气流式喷嘴射流火焰噪声的试验研究

研究气化燃烧反应对射流火焰噪声的影响,并对火焰噪声的特性进行分析.通过快速傅里叶变换(FFT)及声压级计算等方法,对火焰噪声信号进行频谱分析.结果表明,火焰噪声包含燃烧噪声和喷流噪声,主要是300Hz以下的低频波段噪声;火焰噪声以燃烧噪声为主,火焰根部噪声为喷流噪声;氧化剂比燃料更能影响火焰噪声,完全燃烧时,漩涡的大量产生导致更强的噪声.     

气流床撞击火焰声学特性与压力波动分析

利用多喷嘴对置式气流床气化炉热模试验装置,对两喷嘴对置撞击火焰声学特性和压力波动进行了测量与实验研究,并通过Hilbert-Huang变换对火焰噪声信号及压力信号进行频谱分析。结果表明,气化炉内中高频压力波动主要由火焰撞击区内复杂的燃烧状况引发,并产生50~100 Hz频段的火焰噪声。射流火焰噪声受气化炉内低频压力波动影响,并且因来自撞击区的反向流的作用,火焰噪声的幅值和波形都会受到一定的影响。撞击区的火焰噪声信号与燃烧状况有一定的对应关系,可作为气化炉内火焰状况诊断的一种方式。     

气流式雾化火焰声压分布特性试验研究

研究气流式雾化火焰声压分布特性,建立火焰长度与声压分布之间的关系,分析不同火焰长度对火焰噪声辐射特性的影响。结果表明:火焰噪声声压场可根据火焰长度L划分为三个区域:近场区、衰减区和远场区;近场区声压值最大,为气流式雾化火焰噪声的产生区域,燃烧噪声为主要噪声源;衰减区为噪声的最佳采样区域,可测得噪声的总声压值;远场区声压变化幅度很小,可近似等于远场声压;随着火焰长度的增加,火焰噪声的指向性越明显,辐射强度越大。     

某型飞机主起落架结构件气动噪声特性研究

对某型飞机主起落架结构件的气动噪声特性进行了声学风洞试验和数值仿真分析。在声学风洞中,借助麦克风测量获得不同来流速度下噪声频谱特性。采用分离涡方法仿真模拟起落架周围非定常湍流流场,通过涡声理论计算声源的强度和位置,利用FW-H方程积分外推法求解声场,分析噪声的频谱特性、远场指向特性及其产生的机制。结果表明：起落架结构件噪声频谱特性仿真结果与试验结果在低频和中频段吻合较好;起落架结构件噪声呈现宽频噪声的特性;频谱曲线以及频谱中优势频率均随着来流速度的增加而增大;仿真曲线中的能量峰值分别对应于缓冲器与支柱、缓冲器与摇臂的干扰噪声;形成此噪声源的压力脉动位于缓冲器下部表面区域;缓冲器和摇臂是总噪声的主要贡献源,支柱对总噪声的贡献量最小;噪声辐射呈现非对称的指向特性。     

火焰燃烧区域空气声速与温度关系的实验研究

为使声学方法能对火焰温度进行精确测量,实验研究了火焰燃烧区域中空气声速与温度的关系。首先对非火焰气体环境中的声速与温度进行测量,然后在此基础上对不同燃料燃烧的火焰区域进行声速测量实验,并结合热电偶测得火焰温度,进而得到火焰中空气声速与温度的关系。结果表明:在固定距离下,与室温空气环境相比,高温烟气环境会使声波的传播时间减小,火焰环境会使声波的传播时间变长;在非火焰区域,空气声速与温度的关系符合理想气体中声速与温度的关系;在火焰燃烧区域,空气声速与温度关系偏离理想气体的声速与温度方程,与按照理想气体计算的声速结果相比,实际声速测量值偏小;对于同种燃料的火焰,随着火焰温度升高会出现空气中的声速减小的现象。     

汽车外表面气动噪声特性分析

以荣威750为研究对象,通过声学风洞实验手段对车辆后视镜表面、侧窗表面及其附近流场,以及外场的气动噪声特性进行测试分析;在对数值计算结果验证分析之后,通过数值计算手段以流场脉动压力标准差为评价指标并结合速度场特征,分析车辆表面的压力脉动特性及其产生的原因,在此基础上对车辆表面的噪声大小和分布以及频率特性进行计算分析。研究表明车辆的气动噪声主要能量集中在中低频,频带较宽,不同部位特性差异较大;表面压力脉动是表面气动噪声产生的根本原因,压力脉动大的地方气动噪声亦大;气动噪声大的位置是发生气流分离,湍流运动比较剧烈的区域。就该款车而言,气动噪声主要出现在汽车头部上方、前后挡风玻璃边沿、车顶、A柱、侧窗、后视镜以及车尾和轮胎部分位置处。     

整车声学管理过程及关键技术研究

要想根本性地处理好轨道车辆的噪声问题,必须从研发初始阶段就对声学要求和设计进行管控,通过不断平衡用户、车辆制造商、部件供应商三者之间的关系,才能从整体上得到满意的结果。整车声学管理的过程可分为投标、研发、设计和验收四个阶段;提出整车声学管理的系统解决思路和管理过程中的关键技术问题;提出噪声源声学数据信息、声学结构数据信息的程序化管理方法和实现过程;建立基于声学数据库的整车噪声预测工具,其具有操作简便、计算快捷、声学信息全面等优点,可为车辆声学设计提供分析工具和设计参考。     

高铁高架车站候车环境噪声的数值预测

结合某实际高铁高架车站候车厅,分别建立候车厅的有限元、边界元模型与统计能量法模型,采用直接边界元法计算20～200 Hz范围内的低频噪声,用统计能量法计算200～2 000 Hz范围内的高频噪声。计算结果通过实测结果进行验证,结果表明模型能够较准确地预测出候车厅内的噪声水平。进一步对候车厅各部分进行声学板块贡献度分析以及声腔子系统的声学贡献度分析,结果表明候车厅建筑顶棚对于候车厅内噪声的贡献水平最大,为候车厅的降噪提供指导。     

某拖拉机驾驶室声学特性分析与改进

为了降低拖拉机驾驶室内中高频噪声,建立基于统计能量分析(SEA)方法的驾驶室内噪声预测模型。通过理论计算和试验方法确定模型的基本参数和激励输入,通过仿真与试验对比验证了SEA模型的准确性。最后对驾驶室内声腔的功率输入分析,得到对驾驶室内噪声贡献较大的板件子系统,据此提出有针对性的声学包装改进方案,仿真结果表明该声学包装设计方案可以有效降低驾驶室内中高频噪声,总声压级降低1.87 d B(A),为拖拉机驾驶室内噪声控制及声学包装优化提供有效依据。     

VA One在舱室噪声预报中的应用

介绍统计能量法的基本理论,并以某科考船为分析对象,利用VA One软件进行舱室噪声预报,分析声学材料、阻尼材料及空调噪声对舱室噪声的影响。结果发现:添加声学材料后,舱室噪声下降显著;3种阻尼方案总体上来说对舱室噪声影响差别不大,但从重量方面考虑,方案二优于其它2种;选用具有良好降噪效果的布风器,可减小空调噪声对上层建筑舱室噪声的影响。     

Experimental Investigation of Buoyant Laminar Jet Diffusion Flames in an Inverted Configuration

Buoyant laminar jet diffusion flames are studied experimentally in an inverted configuration, where gaseous fuel-stream jets vertically downward into air. Flame shape, thermal structure, soot and stability behaviors are obtained until the blowoff limit is reached. By comparing with conventional jet flames, which are established when the fuel jets upward, the effects of buoyancy on laminar diffusion flames are analysed. Downward flame yields larger flame height, although the non-dimensional flame height increases linearly with the Reynolds number at nozzle exit, which is similar to upward flame. Possible reasons for the increased flame height include flow deceleration within downward buoyant flames and presence of more combustion products surrounded the jet stream, thus slowing mixing process between fuel and air. The different relative directions of buoyant flows and jet streams also result in different temperature distributions in downward and upward flames, and a stagnant interface produced by the balance between buoyant flow and jet stream is particularly observed downstream of downward flame. Downward flames contain more soot and the soot formation region is wider, which are mainly attributed to the modifications of flow field and soot path. In addition, downward and upward flames stabilize at different axial positions relative to the nozzle exit. Because of increased characteristic flame residence time, downward flames have higher blowoff limits. The downward jet flame provides an alternative configuration to upward jet flame in studying buoyant diffusion flames due to the different manifestations of buoyancy effects.     

Spatially resolved laser-induced breakdown spectroscopy in methane-air diffusion flames

A new setup for spatially resolved laser-induced breakdown spectroscopy (SR-LIBS) is used for the first time to analyze methane-air diffusion flames. Using this configuration, background continuum emission is reduced, signal-to-background noise ratio is increased up to eight times, and spatial resolution is enhanced. The local equivalence ratio is also quantitatively estimated and the width of the secondary combustion region at a specified height above the burner is determined for two different methane flow rates. Furthermore, the threshold energy for spark formation is measured for regions inside and outside the flame. The results show that threshold energy is larger at the secondary combustion region, near the border of the flame, than inside the flame.     

Modeling the characteristic types and heat release of stretched premixed impinging flames

 A numerical code has been developed for the simulation of the impingement of a turbulent jet on a plane surface. The performance of three turbulence models is assessed under isothermal conditions. Predictions are compared with experimental data from the literature. Based on the results an appropriate turbulence model is selected to model a premixed jet flame impinging on a solid surface. Mass transfer and combustion are modeled with a two-equation model simulating volumetric and kinetically controlled chemical reaction rates. Modeling of heat transfer accounts for convection and radiation effects. Results show that under high pressure environment turbulent premixed flames are of wrinkled-thickened type near the outlet of the nozzle (free jet region) and of wrinkled reaction sheets in the area near the surface (impingement region and radial wall jet). The results establish that appropriate choice of turbulence and combustion models can lead to accurate prediction of the flow characteristics.     

Asymptotic approach to combustion instability

This paper presents an asymptotic approach to combustion instability in premixed flames under the assumptions of large activation energy and small Mach number. The entire flow consists of four distinct yet fully interactive sub-regions, which accommodate the chemical reaction, heat transport, hydrodynamics and acoustics, respectively. A reduced system was derived to describe the intricate coupling between the flame and acoustics that underlies the combustion instability. The asymptotically reduced system was employed to study the weakly nonlinear interaction between the Darrieus-Landau instability and the longitudinal acoustic mode of the combustion chamber. The general asymptotic formulation includes the influence of enthalpy fluctuation in the oncoming mixture. It is shown that one-dimensional enthalpy fluctuation, through its interaction with flame, produces sound waves, and may cause parametric instability of the flame. The mutual coupling between the sound wave and parametric instability is analysed at the instability thresholds.     

Buoyancy influence on wrinkled premixed V-flames

Flame-flow interaction is crucial to the application of combustion. The flow field in which a flame is embedded depends on not only the coming flow but also the flame itself. Flame-generated flow will determine how the flame changes the flow field. Buoyancy is an instinctive feature of flames and will induce flow. To study how buoyancy influence flames will lead to better understanding on the mechanism of flame-flow interaction. CH₄-Air premixed V-flames of wrinkled flame fronts were studied in 1g₀ and μg experiments to observe buoyancy influence. In assistance to the experiments, the mechanism of buoyancy influence on V-flames was analyzed and the ratios of buoyancy force to corresponding dynamic force in the coming flow were estimated. The experiment was done in the Bremen Drop Tower and OHPLIF was used in both 1g₀ and μg to record the instantaneous flame fronts. The results show that buoyancy effect on the laminar flame differs form that on the turbulent flame. This suggests the existence of the buoyancy influence on flame wrinkling and the coupling between buoyancy and the coming flow. Another observation is that the laminar exhibited wrinkles in μg and this implies that μg is an ideal environment for the study of both buoyancy influence and other mechanisms generating flow in flames.     

Dynamic and scalar turbulent fluctuation in a diffusion flame of an-axisymmetric methane jet into air

 A study of turbulence/combustion interactions in a relatively large turbulent diffusion flame of an axisymmetric methane jet into air is presented. A first order k–ɛ turbulence closure model is used along with two different models (equal scales and non-equal scales) for the submodel describing the scalar dissipation rate. The flamelet concept is used to model the turbulent combustion along with a joint mixture fraction/strain rate probability density function (PDF) for the prediction of the average parameters of the turbulent diffusion flame. The numerical approach is that of Patankar and Spalding, while the flamelet simulations are obtained from the RUN-1DL code of Rogg and co-workers based on a 17 species detailed reaction mechanism. The chosen configuration is that of the experimentally studied turbulent diffusion flame of Streb [1]. A comparison between these experimental results and the obtained numerical ones is thus presented. Relatively good agreements are obtained which show the usefulness of the two-scale model compared to the classical one-scale model for predicting turbulent diffusion flames. Nonetheless some discrepancies are obtained in the outer and downstream regions of the jet, especially in comparison with the experimental data. These are attributed to short coming of the considered turbulence model and soot radiation which is not accounted for. Received: 2 May 2002 / Accepted: 31 January 2003     

Analysis of Flame Extinguishment and Height in Low Frequency Acoustically Excited Methane Jet Diffusion Flame

The exploration of microgravity conditions in space is increasing and existing fire extinguishing technology is often inadequate for fire safety in this special environment. As a result, improving the efficiency of portable extinguishers is of growing importance. In this work, a visual study of the effects on methane jet diffusion flames by low frequency sound waves is conducted to assess the extinguishing ability of sound waves. With a small-scale sound wave extinguishing bench, the extinguishing ability of certain frequencies of sound waves are identified, and the response of the flame height is observed and analyzed. Results show that the flame structure changes with disturbance due to low frequency sound waves of 60–100 Hz, and quenches at effective frequencies in the range of 60–90 Hz. In this range, 60 Hz is considered to be the quick extinguishing frequency, while 70–90 Hz is the stable extinguishing frequency range. For a fixed frequency, the flame height decreases with sound pressure level (SPL). The flame height exhibits the greatest sensitivity to the 60 Hz acoustic waves, and the least to the 100 Hz acoustic waves. The flame height decreases almost identically with disturbance by 70–90 Hz acoustic waves.     

Influence of an acoustic field on flame development and transition to detonation

The work is devoted to experimental and numerical study of flame interaction with acoustic waves in closed and semiclosed pipes filled with preliminarily mixed gaseous mixtures. We analyze the influence of eigenfield (generated by the flame itself) and external acoustic field on the flame dynamics. We show that acoustic field affects the combustion process at all stages. The effect increases with any increase in the energy of initiation of combustion. At later stages of flame development, acoustic waves can initiate the transition to detonation or prevent it. Thus, it is possible to control the combustion modes using external acoustic field.     

空压机排气放空复合消声器设计与性能分析

为降低空压机的高压排气放空气流噪声,给出了节流降压板、片式消声通道及小孔喷注层的消声性能经验公式计算方法,设计了一款适用于排气放空气流的复合消声器。在Virtual. Lab中建立了复合消声器的声学有限元模型,开展了消声器的传递损失性能分析。仿真结果表明:声学有限元仿真与经验公式计算的传递损失结果较为接近;所设计的高压排气放空复合消声器在20 Hz~3 000 Hz宽频段内具有理想的消声效果,平均消声量可达57 dB。