Orientadores: Milton Mori, Marcos Akira D'Ávila / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Química / Made available in DSpace on 2018-08-20T21:16:08Z (GMT). No. of bitstreams: 1 MiillerLopes_MariaFernanda_M.pdf: 2798310 bytes, checksum: d508c014dbb2db75fd6d052fdf5a76c7 (MD5) Previous issue date: 2012 / Resumo: Esta pesquisa tem por objetivo avaliar os resultados numéricos por Fluidodinâmica Computacional (CFD) e experimentais pela técnica de Velocimetria por Imagem de Partícula (PIV) e a técnica de Shadow Imaging em uma coluna de bolhas de 1m de altura, 14,5 cm de diâmetro com um distribuidor de gás de 5 furos de 1 mm de diâmetro cada. O sistema estudado foi o sistema bifásico água-ar. O estudo foi realizado no regime homogêneo de operação para as velocidades superficiais de gás de 0,3 cm/s, 0,5 cm/s e 0,7 cm/s. Os experimentos de PIV e shadow imaging foram realizados nas mesmas condições operacionais. Para uma boa representação do escoamento pela técnica de PIV foi necessário o tratamento de 2500 fotos para as velocidades mais baixas e 3500 fotos para a velocidade mais alta. Para a técnica de shadow imaging foi necessário o tratamento de 2000 fotos para a determinação do diâmetro médio das bolhas. Nos testes numéricos foram avaliados dois modelos de arraste, o de Ishii-Zuber (1979) e o de Zhang-Vanderheyden (2002), para a turbulência foi utilizado o modelo k-?, e o diâmetro de bolha utilizado foi o obtido experimentalmente pela técnica de shadow imaging. Perfis de velocidade média axial de líquido obtidos experimentalmente foram comparados com os dados numéricos. Análises dos tensores de Reynolds, energia cinética turbulenta e intensidade turbulenta também foram avaliados. Para a velocidade de 0,3 cm/s e 0,5 cm/s ambos os modelos de arraste representaram bem o escoamento. Para a velocidade de 0,7 cm/s o modelo de arraste de Zhang-Vanderheyden representou melhor o escoamento / Abstract: This research aims to evaluate the numerical results from Computational Fluid Dynamics (CFD) simulations and experimental results obtained using Particle Image Velocimetry (PIV) and Shadow Imaging in a bubble column of 1m height, diameter of 14,5cm with a gas distributor with 5 holes of 1 mm diameter each one. The system studied was water-air. The study was performed in homogeneous operation regime for the superficial gas velocity of 0.3 cm/s, 0.5 cm/s and 0.7 cm/s. The PIV and shadow imaging experiments were performed under the same operation conditions. For a good flow representation of PIV data it was necessary to treat 2500 photos to the low superficial gas velocities and 3500 photos to the highest superficial gas velocity. For the shadow imaging technique it was necessary to treat 2000 photos for determining the bubble diameters. In the numerical tests two drag models were evaluated: Ishii-Zuber (1979) and Zhang-Vanderheyden (2002); for turbulence the k-? model was used and the bubble diameter used in the simulations was obtained experimentally by the shadow imaging technique. Average axial velocity profiles of fluid obtained experimentally were compared with numerical results. Analysis of Reynolds tensor, turbulent kinetic energy and turbulent intensity were also evaluated. For superficial gas velocities of 0,3 cm/s and 0,5 cm/s both drag models show good agreement with experimental data. For superficial gas velocities of 0,7 cm/s Zhang-Vanderheyden drag model showed better agreement with experimental data / Mestrado / Desenvolvimento de Processos Químicos / Mestra em Engenharia Química

Catalytic converters are widely used in the automotive industry to comply with increasingly stringent emissions regulations. The flow distribution across the catalyst substrate significantly aects its conversion eciency. Measuring the flow in a catalyst system is challenging; computational fluid dynamics (CFD) provides an alternative approach for the assessment of different design concepts and is therefore commonly employed to model flow behaviour. This thesis studies the application of CFD to modelling ow in a two-dimensional system consisting of a catalyst monolith downstream of a wide-angled planar diuser, with total included angle 60°. Computational models are developed using the commercial CFD software STAR-CCM+. Flow predictions are compared to experimental data collected by Mat Yamin, (2012) and also as part of this study. Measurements were obtained on a two-dimensional isothermal flow rig using particle image velocimetry (PIV) and hot-wire anemometry (HWA). Steady flow studies compare different methods of modelling the monolith. Models include the common approach of modelling the monolith as a porous medium and the computationally expensive individual channels model. A hybrid model is developed that combines the two approaches, benefiting from the respective merits of each method. Two monolith lengths are considered, with flow at varying Reynolds numbers. The porous model predicts the downstream velocity prole well for the shorter monolith but overpredicts flow maldistribution for the longer monolith. The inclusion of an entrance effect to account for the pressure losses associated with oblique entry into the monolith channels is studied. Best agreement in downstream velocity is observed when the pressure losses are limited using a critical angle approach. The individual channels model is found to be the most consistently accurate across monolith lengths, attributable to the accurate capture of flow behaviour upon entry into the monolith channels. A novel hybrid model is proposed, which combines the computational efficiency of the porous model with the geometrical accuracy of individual channels. The model is evaluated and is found to provide results similar to the individual channels model, with improved predictions of velocity maxima and minima. Pulsating flow studies present three transient flow regimes with similar inlet pulse shapes and varying Reynolds number and frequency. Predicted velocities in the diuser are in good agreement with PIV flow fields, however CFD predicts higher magnitudes at the shear layer. The model predicts large residual vortices present at the end of the cycle where experimental data shows none; it is concluded that CFD underpredicts turbulence diffusion. Evidence of cyclic variation in experimental data highlights the limitation of URANS turbulence models.

M.Ing. / The characterisation of the flow field and thermal performance of supply air windows (airflow windows operating in supply mode) have been a topic of interest for at least two decades. Computational Fluid Dynamics (CFD) as well as other simulation methods have been used to model and characterise the flow field, temperature distributions and thermal performance of the supply air window in recent years. Where experimental validation of the velocity (only outlet velocity) and temperature predictions has been provided the error between experiment and CFD (and other forms of simulation) is in the order of 50 % and 3 ◦C (10-13 %), respectively. Furthermore, a large part of the literature does not have experimental validation of the simulation results. The significant error in many of the studies, that provide experimental val- idation of the velocity field, is attributed to inappropriate turbulence mod- els, unrealistic boundary conditions, neglecting significant three-dimensional effects, solar radiation effects not entirely accounted for, mesh sensitivity studies neglected and material properties of glass and air assumed constant. The aim of this research was to characterise a supply air window in terms of its velocity field, temperature distributions and thermal performance. This was done by mathematically modelling the fluid dynamics and heat trans- fer processes in a supply air window and solving the model in a commer- cial CFD code, namely ANSYS Fluent 12.1. Furthermore, an experimental rig was designed, constructed and used to measure the flow field and tem- peratures with the aim of validating the CFD models. The CFD models incorporated appropriate turbulence models, realistic boundary conditions, three-dimensional effects, solar radiation, temperature dependent material properties and a mesh sensitivity study. The CFD models and experiments were setup for forced and natural flow conditions. Laser Doppler Velocimetry has not been used for velocity field measure- ments in an airflow window to date. The experimental setup made use of Laser Doppler Velocimetry to measure the velocity field and turbulence in- tensities. The Laser Doppler Velocimeter (LDV) probe was positioned using a three axis computer controlled traversing mechanism. Furthermore, flow visualisation experiments were done to qualitatively capture the flow field. The results from the CFD are partially in good agreement with the exper- imental work. Qualitatively the flow field as predicted by CFD is in good agreement with the results from the flow visualisation experiments. Quan- titatively the results from the CFD are in good agreement with the tem- perature measurements, however, there is noticeable error between the LDV readings and the velocities as well as turbulence intensity values predicted by CFD. The error, with regards to velocity and turbulence intensity, may be attributed to the experimental error caused by problems with flow seeding as well as the isotropic turbulence assumption inherent in the turbulence model (SST k − ω) used.

Abstract The impinging jet ventilation's importance in providing better air distribution and energy-efficient operation, as well as both its heating and cooling flexibility potential cannot be overemphasized. This is because acceptable indoor air quality and its environmental conditions are essential to occupant’s wellbeing, comfort, productivity, and improved cognitive function. Poor air quality conditions could result in sick building symptoms (SBS) and several studies have investigated that the prevalence of sick building symptoms is associated with indoor air quality. Consequently, to the need for high ventilation effectiveness, the impinging jet ventilation system has been developed and applied in different types of buildings as a new ventilation strategy and concept within the last two decades. Therefore, it is important to continuously develop inventive air distribution systems such as IJV with a better location and terminal configuration of the supply device to adequately provide an acceptable indoor environment in an energy-efficient manner. This study aims at reaching a comprehensive understanding of the near field zone of an isothermal turbulent corner impinging jet in a room by using computational fluid dynamics (CFD) simulation tool. Thus, directly investigating the flow field involving the velocity magnitude, velocity decay, and spreading rate along the diagonal of the room.The cases carried out consist of 12 different three-dimensional modeled configurations (room) of the computational domain with the dimension 7.0 (L) x 7.0 (W) x 2.67 (H) m. The cases which comprised different aspects of diffuser geometry (triangle, quadrant, square), diffuser area, discharge height, and flow rates, used the RNG k-ε turbulence model to solve the turbulence flow.The result obtained, from the parametric study in all the cases was detailed to analyze the effect of the different flow rate, discharge heights, diffuser geometry, and its area on the velocity profile development, velocity decay, and spreading rate along the diagonal of the room. This study significantly shows the triangular geometries having greater velocity magnitude and velocity decay along all the line profile positions. Interestingly, a comparison between the quadrant and square geometry illustrates that their characteristics of generating a greater velocity magnitude depend on its discharge height. The result also demonstrated a decrease in jet velocity decay with an increase in jet discharge height. With similar jet spread at higher jet discharge, the square geometry exhibited a higher spreading rate at lower discharge height.

This work is done as a small facet of a much larger study on efficient control of indoor air environments. Halton passive chilled beams are used to cool rooms and the focus of this work is to model the beams. This work also reviews the mesh making process in Gmsh. ANSYS Fluent was used throughout the entire research and this thesis describes the software and a careful description of the case study. / Master of Science

The following study investigates hydrodynamic stability for two-dimensional, incompressible flow past a cylinder and compares it alongside four different variations of a wave-like ground introduced within the wake region of the cylinder wake. These different variations include changing the distance of the cylinder both horizontally from the wave-like structure and vertically from the ground. The geometry and meshes were initially constructed using GMSH and imported into Nektar++. The baseflows were then obtained in Nektar++ using the Velocity Correction Scheme, continuous Galerkin method, and Unsteady Navier Stokes solver. Then, the Implicitly Restarted Arnoldi Method driver was used to retrieve the various eigenvalues/eigenmodes and growth rates. Finally, the results were visualized in Paraview which allowed clear comparisons between the stability of the flow between each case. The findings obtained show a clear effect on stability when considering different cases, for a plain cylinder and for each case there are observations to be made in how the various eigenmodes varied in terms of magnitude and shape, other observations were made in the differing critical Reynolds number and frequencies among the cases. This study is relevant to various natural environments where a blunt object may come in range of a bumpy or wavy ground. In these scenarios it can be important to monitor how instabilities propagate and cause effects such as turbulence or drag. Additionally, investigation like these can detail how to effectively avoid undesirable characteristics of instability.

This work explores the impact of burying a short term thermal storage (STTS) tank within a borehole thermal energy storage (BTES, or borehole field). There is motivation to bury tanks in order to save space on ground level, as well as to improve the overall efficiency of the system by reducing heat losses from the tank. This work mainly looks to understand the impact of the lack of boreholes under the buried tank, as well as the thermal interactions between the tank and boreholes. Computational Fluid Dynamics was used to predict the transient temperature throughout the domain. The long-term performance was assessed by simulation a five year period. Examination of factors that influence the tank-borehole interactions was studied and it was determined that radial stratification of the borehole field as well as the soil properties have the largest influence in terms of increasing the efficiency of the BTES. Other factors, such as tank stratification and tank insulation have little impact and the remainder (tank aspect ratio, and alternate geometries) have some impact. / Thesis / Master of Applied Science (MASc)

Widespread structural damage to critical facilities such as levees, buildings, dams and bridges during hurricanes has exemplified the need to consider multiple hazards associated with hurricanes as well as the potential for unacceptable levels of performance even if failure is not observed. These inadequate standards warrant the use of more accurate methods to describe the anticipated structural response, and damage for extreme events often termed performance based engineering (PBE). Therefore PBE was extended into the field of hurricane engineering in this study. Application of performance-based principles involves collection of the numerous hazards data from sources such as historical records, laboratory experiments or stochastic simulations. However, the hazards associated with a hurricane typically include spatial and temporal variation therefore, more detailed collection of data from each hazard of this loading spectrum is required. At the same time, computational power and computer-aided design have advanced and potentially allows for collection of the structure-specific hazard data. This novel technique, known as computational fluid dynamics (CFD), was applied to the wind and wave hazards associated with hurricanes to accurately quantify the spectrum of dynamic loads in this study. Numerical simulation results are presented on verification of this technique with laboratory experimental studies and further application to a typical Florida building and bridge prototype. Both the time and frequency domain content of random process signals were analyzed and compared through basic properties including the spectral density, autocorrelation, and mean. Following quantification of the dynamic loads on each structure, a detailed structural iv FEM was constructed of each structure and response curves were created for various levels of hurricane categories. Results show that both the time and frequency content of the dynamic signal could be accurately captured through CFD simulations in a much more cost effective manner than laboratory experimentation. Structural FEM models showed the poor performance of two coastal structures designed using deterministic principles, as serviceability and strength limit states were exceeded. Additionally, the response curves created for the prototype structure could be further developed for multiple wind directions and wave periods. Thus CFD is a viable option to wind and wave laboratory studies and a key tool for the development of PBE in the field of hurricane engineering.

A multi-dimensional upwind fluctuation splitting scheme is developed and implemented for two-dimensional and axisymmetric formulations of the Navier-Stokes equations on unstructured meshes. Key features of the scheme are the compact stencil, full upwinding, and non-linear discretization which allow for second-order accuracy with enforced positivity. Throughout, the fluctuation splitting scheme is compared to a current state-of-the-art finite volume approach, a second-order, dual mesh upwind flux difference splitting scheme (DMFDSFV), and is shown to produce more accurate results using fewer computer resources for a wide range of test cases. The scalar test cases include advected shear, circular advection, non-linear advection with coalescing shock and expansion fans, and advection-diffusion. For all scalar cases the fluctuation splitting scheme is more accurate, and the primary mechanism for the improved fluctuation splitting performance is shown to be the reduced production of artificial dissipation relative to DMFDSFV. The most significant scalar result is for combined advection-diffusion, where the present fluctuation splitting scheme is able to resolve the physical dissipation from the artificial dissipation on a much coarser mesh than DMFDSFV is able to, allowing order-of-magnitude reductions in solution time. Among the inviscid test cases the converging supersonic streams problem is notable in that the fluctuation splitting scheme exhibits superconvergent third-order spatial accuracy. For the inviscid cases of a supersonic diamond airfoil, supersonic slender cone, and incompressible circular bump the fluctuation splitting drag coefficient errors are typically half the DMFDSFV drag errors. However, for the incompressible inviscid sphere the fluctuation splitting drag error is larger than for DMFDSFV. A Blasius flat plate viscous validation case reveals a more accurate vertical-velocity profile for fluctuation splitting, and the reduced artificial dissipation production is shown relative to DMFDSFV. Remarkably the fluctuation splitting scheme shows grid converged skin friction coefficients with only five points in the boundary layer for this case. A viscous Mach 17.6 (perfect gas) cylinder case demonstrates solution monotonicity and heat transfer capability with the fluctuation splitting scheme. While fluctuation splitting is recommended over DMFDSFV, the difference in performance between the schemes is not so great as to obsolete DMFDSFV. The second half of the dissertation develops a local, compact, anisotropic unstructured mesh adaption scheme in conjunction with the multi-dimensional upwind solver, exhibiting a characteristic alignment behavior for scalar problems. This alignment behavior stands in contrast to the curvature clustering nature of the local, anisotropic unstructured adaption strategy based upon a posteriori error estimation that is used for comparison. The characteristic alignment is most pronounced for linear advection, with reduced improvement seen for the more complex non-linear advection and advection-diffusion cases. The adaption strategy is extended to the two-dimensional and axisymmetric Navier-Stokes equations of motion through the concept of fluctuation minimization. The system test case for the adaption strategy is a sting mounted capsule at Mach-10 wind tunnel conditions, considered in both two-dimensional and axisymmetric configurations. For this complex flowfield the adaption results are disappointing since feature alignment does not emerge from the local operations. Aggressive adaption is shown to result in a loss of robustness for the solver, particularly in the bow shock/stagnation point interaction region. Reducing the adaption strength maintains solution robustness but fails to produce significant improvement in the surface heat transfer predictions. / Ph. D.

Inspired by the dilute spray regime in spray combustion, this dissertation explores the evaporation and combustion of an isolated droplet. Under a highly convective environment inside a gas combustor, due to imbalance of inertial and surface tension forces, the droplets of larger size in sprays exhibit notable deformations from spherical to non-spherical shapes. Such shape changes are generally observed but not quantified in experimental studies. Therefore, the effect of this deformation on droplet combustion dynamics is unknown yet. To bridge this gap, a comprehensive investigation of an isolated freely deforming droplet can be insightful as it can reveal more about the interaction of droplet shape with its evaporation and combustion. This work attempts to analyze and quantify the impact of such deformations on evaporation and combustion using interface-capturing Direct Numerical Simulation approach. With the focus on small-scale processes involved in evaporation as it is a pre-step for combustion, this dissertation first covers a thorough examination on evaporation of a deformable droplet under both natural and forced convection. A single component jet-fuel surrogate n-decane is chosen. To ensure that the droplet remains stationary throughout its lifetime, a novel numerical method called "gravity update method" is developed and implemented. The results obtained from these two separate studies are validated against experimental results and analytical correlations respectively. The findings from the investigation of droplet evaporation under forced convective flow at moderate Reynolds numbers are noteworthy. The droplet shape under such flow conditions is governed by Weber number (We) which is a ratio of inertial force to surface tension force. The results demonstrated upto 20% en- hancement in total evaporation rate for highly deformed droplets. This improvement is a net results of increased droplet surface area and alteration in the distribution of local evaporation flux ( m'' ). It is found that m'' is proportional to its curvature up to the point of flow separation which agrees with low Re theories on droplet evaporation by Tonini and Cossalli (International Journal of Heat and Mass Transfer 2013), Palmore (Journal of Heat Transfer 2022). Beyond the flow separation point, evaporation flux distribution depends on the boundary layer development and flow evolution downstream of the droplet. For highly deformed droplets, a larger wake region creates favorable fuel vapor gradients and promotes mixing in droplet wake, hence higher evaporation flux. Such positive impact of droplet deformation on total evaporation rate motivated further investigation on droplet combustion under a low Reynolds number convective flow. High pressure and temperature gas flow leads to Damköhler number is higher than 1. This fa- vors the generation of envelope type flame. The results show overall little sensitivity to combustion related parameters despite the droplet shape change and significant (upto 9%) enhancement in total evaporation rate. It is also noted that while burning, droplets do not reach critical deformation conditions and break-up even beyond the critical Weber number, suggesting the suppression of deformation due to faster evaporation rate. The findings presented in these studies provide substantial evidence for the interaction between droplet shape and flow dynamics. Therefore, it demonstrates the potential for enhancing the existing numerical models and analytical correlations by accounting the influence of droplet shape. / Doctor of Philosophy / This work is inspired by the spray combustion in gas turbines where the pressurized liquid fuel jet is injected in the combustion chamber and converted into dilute sprays after undergoing a series of processes. Due to the presence of higher air to fuel ratio for these spray droplets, they become the localized combustion sites with rapid evaporation rates. Understanding the evaporation of these droplets becomes crucial, as it sets the stage for their subsequent combustion. In an attempt to understand this chemically and fluid-dynamically complex phenomenon, abundant experimental studies are available with focus on overall atomization process and velocity field evolution. However, they lack in resolving the small-scale processes which govern the evaporation, therefore combustion. With the intent to investigate in detail about the combustion aspect, this problem is reduced to analyzing behavior of isolated droplets. Despite the sophisticated measurement technologies particle-scale processes such as temperature and species mass fraction evolution are yet unknown. Moreover, the majority of these studies are performed with simplifying assumptions. assumption has been that the droplet remains spherical throughout its lifetime. However, in practical applications, particularly when exposed to convective and turbulent environments, droplets can undergo significant deformation due to the presence of inherent surface tension of liquid. This deformation can influence their evaporation and burning rates. Additionally, the droplet's shape governs the flow field around it, potentially altering droplet-droplet interactions. Direct Numerical Simulation (DNS) approach is one of the numerical methods which can resolve both the phases. It offers a promising approach to reveal these small-scale details, such as droplet shape, vapor and temperature field around a droplet, droplet-droplet interaction, droplet motion etc. With the aim to bridge this gap, this dissertation focuses on the study of evaporation and combustion of an isolated deformable droplet under various conditions.