1,721,027 research outputs found
Modeling of particle-radiation-interaction for the numerical simulation of coal combustion
No full textIn the present dissertation, scattering and absorption of thermal radiation by particles in coal combustion scenarios is investigated, modeling approaches are derived and a parameter variation is performed to quantify the influence of the parameters determining heat transfer. In the first section, scattering and absorption properties of coal particles are investigated, making use of the detailed Mie theory which serves as the reference solution throughout the present work. The complex index of refraction which is the material property that determines the interaction of matter with thermal radiation is introduced. Available data on the complex index of refraction for coal and ash particles in literature are compared, discussed investigated regarding their influence on the scattering and absorption properties. Then, modeling approaches are derived that allow to describe scattering and absorption by coal and ash particles in numerical simulations of coal combustion efficiently. This includes a novel approach to describe the variation of scattering and absorption properties of coal particles undergoing burnout based on a shell model. Also, new approximations for the scattering phase function are presented. A very promising approximation for the scattering phase function is provided by the modified Henyey-Greenstein scattering phase function and forward scattering factors calculated from precise Mie theory calculations. Finally, these models are implemented in a program to solve the radiative transport equation numerically. To be applied in a numerical scheme with an angular discretization, the scattering phase functions are integrated over discrete solid angles based on a customized integration procedure which is introduced in this work. The derived models are tested regarding their ability to describe scattering and absorption by particles. Besides the scattering and absorption properties of the particles, all remaining parameters determining radiative heat transfer in coal combustion scenarios are varied and their influence on heat transfer is investigated and the results are discussed. The most important finding for the present work is that with a good approximate scattering phase function, scattering by coal and ash particles can be described very reasonably. Finally, this thesis can be used as a guide on how to treat radiative heat transfer in coal combustion simulations
Enhancement of inverse thermal analysis to quantify multi-scale heat transfer at non-conforming rough contact interfaces
No full textFor the prediction of temperature fields in machines, the contact heat transfer coefficient is a major parameter to estimate heat flow between individual components. At the interface of two joint components the actual contact area is only a fraction of the nominal area, due to manufacturing-related surface roughness. The roughness causes a thermal resistance, leading to a temperature drop across the interface. Besides the surface roughness, the resistance is influenced by variable contact pressures, interstitial media, and contact partners moving relative to each other. Previous studies have focused on macroscopic planar pairings and neglected the influence of curved contact partners. The curvature induces an additional macroscopic constriction resistance, impacting the overall joint heat transfer and resulting temperature field. In this work, therefore, existing experimental methods are extended to quantify, both, microscopic and macroscopic thermal resistances. Based on the extension, parameter studies are performed to investigate the influence of roughness, pressure, curvature as well as specimen width. The obtained results show a clear influence of pressure and roughness on the microscopic resistance confirming thereby findings outlined in literature. Furthermore, the results suggest an influence of the orientation in surface texture on the microscopic heat transfer. In contrast, the macroscopic resistance is sensitive towards variation in curvature and especially in specimen width. Finally, based on the experimental results an empirical correlation is derived. After the investigations of generic specimens, the developed methodology is successfully transferred to real components such as single roller elements and an entire radial bearing. The components provide an appropriate test scenario to determine the overall resistance. Future work will enhance the experimental dataset and transfer the method to other material combinations
Large eddy simulation of three-dimensional mixed convection on a vertical plate
No full textConvective heat losses on solar central receivers arise from a combination of buoyancy-driven natural and forced convection. This mixed convection is characterized by two factors: First, the direction of gravity. Gravity aligned with the imposed flow results in a two-dimensional flow case. Three-dimensional mixed convection occurs when gravity acts perpendicular to the free stream. The second factor is the relative influence of buoyancy, as described by Gr/Re^2. In this work, three regimes are distinguished: Buoyancy-dominated, interia-dominated, and mixed convection with equipollent mechanisms. This thesis seeks to investigate the thermo-fluid interaction of turbulent three-dimensional mixed convection by using computational fluid dynamics. For that purpose, a large-eddy simulation model has been developed and qualitatively validated. In a series of simulations, a flat, hot, vertical plate was exposed to a horizontal fluid flow. Interpretation of the simulation results proved that buoyancy-induced instabilities are a key factor in the onset of turbulence. Reducing the plate temperature led to a significant reduction in turbulence. It was also found that an increase in free-stream velocity suppresses instabilities, delaying the laminar-turbulent transition. These findings are of high relevance for the heat transfer, as an evaluation of the heat transport mechanisms revealed that most of the transfer is linked to turbulence. Depending on flow conditions, a higher velocity can thus reduce the average heat transfer rate off the plate. In order to understand the underlying mechanisms, boundary layer profiles were evaluated. A three-dimensional mixed convection boundary layer flow can be described by a local flow angle and a local velocity magnitude. In wall proximity, a high-angle region was found, exhibiting a significant vertical updraft. After transition, the turbulent transport of horizontal momentum from the free stream into this region is reflected in a reduction of the angle. Consequently, the highest angles were found in the transition zone. When buoyancy had a high influence, a distinct velocity peak emerged in the velocity magnitude. The turbulent coherent structures showed a characteristic pattern: In the laminar flow, elongated streaks form prior to transition. These streaks soon exhibit secondary instabilities and eventually break up into structures like wall-attached hairpin vortices. Here, the buoyancy-fueled updraft has an impact by partially distorting these vortices. While the simulation results are successful in providing insights into the physics, they also point out challenges in finding physically meaningful modeling approaches. This is true i.e. for heat transfer correlations and numerical wall models
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Optical Efficiency Measurement in Solar Simulators of Receivers for Parabolic Trough Solar Thermal Power Plants
No full textTwo linear focus solar simulator test benches were built for the calorimetric measurement of the optical efficiency of parabolic trough receivers. The test benches, called ElliRec and OptiRec, consist of mirrors of elliptical cylinder geometry, flat end mirrors, and metal-Halide lamps. Water at ambient temperature is directed on a spiralling path at the inner surface of the absorber. Absorbed power of the Receiver is calculated from temperature increase and flow rate of the water and amounts to 6.5 kW or 4.8 kW and is compared to that by a reference to yield the relative optical
efficiency. Expanded uncertainty at k=2 of optical efficiency in the solar Simulator of a receiver relative to that of a reference receiver is 0.6 % for the ElliRec and 0.34 % for the OptiRec; expanded uncertainty at k=2 of the absolute power measurement is 1.2 % for ElliRec and 1.6 % for OptiRec. Several aspects of the transfer of laboratory measurement to the application in the field are investigated. Assuming idealized spectra for absorber and glass systematic deviations of optical efficiency between solar simulator and field of typically up to 4 % were calculated. For comparison of relative measurements, as typically performed, deviations between solar simulator and field are < 0.2 % for measured spectra for both test benches. An experimental comparison of the ElliRec solar simulator with two spectrophotometric test benches showed a disagreement of < 0.6% standard deviation for five out of six compared receivers. Differences between the
ElliRec and OptiRec for seven receivers were measured to < 1.1 %. A 1-d model is used to calculate changes from room temperature to operating temperature yielding
corrections for the hot receiver of up to 0.7 % depending on bellow design. Detailed 3-d-ray-tracing models yield a spatial non-uniformity in longitudinal direction SNE
calculated for −2.03 m to +2.03 m is 1.6 % for ElliRec and 2.3 % for OptiRec. Neglecting source volume and slope deviations of the mirror it is shown, that, in contrast to the field, in the solar simulator angle and position of incidence on the absorber are linked. The investigation of the impact of focus deviation fdx based on the Fresnel Equations shows a decreasing effective absorptance with increasing collector angle of incidence and mean focus deviation. For example, assuming Gaussian radiation distribution of fdx of half of the absorber radius and a collector angle of incidence of 0◦ results in an additional loss in effective absorptance of 1 % due to reduced absorptance. The optical efficiency measurement in the linear focus solar simulator is a fast measurement with high repeatability. The influence of the effects in transfer to the field are small enough to consider the measurement a meaningful assessment of the quality of a receiver and to resolve differences between products. Due to the simple optical geometry the method has the potential to be easily reproduced by other laboratories
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Optischer Wirkungsgrad von Parabolrinnenkollektoren: Modellierung und Messung
No full textDie vorliegende Arbeit wendet sich an Ingenieure und Wissenschaftler im Bereich der solarthermischen Parabolrinnentechnologie, welche aktuell ernorme Wachstumsraten verzeichnet. Ihr Schwerpunkt ist die Modellierung des optischen Wirkungsgrades von Parabolrinnenkollektoren mittels Strahlverfolgung, die auf gemessenen Reflektorgeometrien basiert. Das entwickelte Modell wird anhand eines in der Arbeit vorgestellten Flussdichtemesssystems experimentell verifiziert. Durch Variation von Parametern wie Einstrahlungswinkel, Nachführgenauigkeit, Absorberrohrdurchmesser und Positionsabweichungen von Bauteilen wird deren Einfluss auf den optischen Wirkungsgrad untersucht. Diese Sensitivitätsstudien bieten die Möglichkeit, Spezifikationen für die Fertigung und Qualitätssicherung abzuleiten, wodurch Ertragssteigerungen und Kostensenkungen für Parabolrinnenkraftwerke erreicht werden können
Methane Vapor Bubble Growth due to Depressurization and Boiling
No full textCryogens such as liquid methane enable highly efficient propulsion systems in spacecraft. The low saturation temperature causes the liquid to boil in most technical systems. The resulting vapor bubble growth in a superheated liquid can quickly pass through multiple length scales. Under the conditions of reduced gravity in spaceflight applications, bubbles usually remain close to their point of nucleation. During the bubble growth, caused either by a change in saturation conditions due to a reduction of tank pressure or by heat flowing into the cryogenic tank, the displacement of liquid can pose issues for the operation of the spacecraft. This can manifest in the drying up of liquid reservoirs or the loss of a controlled liquid position in the tank. Therefore, predictions of the growth dynamics of vapor bubbles are required.
The bubble growth caused by depressurization is of special interest for two reasons. Firstly, the depressurization of a saturated liquid causes a spatially uniform superheat. This changes the bubble growth dynamics compared to the common assumption of a thermal boundary layer near a solid body for boiling models. Secondly, depressurization of a cryogenic tank is an important step in a potential procedure to introduce a subcooling inside the liquid in order to allow for transfer without cavitation. In a single species system the decrease in pressure lowers the saturation temperature and causes evaporation and thus a transfer of thermal energy from the liquid phase to the vapor phase. A following pressurization raises the saturation temperature again and thus subcools the liquid, which allows for transfer without cavitation. This is of special relevance for spacecraft which have to rely on cryogenic fuels for extended mission durations. There, thermal conditioning of the propellant after extended coasting phases becomes mandatory because heat is constantly flowing into the propellant from the environment. Only if a subcooling is introduced prior to any pumping of propellant can it be ensured that cavitation will not occur in the pumps.
In this thesis the results of twelve drop tower experiments are presented. These experiments were carried out with the aim of investigating bubble dynamics and providing reference data for numerical simulations. Experimental observations focused on the evaluation of vapor bubble growth caused by a depressurization or by the application of a heat flow. Repeatability was investigated and found to be good. Bubble growth during three sets of depressurization parameters as well as three different magnitudes of heat flow were evaluated. The growth behavior was correlated with the applied stimuli. Based on this, numerical investigations into the bubble growth caused by a depressurization have been performed and compared to the experimental results. The established experimental and numerical data points can be used as a basis for numerical models to perform simulations on the technical scale
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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