Modal decomposition for the analysis of the rotor-stator interactions in multistage compressors

A modal analysis method of the rotor-stator interactions in multistage compressors has been developed by LMFA.This method,based on a double modal decomposition of the flow over space and time,has been applied to nu-merical and experimental results of the high-speed 3?-stage compressor CREATE based at LMFA,Lyon-France.It reveals the presence of a very strong rotor-stator interaction which completely drives the flow at casing behind all the rotors.This modal analysis method applied to an unsteady RANS simulation permits to calculate the en-ergy of the rotor-stator interactions and to plot energetic meridian maps to explain experimental results and to analyze the interaction in the whole machine.     

Impact of clocking on the aero-thermodynamics of a second stator tested in a one and a half stage HP turbine

This paper focuses on the experimental investigation of the time-averaged and time-accurate aero- thermodynamics of a second stator tested in a 1.5 stage high-pressure turbine. The effect of clocking on aerodynamic and heat transfer are investigated. Tests are performed under engine representative conditions in the VKI compression tube CT3. The test program includes four different clocking positions, i.e. relative pitch-wise positions between the fh-st and the second stator. Probes located upstream and downstream of the second stator provide the thermodynamic conditions of the flow field. On the second stator airfoil, measurements are taken around the blade profile at 15, 50 and 85% span with pressure sensors and thin-film gauges. Both time-averaged and time-resolved aspects of the flow field are addressed. Regarding the time-averaged results, clocking effects are mainly observed within the leading edge region of the second stator, the largest effects being observed at 15% span. The surface static pressure distribution is changed locally, hence affecting the overall airfoil performance. For one clocking position, the thermal load of the airfoil is noticeably reduced. Pressure fluctuations are attributed to the passage of the up- stream transonic rotor and its associated pressure gradients. The pattern of these fluctuations changes noticeably as a function of docking. The time-resolved variations of heat flux and static pressure are analyzed together showing that the major effect is due to a potential interaction. The time-resolved pressure distribution integrated along the second stator surface yields the unsteady forces on the vane. The magnitude of the unsteady force is very dependent on the clocking position.     

Pressure measurement on casing wall and blade rows interaction of contra-rotating axial flow pump

An application of contra-rotating rotors has been proposed against a demand for developing higher specific speed axial flow pump. The blade rows interaction between front and rear rotors should be clarified for its stable operation and reduction of unsteady losses. In this paper, the static pressure distributions on casing wall are provided by measuring with the phase locked sampling method. The measurements are carried out for two types of the rear rotors with different blade number and chord length, and it is found that, for both types of rotors, the unsteady pressure fluctuations are more remarkable in the front rotor than in the rear rotor and they are caused by the rear rotor pressure field. The effects of pressure fluctuations will be discussed in more details toward understanding the blade rows interaction in the contra-rotating axial flow pump.     

Unsteady pressure measurements in a high-speed centrifugal compressor

This paper presents the unsteady data acquisition system used to measure the pressure field in high speed compressors. Details and electronic sketches are given for the conditioners developed in-house that have been used to amplify and to filter the pressure signal with the aim of acquiring data up to 150 kHz. A discussion of the experimental results carried out in a centrifugal compressor is proposed. Through different processing of the pressure signals and a comparison with URANS simulations, the excitation of the pressure transducers by the pressure waves generated by shock waves that occur between the impeller and the diffuser is highlighted. The levels of pressure fluctuations measured when entering into surge are also presented and reveal very repetitive behaviour of the flow instabilities.     

Unsteady correlation between pressure and temperature field on impinging plate for dual underexpanded jets

In this paper, dual underexpanded impinging jets are experimentally studied by measuring unsteady wall static pressure and temperature fields using high response semiconductor pressure sensors and infrared imaging camera(100 captures per sec) with dual converging sonic nozzles. The flow field is also visualized by means of high speed schlieren system (up to 70000 frame per second) to clarify the unsteady behavior of the jet structures such as barrel shock wave, and Mach disk. Experiments were performed with a blow down tunnel changing the parameters such as the nozzle to plate distance, the distance between the two nozzles, and wind tunnel pressure ratio. The nozzle to plate distance L and the distance between the two nozzles H are non-dimensionalized by the nozzle diameter D covering H/D=1.5, 2.0, and 3, L/D= 2, 3, and 4, and po/pb=1.0 − 5.0, where po and pb are stagnation pressure and back pressure respectively. Although the response of the infrared camera is quite different from that of the high speed camera, the unsteady correlation between the pressure and temperature fields is confirmed under some conditions. Also the high speed schlieren pictures and the wall static pressure fluctuations suggest that the generation of the Mach disk promote the unsteady behavior of the jets. After the confirmation of the correlation, a simple way to find the severe fluctuating region can be provided according to the two dimensional unsteady temperature images without a lot of unsteady pressure measurements.     

Simplified steady-state modeling for variable speed compressor

This paper presents a detailed analysis of semi-empirical methods to calculate mass flow rate, shaft power and discharge temperature for three types of variable speed compressors: reciprocating, scroll and piston rotary. The proposed methods are an integration of physical-based models for constant speed compressor and the physical characteristics of volumetric efficiency and isentropic efficiency between different speeds. The physical-based models were first validated with good agreement with experimental data from publication for the three types of constant speed compressors. The comparison between modeling results and experimental data from publication for the three types of variable speed compressors shows the RMS errors are less than 3%, 3% and 3 °C for refrigerant mass flow rate, compressor power input and discharge temperature, respectively. The model of variable speed compressor will allow the reduction of the number of experimental data required to characterize variable speed compressor behavior in the modeling of refrigeration systems because of its physical mechanisms.     

Numerical simulation of flow instabilities in high speed multistage compressors

INTRODUCTIONTheusefuloperatingrangeofturbomachinerycompressionsystemsislimitedbytheonsetoffluiddynamicinstabilities.Bothformsofinstabilities,rotatingstallandsurge,willdegradecompressionsystemperformanceanddurabilityll].naditionally,thisundesirablebeh...     

三级轴流式油气混输泵内流场压力脉动特性

为研究轴流式油气混输泵含气工况级间相互影响以及流道内压力脉动特性,应用Fluent软件数值模拟了三级油气混输泵在设计流量下的全流场瞬态,获得了混输泵内两相运动特征和流动部件内压力脉动情况,并分析了压力脉动时域和频域。结果表明,动静干涉是产生静压波动的主要因素,静压波动均值从入口到出口逐渐增大,级内动静交界处耦合作用较小,级间动静交界处耦合作用则较为明显。各级压缩单元叶轮进口均为脉动最剧烈的地方,距离叶轮进口越远,压力脉动幅值越小,在叶轮出口达到最小。在压缩单元内,流动方向的压力脉动主频幅值逐渐降低。首级叶轮出口、二级叶轮中间和出口以及末级叶轮中间位置压力脉动频率主要为2倍叶频,其他监测点压力脉动频率均为1倍叶频。气相对压力脉动频率影响较小,仅影响幅值。研究结果可为混输泵的结构优化设计及流动诱导振动控制提供参考。     

A quasi-one-dimensional CFD model for multistage turbomachines

The objective of this paper is to present a fast and reliable CFD model that is able to simulate stationary and transient operations of multistage compressors and turbines. This analysis tool is based on an adapted version of the Euler equations solved by a time-marching, finite-volume method. The Euler equations have been extended by including source terms expressing the blade-flow interactions. These source terms are determined using the ve- locity triangles and a row-by-row representation of the blading at mid-span. The losses and deviations undergone by the fluid across each blade row are supplied by correlations. The resulting flow solver is a performance pre- diction tool based only on the machine geometry, offering the possibility of exploring the entire characteristic map of a multistage compressor or turbine. Its efficiency in terms of CPU time makes it possible to couple it to an optimization algorithm or to a gas turbine performance tool. Different test-cases are presented for which the calculated characteristic maps are compared to experimental ones.     

Heat release dynamics modeling of kinetically controlled burning

We present a heat release dynamics model which utilizes a well-stirred reactor (WSR) and one-step kinetics to describe the unsteady combustion process. The analysis incorporates the linearized mass and energy equations to describe the response of the reactor to external perturbations and is cast in the form of a first order filter. We are able to predict the phase between the mass flow rate oscillations and the resulting heat release fluctuations, as function of the operating conditions, for example, the mean equivalence ratio and mass flow rate. The model predicts a 180° sudden shift in phase between imposed flow oscillations and resulting heat release between the maximum reaction rate and the blow-out limit. We show that this phase change may trigger combustion instability as the equivalence ratio is lowered or the average mass flow rate is increased. A number of experimental investigations, in which the inlet nozzles were choked to minimize equivalence ratio oscillations, are used to corroborate this conclusion. Next, the heat release-mass flow relationship is coupled with the acoustic field and is applied to predict instability conditions in high swirl combustion. The predictions agree qualitatively with the corresponding experimental observations.     

Unsteady transonic flow control around an airfoil in a channel

Transonic internal flow around an airfoil is associated with self-excited unsteady shock wave oscillation. This unsteady phenomenon generates buffet, high speed impulsive noise, non-synchronous vibration, high cycle fatigue failure and so on. Present study investigates the effectiveness of perforated cavity to control this unsteady flow field. The cavity has been incorporated on the airfoil surface. The degree of perforation of the cavity is kept constant as 30%. However, the number of openings (perforation) at the cavity upper wall has been varied. Results showed that this passive control reduces the strength of shock wave compared to that of baseline airfoil. As a result, the intensity of shock wave/boundary layer interaction and the root mean square (RMS) of pressure oscillation around the airfoil have been reduced with the control method.     

稳态与非稳态下旋风分离器气相流场数值模拟方法研究

为了研究旋风分离器气相流场的特性,在稳态与非稳态下,采用雷诺应力方程模型（RSM）对一台气一固旋风分离器气相流场进行了数值模拟和计算,其中旋风分离器网格划分采用混合网格分块划分技术,对流项分别采用不同阶格式进行离散,将数值模拟结果与试验数据进行了比较．结果表明：在非稳态下,动量（Momentum）方程采用二阶迎风格式（SOU）,湍动能走、耗散率￡采用高阶的Quick离散格式,雷诺应力项（Reynolds stresses）采用一阶迎风格式（FOU）是模拟旋风分离器气相流场的最佳选择．     

考虑动态摩阻的管道内水气耦合瞬变流的数值模拟

输水管道系统中经常会发生水气耦合瞬变流,可能会引起异常压力波动,甚至造成爆管事故。目前针对水气耦合瞬变流的研究很少考虑动态摩阻的影响,往往会低估瞬变过程能量衰减。对此,引入一种高效准确的简化加权类动态摩阻模型,建立考虑动态摩阻的水气耦合瞬变流模型。同时,采用虚拟塞法避免了特征线法在水气交界面的动态追踪时数值插值和求解的复杂。对比分析该模型计算结果与不考虑动态摩阻模型计算结果、试验数据,发现考虑动态摩阻的模型能更好地预测气团压力衰减和耗散,与试验压力峰值更为接近;入口压力越大,初始气团越小,动态摩阻对压力波动衰减的影响越大。     

Study on the performance deterioration of mixed flow impeller due to change in tip clearance

Performance of mixed flow compressor with un-shrouded impeller strongly depends upon unsteady, asymmetrical flow fields in the axial directions. The flow through the mixed flow impeller is complex due to three-dimensional nature of geometry. In mixed flow impeller, there are clearances between the rotating impeller blades and the casing as the high pressure ratio compressors are usually open shrouded impellers. As a result, certain amount of reduction in the performance is unavoidable due to clearance flows. In the present investigations, numerical analysis is performed using a commercial code to investigate tip clearance effects on through flow. The perform- ance of mixed flow impeller with four different clearances between impeller and stationary shroud are evaluated and compared with experimental results. The impeller performance map was obtained for different operating speeds and mass flow rates with different tip clearances. The result shows that the tip leakage flow strongly inter- acts with mainstream and contributes to total pressure loss and performance reduction. The pressure and per- formance decrement are approximately linearly proportional to the gap between impeller and stationary shroud. The analysis showed scope for improvement in design of the compressor for better performance in terms of effi- ciency and operating range.     

NUMERICAL ANALYSIS OF FLUID FLOW AND HEAT TRANSFER CHARACTERISTICS IN THREE-DIMENSIONAL PLATE FIN-AND-TUBE HEAT EXCHANGERS

This numerical study provides three-dimensional (3-D) time-dependent modeling of unsteady laminar flow and heat transfer over single- and multirow plate fin-and-tube heat exchangers. The complex nature of the flow field featuring a horseshoe vortex is investigated for both configurations. The time-dependent evolution of the horseshoe vortex mechanism on the forward part of the tube and its journey to the rear of the tube are studied to provide fundamental information on the local flow structure and the corresponding heat transfer characteristics. The effects of various governing parameters, such as fin spacing, Reynolds number, tube row number, and tube arrangement, on the heat transfer and flow characteristics are also studied for the Reynolds number range investigated. It is found that the local flow structure including formation and evolution of vortex systems and singular-point interactions correlates strongly with the heat transfer characteristics. The numerical results for the integral heat transfer parameters agree well with available experimental measurements.     

一级半轴流式透平级内部若干非定常流动现象的试验研究

设计研制了具有亚音速透平高压级气动特性的一级半轴流式试验透平,采用试验方法对时序效应、叶栅壁面非定常静压幅频特性以及动叶出口非定常速度场进行了研究。结果表明:时序效应具有改善轴流式透平气动性能的潜力;动、静叶排压力有势场干涉引发的基频信号和上游静叶尾迹片段引发的两阶倍频信号,构成了第二列静叶壁面静压非定常分量的基本频率特征,其间还伴随高达六阶的倍频信号,主要由动叶尾缘高频脱落的涡街扰动产生;尚未完成掺混的第一列静叶尾迹片段出现在动叶出口,由其引发的负射流显著改变了动叶出口局部位置处的气流偏转角。     

Numerical Study of Pressure Pulsation of Centrifugal Pumps with the Compressible Mode

The compressible effect of water is often neglected in the simulation of hydraulic machinery. However, based on experimental and numerical study, it is found that the compressibility of water could influence the magnitude of the pressure pulsation at some frequency in the pump. Therefore, in order to investigate the influence of water compressibility, compressible model is established by using Tait equation. The internal flow of centrifugal pump under different conditions is calculated by this model. The calculated results are compared with the incompressible results, and it is indicated that the compressibility of water has little effect on the performance parameters. But it affects the amplitude of pressure fluctuations at some discrete frequency, especially at the outlet of impeller and volute tongue where significant jet-wake and rotor/stator interaction appears respectively. Meanwhile, water compressibility makes greater influence on the flow pulsation under off-design condition. Therefore, it is necessary to consider the compressibility of working medium in the numerical simulation of unsteady flow in centrifugal pumps, especially in area with strong unsteady flow and at off-design condition.     

A study of the unsteady tip flow field of a transonic compressor

An experimental investigation on the unsteady tip flow field of a transonic compressor rotor has been performed.The casing-mounted high frequency response pressure transducers were arranged along both the blade chord and the blade pitch.The chord-wise ones were used to indicate both the ensemble averaged and time varying flow structure of the tip region of the rotor at different operating points under 95% design speed and 60% design speed.The pitch-wise circumferential transducers were mainly used to analyze the unsteadiness frequency of the tip leakage flow in the rotor frame at the near stall condition.The contours of casing wall pressure show that there were two clear low pressure regions in blade passages,one along the chord direction,caused by the leakage flow and the other along the tangential direction,maybe caused by the forward swept leading edge.Both low pressure regions were originated from the leading edge and formed a scissor-like flow pattern.At 95% design speed condition,the shock wave interacted with the low pressure region and made the flow field unsteady.With the mass flow reduced,the two low pressure regions gradually contracted to the leading edge and then a spike disturbance emerged.     

Experimental study of unsteady convective boiling in heated minichannels

Convective boiling in narrow channels may under specific conditions display an unsteady behavior. An experimental set-up has been elaborated to investigate heat and mass transfer and to analyze two-phase flow instabilities in rectangular microchannels with a hydraulic diameter of 889 μm. Depending on the operating conditions two types of behavior are observed: a steady state characterized by pressure drop fluctuations with low amplitudes (from 0.5 to 5 kPa/m) and no characteristic frequency; a non-stationary state of two-phase flow. The pressure signals exhibit fluctuations with high amplitudes (from 20 to 100 kPa/m) and frequencies ranging from 3.6 to 6.6 Hz. Steady and unsteady thermo-hydraulic behaviors depending on the two control parameters (heat flux and mass velocity) are analyzed and given in this paper.