As the oil industry moves from a heuristic of avoidance of hydrates to a heuristic of risk management time dependent phenomena of hydrate formation and plugging must be known. One of the key parameters to this process is the aggregation of hydrate particles, the fractal networks they form, and the effect these two parameters have on flow. Unfortunately the aggregation and fractal structure information is extremely difficult to acquire experimentally, for this reason a three-dimension discrete element method (3D-DEM) model has been implemented. The 3D-DEM model calculates detailed solutions to Newton's equations of motion for individual particles. In addition these particles are coupled with the surrounding fluid through computational fluid dynamics (CFD). This coupled 3D-DEM can be used to investigate what the effects of shear, suspending viscosity, attractive forces, and other relevant variables have on the structure, stresses, and positions of the hydrate particles over time. In addition, the effect on viscosity has been calculated using CFD and compared back to basic hard sphere theory.

Dust and hybrid mixture explosions continue to occur in industrial processes that handle fine powders and flammable gases. Considerable research is therefore conducted throughout the world with the objective of both preventing the occurrence and mitigating the consequences of such events. In the current work, research has been undertaken to help advance the field of dust explosion prevention and mitigation from an emphasis on hazards to a focus on risk. Employing the principles of quantitative risk assessment (QRA) of dust and hybrid mixture explosions, a methodological framework for the management of these risks has been developed. The Quantitative Risk Management Framework (QRMF) is based on hazard identification via credible accident scenarios for dust explosions, followed by probabilistic Fault Tree Analysis (using Relex – Reliability Excellence – software) and consequence severity analysis, represented by maximum explosion pressure, (using DESC – Dust Explosion Simulation Code – software). Identification of risk reduction measures in the framework is accomplished in a hierarchical manner by considering inherent safety measures, passive and active engineered devices, and procedural measures. Dust explosion tests to determine icing and granulated sugar dust explosibility characteristics have been achieved in a 20-L Siwek chamber, and, accordingly, DESC fuel files were built and DESC has validated. Three industrial case studies are presented to show how the QRMF could has been helpful in reducing dust and hybrid mixture explosion risk at the Imperial Sugar refinery, the Semabla grain storage silo, and a hypothetical 400-m3 polyethylene storage silo. DESC simulations and Probit equation for structural damage provide the destructive percentage of each pressure zone in the simulations, followed by probabilistic FTA that were achieved for the first two case studies, before and after applying the framework. Detailed individual and societal risks calculations were made and F-N curves plotted for the two processes. The polyethylene silo case study is presented to show how inherent safety measures can be helpful in reducing dust and hybrid mixture explosion risk. The framework showed significant risk reduction to the point where the residual risks are acceptable for both processes. Finally, assessment results are compared and improvements measured. / The research main objective is: to manage the risks of any expected dust or hybrid mixture explosion in industrial complexes (large-scale).

Twin-screw pumps are economical alternatives to the conventional multiphase system and are increasingly used in the oil and gas industry due to their versatility in transferring the multiphase mixture with varying Gas Void Fraction (GVF). Present work focuses on the experimental and numerical analysis of twin-screw pumps for different operating conditions. Experimental evaluation aims to understand steady state and transient behavior of twin-screw pumps. Detailed steady state evaluation helped form better understanding of twin-screw pumps under different operating conditions. A comparative study of twin-screw pumps and compressors contradicted the common belief that compressor efficiency is better than the efficiency of twin-screw pumps. Transient analysis at high GVF helped incorporate necessary changes in the design of sealflush recirculation loop to improve the efficiency of the pump. The effect of viscosity of the sealflush fluid at high GVF on pump performance was studied. Volumetric efficiency was found to be decreased with increase in viscosity. Flow visualization was aimed to characterize phase distribution along cavities and clearances at low to high GVF. Dynamic pressure variation was studied along the axis of the screw which helped correlate the GVF, velocity and pressure distribution. Complicated fluid flow behavior due to enclosed fluid pockets and interconnecting clearances makes it difficult to numerically simulate the pump. Hence design optimization and performance prediction incorporates only analytical approach and experimental evaluation. Current work represents an attempt to numerically simulate a multiphase twin-screw pump as a whole. Single phase 3D CFD simulation was performed for different pressure rise. The pressure and velocity profile agreed well with previous studies. Results are validated using an analytical approach as well as experimental data. A two-phase CFD simulation was performed for 50% GVF. An Eulerian approach was employed to evaluate multiphase flow behavior. Pressure, velocity, temperature and GVF distributions were successfully predicted using CFD simulation. Bubble size was found to be most dominant parameter, significantly affecting phase separation and leakage flow rate. Better phase separation was realized with increased bubble size, which resulted in decrease in leakage flow rate. CFD results agreed well with experimental data for the bubble size higher than 0.08 mm.

The safety and security of liquefied natural gas (LNG) facilities has prompted the need for continued study of LNG mitigation systems. Water spray systems are widely recognized as an effective measure for dispersing LNG vapor clouds. Currently, there are no engineering guidelines available for water curtain applications in the LNG industry due to a lack of understanding of the complex interactions between the LNG vapor cloud and water droplets. This research applies computational fluid dynamics (CFD) modeling to investigate the forced dispersion of LNG vapor using upward-oriented full-cone spray nozzles. A Eulerian-Lagrangian approach was applied to simulate the energy and momentum exchange between the continuous (gas flow) and discrete (droplets) phases. Discussed are the physical parameters that are essential inputs to the CFD simulation of the water spray-LNG system. The experimental data collected from the Mary Kay O’Connor Process Safety Center’s outdoor LNG spill work in March 2009 at the Brayton Fire Training Field were used to calibrate the physical parameters. The physical mechanisms of the water spray application were investigated using LNG forced dispersion modeling. The effects of momentum imparting from the droplets to the air- vapor mixture, thermal transfer between the two phases (droplet/vapor) and effects of various levels of air entrainment rates on the behavior of the LNG vapors are evaluated. Lastly, the key parametric dependences of the design elements for an effective water curtain system are investigated. The effects of different droplet sizes, droplet temperatures, nozzle cone angles, and installation configurations of water spray applications on LNG vapor behavior are analyzed. This work aims to investigate the complex interaction of the water droplet-LNG vapor system, which will serve in developing guidelines and establishing engineering criteria for a site-specific LNG mitigation system. Finally, the potentials of applying CFD modeling in providing guidance for setting up the design criteria for an effective forced mitigation system as an integrated safety element for LNG facilities are discussed.

Computational fluid dynamics (CFD) has been extensively used in the study of building energy usage and thermal comfort in buildings, however there remains the need to thoroughly evaluate the accuracy of the results given by such CFD methods. The present study involves a numerical investigation of the flow and temperature distribution in the atrium situated in the Concordia University Engineering Building. The study involved a steady-state simulation of the conditions in the atria on August 1, 2007, a date for which experimental data was available for validation of the numerical results. The commercial CFD solver FLUENT was used to solve the equations that govern the flow in the atrium. The realizable k- turbulence model incorporating buoyancy force effects was used. During the period studied a forced airflow through the atrium existed due to a mechanical air supply vent near the floor level. The natural convection in the atrium, induced by the temperature differences resulting mainly from the incoming solar radiation, was modeled using the bousinessq approximation. In general, good agreement was obtained between the numerical and experimental results. The numerical results also predicted the thermal stratification in the atrium relatively accurately. A parametric study was performed to assess the sensitivity of the numerical results to the assumed boundary conditions used in the study. An evaluation of the thermal comfort levels in the atrium was also undertaken using the numerical results. This indicated that while regions of thermal discomfort did exist in the atrium, these regions constituted only a small part of the atrium. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2009-09-25 17:29:46.857

A steady-state simulation of a diesel-fuelled SOFC system was developed using a process simulation software package (VMGSim). The system was studied by conducting a sensitivity analysis of six independent variables (steam to carbon ratio, oxygen to carbon ratio, fuel utilization, air utilization, reformer pre-heater approach temperature and cathode temperature to the SOFC) and their effect on three response variables (net system efficiency, stack efficiency, system exhaust temperature). The steam to carbon ratio, fuel utilization and air utilization were the most influential independent variables and thus affected the greatest change in the performance metrics. Secondly, a multi-variable study was carried out on the most influential variables and constrained optima for the efficiencies (45% net system, 47% stack) and system exhaust temperature (78 degrees Celsius) were obtained. For the second part of this work, a steam reforming heat-exchange reactor was modeled using COMSOL. The reactor performance was assessed on the basis of selectivity and residence time for a given conversion. Both the kinetic models of Parmar et al. (2010) and Shi et al. (2009) for catalytic diesel steam reforming were applied and compared. Differences in performance were attributed to differences in the catalyst support and the reaction mechanisms used for deriving the reforming rate expressions. Finally, a proof of concept multi-scale modeling and design tool was developed by integrating the CFD model with the process simulation. Two-way communication between four different software components; COMSOL, VMGSim, Matlab and Microsoft Excel was achieved. / Thesis (Master, Chemical Engineering) -- Queen's University, 2012-04-18 01:12:27.072

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This dissertation investigates the emulsification of aqueous liquid in immiscible organic liquid in various microfluidic environments, and addresses both experimental characterization and theoretical interpretation of the dynamics and design guidelines, as well as an application of microfluidic emulsification in fabrication of disk-like colloidal particle suspensions for studying its sedimentation behavior. In an attempt to understand the dynamics of drop formation in flow-focusing microfluidic channels, especially for an explanation of a transition from unique drop size to bimodal oscillating drop sizes as observed in the experiments, numerical simulation is developed to use the volume-of-fraction method to model the drop formation, and the simulation results help to interpret the transition in the theory of saddle-node transition in drop formation, as well as show the importance of selecting proper orifice length in flow-focusing microfluidic channel design. The electric technique for controlling of microfluidic emulsification is explored by a detailed study on low-frequency alternating-current electro-flow-focusing (EFF) emulsification in microfluidic channels. It is found that the droplet size variation is not a monotonic function of the electric field as in the case of direct-current EFF emulsification, which originates from the relaxation oscillation of the flow rate through the Taylor cone, and a power-law droplet size distribution was obtained at the voltage ramping-up stage. This emulsification process was modeled in analog to the charge accumulation and release in a resistor-capacitor electric circuit with an adjustable resistor, and the simulated data exhibit good agreement with the experiments. As an application of the microfluidic emulsification, a method of fabricating disk-like wax colloidal particle suspensions using electrospray is reported. Based on this technique, the first measurement of the hindrance function for sedimentation and creaming of disk-shaped colloids via the analytical centrifugation is reported. Disks align with the external flow right above the volume fraction of a few percent and this effect is extremely sensitive to the aspect ratio of disks. Due to this alignment effect, disk sedimentation/creaming demonstrate distinct trends in dilute and semi-dilute region.

Despite advances in the understanding of fire dynamics over the past decades and despite the advances in computational capacity, our ability to predict the behaviour of fires in general and building fires in particular remains very limited. This thesis proposes and studies a method to use measurements of the real event in order to steer and accelerate fire simulations. This technology aims at providing forecasts of the fire development with a positive lead time, i.e. the forecast of future events is ready before those events take place. A simplified fire spread model is implemented, and sensor data are assimilated into the model in order to estimate the parameters that characterize the spread model and thus recover information lost by approximations. The assimilation process is posed as an inverse problem, which is solved minimizing a non linear cost function that measures the distance between sensor data and the forward model. In order to accelerate the optimization procedure, the ‘tangent linear model’ is implemented, i.e. the forward model is linearized around the initial guess of the governing parameters that are to be estimated, thus approximating the cost function by a quadratic function. The methodology was tested first with a simple two-zone forward model, and then with a coarse grid Computational Fluid Dynamics (CFD) fire model as forward model. Observations for the inverse modelling were generated using a fine grid CFD simulation in order to illustrate the methodology. A test case with observations from a real scale fire test is presented at the end of this document. In the two-zone model approach the spread rate, entrainment coefficient and gas transport time are the governing invariant parameters that are estimated. The parameters could be estimated correctly and the temperature and the height of the hot layer were reproduced satisfactorily. Moreover, the heat release rate and growth rate were estimated correctly with a positive lead time of up to 30 s. The results showed that the simple mass and heat balances and plume correlation of the zone model were enough to satisfactorily forecast the main features of the fire, and that positive lead times are possible. With the CFD forward model the growth rate, fuel mass loss rate and other parameters of a fire were estimated by assimilating measurements from the fire into the model. It was shown that with a field type forward model it is possible to estimate the growth rates of several different spread rates simultaneously. A coarse grid CFD model with very short computation times was used to assimilate measurements and it was shown that spatially resolved forecasts can be obtained in reasonable time, when combined with observations from the fire. The assimilation of observations from a real scale fire test into a coarse grid CFD model showed that the estimation of a fire growth parameter is possible in complicated scenarios in reasonable time, and that the resulting forecasts at localized level present good levels of accuracy. The proposed methodology is still subject to ongoing research. The limited capability of the forward model to represent the true fire has to be addressed with more detail, and the additional information that has to be provided in order to run the simulations has to be investigated. When using a CFD type forward model, additional to the detailed geometry, it is necessary to establish the location of the fire origin and the potential fuel load before starting the assimilation cycle. While the fire origin can be located easily (as a first approximation the location of the highest temperature reading can be used), the fuel load is potentially very variable and its exact distribution might be impractical to continually keep track of. It was however shown that for relatively small compartments the exact fuel distribution is not essential in order to produce an adequate forecast, and the fuel load could for example be established based on a statistical analysis of typical compartment layouts.

<p>One of the grand challenges faced by industry is the accurate prediction of unsteady aerodynamics events, including frequency lock-in and forced response. These aeromechanical incidents occurring in airplane engines and gas turbines can cause high-amplitude blade vibration and potential failure of the engine or turbine. During the last decades, the development of computational fluid dynamics has allowed the design and optimization of complex components while reducing the need for expensive engine testing. However, the validation of frequency lock-in and forced response numerical results with experimental data is very incomplete. Despite tremendous advances in computational capabilities, industry is still looking to validate design tools and guidelines to avoid these potentially costly aeroelastic events early in the design process. </p><p>The research efforts presented in this dissertation investigate the aeroelastic phenomena of frequency lock-in and forced response in turbomachinery. First, frequency lock-in is predicted for two structures, namely a two-dimensional cylinder and a single three-dimensional airfoil, and the results are compared to experimental data so that the methods can be extended to more complex structures. For these two simpler structures, a frequency domain harmonic balance code is used to estimate the natural shedding frequency and the corresponding lock-in region. Both the shedding frequencies and the lock-in regions obtained by an enforced motion method agree with experimental data from previous literature and wind tunnel tests. Moreover, the aerodynamic model of the vibrating cylinder is coupled with the structural equations of motion to form a fluid-structure interaction model and to compute the limit-cycle oscillation amplitude of the cylinder. The extent of the lock-in region matches the experimental data very well, yet the peak amplitude is underestimated in the numerical model. We demonstrate that the inclusion of the cylinder second degree of freedom has a significant impact on the cylinder first degree of freedom amplitude. Moreover, it is observed that two harmonics need to be kept in the equations of motion for accurate prediction of the unsteady forces on the cylinder. </p><p>The second important topic covered is a comprehensive forced response analysis conducted on a multi-stage axial compressor and compared with the initial data of the largest forced response experimental data set ever obtained in the field. Both a frequency domain and a time domain codes are used. The steady-state and time-averaged aerodynamic performance results compare well with experimental data, although losses are underestimated due to the lack of secondary flow paths and fillets in the model. The use of mixing planes in the steady simulations underpredicts the wakes by neglecting the important interactions between rows. Therefore, for similar cases with significant flow separation, the use of a decoupled method for forced response predictions cannot yield accurate results. A full multi-row transient analysis must be conducted for accurate prediction of the wakes and surface unsteady pressures. Finally, for the first time, predicted mistuned blade amplitudes are compared to mistuned experimental data. The downstream stator is found to be necessary for the accurate prediction of the modal forces and vibration amplitudes. The mistuned rotor is shown to be extremely sensitive to perturbations in blade frequency mistuning, aerodynamic asymmetry, and excitation traveling wave content. Since this dissertation presents the initial results of a five-year research program, more research will be conducted on this compressor to draw guidelines that can be used by aeromechanical engineers to safely avoid forced response events in the design of jet engines and gas turbines.</p> / Dissertation