Universität Stuttgart
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Item Open Access Particle image velocimetry measurements in accelerated, transonic wake flows(2022) Richter, Judith; Alexopoulos, Charalampos; Weigand, BernhardThis paper reports on particle image velocimetry (PIV) measurements in compressible accelerated wake flows generated by two different central injector types, which are mounted in a convergent-divergent nozzle. The injectors differ by the extent of their trailing edge located either in the subsonic (injector A) or supersonic flow region (injector B). In addition, the undisturbed nozzle flow without injector is studied as a reference case. The PIV results reveal typical wake flow structures expected in subsonic (injector A) and supersonic (injector B) wake flows. They further show that the Reynolds stresses Rexxand Reyysignificantly decay in all three cases due to the strong acceleration throughout the nozzle. Interestingly, in the case of injector A, the flow stays non-isotropic with Reyy>Rexxalso far downstream in the supersonic flow region. These measurements were motivated by the lack of velocity data needed to validate numerical simulations. That is why this paper additionally contains results from (unsteady) Reynolds-averaged Navier-Stokes ((U)RANS) simulations of the two wake flows investigated experimentally. The URANS simulation of the injector A case is able to accurately predict the entire flow field and periodic fluctuations at the wake centerline. However, in the case of injector B, the RANS simulation underestimates the far wake centerline velocity by about 4%.Item Open Access An analytical study on the mechanism of grouping of droplets(2022) Vaikuntanathan, Visakh; Ibach, Matthias; Arad, Alumah; Chu, Xu; Katoshevski, David; Greenberg, Jerrold Barry; Weigand, BernhardThe condition for the formation of droplet groups in liquid sprays is poorly understood. This study looks at a simplified model system consisting of two iso-propanol droplets of equal diameter, Dd0, in tandem, separated initially by a center-to-center distance, a20, and moving in the direction of gravity with an initial velocity, Vd0>Vt, where Vt is the terminal velocity of an isolated droplet from Stokes flow analysis. A theoretical analysis based on Stokes flow around this double-droplet system is presented, including an inertial correction factor in terms of drag coefficient to account for large Reynolds numbers (≫1). From this analysis, it is observed that the drag force experienced by the leading droplet is higher than that experienced by the trailing droplet. The temporal evolutions of the velocity, Vd(t), of the droplets, as well as their separation distance, a2(t), are presented, and the time to at which the droplets come in contact with each other and their approach velocity at this time, ΔVd0, are calculated. The effects of the droplet diameter, Dd0, the initial droplet velocity, Vd0, and the initial separation, a20 on to and ΔVd0 are reported. The agreement between the theoretical predictions and experimental data in the literature is good.Item Open Access Droplet velocity and diameter distributions in flash boiling liquid nitrogen jets by means of phase Doppler diagnostics(2020) Rees, Andreas; Araneo, Lucio; Salzmann, Heiko; Lamanna, Grazia; Sender, Joachim; Oschwald, MichaelDue to current and future environmental and safety issues in space propulsion, typical propellants for upper stage or satellite rocket engines such as the toxic hydrazine are going to be replaced by green propellants like the combination of liquid oxygen and hydrogen or methane. The injection of that kind of cryogenic fluids into the vacuum atmosphere of space leads to a superheated state, which results in a sudden and eruptive atomization due to flash boiling. For a detailed experimental investigation of superheated cryogenic fluids, the new cryogenic test bench M3.3 with a temperature controlled injection system was built at DLR Lampoldshausen. After a first test campaign with high-speed shadowgraphy of flash boiling liquid nitrogen sprays, a laser-based Phase Doppler system was set-up to determine the spatial distributions of droplet velocities and diameters in highly superheated sprays. The spatial distributions revealed a core region with high mean velocities close to the injector orifice. With increasing distance from the injector orifice, the sprays develop a more and more monodisperse pattern. These distributions also showed that atomization due to flash boiling generates finer sprays with growing degrees of superheat. In certain spray regions, two droplet populations varying in their direction of motion, velocity and diameter due to possible recirculation zones were observed. The experimental data of flash boiling liquid nitrogen generated within this study provide a comprehensive data base for the validation of numerical models and further numerical investigations.Item Open Access Investigation of macroscopic nearcritical fluid phenomena by applying laser-induced thermal acoustics(2023) Steinhausen, Christoph; Weigand, Bernhard (Prof. Dr.-Ing. habil.)The political and social aspiration to reduce greenhouse gases together with increasing energy demands are driving the development of new sustainable energy solutions. To achieve long term sustainability both innovative energy sources and improvements in efficiency are essential. Higher process efficiencies have been achieved by raising combustion pressures, reaching values that now exceed the critical pressures of the injected fuels. However, for an efficient and stable combustion a profound understanding of the processes prior to the combustion, such as fluid injection, disintegration and subsequent evaporation is essential. Unfortunately, the fundamental changes in fluid behaviour at near- to supercritical conditions leading to the observed fluid phenomena are not yet fully understood. Besides fluid injection, supercritical fluids themselves have been identified as an innovative path for an efficient energy conversion and heat transfer processes. The Brayton cycle using supercritical carbon dioxide as operating fluid, supercritical water as coolant and process fluid in nuclear reactors, or the application of supercritical methane as new 'green' fuel in rocket propulsion are just a few examples. Laser-induced thermal acoustics (LITA) has been identified as a promising diagnostic tool in near- to supercritical fluid research. The latter is based on the capability to acquire speed of sound data as well as to resolve thermal and acoustic attenuation in both pure fluids and complex mixing processes, such as evaporation and jet disintegration. Moreover, based on the non-linear pressure dependencies, LITA becomes increasingly more effective in high-pressure environments. By analysing the frequency domain of recorded signals, speed of sound data can be directly determined without any equation of state or additional modelling approaches. Furthermore, acoustic damping rates and thermal diffusivities can be acquired by an analytical expression for the temporal-domain of the signals. By combining both evaluations with suitable analytic expressions for the thermodynamic properties, mixing states, such as local mixing temperatures and concentrations, can be determined. Moreover, since in complex fluids at near- to supercritical conditions acoustic damping is related to both sound dispersion and volume viscosity, important insights into the physics of supercritical fluids are provided. Concordantly, the purpose of this thesis is to apply LITA in the investigation of macroscopic fluid phenomena at nearcritical to supercritical fluid conditions. This includes the following major research objectives. First, the significance of volume viscosities in complex fluids at dense gas conditions as well as the dependency of acoustic damping on mixing states are assessed. Second, the feasibility of time-resolved LITA measurements under complex flow conditions is evaluated. To achieve these objectives an experimental test facility has been designed, which enables investigations at high pressure and high temperature conditions in both pure fluids and complex mixing processes. Moreover, the laser-induced thermal acoustics setup of the ITLR has been optimised for high pressure investigations. Also a new high-speed LITA system with an adjustable measurement volume has been developed. Furthermore, a new post-processing methodology capable of analysing both the frequency and time-domain of the signal has been developed and validated. With the developed system and routines investigations in carbon dioxide, nitrogen, and binary mixtures at gas and gas-like states have been conducted to assess acoustic attenuation and volumes viscosities. Additionally, a jet mixing process has been studied to characterise the LITA arrangement and to evaluate the dependency of acoustic damping on mixing concentration. At last, to assess the feasibility of transient LITA measurements in turbulent, physically complex flow conditions and to further characterise the evaporation process, time-resolved LITA measurements have been performed in the wake of a free falling droplet evaporating in a supercritical atmosphere.Item Open Access Modellierung des Wärmeübergangs komplexer Prallstrahlfelder an Turbinengehäusen(2021) Schweikert, Julia; Weigand, Bernhard (Prof. Dr.-Ing. habil.)Die Auslegung effizienter Flugtriebwerke in allen Betriebspunkten ist eine der zentralen Aufgaben der Luftfahrttechnik. Ein wesentlicher Bestandteil liegt dabei in einer optimalen Kühlung thermisch hochbelasteter Triebwerkskomponenten über den gesamten Zyklus einer Flugmission. Für eine optimale Auslegung der eingesetzten Kühlmechanismen müssen die Wärmeübergangscharakteristika möglichst exakt bekannt sein. Unter diesem Aspekt befasst sich die vorliegende Arbeit mit dem Wärmeübergangsverhalten an Turbinengehäusen. Die Kühlung erfolgt in der realen Anwendung über den Einsatz komplexer Prallstrahlfelder, welche derzeit noch Lücken hinsichtlich der theoretischen Beschreibung aufweisen. Im Rahmen dieser Arbeit wurde ein Versuchsstand zur experimentellen Untersuchung der komplexen Prallstrahlfelder aufgebaut. Die Bestimmung des Wärmeübergangsverhaltens erfolgt dabei auf Basis der Infrarot-Thermografie. Neben einer experimentellen Betrachtung dienen numerische Simulationen der Untersuchung der Wärmeübergangs- und Strömungscharakteristika. Eine erfolgreiche Validierung des verwendeten numerischen Setups erfolgt anhand von Referenzdaten aus der Literatur. Das experimentelle Setup wird an einem vereinfachten Prallstrahlmodell verifiziert und den numerischen Verläufen gegenübergestellt. In einer zusätzlichen numerischen Studie wird der Einfluss der thermischen Randbedingung auf die Berechnung der Nusseltzahl an der Prallplatte quantifiziert. Mit den Daten dieser Studie wird eine Methode vorgestellt, den zum Teil deutlichen Einfluss der thermischen Randbedingung zu berücksichtigen. Neben der Entwicklung geeigneter Modelle zur Beschreibung des Wärmeübergangs in realen Anwendungen, bei welchen die thermischen Gegebenheiten nicht gänzlich bekannt sind, sind die Erkenntnisse essentiell für die Betrachtung des Wärmeübergangs an dem in dieser Arbeit betrachteten Modell eines komplexen Prallstrahlfeldes.Item Open Access Evaporation modeling of water droplets in a transonic compressor cascade under fogging conditions(2020) Seck, Adrian; Geist, Silvio; Harbeck, Janneck; Weigand, Bernhard; Joos, FranzHigh-fogging is widely used to rapidly increase the power outputs of stationary gas turbines. Therefore, water droplets are injected into the inflow air, and a considerable number enter the compressor. Within this paper, the primary process of droplet evaporation is investigated closely. A short discussion about the influential parameters ascribes a major significance to the slip velocity between ambient gas flow and droplets. Hence, experimental results from a transonic compressor cascade are shown to evaluate the conditions in real high-fogging applications. The measured parameter range is used for direct numerical simulations to extract evaporation rates depending on inflow conditions and relative humidity of the air flow. Finally, an applicable correlation for the Sherwood number in the form of Sh(Re1/2Sc1/3) is suggested.Item Open Access Characterisation of the transient mixing behaviour of evaporating near-critical droplets(2023) Steinhausen, Christoph; Gerber, Valerie; Stierle, Rolf; Preusche, Andreas; Dreizler, Andreas; Gross, Joachim; Weigand, Bernhard; Lamanna, GraziaWith technical progress, combustion pressures have been increased over the years, frequently exceeding the critical pressure of the injected fluids. For conditions beyond the critical point of the injected fluids, the fundamental physics of mixing and evaporation processes is not yet fully understood. In particular, quantitative data for validation of numerical simulations and analytical models remain sparse. In previous works, transient speed of sound studies applying laser-induced thermal acoustics (LITA) have been conducted to investigate the mixing behaviour in the wake of an evaporating droplet injected into a supercritical atmosphere. LITA is a seedless, non-intrusive measurement technique capable of direct speed of sound measurements within these mixing processes. The used setup employs a high-repetition-rate excitation laser source and, therefore, allows the acquisition of time-resolved speed of sound data. For the visualisation of the evaporation process, measurements are accompanied by direct, high-speed shadowgraphy. In the present work, the measured speed of sound data are evaluated by applying an advection-controlled mixing assumption to estimate both the local mole fraction and mixing temperature. For this purpose, planar spontaneous Raman scattering results measured under the same operating conditions are evaluated using an advection-controlled mixing assumption with the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state. Successively, the resulting concentration–temperature field is used for the estimation of local mixture parameters from the detected speed of sound data. Moreover, models using the PC-SAFT equation of state and the NIST database for the computation of the speed of sound are compared. The investigations indicate a classical two-phase evaporation process with evaporative cooling of the droplet. The subsequent mixing of fluid vapour and ambient gas also remains subcritical in the direct vicinity of the droplet.Item Open Access Characterization of effective diffusion within viscoelastic fluids with elastic instabilities(2022) Hietsch, Valerie; Ligrani, Phil; Su, MengyingWe considered effective diffusion, characterized by magnitudes of effective diffusion coefficients, in order to quantify mass transport due to the onset and development of elastic instabilities. Effective diffusion coefficient magnitudes were determined using different analytic approaches, as they were applied to tracked visualizations of fluorescein dye front variations, as circumferential advection was imposed upon a flow environment produced using a rotating Couette flow arrangement. Effective diffusion coefficient results were provided for a range of flow shear rates, which were produced using different Couette flow rotation speeds and two different flow environment fluid depths. To visualize the flow behavior within the rotating Couette flow environment, minute amounts of fluorescein dye were injected into the center of the flow container using a syringe pump. This dye was then redistributed within the flow by radial diffusion only when no disk rotation was used, and by radial diffusion and by circumferential advection when disk rotation was present. Associated effective diffusion coefficient values, for the latter arrangement, were compared to coefficients values with no disk rotation, which were due to molecular diffusion alone, in order to quantify enhancements due to elastic instabilities. Experiments were conducted using viscoelastic fluids, which were based on a 65% sucrose solution, with different polymer concentrations ranging from 0 ppm to 300 ppm. Associated Reynolds numbers based on the fluid depth and radially averaged maximum flow velocity ranged from 0.00 to 0.5. The resulting effective diffusion coefficient values for different flow shear rates and polymer concentrations quantified the onset of elastic instabilities, as well as significant and dramatic changes to local mass transport magnitudes, which are associated with the further development of elastic instabilities.Item Open Access Numerical investigation of internal two-pass gas turbine cooling channels under the influence of rotation(2020) Göhring, Michael; Wolfersdorf, Jens von (Prof. Dr.-Ing.)Numerical steady-state and transient conjugated simulations are applied to investigate a machine-like cooling channel configuration under the influence of rotation. Among other aspects, the comparison with the experimental data is given special consideration. With the transient thermochromic liquid crystal (TLC) measurement technique, local heat transfer coefficients can be determined. However, the evaluation method assumes that the values are constant over time. Together with the changing thermal conditions and the rotational motion this represents a contradiction, which is investigated in this thesis. As a basis, steady-state simulations are used. For validation, a test case from the literature has been selected to assess fundamental aspects such as turbulence modeling, rotational effects, and the influence of different inlet boundary conditions. This test case is further investigated to analyze the temporal development of heat transfer as it would occur in a transient TLC experiment. The isothermal walls are therefore replaced with a perspex solid body and transient conjugated simulations are performed. In the end, the dependency of heat transfer on the rotational buoyancy can be determined from the temporal development. Based on the preceding findings, the work concentrates on the ribbed channel configuration which has been experimentally investigated on the rotating test rig at ITLR. For an improved comparability with the transient TLC experiment, the color play of the TLCs is imitated from the numerical results. This enables the numerical data to be analyzed with the same evaluation software as the experiments. Although the heat transfer structures are remarkably similar, the heat transfer level of the experimental results is much higher than that of the numerical results. Of particular interest is the fact that the choice of the fluid reference temperature has a considerable influence on the local heat transfer distribution. Finally, the Nusselt numbers are illustrated as the ratio of the rotating case to the non-rotating case. Systematic influencing factors are thereby eliminated, and great similarities can be achieved between numerical simulation and experiment. The simulations not only provide a detailed analysis of the heat transfer characteristics of a machine-like cooling channel configuration, but also make a major contribution to a better understanding of the time-dependent processes in transient TLC experiments.Item Open Access Development of a passively Q-switched microchip laser operating at 914 nm for automotive lidar applications(2022) Nägele, Marco; Dekorsy, Thomas (Prof. Dr. rer. nat.)Die meisten Festkörperlaser besitzen Emissionswellenlängen oberhalb eines Mikrometers und können deshalb nicht für moderne Lidarsysteme in Kombination mit günstiger und weit etablierter siliziumbasierten Detektortechnologie verwendet werden. Ziel dieser Arbeit ist daher die Untersuchung und Realisierung eines passiv gütegeschalteten Nd3+:YVO4 Lasers bei einer Wellenlänge von 914nm für die Anwendung in einem automobilen Lidar Sensor. Zur Untersuchung der für die Lidaranwendung relevanten Laserparameter werden insgesamt drei experimentelle Resonatorkonfigurationen verwendet. Die Konfigurationen sind dabei so gewählt, dass die Laserparameter möglichst entkoppelt vom Gesamtsystem analysiert werden können. Experimentelle Untersuchungen zeigen, dass der quasikontinuierlich gepumpte Nd3+:YVO4 Laser Pulsdauern im einstelligen Nanosekundenbereich und Pulsenergien von knapp 40 μJ erreichen kann. Zudem lässt sich für eine mögliche Lidaranwendung die Repetitionsrate bis ungefähr 60 kHz über die verwendete Pumpleistung skalieren. Der Vergleich mit der Theorie basierend auf Ratengleichungen zeigt eine gute Übereinstimmung zum Experiment, woraus sich das zukünftige Potential des Lasers für mögliche Anwendungen abschätzen lässt. Über alle Untersuchungen hinweg konnte eine sehr gute Strahlqualität beobachtet werden, was in der Lidaranwendung ein hervorragendes Auflösevermögen verspricht. Neben der Betrachtung verschiedener Systemzusammenhänge mittels experimenteller Konfigurationen wird ein kompakter, monolithischer, passiv gütegeschalteter Demonstratoraufbau im Einzelpulsbetrieb bei einer Wiederholrate von 200 Hz präsentiert. Hierbei kommt als Pumplaser ein 808nm Breitstreifen-Diodenlaser zum Einsatz, welcher verglichen mit einem fasergekoppelten Laserdiodenmodul nicht nur ein deutlich kompakteres Gesamtsystem verspricht, sondern ebenfalls eine schmalere Linienbreite besitzt. Folglich kann das Gesamtsystem allein durch Anpassung des Pumplaserstroms und ohne aktive Temperaturstabilisierung in einem Temperaturbereich von 20-50 °C stabil betrieben werden. Zudem liefert der kurze Resonator des monolithischen Laserkristalls nicht nur kurze Pulsdauern, sondern ermöglicht ebenfalls den Betrieb auf einer einzelnen longitudinalen Mode und folglich spektrale Emissionsbandbreiten von wenigen Pikometern. Hierdurch ergibt sich für Langzeitmessungen über 60 Minuten eine hervorragende Stabilität der spektralen Eigenschaften, der Pulsenergie und der Pulsdauer.