Numerical Investigation on Vortex Structure and Aerodynamic Noise Performance of Small Axial Flow Fan

The details of unsteady flow field in small axial fans are described and the relationship between the internal flow char- acteristics and aerodynamic noise of small axial flow fans are explored in the manuscript. Firstly, the broadband noise model is introduced to calculate the distributions of broadband noise sources in fan’s internal flow field, and further fan’s internal flow characteristics affecting broadband noise sources are analyzed by the main distributions of broad- band noise sources. Secondly, the unsteady characteristics of vortex structure in fan’s internal flow field are analyzed by large eddy simulation, and FH-W acoustic model is introduced to calculate aerodynamic noise affected by the unsteady characteristics of vortex structure. Finally, Monitoring points are set up near and far field of small axial fans, at which sound pressure level and spectral characteristics are analyzed. The results show that broadband noise sources are mainly distributed at the tip clearance close to blade trailing edge and one third of chord length of blade trailing edge of small axial fans. The maximum sound power of broadband noise sources at the tip clearance is greater than that at blade trail- ing edge. Sound power level of broadband noise near one third of blade chord length of blade trailing edge is first in- creasing and then decreasing when the distance between radial planes and the center of fan hub increases. Fan’s internal flow characteristics affecting broadband noise sources are the tip leakage vortex and the trailing edge vortex shedding. The tip leakage vortices at the leading edge of blade tip first integrate and then break down, while vortex cores of the trailing edge vortex shedding gradually move from blade hub to one-third of blade chord length and then move to far field within a one-seventh of the rotation cycle. Within a flow passage of fan rotation period, sound pressure level of the monitoring points is relevant to the unsteady feature of vortex structure and the main bands of aerodynamic noise of monitor points are irrelevant to the unsteady feature of vortex structure, relating to the distance between the monitor points and fan. With the above distance increasing, the main bands of aerodynamic noise are widened and moved from the low bands to high bands. All conclusions in the paper will provide significant references for reducing the noise of small axial flow fans.

MEASUREMENT AND VISUALIZATION OF FLOW FIELD IN AXIAL FLOW FAN

An experimental investigation associated with the basic fluid mechanics in an axial flow fan is described in this paper. The flow field in the tip region has been studied by laser Doppler anemometer (LDA) and flow visualization technique. Some experimental data and images are interpreted to understand the complex interactions between the annulus wall boundary layer and the leakage flow. It shows that the vortex inside the blade passage is produced by the separation of annulus wall boundary layer rather than the rolling up of leakage flow.

Off-Design Performance Analysis of Axial Flow Fan in Helicopter

A theoretical calculation method of off-design performance is developed for an axial flow fan of oil cooling system in helicopter,including calculation of aerodynamic parameters and performance parameters.When calculating inlet shock loss,the shock loss coefficient is obtained by comparing results of theoretical calculation,experimental and numerical calculation.The theoretical results and numerical results show that all air velocity components increase from hub to shroud in main flow area at rated condition.Tip leakage vortex moves downstream as flow rate increases.When flow rate decreases,Re decreases,and boundary layer thickness from hub to shroud area all increases gradually.Tip leakage vortex moves upstream,and secondary loss increases.Low speed area in the passage is widened along with high speed area moving to hub area,influenced by boundary layer separation.Consequently wake area and jet area at fan outlet are both larger than rated condition.Therefore optimization design for off-design performance of the fan is required on aerodynamic parameters influencing fan loss.A reliable method is supplied for estimating altitude performance of lubricating system in helicopter.

Design of Half-Ducted Axial Flow Fan Considering Radial Inflow and Outflow

Half-ducted fan and ducted fan have been designed and numerically analyzed for investigating the radial flow effect on the overall performance and the three dimensional flow field in design. Based on quasi-three dimensional flow theory, the meridional flow was calculated by adopting the radial balance equations, while the calculation of the blade to blade flow was obtained by 2D cascade data with the correction by a potential flow theory. Two types of axial flow fan were designed. One is the full ducted case as if it was in the straight pipe and another is the half-ducted case with the radial inflow and outflow. The previous experimental results of authors were used to decide the inclinations of both the inflow and outflow. And the circular arc blade with equal thickness was adopted. The numerical results indicate that both of the designed fans can reach the specified efficiency and also the efficiency surpasses more than 11%. Furthermore, the static pressure characteristic of half-ducted fan is much better than that of ducted fan. In comparison of the three dimensional internal flow of these two fans, the improvement of the flow angle at inlet and outlet, the distributions of velocity in the flow field and the pressure distributions on the blade surfaces can be achieved more successfully in accordance with the design intension on consideration of flow angle in design. The conclusion that half-ducted design with considering radial inflow and outflow is feasible and valid in comparison with ducted design for axial flow fans has been obtained at the end of the paper.

In uence of Endwall Boundary Layer Suction on the Flow Fields of a Critically Loaded Di usion Cascade

Boundary layer suction is an e ective method used to delay separations in axial compressors. Most studies on bound?ary layer suction have focused on improving the performance of compressors,whereas few studies investigated the influence on details of the flow fields,especially vortexes in compressors. CFD method is validated with experi?mental data firstly. Three single?slot and one double?slot endwall boundary layer suction schemes are designed and investigated. In addition to the investigation of aerodynamic performance of the cascades with and without suction,variations in corner open separation,passage vortex,and concentration shedding vortex,which are rarely seen for the flow controlled blades in published literatures,are analyzed. Then,flow models,which are the ultimate aim,of both baseline and aspirated cascades are established. Results show that single?slot endwall suction scheme adjacent to the suction surface can e ectively remove the corner open separation. With suction mass flow rate of 0.85%,the overall loss coe cient and endwall loss coe cient of the cascade are reduced by 25.2% and 48.6%,respectively. Besides,this scheme increases the static pressure rise coe cient of the cascade by 3.2% and the flow turning angle of up to 3.3° at 90% span. The concentration shedding vortex decreases,whereas the passage vortex increases. For single?slot suction schemes near the middle pitchwise of the passage,the concentration shedding vortex increases and the passage vortex is divided into two smaller passage vortexes,which converge into a single?passage vortex near the trailing edge section of the cascade. For the double?slot suction scheme,triple?passage vortexes are presented in the blade passage. Some new vortex structures are discovered,and the novel flow models of aspirated compressor cascade are proposed,which are important to improve the design of multi?stage aspirated compressors.

Effect of Tip-Blade Cutting on the Performance of Large Scale Axial Fan

The effect of tip-blade cutting on the performance of a large scale axial fan was investigated using computational fluid dynamics(CFD)methods.Experiments verified the numerical simulations.The original fan was compared with the one with tip-cutting in terms of dimensionless characteristic and aerodynamic performance in tip region under the conditions of the maximum efficiency point and near-stall point.The results showed that double leakage flow occurred in tip clearance at maximum efficiency point and spillage of leakage flow from leading edge occurred in tip-blade region at near-stall point for the both two fans;and that tip-cutting with 6% of blade height could reduce the intensity of tip-leakage vortex and increase flow capacity in tip blade region,and hold the stall margin almost the same as the original fan.The maximum efficiency of the fan with tip-cutting was improved by1%,and the ability of total pressure rising was obviously greater than the original fan.

Flow Fields with Tip Leakage Vortex in a Small Axial Cooling Fan

In order to improve the fan characteristics, especially efficiency and noise level of a small axial cooling fan with a large tip clearance, the internal flow measurements with tip leakage vortex were carried out at fan rotor outlet us- ing an I-type hot-wire probe. The probe was set toward two directions, parallel and normal to the meridional plane of test fan, and the two directional velocity components were measured. From the result of fan test it was found that the test fan didn＇t have the unstable characteristic with a positive gradient on its pressure - flow-rate curve. From the results of velocity measurement it was observed that the tip leakage vortex exited at maximum efficiency flow-rate and large flow-rate conditions. However, at small flow-rate conditions the tip leakage vortex was not observed and it was found that the flow field were enlarged toward radial outwards

两相出口对轴向涡流分离器的影响研究

为了更好地预测分离器的分离效率和设计出更高效的分离器,应进一步了解其内部流场分布特性及不同结构参数对内部流场的影响。为此,采用Fluent软件对轴向涡流分离器内部流场分布特性和分离器两相出口结构对内部流场及分离性能的影响展开研究。研究结果表明:当水相出口直径增加到40 mm时,轴向涡流分离器水相出口处会形成一个低速回流区域,可通过减小管径收缩程度等方法来缓解该现象的影响;在其他结构参数不变的情况下,分离器的分离效率是关于雷诺数和两相出口与入口直径之比的函数;分离效率正比于油相出口与入口直径之比,与雷诺数、水相出口与入口直径之比均为二次函数关系。研究结论可为进一步优化轴向涡流分离器的分离性能提供理论参考。

COMPUTATIONAL AND EXPERI-MENTAL STUDY ON TIP LEAKAGE VORTEX OF CIRCUMFERENTIAL SKEWED BLADES

In the steady operation condition, the experiments and the numerical simulations are used to investigate the tip leakage flow fields in three low pressure axial flow fans with three kinds of circumferential skewed rotors, including the radial rotor, the forward-skewed rotor and the back- ward-skewed rotor. The three-dimensional viscous flow fields of the fans are computed. In the experiments, the two-dimensional plane particle image velocimetry （PIV） system is used to measure the flow fields in the tip region of three different pitchwise positions of each fan. The results show that the computational results agree well with the experimental data in the flow field of the tip region of each fan. The tip leakage vortex core segments based on method of the eigenmode analysis can display clearly some characteristics of the tip leakage vortex, such as the origination position of tip leak- age vortex, the development of vortex strength, and so on. Compared with the radial rotor, the other two skewed rotors can increase the stability of the tip leakage vortex and the increment in the forward-skewed rotor is more than that in the backward-skewed one. Among the tip leakage vortices of the three rotors, the velocity of the vortex in the forward-skewed rotor is th6 highest in the circumferential direction and the lowest in the axial direction.

基于叶片表面附面层分析的风扇性能研究

为了提高轴流散热风扇效率,本文应用孤立翼型设计法,将高升阻比翼型CAP-12应用到轴流风扇的设计,设计出一款轴流风扇,并对设计出的轴流风扇进行了数值模拟研究,基于附面层理论,分析了风扇的静特性、内部流场与气动特性。计算结果表明:在叶片吸力面叶尖靠近尾缘处会产生有限叶高的诱导旋涡,是导致风扇性能效率降低的主要原因,在对轴流风扇进行结构优化时,应着重考虑该区域,该研究结果可为轴流风扇的性能优化设计提供理论基础。

轴向间距对FBCDZ-10-No20型对旋风机失速过程的影响

对旋风机在小流量工况运行时容易出现失速甚至喘振等不稳定现象,为研究轴向间距对对旋风机失速起始及其发展与传播过程的影响,采用Shear Stress Transport(剪切应力输运)k-ω湍流模型,对一台FBCDZ-10-No20型对旋风机全通道内的非定常流动进行数值模拟,探究了2种轴向间距下对旋风机的失速发展过程。结果表明:轴向间距对对旋风机失速起始及发展过程具有显著影响。在失速起始阶段,当对旋风机两级叶轮的轴向间距为170 mm时,失速起始扰动首先发生于后级叶根吸力面的尾缘区域,之后扰动沿径向从叶根向叶顶发展,沿周向向某个叶片通道聚集,导致该叶片通道堵塞,从而引起对旋风机失速;当轴向间距为70 mm时,失速起始扰动先后发生于前、后级的叶顶区域,之后扰动逐渐增强,最终发生失速。在完全失速阶段,轴向间距对于失速涡团沿周向、轴向及径向的传播都具有显著影响。轴向间距为170 mm时对旋风机进入单涡团全叶高失速,失速涡团沿轴向传播范围限于后级叶轮区域,沿周向以33.3%的后级转速旋转;轴向间距为70 mm时对旋风机进入多涡团部分叶高失速,失速涡团先后在两级叶轮叶顶区域产生,沿轴向向上、下游传播,沿径向分散于各叶片通道的70%叶高以上区域。由于两级叶轮轴向间距的改变,引起了对旋风机的失速起始扰动类型发生了由“局部喘振型”向“突尖型”的转变。

轴向间距对矿用旋风机旋转失速性能的影响

矿用对旋风机在小流量工况运行时极易出现旋转失速现象,严重影响其运行稳定性。采用SST k-ω湍流模型,对FBCDZ-10-No20型对旋风机在5种轴向间距下全流道内的非定常流动进行数值模拟,研究了轴向间距对矿用对旋风机失速过程的影响,揭示了不同轴向间距时的失速起始与发展机理。结果表明:轴向间距对失速起始扰动的首发位置、表现形式和发展过程都具有显著影响。对于70、100 mm两种轴向间距,失速起始扰动均首发于前级叶顶,随后在两级叶轮之间的动−动干涉作用下,后级叶顶也出现失速起始扰动,但与70 mm的轴向间距相比,当轴向间距为100 mm时,动−动干涉效应相对较弱,使得后级出现失速起始扰动需要更长的时间。对于140、170、225 mm三种轴向间距,失速起始扰动均是首先发生在后级叶根区域,不同的是140 mm间距时,前级叶顶泄漏流不能完全随主流流出通道,而是形成了局部叶顶堵塞区;而对于170、225 mm两种间距,前级叶顶则几乎没有堵塞区,最终仅在后级形成了成熟的失速涡团。随着轴向间距的增大,由前级叶顶泄漏流发生前缘溢流及尾缘反流形成的堵塞区逐渐缩小,而由后级叶根吸力面径向涡流形成的堵塞区则逐渐增大,当前级叶顶泄漏流无法形成堵塞区时,失速类型就由前级叶顶泄漏流诱发的“突尖型”转变为由后级叶根径向涡流诱发的“局部喘振型”。

叶根前缘VG对微型轴流风扇性能的影响

为降低微型轴流风扇叶根端壁区域二次流所引起的损失,根据涡流发生器的流动控制思想,提出一种在叶根前缘压力面侧设置微型直板的新型流动控制方法;以某微型轴流风扇为研究对象,采用数值模拟结合实验的方法,重点分析了不同安装角的涡流发生器对轴流风扇气动性能及内部流场的影响;研究结果表明:涡流发生器存在提高风扇静压与静压效率的最佳几何安装角,涡流发生器会对叶轮内部流场产生影响,由涡流发生器所形成的诱导涡与压力侧马蹄涡分支进行掺混,会削弱马蹄涡的强度,在一定程度上抑制了由马蹄涡参与演变成的通道涡的发展,使叶轮流道中流体进行再分配;在宏观方面,结构匹配的涡流发生器可提高风扇的气动性能,当涡流发生器安装角度为15°时,在风扇高效运行区间内同原型风扇相比,安装涡流发生器的风扇其静压最多提高8%,静压效率最大可提升2.4%。对于大轮毂比微型轴流风扇,由通道涡所引起的二次流损失不容忽视,同时在对叶轮进行设计优化时应重视叶根端壁处的结构设计。

Mechanism of Affecting the Performance and Stability of an Axial Flow Compressor with Inlet Distortion

The purpose of the investigation is to obtain the mechanism of affecting the performance and stability of an axial flow compressor with two kinds of multi distortion area.In the investigation,the compressor inlet distortion is made by the distortion generator,which is a column and is placed in the compressor rotor passage upstream.The five-passages unsteady numerical results show that the compressor performances for two kinds of multi distortion area are both lower than that for the clean inlet,and the compressor peak efficiencies for inlet distortions of 6×12 degrees and 6×24 degrees are about 3.5%,and 9.2%lower than that for the clean inlet,respectively.Further,two kinds of multi distortion area both reduce the compressor stability,and the stall margin improvements of–2.93%and–6.11%are provided by inlet distortions of 6×12 degrees and 6×24 degrees,respectively.The flow field analyses show that the flow conditions of the rotor tip inlet upstream deteriorate after the application of multi distortion area.The values of axial velocity and flow angle of incoming flows in some regions,which are near the shroud,become small due to the adverse effect made by the inlet distortion.So,the circulation capacity of the regions becomes low,and inlet blockages appear in some rotor tip passages.The bigger the distorted range is,the greater inlet blockages and corresponding flow losses are.When multi distortion area changes from 6×12 degrees to 6×24 degrees,there are obvious separations of the boundary layer near the blade suction surface in some blade tip passages,and the serious separations prevent the incoming flows from entering the blade tip passage.As a result,overflows occur at the leading edge of the blade tip.Furthermore,as the number of the distortion generator increases by a factor of one,the effects of inlet distortion on the amplitudes of the air relative velocity and static pressure in the rotor inlet upstream at the distortion generator passing frequency of 1076.5 Hz increase by a factor of about one.

直升机用高转速、小流量轴流风扇设计

小流量、高转速、小尺寸轴流风扇的设计是高性能直升机滑油冷却系统的关键技术之一。针对直升机用轴流风扇的特点,进行了基本结构型式的分析和参数的优化计算;采用简化的三元流动设计理论得到叶轮叶片气动设计径向平衡方程式和求解约束条件,进而引入了控制涡设计理论进行风扇叶片造型,获得了优良的气动性能;采用“改进损失模型”的效率计算方法,为叶片气动方案的优化选择提供了理论依据。应用于某型号直升机用滑油冷却轴流风扇的设计计算,所得结果与试验结果吻合较好。

射流对压气机叶栅分离流控制的数值研究

现代先进轴流压气机级负荷不断提高的发展趋势导致流动分离日益严重。借助数值模拟分别对非定常射流和定常射流进行了参数优化研究。结果表明:基于射流的主动流动控制能有效弱化或消除流动分离,不同射流方式存在不同的最优射流参数(射流方向、位置、速度和频率等),这就为利用射流控制轴流压气机分离流动的工程应用奠定了一定的理论基础。

轴向间距对两级轴流风机失速先兆的影响机制

基于节流阀函数以及SSTk-ω湍流模型,对某电站两级动叶可调式轴流风机的非定常失稳过程进行数值模拟,分析了3种轴向间距对风机失速先兆起始位置、表现形式以及失速三维非定常演化过程的影响机制.结果表明:风机稳定运行范围随着轴向间距的缩短得到拓宽;不同轴向间距下,第二级动叶内首先出现失速先兆,且失速先兆表现形式相同,均为突尖型;轴向间距增大时,失速先兆从第二级动叶传播至第一级动叶所用时间增加;轴向间距对叶顶间隙分离涡的产生具有重要影响,轴向间距较小时,分离涡主要由相邻流道的泄漏流绕过尾缘附近反向流入流道并与主流相互作用形成,轴向间距较大时分离涡的形成主要与叶顶间隙的泄漏流有关.

叶尖小翼对轴流风机气动性能及噪声特性影响的数值研究

为了控制轴流风机叶顶泄漏流造成的气动损失和噪声,在轴流风机叶片顶部添加了融合式叶尖小翼结构,并对风机的气动性能及噪声特性进行了数值研究。采用大涡模拟结合声类比方程的数值方法,研究了叶尖小翼对轴流风机流场和声场的影响。通过对风机叶尖流场和声场进行分析,对比不同外倾角的融合式叶尖小翼周围的涡场结构以及表面声压脉动,分析了不同外倾角小翼对叶尖泄漏流的控制作用以及叶尖小翼对风机气动性能和噪声特性产生的影响。结果显示:叶尖小翼结构可以有效抑制叶尖泄漏涡以及叶尖分离涡的发展,降低轴流风机的气动噪声,提高轴流风机静压效率;使用20°外倾角的叶尖小翼,风机静压效率提高了1.1%,噪声降低了5.0dB;叶尖泄漏流造成的气动噪声主要是宽频噪声,叶尖小翼可以明显降低轴流风机的宽频噪声;通过优化叶尖小翼的外倾角可以在不损害风机气动性能的同时实现较好的降噪效果。

基于流场控制的导向叶片式旋风管开缝面积比的优化设计

通过试验与数值模拟相结合的方法,研究了分流型芯管开缝面积比对导向叶片式旋风管内流场及性能的影响关系。研究发现,随着开缝面积比的增加,经芯管侧缝流走的气量Q1逐渐增加,但当开缝面积比大于2.9时,Q1呈现饱和状态;开缝面积比大于1.94时,短路流量保持在总进口流量的5%以下;分离空间内切向速度、轴向速度及向心径向速度随开缝面积比的增加而减小;旋风管内能耗随着开缝面积比的增加而减小。另外,内旋流区及排气管内的能耗受芯管开缝面积比的影响较大;芯管开缝面积比的最优范围为1.94-2.9。实验结果表明：入口流量为2380m3·s^-1,入口颗粒浓度为1g·m^-1的滑石粉,开缝面积比为2.42时,分流型芯管提高分离效率3%;入口为单相常温空气时,分流型芯管减低阻力系数近73%。

空调器室外机轴流风机内部流动的PIV研究

采用粒子图像测速仪PIV,对具有半管道式结构特点的空调器室外机轴流风机内部流场进行了实验研究,并结合实验结果分析了叶片顶部的叶尖涡和叶片出口尾缘涡的流动特性。实验结果显示在轴流风机流道内部叶顶区域存在与叶轮旋转方向相反的叶尖涡结构。叶尖涡产生于叶片前缘叶顶近吸力面侧,在流道内部与主流发生干涉后朝向周向和出口传播并逐渐耗散。叶尖涡涡心轨迹与叶顶弦长方向的夹角为10°,在叶高方向上叶尖涡的径向位置并不固定。与普通管道内部流动不同,叶片顶部与导风罩间的间隙中未捕捉到明显的叶顶泄漏涡现象。叶片出口近尾缘处30%以上叶高明显捕捉到尾缘涡结构,叶片压力面和吸力面侧的径向速度存在明显的方向变化,切向速度在尾迹区增加。