Analysis of Polycyclic Aromatic Hydrocarbon Emissions from a Pilot Scale Silicon Process with Flue Gas Recirculation

Flue gas recirculation (FGR) is a method used in several industries to control emissions and process conditions, such as NOx reduction and temperature levels, and increase the CO2 concentration in the off-gas, to be better suited for methods of carbon capture. In this study, the influence of FGR, varying levels of flue gas flow and oxygen concentration on the emissions of polycyclic aromatic hydrocarbons (PAHs) was investigated during Si alloy production. In addition, computational fluid dynamics (CFD) modeling was performed using OpenFOAM for combustion of C2H2 and H2 with varying O2 levels to simulate FGR and to gain better insight into the impact of furnace operations on the PAH evolution. Experimental results show that increasing FGR (0–82.5%) and decreasing levels of oxygen (20.7–13.3 vol %) increase the PAH-42 concentration from 14.1 to 559.7 μg/Nm3. This is supported by the simulations, where increased formation of all PAHs species was observed at high levels of FGR, especially for the lighter aromatic species (like benzene and naphthalene), due to the lower availability of oxygen and the reduction in temperature. Residence time was identified as another key parameter to promote complete combustion of PAHs. Benzene oxidation can be prevented with temperatures lower than 1000 K and residence times smaller than 1 s, while complete oxidation is found at temperatures of around 1500 K.publishedVersio

Introduction to Surface-Mount Technology

In chapter 1, the surface-mount technology and reflow soldering technology are overviewed. A brief introduction is presented into the type of electronic components, including through-hole- and surface-mounted ones. Steps of reflow soldering technology are outlined, and details are given regarding the properties of solder material in this technology. The rheological behavior of solder pastes is detailed, and some recent advancements in addressing the thixotropic behavior of this material are summarized. The process of stencil printing is detailed next, which is the most crucial step in reflow soldering technology; since even 60–70% of the soldering failures can be traced back to this process. The topic includes the structures of stencils, discussion of the primary process parameters, and process optimization possibilities by numerical modeling. Process issues of component placement are presented. The critical parameter (process and machines capability), which is used extensively for characterizing the placement process is studied. In connection with the measurement of process capability, the method of Gage R&R (repeatability and reproducibility) is detailed, including the estimation of respective variances. Process of the reflow soldering itself is detailed, including the two main phenomena taking place when the solder is in the molten state, namely: wetting of the liquid solder due to surface tension, and intermetallic compound formation due to diffusion. Solder profile calculation and component movements during the soldering (e.g., self-alignment of passive components) are presented too. Lastly, the pin-in-paste technology (reflow solder of through-hole components) is detailed, including some recent advancements in the optimization of this technology by utilizing machine learning techniques

BASIC RESONANT TOPOLOGIES FOR SWITCHING POWER SUPPLIES

The objective of this thesis is to study the basic resonant converter topologies for switching power supplies and to compare their performance under different operating conditions. The series and parallel resonant converters are analyzed in detail. The analysis uses the state plane method, which gives a good insight of the operation of the converter. It is found that these converters have more desirable characteristics when operated at a frequency above the resonant frequency rather than below it. Analytical results are verified by sinlulation and experiment

Interface resolved simulations of continuum scale electrochemical hydrogen evolution

An important aspect of improving the efficiency of water electrolysis is to remove the electrochemically generated hydrogen and oxygen bubbles. The evolution of these gases, which are associated with increased electrical resistance, are driven by electrochemical reactions causing supersaturation of the electrolyte which leads to bubble nucleation, growth, and eventual detachment from the electrode. Due to the different physics as well as the length and time scales associated with the process, referred to as the multiscale and multiphysics nature, predicting the bubble evolution using analytical models is challenging. As numerical modelling approaches, like Computational Fluid Dynamics (CFD), predicts the fluid flow based on the underlying governing equations, it can be used to study electrochemical bubble evolution. The work undertaken during the PhD is primarily to develop and verify a multiphysics CFD framework based on the Volume of Fluid (VOF) method available in OpenFOAM® for continuum scale hydrogen bubbles. In the context of this work, continuum scale bubbles refers to bubble diameters which are larger than a few hundred micrometers. The VOF method is customized by adding the physics and numerical techniques relevant to treating electrochemical reactions, dissolved gas transport, charge transport, interfacial mass transfer and associated bubble growth (from a pre-existing submillimeter bubble). The proposed framework is developed incrementally, with each step corresponding to implementation and verification of a multiphysics module, eventually culminating in the fully coupled multiphysics framework. This modularized approach allows for verification of the implemented functionality with existing theoretical models and/or computational benchmarks. The thesis, in essence, provides context to the undertaken research, review of the various modelling techniques used to treat the multiphysics nature of electrochemical hydrogen evolution and details of developed framework. In addition, the thesis also summarizes knowledge gained during the PhD about the solution procedure used in OpenFOAM® and the VOF method to enable knowledge dissemination for further research

An analytical and numerical study of droplet formation and break-off for jetting of dense suspensions

The jet printing of solder paste from a uid dynamics perspective involves viscosity change due to varying shear rate and eventual break o of the ejected solder paste droplet from the uid in the printer head. The ability to model the jetting process in a simulation package is important as it can be used as a tool for future development of the jetting device. The jetting process is modelled as a two phase (air - solder paste) ow with interface tracking performed using phase eld method and temporal stepping based on a second-order Backward Di erence Formula with relaxed tolerences. This thesis investigates the droplet morphology, volume and speed predictions for three di erent piston actuation modes and solder paste viscosity denitions given by the Carreau- Yasuda model. A Darcy condition with the porosity parameter is calibrated equal to unity such that the droplet speed is within the realistic range of 20 m/s - 30 m/s. The simulations are compared against previous simulation results from IBOFlow, performed within a collaboration between Mycronic AB and Fraunhofer-Chalmers Centre. As the Carreau models cannot capture the dependence of the uid viscosity of ow history, an indirect structure based viscosity model is used to compare the thixotopic behaviour. The expressions for the parameters of the structure based viscosity model are derived based on an analytical model which assumes that shear rate is constant. Experimental data for constant shear rate is curve tted on a Carreau model and an initial estimate of the parameters are obtained. The parameters are then adjusted to match experimental thixotopic behaviour. This method can be used to obtain parameter values for structure based viscosity models for uids with no previous data. Once the solder paste is ejected through the nozzle and the piston retracts, the uid undergoes stretching. Studying lament stretching during jetting is dicult as it can be driven by both droplet and piston motion. The data from an extensional rheometer is analyzed to study the lament stretching phenomenon for solder pastes. An analytical model for the critical aspect ratio is derived for a Newtonian uid lament undergoing a pure extension and modelled as a cylinder whose radius is decreases with time. The exponential decrease of the lament radius predicted by the analytical model is found to reproduce the experimental observations very well. The lament radius calculated based on the lament height from the experiments and analytical model shows that the model captures the stretching process, but the formation of beads usually seen in suspensions is not accounted for

Numerical simulation of continuum scale electrochemical hydrogen bubble evolution

One of the important aspects in improving the efficiency of electrochemical processes, such as water electrolysis, is the efficient removal of bubbles which evolve from the electrodes. Numerical modelling based on Computational Fluid Dynamics (CFD) can describe the process, provide insights into its complexity, elucidate the underlying mechanisms of how bubbles evolve and their effect as well as aid in developing strategies to reduce the impact of the bubble. In this paper, a Volume of Fluid (VOF) based simulation framework to study the evolution of hydrogen bubbles in the order of few hundred micrometers, refered to as continuum scale bubbles, is proposed. The framework accounts for the multiphase nature of the process, electrochemical reactions, dissolved gas transport, charge transport, interfacial mass transfer and associated bubble growth. The proposed solver is verified, for two-dimensional cases, by comparison to analytical solution of bubble growth in supersaturated solutions, stationary bubble, rising bubbles and qualitative analysis based on experimental observations of the variations in current based on static simulations. The proposed solver is used to simulate the evolution of a single bubble under various wetting conditions of the electrode as well as the coalescence driven evolution of two bubbles. The results show that as the bubbles detach, its surface oscillates and the shape of the rising bubble is determined by the balance between drag force and surface tension. These surface oscillations, which causes the bubble to get flattened and elongated, results in temporal variation of the electrical current. The reduction of current due to bubble growth is visible only when these surface oscillations have reduced. The simulations also show the current as a function of the position of the bubble in the interelectrode gap. The framework also predicts the increase in current as a result of bubbles leaving the surface which is larger when the process is coalescence driven. The simulations indicate that bubble coalescence is the underlying mechanism for continuum scale bubble detachment

ON MODELLING ELECTROCHEMICAL GAS EVOLUTION USING THE VOLUME OF FLUID METHOD

In this work we describe the various building block relevant in simulating electrochemical gas evolution using Volume of Fluid (VOF) method. These building blocks are implemented in the VOF solver available in OpenFOAM® and its predictions are compared to the theoretical models reported in literature. The fully coupled solver to model electrochemical gas evolution is used to model the case of a bubble evolving on a vertical electrode under constant potential condition to showcase its ability.publishedVersio

ON MODELLING ELECTROCHEMICAL GAS EVOLUTION USING THE VOLUME OF FLUID METHOD

In this work we describe the various building block relevant in simulating electrochemical gas evolution using Volume of Fluid (VOF) method. These building blocks are implemented in the VOF solver available in OpenFOAM® and its predictions are compared to the theoretical models reported in literature. The fully coupled solver to model electrochemical gas evolution is used to model the case of a bubble evolving on a vertical electrode under constant potential condition to showcase its ability

On sharp surface force model: effect of sharpening coefficient

Amongst the multitude of approaches available in literature to reduce spurious velocities in Volume of Fluid approach, the Sharp Surface Force (SSF) model is increasingly being used due to its relative ease to implement. The SSF approach relies on a user-defined parameter, the sharpening coefficient, which determines the extent of the smeared nature of interface used to determine the surface tension force. In this paper, we use the SSF model implemented in OpenFOAM® to investigate the effect of this sharpening coefficient on spurious velocities and accuracy of dynamic, i.e., capillary rise, and static bubble simulations. Results show that increasing the sharpening coefficient generally reduces the spurious velocities in both static and dynamic cases. Although static millimeter sized bubbles were simulated with the whole range of sharpening coefficients, sub-millimeter sized bubbles show nonphysical behavior for values larger than 0.3. The accuracy of the capillary rise simulations has been observed to change non-linearly with the sharpening coefficient. This work illustrates the importance of using an optimized value of the sharpening coefficient with respect to spurious velocities and accuracy of the simulation