结构及其布置参数对蒸汽水下喷注噪声有一定影响,但影响程度较小。     

不同排烟条件下细水雾扑灭油池火的实验研究

通过全尺寸模拟实验研究了细水雾和机械排烟共同作用下小室火灾烟气特性.实验中通过改变变频器频率来调节风量和风速,通过封堵来改变排烟口位置及其开口大小,研究分析了细水雾和机械排烟共同作用下火场的温度和CO体积分数变化.实验结果表明,随着排烟强度的增加,细水雾灭火时间延长,烟气平均升温速率和平均降温速率均下降,CO体积分数降...     

DNS investigation on flame structure and scalar dissipation of a supersonic lifted hydrogen jet flame in heated coflow

Direct numerical simulation (DNS) of a three-dimensional spatially-developing supersonic lifted hydrogen jet flame has been conducted in this paper. The scalar structure of the lifted flame is investigated through instantaneous images and conditional means of combustion statistics. And then the scalar dissipation rate and its implications on the flamelet-based combustion modeling are analyzed in detail. It can be found that most of the heat release occurs in the subsonic region. However, distributed reaction pockets exist in the sonic mixing layer due to the rolled up vortices. The magnitude of conditional compression or expansion rate of the fluid presents comparable to the corresponding heat release rate, and takes a great influence on the flame temperature in the high speed reacting flow. The probability density functions of mean conditional and unconditional scalar dissipation rate prove to qualitatively agree with the presumed log-normal distribution, while a little skewed to the higher scalar dissipation rate in the sonic mixing layer. The conditional mean scalar dissipation rate presents to be radial dependent at the flame base, especially in the fuel lean mixture. The DNS results show good agreement with the trends of the flamelet calculations; however, the amplitudes of temperature are far lower than the corresponding flamelet statistics due to finite rate reaction and expansion of the high speed reacting flow.     

Experimental and numerical study on heat transfer enhancement in a channel with Z-shaped baffles

The influence of baffle turbulators on heat transfer augmentation in a rectangular channel has been investigated experimentally and numerically. In the experiment, the baffles are placed in a zigzag shape (Z-shaped baffle) aligned in series on the isothermal-fluxed top wall, similar to the absorber plate of a solar air heater channel. The aim at using the Z-baffles is to create co-rotating vortex flows having a significant influence on the flow turbulence intensity leading to higher heat transfer enhancement in the tested channel. Effects of the Z-baffle height and pitch spacing length are examined to find the optimum thermal performance for the Reynolds number from 4400 to 20,400. The Z-baffles inclined to 45° relative to the main flow direction are characterized at three baffle- to channel-height ratios (e/H = 0.1, 0.2 and 0.3) and baffle pitch ratios (P/H = 1.5, 2 and 3). The experimental results show a significant effect of the presence of the Z-baffle on the heat transfer rate and friction loss over the smooth channel with no baffle. The Nusselt number, friction factor and thermal performance enhancement factor for the in-phase 45° Z-baffles are found to be considerably higher than those for the out-phase 45° Z-baffle at a similar operating condition. The in-phase 45° Z-baffle with larger e/H provides higher heat transfer and friction loss than the one with smaller e/H while the shorter pitch length yields the higher Nu, f and TEF than the larger one. The numerical work is also conducted to investigate the flow friction and heat transfer behaviors in the channel mounted with the 45° Z-baffles, and the numerical results are found in good agreement with experimental data.     

Experimental investigation of the effects of heat release on mixing processes and flow structure in a high-speed subsonic turbulent H2 jet

In this paper, we explore the effects of heat release on mixing and flow structure in a high-speed subsonic turbulent H₂ jet in an air coflow. Heat release effects are determined from the comparison of nonreacting and reacting jet behavior, boundary conditions being identical in both cases. Experiments are performed in a wind tunnel specifically designed for this purpose. Planar laser induced fluorescence on OH radicals and on acetone (seeded in the hydrogen jet) are used to characterize the cartography of scalars, and laser Doppler velocimetry is used to characterize velocity profiles in the far field of the H₂ jet. Results show significant effects of heat release on mixing and flow structure, indicating an overall reduction of mixing and entrainment in the reacting jet compared to the nonreacting jet. First, a change is observed in the orientation of coherent structures originating from Kelvin-Helmholtz type instabilities, and responsible for air entrainment within the jet, which appear “flatter” in the jet flame. Then, the flame length is increased over what would be predicted from the intersection of the mean stoichiometric contour with the centerline of the nonreacting jet. And finally, the longitudinal average velocity decrease along the jet axis is quicker in the nonreacting jet, and nondimensional transverse velocity fluctuations are about half as high in the reacting jet as in the nonreacting jet, indicating a reduction of the turbulence intensity of the flow in this direction in the jet flame.