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[pt] O estudo de bolhas de separação laminar em aerofólios que operam a baixos números de Reynolds é importante para diversas aplicações tais como: turbinas a gás, geradores eólicos, veículos aéreos não tripulados, dentre outros. A perda de performance desses equipamentos, normalmente, está associada à presença e à ruptura de bolhas de separação. Neste trabalho buscou-se investigar, experimentalmente, os efeitos relacionados a variação súbita no nível de perturbações do escoamento a montante da região de formação da bolha. A ideia é avaliar como o escoamento se comporta em uma situação onde o nível de turbulência do escoamento livre pode variar. No caso de turbinas, essa condição pode ser observada quando as esteiras dos aerofólios de um estágio influenciam aquelas do estágio seguinte. Apesar da relevância prática, o regime transiente foi pouco investigado na literatura. Para o presente estudo foi utilizado um canal de água do Laboratório de Engenharia de Fluidos. O gradiente de pressão, necessário para a formação da bolha, foi gerado com uma placa convergente-divergente e o nível de perturbações na camada limite foi controlado através de um gerador do tipo fita vibrante. Os campos de velocidades na região da bolha foram medidos com a técnica de Velocimetria por Imagem de Partícula com alta resolução temporal. A geração de perturbações e as medições de velocidade foram sincronizadas, permitindo assim o uso de técnicas de extração de médias de eventos. Caracterizou-se a bolha em um regime estacionário, onde não houve excitação de perturbações controladas a montante da bolha. Os resultados foram comparados com referências da literatura, apresentando boa concordância. No regime transiente, analisou-se o escoamento desde o instante em que o gerador de perturbações foi subitamente desligado até a recuperação completa da bolha. Nessas condições, notou-se que bolha cresceu até atingir um comprimento maior do que o observado no regime estacionário. Somente após o desprendimento de um grande vórtice, a bolha voltou a exibir características similares àquelas do regime estacionário. Esse comportamento se assemelha àquele reportado na literatura para ruptura de bolhas (bursting) mas que ainda não é bem compreendido. / [en] Laminar separation bubbles in Low Reynolds number airfoils is important for several applications such as: gas turbines, wind generators, unmanned aerial vehicles, among others. The performance loss of this airfoils is usually associated with the presence and rupture of separation bubbles. The effects related to the sudden variation of the level of flow disturbances upstream of the bubble formation region are investigated in this work. The idea is to evaluate how the flow behaves when subjected to a sudden variation in the freestream turbulence. This condition can be observed in airfoil cascade of turbines, where the wake of a previous stage can influence the next. Although this scenario is of practical relevance, it has been poorly investigated in literature. For the present study, a water channel from the Fluids Engineering Laboratory was used. The pressure gradient, necessary for the bubble formation, was generated with a convergentdivergent plate and the level of disturbances in the boundary layer was controlled using a vibrating ribbon. Velocity fields in the bubble region were measured using the Particle Image Velocimetry technique with high temporal resolution. The generation of disturbances and measurements were phase locked, thus allowing the use of ensemble average techniques. The bubble was characterized in a so called steady regime, where there was no excitation of controlled disturbances upstream of the bubble. The results were compared with references in the literature, showing good agreement. In the transient regime, the flow was analyzed from the moment the disturbance generator was suddenly turned off until the bubble had completely recovered. In these conditions, it was noted that the bubble grew until reaching a length greater than that observed in the stationary regime. Only after the release of a large vortex did the bubble return to exhibit characteristics similar to those of the stationary regime. This behavior is similar to that reported in the literature for bubble rupture (bursting) but that is still not well understood.

[pt] O fenômeno de retorno de chama em tubos é conhecido e estudado há várias décadas. Sua análise clássica é baseada na determinação do gradiente de velocidade crítico, Gc, que o delimita como função das propriedades das misturas combustíveis. Entretanto, não é conhecido o efeito da diluição por CO2, importante para a previsão da segurança das instalações do pré-sal. Por isto, são aqui desenvolvidos estudos específicos do retorno de chamas pré-misturadas em escoamentos laminares. O objetivo geral deste trabalho é determinar experimentalmente a influência da diluição por CO2 sobre o retorno de chamas (flashback) em misturas de hidrocarbonetos (CH4 ou propano) e de ar. O levantamento do estado da arte permitiu especificar as características da instalação experimental para o estudo deste fenômeno e, também, identificar as principais questões a serem abordadas. Foi projetado e construído um aparato experimental para o estudo do flashback em escoamentos laminares. Os resultados originais obtidos mostram como a propensão ao retorno de chama é influenciada pela natureza do combustível, pela estequiometria da mistura e pela diluição. Misturas de propano possuem maior propensão ao flashback e maiores valores de Gc do que as de metano. Também foi mostrado que há uma redução da propensão ao flashback com o aumento da diluição. Esta propensão foi relacionada aos números adimensionais que caracterizam a combustão, isto é, os números de Lewis, Péclet, Karlovitz e Zel dovich. Para este último, uma proposta original visando sua determinação é apresentada, que envolve uma expressão da taxa de liberação de calor da reação química global controlada por uma variável de progresso. Esta formulação permite resolver o problema da singularidade na região da estequiometria. / [en] The flashback phenomenon in tubes has been known and studied for several decades. Its classical analysis is based on the determination of the critical velocity gradient, Gc, which delimits it as a function of the fuel mixture properties. However, the effect of the CO2 dilution is not known, which is important for predicting the safety of pre-salt facilities. For this reason, specific studies of premixed flame flashback in laminar flows are developed here. The general objective of this work is to experimentally determine the influence of CO2 dilution on flame flashback in mixtures of hydrocarbons (CH4 and propane) and air. The state of the art research made it possible to specify the characteristics of the experimental installation for this phenomenon study and, also, to identify the main issues to be addressed. An experimental apparatus was designed and built to study the flame flashback in laminar flows. The original results obtained show how the propensity of the flame flashback is influenced by the nature of the fuel, the stoichiometry of the mixture, and the dilution. Propane mixtures have a greater propensity for flashback and higher values of Gc than those of methane. It has also been shown that there is a reduction in the propensity of flashback with increasing dilution. This propensity was related to the dimensionless numbers that characterize combustion, that is, the Lewis, Péclet, Karlovitz, and Zel dovich numbers. For the latter, an original proposal aimed at its determination is presented, which involves an expression of the heat release rate from the global chemical reaction controlled by a progress variable.

The main aim of this thesis was to understand the mechanism behind the classical transition scenario inside the boundary layer over an airfoil and eventually attempting to control this transition utilizing passive devices for transition delay. The initial objective of analyzing the transition phenomenon based on TS wave disturbance growth was conducted at 90 Hz using LDV and CTA measurement techniques at two different angles of attack. This was combined with the studies performed on two other frequencies of 100 and 110 Hz, in order to witness its impact on the neutral stability curve behavior. The challenges faced in the next phase of the thesis while trying to control the transition location, was to understand and encompass the effect of adverse pressure gradient before setting up the passive control devices, which in this case was miniature vortex generators. Consequently, several attempts were made to optimize the parameters of the miniature vortex generators depending upon the streak strength and stability. Finally, for 90 Hz a configuration of miniature vortex generators have been found to successfully stabilize the TS wave disturbances below a certain forcing amplitude, which also led to transition delay.

Thesis: M.S., Massachusetts Institute of Technology, Department of Mechanical Engineering, 1981 / Includes bibliographical references. / by Mark Charles Sellnau. / M.S. / M.S. Massachusetts Institute of Technology, Department of Mechanical Engineering

Scientific research in areas of physics, chemistry, and biology traditionally depends purely on experimental and theoretical methods. Recently numerical simulation is emerging as the third way of science discovery beyond the experimental and theoretical approaches. This work describes some general procedures in numerical computation, and presents several applications of numerical modeling in bioheat transfer and biomechanics, jet diffusion flame, and bio-molecular interactions of proteins in blood circulation. A three-dimensional (3D) multilayer model based on the skin physical structure is developed to investigate the transient thermal response of human skin subject to external heating. The temperature distribution of the skin is modeled by a bioheat transfer equation. Different from existing models, the current model includes water evaporation and diffusion, where the rate of water evaporation is determined based on the theory of laminar boundary layer. The time-dependent equation is discretized using the Crank-Nicolson scheme. The large sparse linear system resulted from discretizing the governing partial differential equation is solved by GMRES solver. The jet diffusion flame is simulated by fluid flow and chemical reaction. The second-order backward Euler scheme is applied for the time dependent Navier-Stokes equation. Central difference is used for diffusion terms to achieve better accuracy, and a monotonicity-preserving upwind difference is used for convective ones. The coupled nonlinear system is solved via the damped Newton's method. The Newton Jacobian matrix is formed numerically, and resulting linear system is ill-conditioned and is solved by Bi-CGSTAB with the Gauss-Seidel preconditioner. A novel convection-diffusion-reaction model is introduced to simulate fibroblast growth factor (FGF-2) binding to cell surface molecules of receptor and heparan sulfate proteoglycan and MAP kinase signaling under flow condition. The model includes three parts: the flow of media using compressible Navier-Stokes equation, the transport of FGF-2 using convection-diffusion transport equation, and the local binding and signaling by chemical kinetics. The whole model consists of a set of coupled nonlinear partial differential equations (PDEs) and a set of coupled nonlinear ordinary differential equations (ODEs). To solve the time-dependent PDE system we use second order implicit Euler method by finite volume discretization. The ODE system is stiff and is solved by an ODE solver VODE using backward differencing formulation (BDF). Findings from this study have implications with regard to regulation of heparin-binding growth factors in circulation.

<p>Numerical simulations have been done with the CHEMKINsoftware to study different aspects of wall effects in thecombustion of lean, laminar and premixed flames in anaxisymmetric boundary-layer flow.</p><p>The importance of the chemical wall effects compared to thethermal wall effects caused by the development of the thermaland velocity boundary layer has been investigated in thereaction zone by using different wall boundary conditions, walltemperatures and fuel/air ratios. Surface mechanisms include acatalytic surface (Platinum), a surface that promotesrecombination of active intermediates and a completely inertwall with no species and reactions as the simplest possibleboundary condition.</p><p>When hydrogen is the model fuel, the analysis of the resultsshow that for atmospheric pressure and a wall temperature of600 K, the surface chemistry gives significant wall effects atthe richer combustion case (f=0.5), while the thermal andvelocity boundary layer gives rather small effects. For theleaner combustion case (f=0.1) the thermal and velocityboundary layer gives more significant wall effects, whilesurface chemistry gives less significant wall effects comparedto the other case.</p><p>For methane as model fuel, the thermal and velocity boundarylayer gives significant wall effects at the lower walltemperature (600 K), while surface chemistry gives rather smalleffects. The wall can then be modelled as chemically inert forthe lean mixtures used (f=0.2 and 0.4). For the higher walltemperature (1200 K) the surface chemistry gives significantwall effects.</p><p>For both model fuels, the catalytic wall unexpectedlyretards homogeneous combustion of the fuel more than the wallthat acts like a sink for active intermediates. This is due toproduct inhibition by catalytic combustion. For hydrogen thisoccurs at atmospheric pressure, but for methane only at thehigher wall temperature (1200 K) and the higher pressure (10atm).</p><p>As expected, the overall wall effects (i.e. a lowerconversion) were more pronounced for the leaner fuel-air ratiosand at the lower wall temperatures.</p><p>To estimate a possible discrepancy in flame position as aresult of neglecting the axial diffusion in the boundary layerassumption, calculations have been performed with PREMIX, alsoa part of the CHEMKIN software. With PREMIX, where axialdiffusion is considered, steady, laminar, one-dimensionalpremixed flames can be modelled. Results obtained with the sameinitial conditions as in the boundary layer calculations showthat for the richer mixtures at atmospheric pressure the axialdiffusion generally has a strong impact on the flame position,but in the other cases the axial diffusion may beneglected.</p><p><strong>Keywords:</strong>wall effects, laminar premixed flames,platinum surfaces, boundary layer flow</p> / QC 20100504

Fully understanding soot formation in flames is critical to the development of practical combustion devices, which typically operate at high pressures, and fire suppression systems in space. Flames display significant changes under microgravity and high-pressure conditions as compared to normal-gravity flames at atmospheric pressure, but the exact causes of these changes are not well-characterized. As such, the effects of gravity and pressure on the stability characteristics and sooting behavior of laminar coflow diffusion flames were investigated. To study these effects, a new highly-scalable combustion modelling tool was developed specifically for use on large multi-processor computer architectures. The tool is capable of capturing complex processes such as detailed chemistry, molecular transport, radiation, and soot formation/destruction in laminar diffusion flames. The proposed algorithm represents the current state of the art in combustion modelling, making use of a second-order accurate finite-volume scheme and a parallel adaptive mesh refinement algorithm on body-fitted, multi-block meshes. An acetylene-based, semi-empirical model was used to predict the nucleation, growth, and oxidation of soot particles. Reasonable agreement with experimental measurements for different fuels and pressures was obtained for predictions of flame height, temperature and soot volume fraction. Overall, the algorithm displayed excellent strong scaling performance by achieving a parallel efficiency of 70% on 384 processors. The effects of pressure and gravity were studied for flames of two different fuels: ethylene-air flames between pressures of 0.5–5 atm and methane-air flames between 1–60 atm. Based on the numerical predictions, zero-gravity flames had lower temperatures, broader soot-containing zones, and higher soot concentrations than normal-gravity flames at the same pressure. Buoyant forces caused the normal-gravity flames to narrow with increasing pressure while the increased soot concentrations and radiation at high pressures lengthened the zero-gravity flames. Low-pressure flames at both gravity levels exhibited a similar power-law dependence of the maximum carbon conversion on pressure which weakened as pressure was increased. This dependence decayed at a faster rate in zero gravity when pressure was increased beyond 1–10 atm.

The arising environmental concerns make emission reduction from combustion devices one of the greatest challenges of the century. Modern dry low-NOx emission combustion systems often operate under lean premixed turbulent conditions. In order to design and operate these systems efficiently, it is necessary to have a thorough understanding of combustion process in these devices. In premixed combustion, flame speed determines the conversion rate of fuel. The flame speed under highly turbulent conditions is defined as turbulent flame speed. Turbulent flame speed depends on laminar flame speed, which is a property of the combustible mixture. The goal of this thesis is to estimate laminar flame speed and NOx emissions under certain conditions for specific industrial gas turbines. For this purpose, an in-house one-dimensional code, GENE-AC, is used. At first, a data validation is performed in order to select an optimized chemical reaction mechanism which can be used safely with the fuels of interest in gas turbines. Results show that GRI-Mech 3.0 performs well in most cases. This mechanism is selected for further simulations. Secondly, laminar flame speed is calculated using GRI-Mech 3.0 at SGT-800 conditions. Results show that at gas turbine conditions, increasing ambient temperature and fuel to air ratio enhances flame speed, mainly due to faster reaction rates. Moreover, laminar flame speed is highly affected by fuel composition. In particular, adding hydrogen to a fuel changes chemical processes significantly, because hydrogen is relatively light and highly diffusive. Calculations are conducted over a range of equivalence ratios and hydrogen fractions in methane at atmospheric as well as gas turbine operating conditions. Results reveal some trends for changes in laminar flame speed, depending on hydrogen content in the mixture. The final part of the thesis involves the development of a reactor network model for the SGT-700 combustor in order to predict NOx emissions. The network model is built in GENE-AC based on results from available computational fluid dynamics (CFD) simulations of the combustor. The model is developed for full load conditions with variable pilot fuel ratios. The NOx emissions are predicted using GRI-Mech 3.0 mechanism. A parametric study shows the dependency of NOx emissions on equivalence ratio and residence time. For SGT-700 running on natural gas, NOx emissions are fitted to measurement data by tuning equivalence ratio and residence time. The model is then tested for a range of ambient temperatures and fuel compositions. It is found that, although the model can correctly predict the trends of ambient temperature and fuel effects on NOx emissions, these effects are to some extent over-estimated. Using future engine tests and amending calibration can improve the results.

The laminar burning velocity of a fuel is the rate of normal propagation of a 1D flame front relative to the movement of the unburned gas. This is a fundamental property of a fuel that affects many aspects of its combustion behaviour. Experimental values are required to validate kinetic simulations, and also to provide input for models of flashback, minimum ignition energy and turbulent combustion. Burning velocity affects burn duration and consequently power output in spark ignition engines. Burning velocities are affected by pressure, temperature, equivalence ratio, residuals, additives, and stretch rates. The constant volume vessel has been used as it is considered both the most versatile and accurate method of measuring laminar burning velocities. An existing combustion vessel and oven were refurbished and new systems built for fuel injection, ignition, experiment control, data acquisition and high speed schlieren photography. An existing multi-zone model was used to allow calculation of burning velocity from pressure and schlieren data, allowing the user to select data uncorrupted by heat transfer or cellularity. A twelve term correlation for burning velocity was validated using methane modelling data. The chosen data from all the experiments was then fitted to the correlation. Methane, n-butane, n-heptane, iso-octane, toluene, ethylbenzene and ethanol were tested over a wide range of initial pressures (0.5, 1, 2 and 4 barA), temperatures (289-450 K) and equivalence ratios (0.7-1.4). For liquid fuels, tests with real residuals at mole fractions of up to 0.3 were also conducted. Stoichiometric mixture tests were performed at two initial temperatures (380 and 450 K) and the same four initial pressures. For mixtures of iso-octane and ethylbenzene, percentage volumes of 12.5, 25, 50 and 75% iso-octane were tested. It was found that the the percentage of iso-octane affected burning velocity non-linearly. For iso-octane/ethanol, a single 50:50% mixture was tested.