Nonlinear uncertainty impact of geometric variations on aerodynamic performance of low-pressure turbine blades with ultra-high loading under extreme operational conditions

Uncertainty impact of random geometric variations on the aerodynamic performance of low-pressure turbine blades is considerable,which is further amplified by the current ultra-high-lift design trend for weight reduction.Therefore,this uncertainty impact on ultra-highly loaded blades under extreme operational conditions near the margins with potential large-scale open separation is focused on in this study.It is demonstrated that this impact is significant,unfavourable,and nonlinear,which is clearly severer under extreme conditions.In addition to the overall attenuation and notable scattering of specific performance,the operational margins with open separation are also notably scattered with great risk of significant reduction.This scattering and nonlinearity are dominated by the variations in leading-edge thickness.The thinning of leading edge triggers local transition,enhancing downstream friction and reducing resistance to open separation,which is further exacerbated by operational deterioration.However,the opposite thickening yields less benefit,implying nonlinearity.This unfavourable impact highlights the need for robust aerodynamic design,where both a safer operational condition and a more robust blade are indispensable,i.e.,a compromise among performance,weight,and robustness.Besides the necessary limitation of loading levels,a mid-loaded design is recommended to reduce adverse pressure gradients in both the leading edge and rear region of the suction side,which helps to decrease the susceptibility of the transition and open separation to random perturbations.Similar improvements can also be achieved by appropriately thickening the leading edge.

Insights into thermodynamic performance of a hypersonic precooled air-breathing engine with a complicated multi-branch closed cycle

An advanced precooled airbreathing engine with a closed Brayton cycle is a promising solution for high-speed propulsion,of which the Synergetic Air Breathing Rocket Engine(SABRE)is a representative configuration.The performance of the latest SABRE-4 cycle was analyzed in this paper.Firstly,a relatively complete engine performance model that considers the characteristics of turbomachinery and heat exchangers was developed.Then,Sobol’global sensitivity analysis of key performance parameters was carried out to identify the most influential design variables.Optimal specific impulses under different target specific thrusts were obtained by particle swarm optimization,of which the thermodynamic parameters corresponding to a specific thrust of 1.12 kN·s·kg^(-1)and a specific impulse of 3163 s were chosen as the design values.Four different control laws were analyzed in contrast,and the charge control method had the strongest ability of thrust regulation as well as maintaining a favorable specific impulse performance.Finally,working characteristics under the charge control and over a typical flight envelope were calculated,in which the average value of the maximum specific impulse was as high as 5315 s.This study would help to deepen the understanding of SABRE-4 thermodynamic characteristics and other precooled airbreathing engine cycles with similar layouts.

低压涡轮铸造叶片几何不确定性统计

精铸涡轮叶片的几何偏差诱因较多,几何偏差的统计对认识偏差来源及统计建模、评估叶片几何精度等均具有重要意义。首先初步分析千套低压涡轮真实叶片几何偏差的基本特征,通过主元分析提取偏差模态并识别主要偏差来源,发现该涡轮叶片存在明显的偏移、扭转和叶型误差。之后介绍一种基于优化策略的高效高精度叶片几何偏差分解方法,分离出偏移误差、扭转误差和叶型误差,统计发现总体几何偏差的概率密度函数(PDFs)接近高斯分布。最后对叶型误差进行统计分析并发现相对于真实叶片,偏差分解后叶型轮廓度误差的统计均值和标准差均明显下降,叶片合格率明显上升;此外,通过叶表特殊位置的轮廓度统计发现叶型轮廓度的概率密度函数也近似满足高斯分布。

Micromanufacturing technologies of compact heat exchangers for hypersonic precooled airbreathing propulsion: A review

The Hypersonic Precooled Combined Cycle Engine(HPCCE), which introduces precooler into traditional hypersonic engine, is regarded as the most promising propulsion system for realizing a single-stage-to-orbit vehicle. The unique demands lead to the application of the compact heat exchangers, which can realize high thrust-to-weight ratio, sufficient specific impulse and high compression ratio. However, it is challenging to accurately manufacture the compact heat exchanger due to its extremely high heat dissipation capacity, remarkable compactness, superior adaptability and harsh operating condition. This review summarizes the precooling schemes of combined cycle propulsions and describes the demands and key issues in the fabrication of a compact heat exchanger for HPCCE. The investigation focuses on the application of various micromanufacturing methods of heat exchangers constructed from tubes of less than 1 mm in diameter and microchannels of less than 200 micrometers. Various micromanufacturing processes, which include microforming, micromachining, stereolithography, chemical etching, 3 D printing, joining and other advanced microfabricating processes, were reviewed. In addition, the technologies are compared in terms of dimensional tolerance, material compatibility, and process applicability. Furthermore, the boundaries of the micromanufacturing constraints are specified as references for the design of compact heat exchangers. Ultimately, the technological difficulties and development trends are discussed for the fabrication of compact heat exchangers for HPCCE.

An experimental method for squealer tip flow field considering relative casing motion

Squealer tip is widely used in turbines to reduce tip leakage loss.In typical turbine environment,the squealer tip leakage flow is affected by multiple factors such as the relative casing motion and the wide range of variable incidence angles.The development of experimental methods which can accurately model the real turbine environment and influencing factors is of great significance to study the squealer tip leakage flow mechanism.In the present paper,a low-speed turbine cascade test facility which can model the relative casing motion and wide range of variable incidence angles(-25°to 55°)is built.Based on the similarity criteria,a high-low speed similarity transformation method of the turbine cascade is established by considering the thickness of the turbine blade.A combined testing method of Particle Image Velocimetry(PIV)and local pressure measurement is proposed to obtain the complex flow structures within the tip cavity.The results show that the experimental method can successfully model the relative casing motion and the wide range of variable incidence angles.The low-speed cascade obtained by the similarity transformation can model the high-speed flow accurately.The measurement technique developed can obtain the complex flow field and successfully capture the scraping vortex within the squealer tip.

Uncertainty analysis of measured geometric variations in turbine blades and impact on aerodynamic performance

Inevitable geometric variations significantly affect the performance of turbines or even that of entire engines;thus,it is necessary to determine their actual characteristics and accurately estimate their impact on performance.In this study,based on 1781 measured profiles of a typical turbine blade,the statistical characteristics of the geometric variations and the uncertainty impact are analyzed,and some commonly used uncertainty modelling methods based on Principal-Component Analysis(PCA)are verified.The geometric variations are found to be evident,asymmetric,and non-uniform,and the non-normality of the random distributions is non-negligible.The performance is notably affected,which is manifested as an overall offset,a notable scattering,and significant deterioration in several extreme cases.Additionally,it is demonstrated that the PCA reconstruction model is effective in characterizing major uncertainty characteristics of the geometric variations and their impact on the performance with almost the first 10 PCA modes.Based on a reasonable profile error and mean geometric deviation,the Gaussian assumption and stochasticprocess-based model are also found to be effective in predicting the mean values and standard deviations of the performance variations.However,they fail to predict the probability of some extreme cases with high loss.Finally,a Chi-square-based correction model is proposed to compensate for this deficiency.The present work can provide a useful reference for uncertainty analysis of the impact of geometric variations,and the corresponding uncertainty design of turbine blades.

Effect of surface roughness on the aerodynamic performance of turbine blade cascade

The effect of surface roughness on the boundary development and loss behavior of turbine blades is investigated with different Reynolds numbers in this paper.The result shows that the velocity profile in boundary layer is plumper on rough surface than on smooth blade.The aerodynamic loss is lowered at low Reynolds number,but becomes significantly large at high Reynolds number.The total pressure loss coefficient of cascade can reach a top increase of 129%for rougher blades comparing with smooth blades at Re=300000.

A refined design method for precoolers with consideration of multi-parameter variations based on low-dimensional analysis

The precooler is a distinctive component of precooled air-breathing engines but constitutes a challenge to conventional thermal design methods.The latter are based upon assumptions that often reveal to be limited for precooler design.In this paper,a refined design method considering the variations of fluid thermophysical properties,flow area and thermal parameters distortion,was proposed to remediate their limitations.Firstly,the precooler was discretized into a fixed number of sub-microtubes based on a new discretization criterion.Next,in-house one-dimensional(1D)and two-dimensional(2D)segmented models were established for rapid thermal design and precooler rating with non-uniform airflow,respectively.The heat transfer experimental studies of supercritical hydrocarbon fuel were performed to verify the Jackson correlation for precooler design and the in-house models were validated against the reported data from open literature.On this basis,the proposed method was employed for the design analysis of hydrocarbon fuel precoolers for precooled-Turbine Based Combined Cycle(TBCC)engines.The results show that the local performance of precoolers is intrinsically impacted by the aforementioned three variations.In the case study,the local heat transfer performance is drastically affected by coolant flow transition.While the circumferential temperature distortion of airflow is weakened by heat transfer.With consideration of additional parameter variations,this novel method improves design accuracy and shortens the design time.

Conjugate heat transfer investigation of cooled turbine using the preconditioned density-based algorithm

The preconditioned density-based conjugate heat transfer(CHT)algorithm was used to investigate the heat transfer characteristics of a cooled turbine vane.Fluid domain provided boundary heat flux for solid domain and obtained boundary temperature from it for the coupling strategy.The governing equations were solved by the preconditioned density-based finite-volume method,with preconditioning matrix,improved Abu-Gharmam Shaw(AGS)transition model,matrix dissipation scheme and four kinds of turbulence models.The grid system is multi-block structured grids for fluid domain and unstructured grids for solid domain,with full-matched grids at the fluid-solid interfaces.The effects of turbulence model,outlet Mach number,outlet Reynolds number,inlet turbulence intensity and the temperature ratio of blade surface/gas on the local heat transfer performance were studied.Results indicate that the k-o shear-stress transport(SST)and AGS model can predict the conjugate heat transfer better than others.The Mach number and Reynolds number have relatively obvious influences on the heat transfer,while the turbulence intensity and temperature ratio only have slight influences.Comparisons with experimental data demonstrate the applicability and accuracy of the numerical algorithm.

Impact of Stagger Angle Nonuniformity on Turbine Aerodynamic Performance

The assembling error may lead to variation in stagger angles,which would affect the aerodynamic performance of the turbine.To investigate this underlying effect,two parallel numerical experiments on two turbines with the same profile,but uniform and nonuniform vane stagger angle respectively,were conducted in both steady and unsteady methods.The results indicate that certain changes in the detailed flow field of the turbine occur when the stagger angles are nonuniform,further,the blade loading distribution of the vane and rotor become markedly different from that in uniform vane stagger angle situation.Then these consequences caused by nonuniformity mentioned above enhance the unsteadiness of the flow,finally,the aerodynamic performance changes dramatically.It also shows that,compared with steady simulation,the unsteady numerical simulation is necessary in this investigation.

Robust and Quality Boundary Constrained Tetrahedral Mesh Generation

A novel method for boundary constrained tetrahedral mesh generation is proposed based on Advancing Front Technique(AFT)and conforming Delaunay triangulation.Given a triangulated surface mesh,AFT is firstly applied to mesh several layers of elements adjacent to the boundary.The rest of the domain is then meshed by the conforming Delaunay triangulation.The non-conformal interface between two parts of meshes are adjusted.Mesh refinement and mesh optimization are then preformed to obtain a more reasonable-sized mesh with better quality.Robustness and quality of the proposed method is shown.Convergence proof of each stage as well as the whole algorithm is provided.Various numerical examples are included as well as the quality of the meshes.