Centrifugal compressor tip clearance and impeller flow

Compressors consume a considerable portion of the electricity used in the industrial sector. Hence, improvements in compressor efficiency lead to energy savings and reduce environmental impacts. The efficiency of an unshrouded centrifugal compressor suffers from leakage flow over the blade tips. The effect of tip leakage flow on the passage flow differs between the full and splitter blade passages. In this study, the differences in the flow fields between the full and splitter blade passages were studied numerically in detail. An industrial high-speed compressor with a design pressure ratio of 1.78 was modelled. Numerical studies were conducted with six different tip clearances and three different diffuser widths. The results show that increasing tip clearance considerably increases the reversed flow into the impeller with an unpinched diffuser. The reversed flow then partly mixes into the flow in the same blade passage it entered the impeller and the rest migrates over the blade, mixing with the tip clearance flow. Furthermore, as the reversed and clearance flow mix into the wake, the wake is weakened. As pinch reduces both the reversed flow and clearance flow, the passage wakes are stronger with pinches. However, the pinch is beneficial as the losses at the impeller outlet decrease.

     

基于机器学习的斜流压气机多目标优化

针对100公斤级涡喷发动机,为了提高斜流压气机性能,利用编码搭建的BP神经网络-遗传算法平台,对叶轮入口轮毂半径、叶轮叶片数、扩压器轴向长度和扩压器叶片数进行压气机级的多目标优化。结果表明:优化后的压气级总压比提高了3.2%,等熵效率提高了7.9%,工作稳定性大幅度提高;优化后的斜流压气机叶轮出口处的泄漏流得到明显的改善,扩压器转弯段逆压梯度和附面层影响减小。     

Static pressure recovery analysis in the vane island diffuser of a centrifugal pump

The overall performance of a vane-island type diffuser of a centrifugal pump model was obtained by means of directional probe traverses. These measurements were performed in an air model of a real hydraulic pump for five volume flow rates. Directional probe traverses are performed with a classical three-hole probe to cover most of the complete inlet section of the diffuser from hub to shroud and from pressure to suction side. Existing Particle image velocimetry (PIV) measurement results are also used to compare probe measurement results between the inlet and outlet throats of vane island diffuser at mid-span. Some assistance from already existing unsteady calculation, including leakage effects, is used to evaluate the numerical approach capability and to correctly define the mean initial conditions at impeller’s outlet section. Pressure recovery and the measured total pressure loss levels inside this particular vane diffuser geometry are then calculated. Detailed analysis of the flow structure at the inlet section of the vane island diffuser is presented to focus on pressure evolution inside the entire diffuser section for different flow rates. The combined effects of incidence angle and blockage distributions along hub to shroud direction are found to play an important role on loss distribution in such a diffuser.     

EXPERIMENTAL INVESTIGATION OF ROTATING STALL FOR A LOW-SPEED CENTRIFUGAL COMPRESSOR

Unsteady flows and rotating stall of a low-speed centrifugal compressor are investigated by measuring vaneless diffuser wall static pressure fluctuation and internal flow fields at different small flow fluxes. During the experiment, firstly the real time static pressure fluctuations on the vaneless diffuser shroud at different circumferential and radial position were acquired by high-frequency dynamic pressure transducers. Discrete Fourier transformation analysis and cross-correlation analysis were applied to the experimental results to ascertain the rotating stall beginning operation conditions and stall cells numbers and rotating speed. Secondly, the vaneless diffuser inlet flow angle distribution along diffuser width direction was acquired by single hotwire, which was compared with SENOO＇s analysis results. At last, the internal flow fields of the centrifugal compressor were investigated with a particle image velocimetry （PIV） system at different small flow fluxes. The flow field development of vaneless diffuser and blade flow passage are given at rotating stall conditions. The experiments enrich the understanding of rotating stall flow phenomenon of the low-speed centrifugal compressor and provide full experiment data for designing high performance centrifugal compressor.     

离心压缩机末级最佳D_4/D_2值的数值研究

考虑到离心压缩机末级各通流元件之间的相互影响,将末级叶轮、无叶扩压器和排气蜗壳组合一起对扩压器出口直径与叶轮直径的比值D4/D2进行研究,通过对不同D4/D2条件下的整级模型进行数值模拟,研究扩压器流道长短对整级性能的影响,对离心压缩机末级流场及D4/D2值变化与离心压缩机整级性能的变化关系进行了详细的对比分析.研究结果表明,D4/D2值的变化对末级整级性能有较大的影响,并存在一个使级效率为最大值的最佳D4/D2值,D4/D2过大或过小都将使整级效率下降.     

低稠度半高叶片扩压器的模型级性能研究

通过数值模拟的方法研究了低稠度半高叶片扩压器在大流量离心压缩机模型级中的应用,并比较了其与无叶扩压器对模型级气动性能影响的差异.研究结果表明:采用低稠度半高叶片扩压器的模型级基本消除了盖侧附近低总压高熵值的区域,气动性能有较大提高.     

Exit flow measurements of a centrifugal pump impeller

Discharge flows from a centrifugal pump impeller with a specific speed of 150 [rpm, m³/min, m] were experimentally investigated. A large axisymmetric collector instead of a volute casing was installed to obtain circumferentially uniform flow, i.e. without interaction of the impeller and the volute. The unsteady flow was measured at the impeller exit and vaneless diffuser using a hot film probe and a pressure transducer. The flow at impeller exit showed pronounced jet-wake flow patterns. The wake, which was on the suction/hub side at high flow rate, became enlarged pitchwisely on both the hub and the shroud side as the flow rate decreases. The pitchwise non-uniformity of the flow rapidly decreased along the downstream and the nonuniformity almost disappeared at radius ratio of 1.18 for medium flow rate. The mean vaneless diffuser flow was reasonably predicted using a one dimensional analysis when an empirical constant was used to specify the skin friction coefficient. The data can be used for a centrifugal pump impeller design and validation of CFD codes and flow modeling.     

高轮毂比离心压气机改型设计及分析

以某高轮毂比离心压气机为研究对象,在给定的轴向尺寸限定下,进行离心压气机改型设计,提出了离心叶轮和径向扩压器子午流道倾斜的设计方法以提高离心压气机性能,并借助数值模拟手段,揭示了该设计方法提高高轮毂比离心压气机性能的机理,可为高轮毂比离心压气机改型气动设计提供借鉴。     

离心压气机改型设计及扩压器前掠分析

本文主要研究单级离心压气机的改型设计,并采用数值方法对楔形扩压器前掠展开了详细研究。在保持原有压气机叶轮外径不增加、流量和设计转速不变的条件下,将压比由4.0提高到4.5。在设计离心叶轮时,首先需要确定叶轮后弯角,为了真实评估叶轮后弯角对离心压气机效率和稳定性的影响,详细设计了后弯角为20°,25°,30°和35°的三维叶轮以及相匹配的楔形扩压器,最终选择35°后弯角叶轮。其次,后弯角为35°叶轮的设计点压比相对于设计目标偏低,将hub处弯角减小到30°达到增加设计点压比。最后,考虑到楔形扩压器各截面载荷的不同,研究了楔形扩压器前掠对性能影响。     

混流式核主泵叶轮叶片厚度分布规律对能量性能的影响

采用数值计算方法,开展混流式核主泵叶轮叶片自前盖板至后盖板方向厚度分布规律对能量性能影响的研究。通过对5种具有不同厚度分布规律叶片的叶轮进行比较分析,探索最佳叶片厚度比值和其对叶片表面静压力分布规律的影响。结果表明:当叶片后盖板处厚度与前盖板处厚度比值L=1.5时,叶轮的水力效率达到最高,当L=1.25时叶片做功能力低,叶轮内能量损失大,水力效率较低;在小流量工况下,随着比值L的增加,相同流量下扬程和水力效率都随之升高;在大流量工况下,扬程和水力效率都随着L的增大而降低;叶轮流道内压能沿着前盖板至后盖板方向增大并且最大压能位置向中间移动。     

Multi-objective optimization for mixed-flow pump with blade angle of impeller exit and diffuser inlet

During design optimization, the impeller and diffuser of a mixed-flow pump are generally optimized separately. In such cases, the total head can be overdesigned. In this study, the designs of the impeller and diffuser were optimized simultaneously by using computational fluid dynamics and the Response surface method (RSM). Design variables were defined according to the vane plane development of the impeller and diffuser. Three-dimensional Reynolds-averaged Navier–Stokes equations for the shear stress transport turbulence model were discretized by finite volume approximations and solved on hexahedral grids to analyze the flow in the pump. The total head and total efficiency were selected as objective functions, with four design variables related to the impeller outlet angles and diffuser inlet angles used for optimization. The RSM was constructed based on the objective functions with design points generated from the central composite method. The hydraulic performance of the optimum model was analyzed.     

Experimental and numerical investigations of radial flow compressor component losses

This research numerically and experimentally investigates a small turbocharger radial flow compressor with a vane-less diffuser and volute. The geometry of the compressor is obtained via component scanning, through which a 3D model is prepared. The flow inside this model is numerically analyzed by using a Navier-Stokes solver with a shear-stress transport turbulence model. The characteristic curves of the compressor and the contributions of its components to total pressure drop are acquired by measuring the static and total pressures at different cross sections of the compressor. Numerical results are verified with the experimental test results. The model results exhibit good agreement with the experimental results. In particular, the results show that the losses related to the impeller are higher than those related to the stationary components at different conditions, with the former causing a decline of at least 15% in compressor isentropic efficiency. The contribution of stationary components to efficiency decrease is approximately 4.8% at maximum efficiency mass flow rate and is limited to 7.1%. At low mass flow rates, the contribution of the diffuser to efficiency decline is higher than that of the volute. This finding is reversed at high mass flow rates. The performances of the diffuser and the volute are also studied by exploring total pressure and static pressure recovery coefficients, as well as the net radial force on the impeller shaft under a wide operating range.     

Influence of tip clearance on pressure fluctuations in an axial flow pump

Rotor-stator interaction in axial pumps can produce pressure fluctuations and further vibrations even damage to the pump system in some extreme case. In this paper, the influence of tip clearance on pressure fluctuations in an axial flow water pump has been investigated by numerical method. Three-dimensional unsteady flow in the axial flow water pump has been simulated with different tip clearances between the impeller blade tip and the casing wall. In addition to monitoring pressure fluctuations at some typical points, a new method based on pressure statistics was proposed to determine pressure fluctuations at all grid nodes inside the whole pump. The comparison shows that the existence of impeller tip clearance magnifies the pressure fluctuations in the impeller region, from the hub to shroud. However, the effect on pressure fluctuation in the diffuser region is not evident. Furthermore, the tip clearance vortex has also been examined under different tip clearances.     

基于环量分布的离心叶轮S2流面设计与优化

采用准三维气动设计方法,用Bezier曲线表示环量沿轮毂和轮盘的流向分布,子午面环量分布由拉普拉斯方程确定,设计了3种不同负荷分布的离心式叶轮。采用CFD模拟,对比叶轮内流动和气动性能的优劣,结果表明:均匀加载型式的叶轮性能优于前加载或后加载的叶轮,最优设计的叶轮负荷分布是"两端小,中间大"。通过Laplace方程和Bezier多项式控制环量可以达到灵活控制叶片载荷的目的。     

Analysis and measurement of the impact of diffuser width on rotating stall in centrifugal compressors

In compression systems, instability has long been an important issue. However, compared to axial machines, relatively little work has been done on the stability of centrifugal machines. Especially, many analytical models of stabilities have been developed to predict and control rotating stall, using compressor characteristic. However, stability models for centrifugal compressors are not scarce. Much research on compressor stability has focused on stalling flow coefficient and rotating stall phenomenon at the stalling flow coefficient. Given this situation, this paper presents a stability analysis of centrifugal compressors to predict rotating stall inception as well as the speed and number of cells. This analysis involves the use of compressor geometries, a steady compressor characteristic, and threedimensional flow analysis in the diffuser. The flow field perturbations at the axial inlet duct, impeller, and radial exit duct are determined via an eigenvalue analysis. The predictions are validated against experimental results from compressors with three different diffuser widths. The model accurately predicts the rotating stall inception flow coefficient. As the compressor characteristic becomes less steep with increasing diffuser width, the stalling flow coefficient increases. Also, experiment validates the model prediction that, depending on the dominant mode of flow perturbation, the number of rotating stall cells can be changed from three to two cells in the tested configurations. Furthermore, the cell speed increases as the flow coefficient decreases for a given number of stall cells. However, when the stall cell number is reduced, the cell speed decreases.     

带楔形扩压器的跨声速离心压气机设计及内部流场计算

采用离心压气机计算机辅助集成设计系统设计了带楔形扩压器的跨声速离心压气机。采用混合平面法对设计的跨声速离心压气机叶轮和扩压器内部流场进行了三维粘性计算,给出了叶轮和扩压器内部的计算结果。计算结果表明,在流量为2．45 kg/s工况下,压气机压比为6．22,压气机总效率为75％,叶轮内部出现典型的二次涡系结构。扩压器内的流场参数分布表明,扩压器前缘出现激波,在楔形扩压器内存在复杂的涡系结构,二次流动涡在扩压器内部经历了一个发生、发展和消失的过程。     

不同扩压器形式对叶轮内流动影响的研究

针对离心式压缩机模型级进行了数值模拟,与试验结果对比验证了计算模型的正确性,用无叶扩压器代替叶片扩压器在相同的边界条件下进行流场数值模拟,根据模拟结果对比分析了两种情况下叶轮内部的流动特征,揭示了叶片扩压器和无叶扩压器对离心式压缩机级内流场的影响规律.     

采用叶片扩压器时单级离心叶轮内流分析

对采用滑移网格技术计算得到的,带有叶片扩压器的单级高速离心压缩机,非定常流动计算结果的叶轮内流进行分析,研究了叶片扩压器的存在对叶轮内流的影响。     

离心式压气机椭圆叶型成型方法

随着高性能、高压比离心式压气机的发展、文献[1]指出：椭圆成型是一种最佳叶型。本文提出了一种椭圆叶型成型方法,该法采用极坐标系代替笛卡儿坐标系,建立了以椭圆中心为极点的简单极坐标方程。直接推导了叶片翘曲角β s,β h[2][3]与极角ψ之间关系式,并给出了β s,β h沿轮盖、轮毂回转面的分布规律。通过95mm轮径压气机示例计算,证实了本文所提出方法是简单而可行的。     

离心压缩机机壳在基频气动力激励下的振动噪声研究

研究了离心压缩机机壳在内部基频气动力激励下的振动辐射噪声。考虑机壳与轮盘、轮盖之间的间隙,采用SSTk-co湍流模型模拟了离心压缩机的整机三维非定常流场,得到蜗壳内表面的脉动压力,然后将脉动压力的基频部分加载到蜗壳上,模拟离心压缩机蜗壳在非定常气动力激励下的振动噪声。研究表明：机壳内壁面基频压力脉动主要分布在无叶扩压器靠近叶轮出口一侧;机壳主要基频振动噪声源位于蜗壳靠近出口管道一侧;该离心压缩机机壳基频振动位移幅值很小,最大振动位移仅为11．8×10^-9m,因此可以忽略机壳的基频振动对离心压缩机运行安全性的影响。