脉冲进气条件下弹性约束导叶可变几何涡轮性能研究

文中基于可调涡轮弹性约束导叶的概念,采用流固耦合方法研究了在脉冲进气条件下,弹性约束导叶涡轮的非稳态特性以及脉冲幅值和频率对其性能的影响,通过与传统可调涡轮对比,分析了弹性约束导叶可调涡轮性能发生变化的原因.结果表明,脉冲进气幅值越小,频率越低,弹性约束导叶可调涡轮相对传统可调涡轮流量提升率越大,输出功率提升率越大,效率下降率越低.通过对转静子进口气流角以及导叶出口压力损失分布的分析,论证了弹性约束导叶涡轮性能对脉冲幅值和频率发生变化的原因.

动静干涉下压气机叶轮气动激励与振动分析

为研究离心压气机叶轮的气动激励特征及激励作用下的振动特征,建立了单向瞬态流固耦合模型进行计算分析.利用模态分析确定了叶轮的共振工况,在此工况下进行单向瞬态流固耦合计算.讨论了叶轮与进气弯管和出口蜗壳发生动静干涉时,其表面非定常气动激励的时域与频域特征,并进一步讨论了叶轮在此气动激励下振动时的时域与频域特征.结果表明,叶轮共振时的主要振动分量是基频与共振频率处的振动分量,共振频率处较小的气动激励引发了较大的振动幅值;长叶片吸力面前缘等位置处动应力及离心应力较大,是疲劳失效危险点,叶轮易从此处萌生疲劳裂纹并发生疲劳断裂.

车用涡轮增压器瞬态响应性测试方法

为了实现对涡轮增压器瞬态响应性的评估,本文通过分析内燃机瞬态响应与涡轮迟滞之间的关系,提出涡轮增压器瞬态响应能力的测试方法。根据所提出的测试方法,建立了测试过程仿真模型,在对仿真模型校核后,利用该模型分析了不同起止过渡工况、转子转动惯量及涡轮增压器效率对加速性的影响。结果表明:本文方法操作性强,可独立于内燃机评价涡轮增压器的瞬态响应能力。

涡轮增压器涡轮叶轮有意失谐强迫振动特性研究

通常希望增压器涡轮叶轮的每个叶片都制造得完全相同,形成严格的循环对称结构,但由于制造误差和工作磨损等因素会造成涡轮叶轮存在失谐。为了在设计中降低叶轮对随机失谐的敏感性,以某型号涡轮增压器涡轮叶轮为研究对象,使用CMM缩减模型和加速Monte Carlo模拟,对使用3种有意失谐方案设计的涡轮随机失谐的敏感性进行了分析。研究表明,方案Ⅰ的叶轮强迫振动敏感性曲线仍然具有峰值特征,但振动响应幅值得到一定程度的降低。方案Ⅱ的叶轮强迫振动敏感性曲线的响应峰值不再明显,最大响应幅值放大系数相比谐调叶轮由1.86降到1.58。方案Ⅲ的叶轮强迫振动敏感性曲线上的响应峰值有较大下降,最大响应幅值放大系数为1.49,微小失谐时响应值较低,对失谐不敏感。

复杂进口边界对高压级压气机性能影响分析

建立了带有级间管道的高压级压气机模型,对比分析了在均匀进气条件和非均匀进气条件下,下游高压级压气机的流场变化。结果表明:不论是均匀进口边界条件还是不均匀进口边界条件,高压级进口都出现了明显的周向畸变,压力呈现“外高内低”的分布形式,在弯管和直管的转换处,存在流动漩涡,说明高压级进口的周向畸变主要是由于高压级进口弯扭形状的管路引起的。此外,通过对高压级压气机叶间流场进行分析,发现50%槽道展向高度位置的流场明显受到蜗舌引起的周向不均匀压力场的作用,即蜗舌的左侧都存在明显的高熵区域,这可能是由于蜗舌处高静压的作用,导致蜗舌左侧气流流动受阻,从而诱发流动分离所致。在90%槽道展向高度位置,蜗舌的作用减弱,间隙泄漏流动引起的损失更加明显。

变海拔环境下离心压气机叶轮多场应力响应

针对车用涡轮增压器离心压气机叶轮在变海拔环境下可能出现的静强度失效问题,开展了叶轮在变海拔环境下多场载荷及应力响应的变化规律研究。采用单向稳态流体与固体耦合的方法,计算了叶轮在气动载荷、热载荷以及离心载荷作用下的单场应力以及多场耦合应力。结果表明:在真实流量、真实转速不变,仅根据海拔高度不同改变进气压力与进气温度时,叶轮进口流动角随着海拔升高而增加,导致高海拔环境下长叶片前缘静压差较大,最大气动应力从长叶片尾缘转移到前缘,叶轮的热载荷、热应力以及最大多场耦合应力随海拔升高而减小;在增压发动机工况不变时,随着海拔的升高,叶轮气动应力与热应力的绝对变化较小,压气机转速上升带来较大的离心应力增幅,进而导致耦合应力的增加。

The Change of the Inlet Geometry of a Centrifugal Compressor Stage and its Influence on the Compressor Performance

The impact on the compressor performance is important for designing the inlet pipe of the centrifugal compressor of a vehicle turbocharger with different inlet pipes. First, an experiment was performed to determine the compressor performance from three cases: a straight inlet pipe, a long bent inlet pipe and a short bent inlet pipe. Next, dynamic sensors were installed in key positions to collect the sign of the unsteady pressure of the centrifugal compressor. Combined with the results of numerical simulations, the total pressure distortion in the pipes, the pressure distributions on the blades and the pressure variability in the diffuser are studied in detail. The results can be summarized as follows: a bent pipe results in an inlet distortion to the compressor, which leads to performance degradation, and the effect is more apparent as the mass flow rate increases. The distortion induced by the bent inlet is not only influenced by the distance between the outlet of the bent section and the leading edge of the impeller but also by the impeller rotation. The flow fields in the centrifugal impeller and the diffuser are influenced by a coupling effect produced by the upstream inlet distortion and the downstream blocking effect from the volute tongue. If the inlet geometry is changed, the distributions and the fluctuation intensities of the static pressure on the main blade surface of the centrifugal impeller and in the diffuser are changed accordingly.

导叶端隙密封结构对可调涡轮性能的影响

基于传统可调涡轮导叶,设计了一种带有端隙密封结构的喷嘴环导叶(end-clearance sealed guide vane, ESGV)。在导叶不同开度下,对ESGV可调涡轮进行数值模拟,分析了ESGV对可调涡轮性能的影响,结果表明:ESGV可以有效的抑制导叶端隙泄漏流,能够明显改善喷嘴环和转子通道的流动状态,进而提高涡轮效率。对ESGV可调涡轮和原始涡轮进行涡轮特性试验,结果显示:两型涡轮流量特性大致相同时,在导叶中开度下,ESGV涡轮效率较原始涡轮提高了5%,验证了ESGV方案的有效性。

脉冲频率对径向涡轮转子进口流动特性的影响

可调涡轮增压器涡轮转子进口的动静干涉效应以及脉冲气流的波动使得转子进口流动更为复杂,进而影响转子通道内的气体流动。为了更好地了解脉冲频率对涡轮转子进口流动特性的影响,应用ANSYS CFX软件对不同频率(20、40、60 Hz和80 Hz)脉冲进气条件下的全周径向可调涡轮模型进行数值模拟,并通过试验数据对数值模型进行了验证,得出转子进口平均气流角在一个脉冲周期内的分布规律以及气流角在周向和叶高方向的分布规律。发现脉冲频率对低压侧转子进口入射角影响更大,脉冲频率越低,低压侧的转子进口负入射角越大;周向绝对气流角分布规律相似,波动幅度基本相同;相对气流角在压力下降沿波动更稳定。

Waste heat recovery from heavy-duty diesel engine exhaust gases by medium temperature ORC system

A medium-temperature waste-heat recovery system based on the organic Rankine cycle (ORC) is designed to recover the exhaust energy from a heavy-duty diesel engine. Analysis of the 1st law of thermodynamics for an ORC system is performed. This analysis contains two parts. The first part is an analysis with undefined heat exchangers to gain an understanding of the ORC and find out suitable organic fluid parameters for a better ORC efficiency. The second part of the analysis uses combined engine test results and two designs of heat exchangers. By comparing the two designs, an improved system of heat exchangers is described. This analysis also quantifies the effect of engine parameters on ORC system. The study concludes that the supercritical Rankine cycle is a better approach towards waste heat recovery. The ORC system is found to perform better under part-load conditions if the medium-high power condition rather than rated working point of the engine is used as the design parameter. The ORC system achieves the highest waste-heat recovery efficiency of up to 10-15% for the optimised heat ex-changer design.

Forced Responses on a Radial Turbine with Nozzle Guide Vanes

Radial turbines with nozzle guide vanes are widely used in various size turbochargers.However,due to the interferences with guide vanes,the blades of impellers are exposed to intense unsteady aerodynamic excitations,which cause blade vibrations and lead to high cycle failures(HCF).Moreover,the harmonic resonance in some frequency regions are unavoidable due to the wide operation conditions.Aiming to achieve a detail insight into vibration characteristics of radial flow turbine,a numerical method based on fluid structure interaction(FSI) is presented.Firstly,the unsteady aerodynamic loads are determined by computational fluid dynamics(CFD).And the fluctuating pressures are transformed from time domain to frequency domain by fast Fourier-transform(FFT).Then,the entire rotor model is adopted to analyze frequencies and mode shapes considering mistuning in finite element(FE) method.Meanwhile,harmonic analyses,applying the pressure fluctuation from CFD,are conducted to investigate the impeller vibration behavior and blade forced response in frequency domain.The prediction of the vibration dynamic stress shows acceptable agreement to the blade actual damage in consistent tendency.

A review of fundamental factors affecting diesel PM oxidation behaviors

Diesel particulate matter(DPF) is usually employed to meet the stringent regulations on particulate matter(PM) emissions for diesel engine. To resolve the DPF regeneration problem, comprehensive information about the factors influencing PM oxidation behaviors must be understood. Large amounts of factors related to PM oxidation activity have been investigated, however, some relations are still ambiguous. This paper reviews the factors related to PM oxidation activity that the factors are divided into the engine-correlated and engine-uncorrelated factors. The methods with both advantages and disadvantages to test the oxidation behaviors are introduced. The microstructure and ingredient being fundamental factors affecting PM oxidation behaviors are as the principle line to correlate PM oxidation behaviors and engine-correlated factors. The relations of engine-correlated factors with oxidation behaviors are obtained though advanced technologies that are mutual complementation. The engine-uncorrelated factors are also reviewed that these factors are vital to oxidation activity changes. Multiple-factor analysis rather than single-factor analysis should be developed to make the oxidation behaviors of diesel PM more clear.

Investigation on the Forced Response of a Radial Turbine under Aerodynamic Excitations

Rotor blades in a radial turbine with nozzle guide vanes typically experience harmonic aerodynamic excitations due to the rotor stator interaction. Dynamic stresses induced by the harmonic excitations can result in high cycle fatigue(HCF) of the blades. A reliable prediction method for forced response issue is essential to avoid the HCF problem. In this work, the forced response mechanisms were investigated based on a fluid structure interaction(FSI) method. Aerodynamic excitations were obtained by three-dimensional unsteady computational fluid dynamics(CFD) simulation with phase shifted periodic boundary conditions. The first two harmonic pressures were determined as the primary components of the excitation and applied to finite element(FE) model to conduct the computational structural dynamics(CSD) simulation. The computed results from the harmonic forced response analysis show good agreement with the predictions of Singh's advanced frequency evaluation(SAFE) diagram. Moreover, the mode superposition method used in FE simulation offers an efficient way to provide quantitative assessments of mode response levels and resonant strength.

Numerical Investigation & Comparison of a Tandem-Bladed Turbocharger Centrifugal Compressor Stage with Conventional Design

Extensive numerical investigations of the performance and flow structure in an unshrouded tandem-bladed centrifugal compressor are presented in comparison to a conventional compressor.Stage characteristics are explored for various tip clearance levels,axial spacings and circumferential clockings.Conventional impeller was modified to tandem-bladed design with no modifications in backsweep angle,meridional gas passage and camber distributions in order to have a true comparison with conventional design.Performance degradation is observed for both the conventional and tandem designs with increase in tip clearance.Linear-equation models for correlating stage characteristics with tip clearance are proposed.Comparing two designs,it is clearly evident that the conventional design shows better performance at moderate flow rates.However;near choke flow,tandem design gives better results primarily because of the increase in throat area.Surge point flow rate also seems to drop for tandem compressor resulting in increased range of operation.