256 research outputs found

    The Effect of Curvature Ratio on Flow Structureand Fluids Mixing in 90o bent square duct.

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    كثير من المشاكل الميكانيكة والكيمياوية تعتمد بالأساس على خصائص الأختلاط لسائل مرشوش مع الغاز الحامل له وهي عملية تتأثر بقوة بمعدلات تبادل الكتلة والحرارة. الجريان الثانوي الناتج عن قوى الطرد المركزي المصاحبة لتغير جوهري في اتجاه الجريان يسبب تكون دوامات متعاكسة الدوران. تهدف الدراسة الى التحقق من أثر نسبة الأنحناء على بنية الجريان وشدة الاضطراب خلال عملية خلط سائل مع غاز لمنظومة حقن ماء مسبقة لمجرى منحني. تستثمر الدراسة تقنية المعالجة الصورية التجريبية PIV بهدف تعقب عملية تكون جريان ثانوي اثناء انتقال خليط ماء-هواء عبر جزء منحني من المجرى. أعتمدت الدراسة ثلاث نسب أنحناء 0.25,0.5 و0.75  لمتوسط سرع 2.5  و 5 m/s لجريان هواء عبر مجرى مربع. تظهر الصور تكون زوج من دوامات دوارة (رباعية-الفصوص) لكل نسب الأنحناء المدروسة مع اندفاع  الدوامات القريبة من الجدار الداخلي للمنحني نحو الخارج مع تناقص نسبة الأنحناء كنتيجة لتأثير الطرد المركزي وأنفصال الجريان. تصميم وترتيب مصفوفة حاقنات الماء تقرر أستنادا الى المحاكاة العددية بأستخدام الحزمة البرمجية ANSYS FLUENT 19.R1 مع اعتماد نموذج الأضطراب RNG-k-. النتائج العددية تظهر ان شدة التدويم لها تأثير صغير على عملية الخلط عند تغير عدد رينولدز ولكنها تتأثر بقوة مع تغيير نسبة الأنحناء. المقارنة الظواهرية بين النتائج التجريبية والعددية قدمت تقاربا جيدا للدراسة الحالية، حيث أن الحد الأقصى للانحراف المسجل كان (7,1)%.Many mechanical and chemical problems rely mainly on the mixing characteristics of a dispersed liquid and the carrier gas which is strongly affected by the rates of mass and heat exchanged. The secondary flow generated by the centrifugal forces accompany a substantial change in flow direction leads to the presence of counter rotating vortices. The study investigates the effect of curvature ratio on the flow structure and turbulence intensity during a liquid-gas mixing process prior to a bent duct. The study employs the experimental Particle Image Velocimetry technique (PIV) in purpose tracking the secondary flow structure when the water-air mixture travelling through a bent duct. The curvature ratios were taken to be (0.25, 0.5, 0.75) at average velocities of 2.5 and 5m/s for air flowing through a square duct. The PIV images illustrate the appearance of a Pair of rotating Dean vortices (four-cell pattern) generated for all curvature ratios with the vortices near the inner side of the bend moved outward while decreasing the curvature ratio as a result of centrifugal effect and flow separation. The design and the configuration of the water nozzles matrix is decided according to the numerical simulation using ANSYS FLUENT 19.R1, with RNG-k-ε turbulent model. The numerical analysis showed that the swirl intensity has little effect on mixing due to changing Reynolds number and was more influenced by the changing of the curvature ratio. The phenomenal comparison between experimental and numerical results showed good agreement as the maximum deviation recorded is about (7.1%).&nbsp

    Thermal simulation of ventilated PV-facades

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    The application of double glazed facades, especially in administration buildings is becoming more and more popular. Aside from the architectural aspects, the energetic consequences with respect to the building into which it is integrated have been discussed much over the past few years. In order to quantify the energy balance of such facades, the heat transfer rate between the inner facade layers and the gap temperature are important factors and constitute the core of this thesis. In contrast to experimental estimations of heat transfer rates, which are measured using heat flux sensors, in this study the energy balance within the facade was determined primarily by means of computational fluid dynamics (CFD). For the purpose of verifying the CFD results, simulation results were assessed through comparison with experimental flow data obtained using particle image velocimetry (PIV). Comparison of CFD simulations and PIV measurements showed good agreement for different symmetric and asymmetric plate temperatures as well as for different forced flow rates. A new Nusselt correlation was developed, which was derived from a CFD parameter study. The suggested correlation includes plate distances which vary from 0.05 to 0.5m, surface temperatures from -10 to 60 degrees C, inlet temperatures from -10 to 30 degrees C and Reynolds numbers (Red) between 500 and 6500. In order to estimate the thermal behaviour of a ventilated facade at an early stage of building planning, a transient simulation program was developed which is able to calculate the dynamic energy balance that occurs in a double facade. To facilitate integration of the calculation method into the commercial building simulation program TRNSYS 15, a new Type (Type 111) was written. This Type 111 can be used to connect an arbitrary facade construction to the existed building model Type56. Comparisons between calculated results from the developed model and measurements on real facades(a hybrid, mechanically-ventilated PV fagade and a naturally-ventilated, double glazed facade) provided sufficiently good agreement. The total energy rate through a window (g-value), estimated by the special g-value test rig at the Stuttgart University of Applied Sciences could also be reproduced accurately using the developed program.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

    Tracer gas techniques for airflow characterization in double skin facades

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    Monitoring airflow rates and fluid dynamics phenomena in the ventilated cavity is a challenging aspect of the experimental assessment of the performance of double-skin facades (DSF). There are various methods to characterize the fluid-dynamics behavior of DSF, but each of these has its advantages and drawbacks. This paper presents the airflow characterization in the cavity of a double-skin façade installed in a full-scale outdoor facility through various methods, and, more specifically, it compares two tracer gas methods with the velocity traverse method. In the paper, we highlight how different characterization results can be explained by considering the features of each method, and how these differences are linked to velocity ranges and airflows in the cavity. By discussing (i) the challenges of these methods and their applicability, (ii) the requirements in terms of experimental set-up and (iii) the limitations linked to instrumentation, we aim to enhance the discussion on experimental methods for advanced building envelope characterization and contribute to a more grounded understanding of the suitability of tracer gas methods for in-field characterization of airflows in facades

    The school of the turbulent swirling flow at the faculty of mechanical engineering university of Belgrade

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    This review paper provides data about research activities at the School of the turbulent swirling flow at the Faculty of Mechanical Engineering, University of Belgrade, conducted in the period 1941 up to date. An overview is provided of the main directions in this research area. First papers dealt with the turbulent swirling flow in hydraulic turbines to be continued by the experimental and analytical approaches on the axial fans pressure side. The complexity of 3-D, non-homogeneous, anisotropic turbulent velocity fields required complex experimental and theoretical approach, associated with the complex numerical procedures. Analytical approaches, complex statistical analyses and experimental methods and afterwards CFD employed in the research are presented in this paper. The 150 scientific papers, numerous diploma works, several master of science (magister) theses, six Ph. D. theses and two in progress, 40 researchers, national and international projects are the facts about the School. Scientific references are chronologically presented. Numerous abstracts from scientific conferences, presentations, projects with industry and lectures are not given here

    Flow field analysis in an expanding healthy and emphysematous alveolar model using particle image velocimetry

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    Particle deposition in the acinus region of the lung is a significant area of interest, because particles can potentially travel into the bloodstream through the capillaries in the lung. Drugs, in the form of aerosols, small particulates in a volume of air, may be delivered through the respiratory system. Also, toxic, airborne, particles could enter the body through the pulmonary capillaries in the acinus region of the lung. In order to accurately predict particle deposition, the aspects that influence deposition needs to be understood. Many physiological features may influence flow and particle deposition in the lung; the geometry of the acinus, expansion and contraction of the alveolar walls due to breathing mechanics, heterogeneities in the lung, breathing flow rate, and the number of breaths. In literature, streamlines and pathlines have been examined, both experimentally and computationally, in models representing the alveolar region of the lung. Some of these studies suggest the presence of irreversible flow, which would significantly influence particle deposition. However, none of these models incorporated all significant features: non-symmetric, three dimensional, expanding geometry. Therefore, flow mechanics, behind particle deposition, in the alveolar region are not well understood. Furthermore, lung disease influences the physiological factors that impact particle deposition. Emphysema physically changes the structure of the alveolar region of the lung. How particle deposition changes with emphysema is not fully understood. In this work, two different alveolar geometries were examined using Particle Image Velocimetry (PIV). The first model represented a healthy alveolar sac, while the second model represented an emphysematous alveolar sac. The same, realistic flow rate was used for both models, which allowed for the fluid flow to be examined as only a function of geometry. The PIV technique was validated by comparing to CFD results, using a simple balloon geometry. Pathlines were plotted in the models in order to examine the fluid flow with respect to time. The fluid was examined, by use of streamlines and pathlines, at the entrance of the alveolar sacs and in areas of high probability for irreversible flow. It was found that the fluid flow inside both alveolar sac geometries was completely reversible, and therefore no mixing was taking place. The comparison between the healthy and emphysematic alveolar sac models showed that the pathlines in health traveled closer to the alveolar walls. Particle deposition by Brownian Diffusion was estimated for particle diameter range of 0.1 μm to 0.01 μm. For the pathlines that began at the duct entrance, the pathlines came approximately 1.5 times closer to the wall in the healthy case when compared to emphysema. Because the pathline traveled closer to the alveolar walls, the particle diffusion was greater in the healthy then emphysema. In the healthy geometry particles with a diameter less then 0.02 μm were estimated to diffuse to all of the alveolar walls within a 5 second time frame, where in emphysema 7 seconds would be needed. It was also determined that if a particle diffuses off of the original streamline, it will remain in the alveolar sac, therefore allowing it to deposit in later breaths

    Experimental and Numerical Study of the Air Distribution in an Airliner Cabin

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    Nowadays, more people, including those with impaired health or who are otherwise potentially sensitive to the cabin environment, are traveling by air than ever before. The flying public demands a higher comfort level and a cleaner environment because they encounter a combination of environmental factors including low humidity, low air pressure, and sometimes, exposure to air contaminants such as ozone, carbon monoxide, various organic chemicals, and biological agents. Moreover, international air travel has increased the potential risks associated with airborne disease transmission and the release, whether accidentally or intentionally, of noxious substances during flight. Many studies suggest that the risk of infection during air travel is related to the cabin environment. In commercial airliner cabins, a thermally comfortable and healthy cabin environment is created by air distributions that are used to regulate air temperature and air velocity and to provide adequate ventilation for reducing gaseous and particulate concentrations of contaminant. The facts shown above leave an element of doubt whether the air distribution in airliner cabins is acceptable. Therefore, it is essential to study how the air is distributed in the air cabins to ensure that the cabin environment is safe, healthy, and comfortable for the flying public. This investigation firstly reviewed the methods used in predicting, designing, and analyzing air distributions in the cabins. Two popular methods are experimental measurements and numerical simulations. The experimental measurements have usually been seen as more reliable although they are more expensive and time consuming. Most of the numerical simulations use Computational Fluid Dynamics (CFD) that can effectively provide detailed information. Numerous applications using the two methods can be found in the literature for studying air distributions in aircraft cabin, including investigations on more reliable and accurate turbulence models. Our review shows that studies using both experimental measurements and computer simulations are becoming popular. Our review also found that it is necessary to use a full-scale test rig to obtain reliable and high quality experimental data, and that the hybrid CFD models are rather promising for simulating air distributions in airliner cabins. This investigation then experimentally studied the air distributions in the first-class cabin of a functional MD-82 aircraft and compared it at unoccupied and fully-occupied conditions. Heated manikins were used to simulate seated passengers. The experiment applied ultrasonic anemometers (UA) to measure the three-dimensional air velocity field and 64 thermo-couples to obtain air temperature field. UA works at 20 Hz, so the measured data could also be used to determine the turbulence intensity of the air. It was found that the flow fields were of low speed and high turbulence intensity. A combination of hot-sphere anemometers (HSA) and UA were applied to obtain the air velocity magnitude, air velocity direction, and turbulence intensity at the diffusers. The measured results indicate that the flow boundary conditions in this real aircraft cabin were rather complex and the velocity magnitude, air velocity direction, and turbulence intensity varied significantly from one slot opening to another. This study compared the flow fields of different occupation conditions in a real commercial airplane and provided high quality data for evaluating Computational Fluid Dynamics (CFD) models, including boundary conditions of diffusers and high-resolution flow and temperature fields. The third part of this investigation evaluated three turbulence models in different categories: the Re-Normalization Group (RNG) k-[varepsilon] model, Large Eddy Simulation (LES), and Detached Eddy Simulation (DES) based on the measured steady-state flow fields under unoccupied and fully-occupied conditions in the first-class cabin of the functional MD-82 commercial airliner. By comparing the data of the two experimental conditions with the computed results from these three turbulence models, this study found that the RNG k-[varepsilon] model gave acceptable accuracy in predicting the airflow in the unoccupied cabin where the flow was simple, but not for the complicated flow in the fully-occupied cabin. The DES gave acceptable flow fields for both conditions. The LES performed the best and the results agreed well with the experimental data. The comparisons also showed that the errors in the experimental data were more significant than that in the turbulence models

    An efficient multi join query optimization for relational database management system using swarm intelligence approaches

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    Currently, it is fairly obvious that the Multi Join Query Optimization (MJQO) is becoming the centre of attention in the context of Database Management System (DBMS). The functions consist of combination of data from multiple tables, reducing the number of needed queries, optimizing the Query Execution Plan (QEP}, and moving processing abounded database servers to enhance both data integrity and performance. MJQO is an optimization task, which serves to locate the optimal QEP of a RDBMS in query processing. A major problem associated with RDBMS is the fact that they are still unable to fully meet the demands of big data. The majority of MJQO techniques encompass solution space at an extremely reduced pace. Many queries attempted to gather information from multiple sites or correlations, while every relation are compelled to answer these query via their limited resources. This lead to the access of data from many locations that are limited in their memory retention capabilities, which inevitably increase the size of the database, the number of the join, and Query Execution Time (QET}. In order to eschew trapping and slow coverage difficulties in the quest to discover the optimal QEP and slow query execution time, this work proposes a total of three optimization algorithm that are based on Particle Swarm Optimization (PSO), Ant Colony Optimization (ACO), and Two-Phase Artificial Bee Colony (TPAPC) to solve the optimization problem in RDBMS Framework. The TP ABC algorithm can be utilized to solve MJQO problems via simulation and increasing exploration and exploitation whilst balancing them for optimal results from giving queries. A directed acyclic graph, based on materialized query graph, aids in the optimization of algorithms and solving MJQO by removing non-promising QEP, which decreases the QEP combination space. Finally, experimental results demonstrate that the performance of TP ABC, when compared to PSO, ACO, and native technique in the context of computational time, is very promising, which is indicative of the fact that the TP ABC algorithm is capable of solving MJQO problems in shorter amounts of time and at lower costs compared to other approache

    Experimental analysis of fire-induced flows for the fire-safe design of double-skin facades

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    Today, ever changing and advancing techniques of construction are constantly pushing the envelope of structural possibilities in the built environment. Although not new, the concept of Double-Skin Façades (DSF) finds increasing implementation with the advent of sustainable construction, aiming to reduce energy consumption to condition buildings whilst improving indoor air quality. As is the case with the traditional concept of the compartment fire, methodologies and assumptions on which our general understanding of the fire problem is based, did fundamentally not change. Inherently bound to this, is the concept of compartmentalisation, prescribing measures to avoid horizontal and vertical fire spread in buildings. A DSF, most commonly featuring a ventilated cavity between curtain wall and the secondary glass façade at an offset, is prone to drastically alter fire and smoke behaviour once able to enter. Unlike curtain walls, the chimney-like aspect ratio of such façades is able to trap fire and combustion gases within the cavity, potentially compromising the integrity of the building perimeter above the fire. The current approach to this issue tends to focus on using non-combustible construction materials and the installation of sprinkler systems to avoid breakage of window panes in the first place. Another topic of interest is the weak connection between floor slab and curtain wall which can allow vertical fire spread to adjacent floors. Research has also been discussing the use of mullions to deflect the fire plume away from the façade. Even if useful in DSF’s, aesthetics and problems with functionality will most likely prevent mullions from being introduced into the DSF. However, very little relevant research actually investigated the fire-induced flow structure under these conditions so that properly informed design decisions can be made. The project at hand aims to understand hazards to the floors above and below the fire floor by experimentally investigating the governing processes by means of large-scale fire testing and small-scale salt-water modelling (SWM). The gathered data shall serve as a basis to discuss current spandrel and cavity design decisions. Results have been compared in terms of dimensionless numbers and demonstrate complex interactions between DSF cavity width and spandrel height, encouraging a discussion about the need of further research of this topic

    Flow over rough surfaces, and conjugate heat transfer, in engineering applications

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    This thesis focuses on two important topics in the field of fluid mechanics and heat transfer that were treated separately. On the one hand, the flow over rough surfaces, which is not yet completely understood, despite of the fact that it has an important implication in many engineering applications, especially in the naval industry for ships and boats because the friction drag caused by fouling, antifouling coatings and roughness in general impacts the fuel consumption and toxic emissions to the atmosphere. In this thesis, new ways to measure and predict the friction drag of rough surfaces are presented, using both numerical simulations and experimental techniques.On the other hand, conjugate heat transfer processes are also important in many applications, but a very relevant one is the electric generators for hydropower, since there is a current need of increasing the efficiency of these machines, which depends a lot on how they are cooled and therefore, on the rate at which the heat that is generated during their operation is dissipated (transferred to the cooling fluid). In this thesis, an experimental method for studying the heat transfer in these machines is presented and validated. Also, new correlations to improve the design phase of the electric generators are also evaluated

    Flow Field Dynamics in a High-g Ultra-Compact Combustor

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    The Ultra Compact Combustor (UCC) presents a novel solution to the advancement of aircraft gas turbine engine performance. A high-g UCC design operates by diverting a portion of the axial compressor flow into a circumferential combustion cavity positioned about the engine outer diameter. The circumferential cavity (CC) provides the necessary residence length and time for combustion within reduced axial lengths; furthermore, high rates of centrifugal acceleration termed high-g loading are imposed upon the swirling cavity flow. These high-g conditions are hypothesized to increase flame speed, reduce flame length, and improve lean blow-out performance. Work at AFIT was sponsored by the Air Force Office of Scientific Research to study high-g combustion. This research capitalized on the availability of advanced flow diagnostic data coupled with a computational fluid dynamics (CFD) model to provide detailed insight into the high-g flow field and combustion dynamics. Results indicated that combustion could be sustained and controlled in a manner suitable for integration into modern gas turbine engine architecture
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