大气环境下柴油机尾排气管口边界条件及声辐射

在用不稳定流动方法模拟柴油机大气环境条件下排气噪声中，本给出了考虑尾管口处压力波动的非等熵准定常边界条件，而在排气声辐射采用Ｈｏｗｅ公式。两相结合，能够从管口透射声功率的角度从管口辐射噪声进行更为合理的描述。     

柴油机水下排气管口边界模型

柴油机废气排入水中所形成的尾管口边界比较复杂。在柴油机水下工况数值模拟中,一般采用定压模型。本文从流体力学的基本方程出发,根据对水下排气管口两相流动状态的观察和管口气泡形成过程的研究,建立了柴油机水下排气不稳定流动管口边界模型。与定压模型相比,该模型不仅能反映出管口的压力波动,而且可以模拟管口处气泡的形成过程及其与管口内气体之间的相互作用。与1维不稳定流动方法相结合可以进行柴油机水下工况的特性分析,特别是水下排气管口声辐射的数值模拟。     

Rijke管声学特性数值研究

应用计算流体动力学(CFD)技术对Rijke管内的声学特性进行了研究.采用内外流场耦合建模方法研究Ri-jke管内流场,得到了相应的管口声压和声泄漏值.提出热源周围空气非定常热流量与声压、脉动速度之间关系的假设.依靠自身参量如声压、速度和温度的波动来激发Rijke管振荡,模拟了Rijke管的声学特性,得到管口声压分布云图,分析了模拟中出现的物理现象,解释了Rijke管自激励振荡的机理.进行了Rijke管管口截面收缩的模拟实验,得到了管口收缩能使噪声降低的结论.     

动力设备中蒸汽喷注噪声的研究

探索了动力设备中蒸汽在水下有限空间内喷注时喷注噪声的声源与辐射规律，发现蒸汽喷注噪声声压级及频谱和水的过冷度有直接关系；喷头孔径对最大喷注噪声声压级的影响不大；但较大的孔径可以使最大声压级出现的位置向低温处移动，而较小的孔间距可降低喷注噪声。同时发现蒸汽水下喷注噪声的产生机理和空气动力性噪声的产生机理是不完全相同的，因此蒸汽或空气在大气中喷注噪声的研究结果不能直接应用于蒸汽水下喷注噪声。图９参１１     

里克管的数值模拟

提出热源周围空气不稳定的热量与声压、脉动速度之间关系的新假设.在无外加强制作用下实现里克管脉动工况数值模拟,与里克管作为自激热声装置的实际情况一致.以外流场处理方法研究里克管内流场问题,进一步研究里克管的管口效应,以降低由它引起的高强度噪声,为里克管型强化脉动燃烧、强化传热装置产品化打下基础.通过实验,验证了模拟的正确性,并分析了模拟中出现的物理现象.     

水平布置水下排气管倒流界限试验及分析

为防止水下排气柴油机发生海水倒灌事故,对水下排气管内发生水倒流的条件及影响因素进行了试验和分析。由水平管内气液逆向流动特性研究现状分析,指出气相惯性力与液相重力的相对大小是排气管内水倒流发生的主要物理机制。由内径90mm水平管在3m水深下排气的试验结果发现:防止水下排气管内水倒流发生的界限气速存在一个与管内滞留水长相关的范围,随管内滞留水长由0.3m增至1.8m,界限表观气速由13.0m/s降至7.8m/s,界限气相Wallis数由0.72降至0.55。最后以某柴油机排气参数为基础,按界限气相Wallis数等于0.60计算分析了排气管管径、排气口水位深度及排气温度等因素对管内倒流界限气速的影响,结果表明:水平管的倒流界限气速随管径增大而增大;排气口水位深度和排气温度的变化影响排气密度大小,也对排气管倒流界限气速有影响。     

提高发动机性能的双自由度排气系的开发

本文研究了排气支管长度和叉形管长度对排气口处压力脉动特性的影响,并开发出最佳的双自由度排气系统。通过优化支管和叉形管的长度,大大提高了发动机在宽广范围内的扭矩特性。由于排气管内的驻波引起了冲击波,发动机在部分转速下会辐射异常噪声。通过优化支管和叉形管长度,可显著地降低这种噪声。     

柴油机水下排气口气液两相流动状态研究

利用摄像机拍摄和重放，观察一研究柴油机水下排气从水面气液两相流动状态有管口气泡形成、上浮和破碎整个物理过程。发现，整个两相流动状态可明显地分成气泡形成、自由气泡、气及水面喷涌四个区域。若对两相流动轮廓平均，则类似于浮力射流结构。水介质以一定速度流动时，从水下排气口到水面气液两相流的流动状态相似，而流动轨迹与静水中的情况不同。     

高压比离心压气机气动噪声数值预测

采用混合计算气动声学方法研究了高压比离心压气机气动噪声,首先计算了压气机非定常流动,获取了声源面上的时域脉动压力,进而通过间接声学边界元法(IBEM)预测了压气机气动噪声,并在增压器性能试验台上完成了相应的噪声测试.结果表明:压气机气动噪声主要由叶片通过频率及其倍频出现的离散单音噪声与宽频噪声组成,且总声压级由离散单音噪声决定;对比计算和试验得到的监测点声压级频谱可以看出二者基本吻合,说明数值仿真具有较高的精确度,使用的仿真方法可应用于高压比离心压气机的噪声预测;压气机气动噪声自进气管口向外辐射时,声压级分布并不均匀;且受频率影响,不同频率噪声的传播能力存在明显差异.     

消声元件安装位置对消声效果的影响

分别以消声量和管口阶次噪声作为评价指标,研究消声元件安装位置对消声效果的影响规律,设计在直管不同位置安放四分之一波长管的消声量测量试验对消声量评价。结果表明:设计频率声模态下,在反节点处安装波长管消声量最大,与理论分析完全吻合;进一步对消声量进行仿真计算,结果与试验完全相符。对于管口噪声指标评价,设计频率为267 Hz的谐振腔解决某发动机4000 r/min时进气管口存在4阶噪声引起的噪声峰值问题,对比谐振腔安装在进气管路该频率附近的声模态反节点和节点位置的管口4阶噪声,可知谐振腔布置在反节点位置比节点位置噪音降低了24 dB。两个案例充分证明声学系统模态反节点位置是消声元件的最佳布置位置。     

Jet characteristics from a submerged combustion system

The characteristics of a combustor operating under submerged conditions are affected by the two phase interaction of exhaust gas jet from the combustor with surrounding liquid. The characteristics of combustion gases are simulated with air and helium to represent combustor operation under different conditions. The exhaust gas signatures under submerged conditions are examined using different nozzle exit cross-sections (circular, square, triangular and elliptical with aspect ratio of 1.5 and 2.5) for their effect on sound pressure levels and pressure fluctuations in the combustion chamber. High-speed cinematography is used to examine the two-phase region and the associated instabilities by the gas jet. Dynamic pressure sensor is used to study the effect of submerged jet on the pressure fluctuations in the upstream gas chamber. The sound pressure level from the elliptical nozzle is found to be lower than the circular, square and triangular nozzles. The frequency of jet instabilities is observed to increase with increase in gas jet momentum but independent of nozzle exit cross-section. The pressure fluctuation in the gas chamber is closely coupled with two phase instabilities downstream of the jet region. At lower jet momentum bubbling regime is present but it transitions to more jet like behavior with increase in the jet momentum, representing deep water and shallow water propulsion applications, respectively. These studies provide valuable fundamental information for range of applications in energy systems extending from underwater propulsion, evaporator, heater, desalination and waste water treatment.     

基于CFD分析的车辆冷却风道综合性能评价

为了优化车辆冷却风道,提出了基于计算流体力学(CFD)分析的车辆冷却风道综合性能评价方法,建立了风道CFD模型并对模型精度进行了试验验证,确定了由CFD分析可获得的风道总压降、散热器压降、风扇消耗功率和排气百叶窗出口温度作为风道综合性能评价指标,应用隶属度线性加权规划法建立了评价模型。实例应用显示,在动力舱空间尺寸允许并保证满足设计散热量的前提下,增大散热器芯体的宽度和长度、蜗壳出口高度和侧出口高度、排气百叶窗尺寸和减少进气百叶窗叶片,可以使冷却风道的综合性能得到提高。     

基于三维数值分析的柴油机排气消声器性能改进

针对某柴油机排气噪声过大问题,设计适合独立测试排气噪声的台架试验,基于测试的排气频谱,结合数值仿真,研究了原消声器的性能缺陷,其中高频是重点针对的频段．通过减小穿孔直径、减少穿孔结构、增加共振腔和设计理想扩张腔,综合考虑改进方案的压力损失、流体再生噪声和声学性能,其仿真性能表现较原消声器有较大提高．最终,通过发动机台架...     

柴油机工作过程的频谱分析和相关分析

本文应用计算机程序对柴油机工作过程信号进行了频谱分析和相关分析。说明了它们在气缸压力、进排气压力的频域分析,燃烧噪声、进排气噪声分析,示功图测压通道效应、各缸工作过程均衡性分析,发火间隔检测,工况监测等方面的应用。     

柴油机排气噪声有源控制的试验研究

尝试利用自适应有源噪声控制技术对４－８５柴油机低频噪声进行空间 消声,取得了一定的消声效果。首先对４－８５柴油机排气噪声频谱进行了分析,然后提出了针对此低频排气噪声应用有源噪声控制技术的可能性。其次讨论了妨碍应用有源噪声控制技术的三大困难,并找出了解决方法。引入次级声场的理论推导并完成了引入次级声场装置的设计、制造。通过对排气噪声信号与排气管上不同位置振动信号的相干性分析,选取参考输入信号,提出了采用自适应有源噪声控制技术,为试验提出了依据,对系统进行了建模并编程,对试验结果了记录并分析。     

Influence of confinement on combustion instabilities of premixed flames stabilized on axisymmetric baffles

The frequency and strength of combustion-induced oscillations have been measured for premixed flames stabilized on baffles located on the axis of a pipe. With the baffle located between 5 and 20 diameters upstream of the pipe exit, the frequency corresponds to longitudinal (standing quarter-wave) acoustic waves in the cold gas column upstream of the baffle and the strength is only weakly dependent on axial location, area blockage ratio (0.25 and 0.5), forebody shape (disk or cone), and Reynolds number (<70,000); with a 0.9 blockage ratio, a half-wave mode was observed. The instability is present in the range of equivalence ratios (defined as the fuel-to-air ratio divided by the stoichiometric fuel-to-air ratio) from 0.8 to 1.2. The pressure coefficient based on the difference in mean pressure between the center of the baffle and pipe wall is a sensitive measure of the onset of oscillations.Combustion-induced oscillations, with a predominant frequency and large amplitude, were not observed when the baffle was located between approximately 1.5 and 5 pipe diameters from the pipe exit. As the baffle was moved from the 1.5 diameter location toward the pipe exit, increasingly strong oscillations were observed with equivalence ratios between 1.1 and 1.5. The frequency of these oscillations increased with decreasing equivalence ratio and increasing Reynolds number. Instability could be obtained with the baffle between 1.5 and 5 diameters of the pipe exit by constricting the diameter of the exit plane.The difference in behavior between confined and unconfined flames is ascribed to the existence of a self-induced favorable pressure gradient in the confining duct. It is proposed that the instantaneous value of this gradient influences the rate of heat released by the flame through changes in the rate of turbulent mixing due to the preferential acceleration of products of combustion over reactants.     

Time periodic flow boiling heat transfer of R-134a and associated bubble characteristics in a narrow annular duct due to flow rate oscillation

An experiment is conducted here to investigate the effects of the imposed time periodic refrigerant flow rate oscillation in the form of nearly a triangular wave on refrigeriant R-134a flow boiling heat transfer and associated bubble characteristics in a horizontal narrow annular duct with the duct gap fixed at 2.0 mm. The results indicate that when the imposed heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears in which nucleate boiling on the heated surface does not exist in an entire periodic cycle. At somewhat higher heat flux persistent boiling prevails. Besides, the refrigerant flow rate oscillation only slightly affects the time-average boiling curves and heat transfer coefficients. Moreover, the heated wall temperature, bubble departure diameter and frequency, and active nucleation site density are found to oscillate periodically in time as well and at the same frequency as the imposed mass flux oscillation. Furthermore, in the persistent boiling the resulting heated wall temperature oscillation is stronger for a longer period and a larger amplitude of the mass flux oscillation. And for a larger amplitude of the mass flux oscillation, stronger temporal oscillations in the bubble characteristics are noted. The effects of the mass flux oscillation on the size of the departing bubble and active nucleation site density dominate over the bubble departure frequency, causing the heated wall temperature to decrease and heat transfer coefficient to increase at reducing mass flux in the flow boiling, opposing to that in the single-phase flow. But they are only mildly affected by the period of the mass flux oscillation. However, a short time lag in the wall temperature oscillation is also noted. Finally, a flow regime map is provided to delineate the boundaries separating different boiling regimes for the R-134a flow boiling in the annular duct.     

增压器对排气管噪声频率特性的影响

测量了增压和非增压6-135柴油机排气管噪声的特征参数。通过对测试结果进行对比分析,得出了增压器对柴油机排气管噪声频率特性的影响规律,并从气体动力噪声和辐射噪声两方面分析了增压器对噪声频率特性的影响。结果表明:与非增压柴油机相比,增压柴油机排气管噪声的频谱主要由一次谐波和一次谐波的多次分量构成,主频频率等于基频频率。     

排气噪声预测的改进研究

作者曾用Benson发展的特征线法模拟内燃机排气系统中的不稳定流动,作为噪声预测的基础,但预测的排气噪声频谱在1kHz以上不能令人满意。针对于此,本文提出了一种改进的特征线算法,旨在改善高频段噪声频谱的预测精度。计算与实测对比表明,其他参数(压力、速度等)与Benson法的结果一致,而噪声预测精度有很大提高,在3kHz以下均能较好地反映实测值。这种算法可作为气流声学分析、排气噪声预测的有效手段。