Добірка наукової літератури з теми "Rotative cascade impactor"

Abstract. A widely used instrument for collecting size-segregated particles is the micro-orifice uniform deposit impactor (MOUDI). In this work, a 10-stage MOUDI (cut-point diameter of 10 µm to 56 nm) was used to collect samples in Ljubljana, Slovenia, and Martinska, Croatia. Filters, collected with and without rotation, were cut in half and analyzed for nine elements (As, Cu, Fe, Ni, Mn, Pb, Sb, V, Zn) using laser ablation ICP-MS. Elemental image maps (created with ImageJ) were converted to concentrations using NIST SRM 2783. Statistical analysis of the elemental maps indicated that for submicron particles (stages 6–10), ablating 10 % of the filter (0.5 cm2, 20 min ablation time) was sufficient to give values in good agreement (±10 %) to analysis of larger parts of the filter and with good precision (RSE < 1 %). Excellent sensitivity was also observed (e.g., 20 ± 0.2 pg m−3 V). The novel use of LA-ICP-MS, together with image mapping, provided a fast and sensitive method for elemental analysis of size-segregated MOUDI filters, particularly for submicron particles.

Ce travail de thèse vise à améliorer notre connaissance des sources proches et lointaines contrôlant le dépassement des seuils réglementaires de qualité de l’air, tels qu’ils peuvent être appréciés par les réseaux de mesure existants, sur le site fortement industrialisé et urbain du Grand Dunkerque. Il s’agit notamment de s’appuyer sur le calcul d’un indice d’état de mélange des particules, prenant en compte l’hétérogénéité de leur composition élémentaire, celle-ci étant liée à leur temps de séjour dans l’air et à la distance entre les sources et le site récepteur étudié. Pour répondre à cette problématique, il a fallu dans un premier temps développer un impacteur séquentiel de particules à haute résolution temporelle, nommé TRAPS, qui répondait au besoin de suivre les changements rapides observés au sein des particules atmosphériques lors d’épisodes de pollution. Couplé à un granulomètre et après analyse individuelle des particules prélevées par microscopie électronique (MEB-EDX), le TRAPS permet de rendre compte de l’évolution physico-chimique des particules atmosphériques au cours du temps. Dans la première partie de cette thèse, des expériences menées en laboratoire et une campagne de terrain ont permis de valider notre prototype, de rendre compte de la dynamique de dépôt des particules sur les zones d’impaction et de vérifier les diamètres de coupure des étages grossier et fin du TRAPS, déterminés respectivement à 1.32µm et 0.13µm. Une étude statistique des épisodes de pollution aux PM₁₀ survenus sur le grand dunkerquois a ensuite été réalisée sur 3 ans, entre 2018 et 2020. Elle nous a permis d’identifier 12 principaux types d’épisodes sur la base de leur étendue spatiale, mais aussi des conditions locales de dispersion des polluants. On a pu ainsi identifier des épisodes locaux et des épisodes régionaux observés, soit en conditions atmosphériques stationnaires ou au contraire en conditions de dispersion des pollutions à plus grande échelle. Alors que 78 % des jours de dépassement du seuil réglementaire des PM₁₀ correspondent à des épisodes locaux, les 22.4% restant correspondent à des panaches de pollution d’étendue au moins régionale, avec une proportion égale des jours de dépassements en condition de dispersion et en conditions stationnaires. Hormis les épisodes très localisés, une étude fine de la variabilité temporelle des concentrations en particules fines (PM₂.₅) montre la présence systématique d'une période d'accumulation progressive des polluants, pouvant atteindre une dizaine d'heures et caractérisée par une contribution importante de ces particules. La dernière partie de ce travail a consisté en l'étude de la composition et l'état de mélange des particules individuelles collectées lors d'évènements de pollution sur la zone du Grand Dunkerque en 2021. La campagne a permis l'échantillonnage et la caractérisation de 5 épisodes de pollution durant lesquels le TRAPS était déployé en parallèle d'autres instruments fournissant des informations complémentaires sur la granulométrie des aérosols, la météorologie ou la dynamique atmosphérique. Près de 28000 particules individuelles ont été caractérisées par MEB-EDX. Avec plus de 90% des échantillons associés à des valeurs de l'indice d'état de mélange chimique supérieures à 0.5, il est possible d'affirmer que les particules collectées sur la zone du Grand Dunkerque, durant ces épisodes de pollution, sont, en général, de composition très hétérogène à l'échelle de la particule individuelle (particules dites "en mélange interne"). Les résultats obtenus montrent en outre une influence de l'origine, locale ou transportée, des particules sur leur composition chimique et par là même sur l'indice d'état de mélange chimique de la population de particules échantillonnées. Une évolution croissante de l'indice d'état de mélange avec le temps de résidence des particules dans l'atmosphère lors de ces évènements est notamment observée
This thesis aims at improving our knowledge of the near and distant sources controlling the exceedance of the regulatory thresholds of air quality, as detected by the air quality monitoring networks, at the strongly industrialized and urban site of Great Dunkirk Area (GDA). This appreciation is notably based on the calculation of a mixing state index of the particles, taking into account the heterogeneity of their elementary composition, this one being related to their residence time in the atmosphere and the distance between the sources and the studied receptor site. To do that, it was firstly necessary to develop a time resolved cascade impactor with high temporal resolution, named "TRAPS", which answered the need to follow the rapid changes observed within the atmospheric particles during pollution episodes. Coupled with a particle size analyser and after individual analysis of the collected particles by electron microscopy (SEM-EDX), TRAPS allows to report the physicochemical evolution of atmospheric particles over time. In the first part of this thesis, laboratory experiments and a field campaign allowed to validate our prototype, to report the dynamics of particle deposition on the impaction stages and to verify the cut-off diameters of the coarse and fine stages of TRAPS, determined respectively at 1.32µm and 0.13µm. A statistical study of PM₁₀ pollution episodes occuring over the GDA was then carried out over 3 years, between 2018 and 2020. It allowed us to identify 12 main types of episodes based on their spatial extent, but also on the local conditions of pollutant dispersion. We were able to identify local episodes and regional episodes observed, either in stationary or dispersive atmospheric conditions. While 78% of the PM₁₀ exceedance days correspond to local episodes, the remaining 22% correspond to pollution plumes with at least a regional extent, with an equal proportion of exceedance days in dispersion and stationary conditions. Except for very localized episodes, a detailed study of these pollutions episodes shows the systematic presence of a period of pollutant accumulation, of about 10 hours, characterized by an important contribution of fine particles (PM₂.₅) except for episodes of limited spatial coverage. The last part of this work consisted in the study of the composition and mixing state of the individual particles collected during pollution events in the GDA in 2021. The campaign allowed the sampling and characterization of 5 pollution episodes, during which TRAPS was deployed in parallel with other instruments providing complementary information on aerosol granulometry, or atmospheric dynamics. Nearly 28000 individual particles were characterized by SEM-EDX. With more than 90% of the samples associated with values of the mixing state index higher than 0.5, it can be said that the particles collected in the GDA during pollution episodes are in general of very heterogeneous composition at the particle scale (internal mixing). Nevertheless, the results show an influence of the local or transported origin of the particles on their chemical composition, but also on the mixing state index. An increasing evolution of this index with the particles residence time during these events is observed

Abstract In turbomachines, rotors and stators differ by the rotation of the former. Hence, half of each stage is directly influenced by rotation effects. The influence of rotation on the flow structure and its impact on the performance is studied through Wall-Resolving Large Eddy Simulations of a rotor with large relative tip gap size. The simulations are performed in a rotating frame with rotation accounted for through a Coriolis force term. In a first step experimental results are used to provide validation. The main part of the study is the comparison of the results from two simulations, one representing the rotating configuration, one with the Coriolis force removed, without any other change. This setup allows very clean assessment of the influence of rotation. The turbulence-resolving approach ensures that the turbulent flow features are well represented. The results show a significant impact of rotation on the secondary flow. In the tip region the Tip Leakage Vortex is enlarged and destabilised. Inside the tip gap the flow is altered as well, with uniformization in the rotating case. At the blade midspan, no significant effects are observed on the suction side, while an earlier transition to turbulence is found on the pressure side. Near the hub, rotation effects are shown to reduce the corner separation significantly.

Abstract A computational analysis is performed to determine if particulate impact events on the external surfaces of gas turbine engine rotor blades can be faithfully replicated in an experimental rotor cascade. The General Electric (GE) Energy Efficient Engine (E3) first-stage turbine flow-field at cruise conditions is first solved using a steady state explicit mixing plane approach with non-reflecting treatment. To model flow in the cascade, a single E3 rotor periodic domain is then constructed with an inlet section matching the relative flow incidence angle from the mixing plane calculation. The mass-averaged relative flow conditions at the inlet and outlet of the mixing plane rotor section are imposed on the cascade boundaries and a steady solution is found. Particles with diameters ranging from 1 to 25 μm are tracked through each fluid domain using a Lagrangian approach, and the OSU Deposition Model is implemented to dictate the sticking and rebounding action when particles interact with solid surfaces. The impact locations on the blade are compared between the rotating (mixing plane) and stationary (cascade) cases. It is discovered that both the locations and parameters of the particle impacts in the cascade vary significantly from the engine environment. For smaller particles, this deviation is credited to a stronger upstream influence of the blade on the cascade flow-field. As particle size increases, this effect tapers off, and the differences in deposition are instead driven by the interaction of the full-stage vane with the particles. The lack of a vane in the cascade causes drastically different particle inlet vectors over the rotor than are seen in the engine setting. The radial differences of particle impact locations are explored, and the role that absolute pressure plays is considered.

The accuracy of flutter or forced response analyses of turbomachinery blade assemblies strongly depends on the correct prediction of the unsteady aerodynamic loads acting on the vibrating blades. This paper presents the aeroelastic numerical results of an annular transonic compressor cascade subjected to harmonic oscillation conditions. The measurements associated were performed in an annular test facility for non-rotating cascades. The aim of this investigation is to get a deeper understanding of the specific characteristics of this test facility as well as improving the flutter prediction procedure and accuracy. For a subsonic and a transonic flow condition, the steady-state blade surface pressure distributions were predicted with two mesh configurations and results were compared to the experimental results. The first configuration omits the geometrical complexity of the experimental model and only models the blade passage. The second mesh configuration includes the cascade’s detailed geometry and cavities. The presence of leakage flows arisen due to the cascade’s slits and cavities are identified and their impact on the main flow field is analyzed and discussed. For the flutter computations, two mesh resolutions were investigated. The global damping predicted with a fine and a coarse mesh was compared, as well as the local pressure amplitudes and phases predicted with both configurations. Results show that even though similar global damping curves are predicted with both mesh resolution, for some IBPAs, local differences exist on the pressure amplitudes and phases. This highlights that only comparing the global damping coefficient, is not sufficient for code validation.

This paper is focused on the influence of stator-rotor purge flow injection angle on the aerodynamic and thermal performance of a rotor blade cascade. Tests were performed in a seven-blade cascade of a high-pressure gas turbine rotor at low Mach number (Ma2is = 0.3) under different blowing conditions. A number of fins were installed inside the upstream slot to simulate the effect of rotation on the seal flow exiting the gap in a linear cascade environment. The resulting coolant flow is ejected with the correct angle in the tangential direction. Purge flow injection angle and blowing conditions were changed in order to identify the best configuration in terms of end wall thermal protection and secondary flows reduction. The 3D flow field was surveyed by traversing a 5-hole miniaturized pressure probe in a downstream plane. Secondary flow velocities, loss coefficient and vorticity distributions are presented for the most significant test conditions. Film cooling effectiveness distributions on the platform were obtained by Thermochromic Liquid Crystals technique. Results show that purge flow injection angle has an impact on secondary flows development and thus on the end wall thermal protection, especially at high injection rates. Passage vortex is enhanced by a negative injection angle, which simulates the real counter rotating purge flow direction.

The measurement accuracy of the temperature/pressure probe mounted at the leading edge of a turbine/compressor blade is crucial for estimating the fuel consumption of a turbo-fan engine. Apart from the measurement error itself, the probe also introduces extra losses. This again would compromise the measurement accuracy of the overall engine efficiency. This paper utilizes high-fidelity numerical analysis to understand the complex flow around the probe and quantify the loss sources due to the interaction between the blade and its instrumentation. With the inclusion of leading edge probes, three dimensional flow phenomena develop, with some flow features acting in a similar manner to a jet in cross flow. The separated flow formed at the leading edge of the probe blocks a large area of the probe bleed-hole, which is one of the reasons why the probe accuracy can be sensitive to Mach and Reynolds numbers. The addition of 4% free stream turbulence is shown to have a marginal impact on the jet trajectory originated from the probe bleedhole. However, a slight reduction is observed in the size of the separation bubble formed at the leading edge of the probe, preceding the two bleedhole exits. The free stream turbulence also has a significant impact on the size of the separation bubble near the trailing edge of the blade. With the addition of the free stream turbulence, the loss observed within the trailing edge wake is reduced. More than 50% of the losses at the cascade exit are generated by the leading edge probe. A breakdown of the dissipation terms from the mean flow kinetic energy equation demonstrates that the Reynolds stresses are the key terms in dissipating the counter rotating vortex pairs with the viscous stresses responsible for the boundary layer.

One of the major concerns of the centrifugal compressor designer is the trade-off between the compressor’s efficiency and its range. The compressors equipped with vaneless diffusers benefit from a broader operational range than conventional vaned diffusers, but with decreased compression efficiency. One solution found in recent years is the Low Solidity Diffuser, which displays a range greater than conventional vaned diffusers, by preventing rotating stall, and efficiency higher than vaneless diffusers. The study focuses on the impact of geometry elements on the flow field through the vanes’ passages, and on the change of performance with the location of the vanes relative to the scroll tongue. This paper presents results of experimental and Computational Fluid Dynamics (CFD) investigations — for centrifugal compressors with low solidity cascade diffusers. The vanes are based on a NACA airfoil profile and have been designed using commercial software from ConceptsNREC: Compal and AxCent. The analysis part was performed using CFX software from Ansys, Inc. The performance parameters recorded are pressure ratios, isentropic efficiency, flow angles and velocity field profiles.

Rim seals are used to prevent the ingress of hot gas into the cavities beneath turbine platforms. As these cavities are not actively cooled, high-pressure air, known as purge flow, is taken from the compressor and introduced beneath the platform to prevent hot gas from penetrating through the gaps between stationary and rotating hardware. Improving the rim-seal geometry however, is made difficult by a lack of understanding of the salient fluid mechanics associated with this region. This study investigates both the impact of a vane-induced static pressure distortion as well as the influence of the pressure distortion of a downstream blade row on an engine-relevant rim seal in a stationary, linear cascade. Vane alterations resulted in minimal change to rim seal performance; however, adding the pressure distortion of a downstream blade row was found to disturb the trench flow resulting in poorer performance of the seal.

This paper presents both experimental and numerical results of tip leakage effects in a High Speed Annular Cascade. The influence of the clearance size has been investigated. For validation purposes and a better understanding of the 3-D phenomena, a comparison of the measurements with three-dimensional Navier-Stokes computations using a k-ε closure with a low-Reynolds approach has been performed. The high speed annular cascade facility has been specifically designed for studying tip leakage flows at the rear of high pressure compressors. In the vicinity of the clearance, the inlet swirl angle, created by a scroll, is of 60° from the axial direction and the Mach number is about 0.60. The blades are cantilevered (fixed on the casing with a clearance gap at the rotating inner wall). In order to obtain a picture of the flow field as complete as possible, different kinds of measurements have been used. 3D velocity measurements within the blade passages have been performed by means of a 3-D Laser Doppler Anemometer system. Furthermore, a five-hole probe with long stem was also used in the blade passage. The experimental data are quite detailed and self-consistent. A leakage vortex can clearly be identified and, within the blade passage, seems to be responsible for a region of overturning above it. The clearance size has a direct impact on the inception point of the phenomena and on the direction and strength of the leakage vortex. The calculation was found to reproduce the same trends as the experiment and give good quantitative comparisons, although it overestimates the leakage effect at blade exit.

Flow distortions in front of turbomachines can lead to additional mechanical and thermal loading of the blades, and may influence the operational stability of the engine and the lifetime of its parts. In current design practice the impact of inflow distortions is accounted by safety margins, which are often conservative. Therefore the knowledge of real limits helps to increase operational flexibility and recommended service intervals. The distortions inside turbomachines have a wide spectrum of sizes and frequencies depending on their origin. Some of them were extensively studied in the recent years, including wakes interaction with blades, hot streaks propagation in turbines, and some other phenomenon. However, there are areas of practical importance, which are not yet covered by available studies. One of this distortion types is investigated in the present work. These are distortions of a size, comparable with the inlet guide vane pitch of a compressor or the nozzle guide vane pitch of a turbine, both steady in the absolute frame of reference. Such distortions in front of a compressor can be caused, for example, by fouling of the intake, or by some structures purposely installed inside the intake. The distortions in front of the expansion turbine can be caused by structures. Main purpose of the study is to provide an understanding of the decay mechanism of distortions considered. For this purpose the numerical and theoretical analysis of inflow distortion propagation inside the set of stationary and rotating blades cascades is performed. These cascades represent the mid-section of a typical multistage compressor and turbine. The analysis showed that the decay mechanism is different for temperature distortions and total pressure distortions, and that the latter one is stronger affected by the blading configuration.

This investigation discusses the impact of a non-steady outflow condition on the compressor stator flow in an annular cascade which is periodically chocked through a rotating disc in the wake, to simulate the expected conditions for a pulsed detonation engine (PDE). A 2D controlled diffusion airfoil of the highly loaded linear stator cascade by [1] has been transferred to the annular compressor test rig to compare results under non-steady conditions via multi-colored oil flow visualization on the suction side and pressure measurements in the wake of the blades. Three different Strouhal numbers of the choking device are investigated and analyzed by phase averaged pressure measurements downstream of the stator to visualize the unsteady flow characteristics. Triggered by the changed incidence angle due to the choking, separation on the suction side and in the hub region form a periodic event depending on the position of the blockage device. Active flow control (AFC) is implemented by means of side wall actuation at the hub to improve flow conditions. Pressure measurements show that the turning of the blades can be raised and a static pressure rise is gained by the AFC while periodic choking is active.