计及几何参数变化的离心压气机特性分析

本文采用CFD软件对微型燃机的离心叶轮进行数值模拟,讨论了叶片数及分流叶片位置对叶轮性能的影响,并进行了流场分析。在本文研究的情况中,叶片数增加使得性能曲线左移,单个叶片载荷减小,损失增加,叶轮效率下降, 但是增压效果得到改善。分流叶片位置靠近主叶片压力面时,性能曲线右移,流通能力提高,同时会使分流叶片的载荷增大,当分流叶片靠近主叶片压力面时,则情况相反。     

Experiments on the unsteady flow field and noise generation in a centrifugal pump impeller

This paper reports on an experimental investigation of large-scale flowfield instabilities in a pump rotor and the process of noise generation by these instabilities. Measurements of the fluctuating components of velocity and surface pressure were made with hot-wire probes and surface mounted pressure transducers on a seven bladed back swept centrifugal water pump impeller operating with air as the working fluid. The impeller was operated without a volute or scroll diffuser, thereby eliminating any sound generation from pressure fluctuations on the volute cutoff. Thus the study focused on flow field and noise components other than the blade passage frequency (and its harmonics). The primary goal of the study was to provide fundamental information on the unsteady flow processes, particularly those associated with the noise generation in the device. It was further anticipated that detailed flow measurements would be useful for the validation of future computational simulations.The measured data at the discharge show a jet-wake type of flow pattern which results in a strong vorticity field. The flow with high velocity found on the pressure side of the impeller tends to move to the low-pressure region present at the suction side of the passage as a form of roll-up around the blade trailing edge. This motion causes an unsteady flow separation at the suction side of the blade and consequently disturbs the flow in the adjacent passage. By interacting with the impeller blades near the trailing edges, this instability flow causes a periodic pressure fluctuation on the blade surface and generates noise by a trailing edge generation mechanism. The spectrum of surface pressure measured at the trailing edge of each blade reveals a cluster of peaks which were identified with azimuthal mode numbers. The correlation between the acoustic farfield pressure and the surface pressure on the impeller blade has proven that the azimuthal modes synchronized with the number of impeller blades generate noise much more efficiently than the other modes. The paper also clarifies the correlation between unsteady flowfield measurements, in both impeller and laboratory co-ordinates, with the radiated noise properties. Thus some light is shed on the noise generation mechanisms of this particular device.     

空调风机叶道内旋涡流动分析及进气口偏心的影响

本文采用CFD方法,详细分析了空调用多翼离心风机叶道中的气流分布以及进气口偏心的影响。研究表明,风机叶道内存在大范围的气流分离现象。在后盘附近,存在分离现象的叶道约占2/3,主要分布在蜗壳内侧;而在“前盘”附近,几乎所有叶道都存在分离现象。在“前盘”附近,蜗舌下方的叶道中气流几乎停滞,蜗舌下游叶道为回流和尾缘旋涡所充满,至临近蜗壳出口侧,前缘旋涡逐步形成、发展并融合尾缘旋涡,最后衰减、消失。风机进气口向蜗壳内侧偏置适当距离, “前盘”附近叶道旋涡分布范围明显减小。     

冲压转子流场和轮毂抽吸处理分析

通过数值计算的方法对冲压转子中的三维流场以及轮毂抽吸后的流场进行了分析。研究发现,冲压转子中,轮毂处的分离影响了叶根至70%叶高截面的流动,轮毂处的角区分离限制了冲压转子的性能,使之只能工作在较低的背压工况下。而通过轮毂抽吸可以改善角区流动,提高冲压转子的压比和效率,并且抽吸位置在分离起始点位置时,效果最好。在抽吸流量...     

叶片开缝的离心风机流场研究

本文提出在离心叶轮轮盖与叶片之间开缝,使得叶片压力面侧的高压气体射流到吸力面侧的低速尾迹区,从而直接给叶轮内的低速流体提供能量,减弱了叶轮内由于二次流所导致的射流-尾迹结构,并可用于消除或解决部分负荷时常发生的离心叶轮积灰问题.对某离心风机整机的数值模拟表明,在设计流量和小流量情况下,叶轮开缝后使叶片表面分离区域减小,整个流道速度分布更为均匀,改善了叶轮内部流场的流动状况,提高了离心叶轮性能和整机性能.研究表明,本方法对于提高离心式流体机械性能是有效的.     

一个高压比离心叶轮的CFD结果确认

本文采用NUMECA的FINE/Turbo软件包数值分析了一个高压比离心叶轮的三维定常粘性流场。选用二阶精度的中心格式和Spalart-Allmaras的一方程湍流模型。通过与现有实验结果(包括性能曲线、外环壁静压和截面马赫数分布等)的比较对计算结果就行了详细的确认,同时讨论了网格密度和间隙尺度、以及不同的数据处理方法对计算结果确认的影响。     

Qualitative comparison between numerical and experimental results of unsteady flow in a radial diffuser pump

Comparison between numerical simulation and experimental results for unsteady flow field in a radial diffuser pump is presented for the design operating point. The numerical result is obtained by solving three-dimensional, unsteady Reynolds-averaged Navier-Stokes equations by the commercial CFD code CFX-10 withk-ω based shear stress transport turbulence model. Two-dimensional PIV measurements are conducted to acquire the experiment result. The phase-averaged velocity and turbulent kinetic energy fields are compared in detail between the results by the two methods in the impeller, diffuser and return channel regions. The qualitative comparison between CFD and PIV results is quite good in the phase-averaged velocity field. Although the turbulence level by PIV is higher than that by CFD generally, the main turbulence features are nearly the same. Furthermore, the blade orientation effect and other associated unsteady phenomena are also examined, in order to enhance the understanding on impeller-diffuser interaction in a radial diffuser pump.     

离心压气机叶轮内部流动的数值研究:分流叶片的作用

本文对一台单级压比为4/1的离心压气机叶轮内部流动,进行了数值模拟的研究。计算方法基于Jameson格式,湍流模型选择Baldwin-Lomax模型。计算结果为分析叶轮流道内二次流的形成与发展,提供了详细的流动结构。通过对不同通流截面速度分布的比较,发现分流叶片可以延缓横向二次流的发展,降低叶片吸力面扩压程度,减小叶轮出口尾迹的强度与范围,对提高叶轮效率起到决定性的作用。     

Simulations of turbulent asymptotic suction boundary layers

A series of large-eddy simulations of a turbulent asymptotic suction boundary layer (TASBL) was performed in a periodic domain, on which uniform suction was applied over a flat plate. Three Reynolds numbers (defined as ratio of free-stream and suction velocity) of Re = 333, 400 and 500 and a variety of domain sizes were considered in temporal simulations in order to investigate the turbulence statistics, the importance of the computational domain size, the arising flow structures as well as temporal development length required to achieve the asymptotic state. The effect of these two important parameters was assessed in terms of their influence on integral quantities, mean velocity, Reynolds stresses, higher order statistics, amplitude modulation and spectral maps. While the near-wall region up to the buffer region appears to scale irrespective of Re and domain size, the parameters of the logarithmic law (i.e. von Kármán and additive coefficient) decrease with increasing Re, while the wake strength decreases with increasing spanwise domain size and vanishes entirely once the spanwise domain size exceeds approximately two boundary-layer thicknesses irrespective of Re. The wake strength also reduces with increasing simulation time. The asymptotic state of the TASBL is characterised by surprisingly large friction Reynolds numbers and inherits features of wall turbulence at numerically high Re. Compared to a turbulent boundary layer (TBL) or a channel flow without suction, the components of the Reynolds-stress tensor are overall reduced, but exhibit a logarithmic increase with decreasing suction rates, i.e. increasing Re. At the same time, the anisotropy is increased compared to canonical wall-bounded flows without suction. The reduced amplitudes in turbulence quantities are discussed in light of the amplitude modulation due to the weakened larger outer structures. The inner peak in the spectral maps is shifted to higher wavelength and the strength of the outer peak is much less than for TBLs. An additional spatial simulation was performed, in order to relate the simulation results to wind tunnel experiments, which – in accordance with the results from the temporal simulation – indicate that a truly TASBL is practically impossible to realise in a wind tunnel. Our unique data set agrees qualitatively with existing literature results for both numerical and experimental studies, and at the same time sheds light on the fact why the asymptotic state could not be established in a wind tunnel experiment, viz. because experimental studies resemble our simulation results from too small simulation boxes or insufficient development times.     

Adaptation of mesoscale turbulence parameterisation schemes as RANS closures for ABL simulations

The present study presents different k-ε turbulence closures for atmospheric boundary layer flows using computational fluid dynamics (CFD) simulations that are consistent with inflow conditions from numerical weather prediction (NWP) simulations. Eight different mesoscale turbulence parameterisation schemes of the Weather Research and Forecasting (WRF) model are covered. To ensure consistency between the NWP and CFD simulations, different closure coefficients of the k ? ε turbulence model for each NWP scheme are proposed. This is achieved by combining production–dissipation closure coefficient relationships based on the Monin–Obukhov similarity theory and the formulation based on the Coriolis parameter proposed by Detering and Etling. The proposed methodology has been implemented in the open source CFD toolbox OpenFOAM and is demonstrated at near-neutral stability conditions for the classical Askervein Hill case.     

旋转离心叶轮内部非定常流动实验研究

利用激光成像速度仪(PIV)测量了旋转离心叶轮内部的非定常流场,获得了旋转离心叶轮内部相对速度的非定常流场分布。详细分析了叶轮内部非定常流动现象和流动规律。通过实验研究发现旋转离心叶轮内部的流动是非定常,在叶轮出口处,叶片的吸力面与轮盖的夹角区存在一个低速区,并观察到了明显的射流/尾迹结构。射流区和尾流区的大小和范围在沿盘盖方向和跨叶片方向上是不同的。射流区和尾流区之间不存在明显的分界线。     

近失速状态轴流压气机转子内尖区三维流动结构

用激光测速系统测量了低速大尺寸单级压气机近失速状态转子内尖区三维流场。结果表明泄漏流在转子进口开始产生,泄漏涡约在１０％弦长最强,并迅速向压力面和低叶高方向移动,沿程造成高紊流和高阻滞。叶尖吸力面附面层发生分离,迫使角区低能物质和旋涡在下游逐渐向通道中部移动,造成转子出口尖部通道中部大面积流动阻塞和紊流脉动。角区旋涡及泄漏涡影响区域紊流强度较高,其中径向分量最高,远大于轴向和切向分量。前缘马蹄涡压力面分支存在于转子进口叶尖压力面角区,并迅速向低叶高和通道中部移动,约在２０％弦长和泄漏涡交汇。     

An experimental study of the forerunners of boundary-layer localized disturbances at an enhanced turbulence level

Turbulence production processes in boundary layer at a high level of free-stream turbulence have been studied. The tests were carried out in the MT-324 subsonic wind tunnel of ITAM, SB RAS, on models of straight and 45° swept wings at Reynolds numbers Re_c1 = 97000 and Re_c2 = 137000, and also at low (Tu = 0.18 % U _∞) and high (Tu = 0.79 and 2.31 % U _∞) levels of free-stream turbulence. The longitudinal localized disturbances developing in the boundary layer under the action of free-stream turbulence were artificially modeled using local air suction through a slot on the model surface. Wave packets, or forerunners, produced in the boundary layer, in the region preceding the abrupt local change of flow velocity near the localized-disturbance fronts, were examined. The high level of free-stream turbulence was found to accelerate the downstream evolution of the wave packets and their transformation into turbulent spots.     

DYNAMIC RESPONSE OF WIND-EXCITED BUILDING USING CFD

Computational wind engineering as a new branch of computational fluid dynamics (CFD) has been developed recently to evaluate the interaction between wind and buildings numerically. In the present study, a systematic examination of wind effects on tall buildings and flow condition around buildings has been carried out using commercially available CFD software FLUENT 5. Both renormalization group (RNG) k-ε method and large eddy simulation (LES) with the Smagorinsky model are adopted as turbulence models and the results are compared with the wind-tunnel measurements. The weighted amplitude wave superposition (WAWS) method is used to generate atmospheric wind turbulence. The RNG k-ε method can predict the vortex shedding phenomenon well when compared with experiments for uniform flow input, but fails to predict the shedding frequency accurately for fluctuating incoming flow. On the other hand, the LES model shows reasonably good agreement with experiment in predicting vortex-shedding phenomenon for both uniform and fluctuating flows at inlet. Random-vibration based theory is employed for estimating r.m.s. response of tall buildings and the results compared well with the experimental results for a square building.     

Large-eddy simulations of temporally accelerating turbulent channel flow

The unsteady turbulent channel flow subject to the temporal acceleration is considered in this study. Large-eddy simulations were performed to study the response of the turbulent flow to the temporal acceleration. The simulations were started with the fully developed turbulent channel flow at an initial Reynolds number of Re₀ = 3500 (based on the channel half-height and the bulk-mean velocity), and then a constant temporal acceleration was applied. During the acceleration, the Reynolds number of the channel flow increased linearly from the initial Reynolds number to the final Reynolds number of Re₁ = 22,600. The effect of grid resolution, domain size, time step size on the simulation results was assessed in a preliminary study using simulations of the accelerating turbulent flow as well as simulations of the steady turbulent channel flow at various Reynolds numbers. Simulation parameters were carefully chosen from the preliminary study to ascertain the accuracy of the simulation. From the accelerating turbulent flow simulations, the delays in the response of various flow properties to the temporal acceleration were measured. The distinctive features of the delays responsible for turbulence production, energy redistribution, and radial propagation were identified. Detailed turbulence statistics including the wall shear stress response during the acceleration were examined. The results reveal the changes in the near-wall structures during the acceleration. A self-sustaining mechanism of turbulence is proposed to explain the response of the turbulent flow to the temporal acceleration. Although the overall flow characteristics are similar between the channel and pipe flows, some differences were observed between the two flows.     

A one-equation turbulence model for recirculating flows

A one-equation turbulence model which relies on the turbulent kinetic energy transport equation has been developed to predict the flow properties of the recirculating flows. The turbulent eddy-viscosity coefficient is computed from a recalibrated Bradshaw’s assumption that the constant a₁ = 0.31 is recalibrated to a function based on a set of direct numerical simulation (DNS) data. The values of dissipation of turbulent kinetic energy consist of the near-wall part and isotropic part, and the isotropic part involves the von Karman length scale as the turbulent length scale. The performance of the new model is evaluated by the results from DNS for fully developed turbulence channel flow with a wide range of Reynolds numbers. However, the computed result of the recirculating flow at the separated bubble of NACA4412 demonstrates that an increase is needed on the turbulent dissipation, and this leads to an advanced tuning on the self-adjusted function. The improved model predicts better results in both the non-equilibrium and equilibrium flows, e.g. channel flows, backward-facing step flow and hump in a channel.     

Effects of superficial gas velocity on process dynamics in bioreactors

Present work analyzes the flow hydrodynamics and mass transfer mechanisms in double Rushton and CD-6 impeller on wide range (0.0075–0.25 m/s) of superficial gas velocity (v _g) in a gas-liquid phase bioreactor by employing computational fluid dynamics (CFD) technique. The volume averaged velocity magnitude and dissipation rate are found higher with increasing superficial gas velocity. Higher relative power draw (P _g/P ₀) is predicted in CD-6 than the Rushton impeller but no significant difference in volume averaged mass transfer coefficient (k _L a) observed between these two types of impeller. The ratio of power draw with mass transfer coefficient has been found higher in CD-6 impeller (25–50 %) than the Rushton impeller.     

雷诺数对涡轮叶栅流动的影响

本文利用数值模拟手段模拟了涡轮叶栅内部三维流动,分析了雷诺数对涡轮叶栅内部流动图画的影响。结果表明在低于自模化雷诺数条件下,雷诺数的降低将对涡轮叶栅吸力面边界层的发展产生严重的不利影响。雷诺数降低至一定程度,将导致吸力面出现严重的分离,端部二次流流动相应加剧,使低雷诺数情况下涡轮叶栅性能恶化,在设计中应予充分重视。     

单转子压气机设计状态和近失速状态出口三维紊流流场

用单斜丝详细测量了单转子压气机设计状态和近失速状态转子出口的三维素流流场。结果表明，设计状态叶尖泄漏涡和端壁附面层的掺混是造成尖部流动损失、气流阻塞和亲流脉动的主要原因。近失速状态流动三维性和非定常性较强；尖部吸力面角区轴向速度最低、相对动能损失最大；吸力面附面层径向潜移、叶尖吸力面角区低能团周向潜移及其输运的低能物质在尖部通道中部与叶尖泄漏流、泄漏涡、刮削涡发生掺混，造成尖部大范围的高损失区；根部和尖部吸力面阻面层局部发生分离。     

Numerical and experimental investigations of the unsteady aerodynamics and aero-acoustics characteristics of a backward curved blade centrifugal fan

A numerical study of the aerodynamic and aeroacoustic behaviors of a backward curved blade centrifugal fan was conducted under two important flow conditions: BEP and 1.3 × BEP. Three-dimensional numerical simulations of the complete unsteady flow field for the whole impeller-volute configuration were used to determine the aeroacoustic sources. To locate the unsteady flow and perturbations, the near field wall pressure fluctuations at different strategic points on the volute were computed using the URANS approach. Thus the intensities and positions of the aeroacoustic sources were identified by analyzing frequency spectra. The aeroacoustic sources caused by fluctuations in the interactions of the flows leaving the impeller and volute were close to the volute tongue, and the most effective noise sources related to the flow rate were near the impeller shrouds. In addition, the unsteady flow variables provided by CFD calculations were used as inputs in the Ffowcs Williams-Hawkings equation to estimate the noise tones of the fan. The aeroacoustic calculation results showed that the volute noise was much larger than the blade noise, and the noise mainly propagated from the outlet duct of the fan. Moreover, to account for the noise propagation, three calculation methods were used by applying different solid boundaries. Compared with the other methods, the FEM method, which accounted for the complex solid boundaries, produced good agreement and showed that the complex solid boundaries cannot be neglected in aeroacoustic predictions. The calculation results showed good agreement with the experimental results.