A large number of organizations that offer products today are experiencing returns; whether it is the return of a book from an online book store, the return of a television to the electronic retailer or a garment to a fashion retailer. How organizations handle product returns (reverse logistics) differs and also how much focuses each organization places on it, be-cause after all; it is extremely difficult to actually make revenue on reverse logistics. Why spend time and money on it? This study focuses on comparing an electronic and a fashion organization, how they both are conducting reverse logistics in regards to e-commerce. This is of interest to examine and add to the literature based on research focusing on a comparison between two organizations of a different nature in terms of their reverse logistics. Furthermore, it was of interest to study how each organization operates internally. For example, what are the barriers and drivers of reverse logistics, do they work proactively or reactively and what is the focus in regards to recycling of products? A case study research strategy was applied with an inductive approach. Data was collected through semi-structured interviews with each organization where qualitative data was gathered. Secondary data was collected from literature sources such as academic journals and books. Data was analysed in order to structure the large amount of data to be able to compare the two organizations and draw conclusions. From analysing the data it is concluded that both the electronic and the fashion retailer are experiencing a great amount of returns, which generally follows the sales trend. Furthermore, the two organizations are similar in several aspects regarding reverse logistics although they are selling different products. One of the main drivers for both organizations in regards to reverse logistics is satisfying their customers. Finally, one of the main barriers for each organization was the costs that play a major role in reverse logistics. Overall, the study shows that it greatly depends on the nature of the products how reverse logistics processes are handled in the organization.

The first part of this two-part thesis examines the reverse-flow operation of auto-thermal methane reforming in a microreactor. A theoretical study is undertaken to explain the physical origins of the experimentally observed improvements in the performance of the reverse-flow operation compared to the unidirectional operation. First, a scaling analysis is presented to understand the effect of various time scales existing within the microreactor, and to obtain guidelines for the optimal reverse-flow operation. Then, the effect of kinetic parameters, transport properties, reactor design and operating conditions on the reactor operation is parametrically studied through numerical simulations. The reverse-flow operation is shown to be more robust than the unidirectional operation with respect to both optimal operating conditions as well as variations in hydrogen throughput requirements. A rational scheme for improved catalyst placement in the microreactor, which exploits the spatial temperature profiles in the reactor, is also presented. Finally, a design modification of the microreactor called "opposed-flow" reactor, which retains the performance benefits of the reverse-flow operation without requiring the input / output port switching, is suggested. In the second part of this thesis, a novel simulation-based Approximate Dynamic Programming (ADP) framework is presented for optimal control of switching between multiple metabolic states in a microbial bioreactor. The cybernetic modeling framework is used to capture these cellular metabolic switches. Model Predictive Control, one of the most popular advanced control methods, is able to drive the reactor to the desired steady state. However, the nonlinearity and switching nature of the system cause computational and performance problems with MPC. The proposed ADP has an advantage over MPC, as the closed-loop optimal policy is computed offline in the form of so-called value or cost-to-go function. Through the use of an approximation of the value function, the infinite horizon problem is converted into an equivalent single-stage problem, which can be solved online. Various issues in implementation of ADP are also addressed.

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The aerodynamics of modern rotorcraft is highly complex and has proven to be an arduous challenge for computational fluid dynamics (CFD). Flow features such as massively separated boundary layers or transition to turbulence are common in engineering applications and need to be accurately captured in order to predict the vehicle performance. The recent advances in numerical methods and turbulence modeling have resolved each of these issues independent of the other. First, state-of-the-art hybrid RANS-LES turbulence closures have shown great promise in capturing the unsteady flow details and integrated performance quantities for stalled flows. Similarly, the correlation-based transition model of Langtry and Menter has been successfully applied to a wide range of applications involving attached or mildly separated flows. However, there still lacks a unified approach that can tackle massively separated flows in the transitional flow region. In this effort, the two approaches have been combined and expended to yield a methodology capable of accurately predicting the features in these highly complex unsteady turbulent flows at a reasonable computational cost. Comparisons are evaluated on several cases, including a transitional flat plate, circular cylinder in crossflow and NACA 63-415 wing. Cost and accuracy correlations with URANS and prior hybrid URANS-LES approaches with and without transition modeling indicate that this new method can capture both separation and transition more accurately and cost effectively. This new turbulence approach has been applied to the study of wings in the reverse flow regime. The flight envelope of modern helicopters has increased significantly over the last few decades, with design concepts now reaching advance ratios up to μ = 1. In these extreme conditions, the freestream velocity exceeds the rotational speed of the blades, and a large region of the retreating side of the rotor disk experiences reverse flow. For a conventional airfoil with a sharp trailing edge, the reverse flow regime is generally characterized by massive boundary layer separation and bluff body vortex shedding. This complex aerodynamic environment has been utilized to evaluate the new hybrid transitional approach. The assessment has proven the efficiency of the new hybrid model, and it has provided a transformative advancement to the modeling of dynamic stall.

This work studies the effects of radial flow on the aerodynamic phenomena occurring on a retreating blade with a focus on dynamic stall and reverse flow as applied to both a helicopter rotor in forward flight and a wind turbine operating at a yaw angle. While great progress has been made in understanding the phenomenon of two-dimensional dynamic stall, the effect of rotation on the dynamic stall event is not well understood. Experiments were conducted on a rigid two bladed teetering rotor at high advance ratios in a low speed wind tunnel. Particle image velocimetry (PIV) measurements were used to quantify the flow field at several azimuthal angles on the rotating blade during the dynamic stall event. The effect of centrifugal forces induced ``pure'' radial velocity on the dynamic stall event at 270 degrees azimuth was studied in detail. Further investigation of the radial flow field suggested that the mean radial velocity attenuated on moving outboard due to an apparent shear layer instability and it was demonstrated to be of first order importance in the flow field. These radial flow results prompted an exploration of the flow over a rotating disk to establish similarities of the radial flow over rotating blade in separated flow to that over a rotating disk in separated flow. While a greater part of this work focused on aspects of dynamic stall on the retreating blade, the final parts focus on the exotic flow regime of reverse flow (characterized by flow from the trailing edge to the leading edge of the blade). Aerodynamic loads measurement and surface flow visualization via tufts are used to first quantify the behavior of a static yawed blade in reverse flow. PIV measurements are then used on a static yawed blade and a rotating blade in reverse flow conditions to ascertain the effects of rotation on reverse flow.

Baculoviruses are versatile viruses that can be used as biopestisides, or for the production of recombinant protein and vaccines. Baculoviruses have also been found to be able to transfer genes to mammalian cells. This finding opened the door for the application of baculovirus vectors in human gene therapy. However, the mass production of clinical grade baculovirus vectors is challenging. Downstream processing has now become the bottle-neck of the manufacturing process. In this work, an anion exchange chromatography-based process was investigated for the purification of recombinant baculovirus vectors using a novel hydrogel based membrane (Natrix Separations Ltd.). Crude recombinant baculovirus supernatant from infected insect cell cultures was first subjected to a clarification process consisting of centrifugation and filtration. The pH of the viral solution was adjusted and then passed through a fast protein liquid chromatography system consisting of the ion exchange membrane. After washing weakly bound impurities, the captured baculoviruses are recovered by an elution step. Overall, baculoviruses strongly associated with the membrane; however, this interaction which was much physical as it was chemical, could not be entirely reversed and baculovirus was lost in the process. Product purity has also been evaluated and up to 85% of total protein reduction was determined. The significant losses of baculovirus observed have indicated major limitations in using this membrane for the purification of baculovirus.

A new combustor, referred to as Stagnation Point Reverse Flow (SPRF) combustor has been developed at Georgia Tech to meet increasingly stringent emission regulations. The combustor incorporates a novel design to meet the conflicting requirements of low pollution and high efficiency in both premixed and non-premixed modes. The objective of this thesis is to perform Large Eddy Simulations (LES) on this lab-scale combustor and explain the underlying physics. To achieve this, numerical simulations are performed in both the premixed and non-premixed combustion modes. The velocity field, species field, entrainment characteristics, flame structure, emissions and mixing characteristics are then analyzed. Simulations have been carried out first for a non-reactive case and the flow features in the combustor are analyzed. Next, the simulations have been extended for the premixed reactive case by employing different sub-grid scale combustion chemistry closures - Eddy Break Up (EBU), Artificially Thickened Flame (TF) and Linear Eddy Mixing (LEM) models. Only LEMLES which is an advanced scalar approach is able to accurately predict both the velocity and species field in the combustor. The results from LEM with LES (LEMLES) using a reduced chemical mechanism have been analyzed in the premixed mode. The results showed that mass entrainment occurs along the shear layer in the combustor. The entrained mass carried products into the reactant stream and provided preheating. The product entrainment enhances the reaction rates and stabilizes the flame even at very lean conditions. These products are shown to enter into the flame through local extinction zones present on the flame surface. The flame structure is further analyzed and the combustion mode is found to be primarily in thin reaction zones. The emissions in the combustor are studied using simple global mechanisms for NOx. Computations show extremely low NOx values comparable to the measured emissions. These low emissions are shown to be primarily due to the low temperatures in the combustor. LEMLES computations are also performed with detailed chemistry to capture more accurately the flame structure. The flame in the detailed chemistry case is more sensitive to strain effects and show more extinction zones very near to the injector. LEMLES approach is also used to resolve the combustion mode in the non-premixed case. The studies indicate that mixing of fuel and air close to the injector controls the combustion process. The predictions in the near field are shown to be very sensitive to the inflow conditions. Analysis shows that fuel and air mixing occurs to lean proportions in the combustor before any burning takes place. The flame structure in the non-premixed mode is very similar to the premixed mode. Along with fuel-air mixing, the products also mix with the reactants and provide the preheating effects to stabilize the flame in the downstream region of the combustor.

In this study, numerical simulations of a low emission lab-scale non-premixed combustor are conducted and analyzed. The objectives are to provide new insight into the physical phenomena in the SPRF (Stagnation Point Reverse Flow) combustor built in the Georgia Tech Combustion Lab, and to compare three Large Eddy Simulation (LES) combustion models (Eddy Break-Up [EBU], Steady Flamelet [SF] and Linear Eddy Model [LEM]) for non-premixed combustion. The nominal operating condition of the SPRF combustor achieves very low NOx and CO emissions by combining turbulent mixing of exhaust gases with preheated reactants and chemical kinetics. The SPRF numerical simulation focuses on capturing the complex interaction between turbulent mixing and heat release. LES simulations have been carried out for a non-reactive case in order to analyze the turbulent mixing inside the combustor. The LES results have been compared to PIV experimental data and the code has been validated. The dominating features of the operational mode of the SPRF combustor (dilution of hot products into reactants, pre-heating and pre-mixing) have been analyzed, and results from the EBU-LES, SF-LES and LEM-LES simulations have been compared. Analysis shows that the LEM-LES simulation achieves the best agreement with the observed flame structure and is the only model that captures the stabilization processes observed in the experiments. EBU-LES and SF-LES do not predict the correct flow pattern because of the inaccurate modeling of sub-grid scale mixing and turbulence-combustion interaction. Limitations of these two models for this type of combustor are discussed.

A novel combustor design, referred to as the Stagnation Point Reverse-Flow (SPRF) combustor, was recently developed that is able to operate stably at very lean fuel-air mixtures and with low NOx emissions even when the fuel and air are not premixed before entering the combustor. The primary objective of this work is to elucidate the underlying physics behind the excellent stability and emissions performance of the SPRF combustor. The approach is to experimentally characterize velocities, species mixing, heat release and flame structure in an atmospheric pressure SPRF combustor with the help of various optical diagnostic techniques: OH PLIF, chemiluminescence imaging, PIV and Spontaneous Raman Scattering. Results indicate that the combustor is primarily stabilized in a region downstream of the injector that is characterized by low average velocities and high turbulence levels; this is also the region where most of the heat release occurs. High turbulence levels in the shear layer lead to increased product entrainment levels, elevating the reaction rates and thereby enhancing the combustor stability. The effect of product entrainment on chemical timescales and the flame structure is illustrated with simple reactor models. Although reactants are found to burn in a highly preheated (1300 K) and turbulent environment due to mixing with hot product gases, the residence times are sufficiently long compared to the ignition timescales such that the reactants do not autoignite. Turbulent flame structure analysis indicates that the flame is primarily in the thin reaction zones regime throughout the combustor, and it tends to become more flamelet like with increasing distance from the injector. Fuel-air mixing measurements in case of non-premixed operation indicate that the fuel is shielded from hot products until it is fully mixed with air, providing nearly premixed performance without the safety issues associated with premixing. The reduction in NOx emissions in the SPRF combustor are primarily due to its ability to stably operate under ultra lean (and nearly premixed) condition within the combustor. Further, to extend the usefulness of this combustor configuration to various applications, combustor geometry scaling rules were developed with the help of simplified coaxial and opposed jet models.

Le réacteur à inversion de flux pour retenir et recycler la chaleur est une technologie efficace qui intègre la réaction chimique et la réutilisation de la chaleur dégagée lors de la réaction. Cette technologie a été considérablement commercialisée et industrialisée, vue l'amélioration de la productivité du réacteur et vue l'intensification du procédé qu'elle offrirait. Par analogie chaleur-matière, il a été voulu, par l'actuel travail de doctorat, évaluer la technologie d'inversion de flux et la combinaison de la réaction chimique et de la séparation et la rétention du catalyseur (matière) dans un seul réacteur adsorbeur multifonctionnel. La métathèse des oléfines qui utilise des catalyseurs moléculaires -qui ne sont pas/ ne peuvent pas être efficacement et parfaitement immobilisés et hétérogénéisés sur un support solide- peut être réalisée dans le réacteur adsorbeur à inversion de flux, qui prétend permettre de séparer, de recycler et de retenir les catalyseurs. La réaction cible dans cette thèse est l'homo-métathèse de l'oléate de méthyle. D'abord, un modèle cinétique pour décrire cette réaction et la transformation du réactif a été développé et proposé en suivant une approche micro-cinétique. Ensuite, ce modèle a été ajusté par rapport aux résultats expérimentaux pour optimiser et accéder aux paramètres cinétiques de la réaction. Le modèle cinétique, ainsi obtenu, permettra après son intégration dans le modèle du réacteur adsorbeur à inversion de flux, de prédire la conversion du réactif et sa concentration à la sortie. Le réacteur adsorbeur à inversion de flux, a été étudié et évalué, par la modélisation et par une étude théorique calculatoire, pour avoir une meilleure compréhension de son comportement, et aussi de l'influence des conditions opératoires sur le procédé (perte de catalyseur, conversion, productivité, etc.). Le design de réacteur qui a été proposé dans cette étude et ce travail est une colonne à lit fixe d'adsorbant, avec inversion de flux. Les résultats de simulation du réacteur adsorbeur à inversion de flux pour retenir et recycler les catalyseurs moléculaires ont montré l'intensification du procédé que peut offrir cette technologie en se référant à un réacteur continu conventionnel (réacteur tubulaire monophasique). Il a été montré aussi qu'il n'est pas possible d'atteindre un régime permanent, sans appoint et rajout du catalyseur pour compenser les pertes, dues aux phénomènes physiques indissociables au réacteur. Par la suite, un dispositif expérimental a été construit pour vérifier et valider les résultats de simulations ainsi obtenus, et aussi pour démonter, à l'échelle pilote, la faisabilité de la technologie d'inversion de flux avec adsorption pour séparer l'adsorbé et le retenir à l'intérieur de la colonne. Et similairement au réacteur à inversion de flux pour le recyclage et la réutilisation de la chaleur, le réacteur adsorbeur à inversion de flux pour séparer et recycler le catalyseur, peut lui aussi, avoir un comportement asymptotique sous certaines conditions opératoires, et s'approcher du fonctionnement du réacteur adsorbeur à contre-courant. Par la modélisation et le calcul numérique, il a été établi le domaine des conditions opératoires dans lesquelles, les réacteurs adsorbeurs à inversion de flux et à contre-courant sont équivalents. Le modèle asymptotique à contre-courant permet de calculer et de pré-dimensionner plus rapidement le réacteur à inversion de flux / The reverse flow reactor for heat trapping is an efficient technology that integrates the chemical reaction and the recovery of the heat of the reaction. This technology was widely commercialized and applied in industry because of the reactor productivity enhancement and the process intensification it offers. By heat-matter analogy, we wanted, in this thesis, to evaluate the reverse flow technique and the combination of the chemical reaction with the trapping and the recycling of the catalyst (matter) in one single multi-functional adsorber reactor. The metathesis of olefins that uses molecular catalysts -which are imperfectly immobilized and heterogeneized on the solid support- can be performed in the reverse flow adsorber reactor that claims to separate, recycle and trap the catalyst. The targeted reaction is the self-metathesis of methyl oleate. Thus, a kinetic model of the reaction was developed using a micro-kinetic approach. The obtained kinetic model was fitted to the experiences to get the kinetic parameters values. Then, the kinetic model can be integrated in the reverse flow adsorber reactor model to predict the conversion and the outlet methyl oleate concentration. The reverse flow reactor adsorber was evaluated and studied (by modeling and theoretical study) to have a better understanding of its behavior, and of the operating parameters influence on the process (catalyst leaching, conversion, productivity, etc.). A single fixed bed adsorption column is proposed as a design for the reverse flow adsorber reactor. The simulation results show the process intensification that offers the reverse flow adsorber reactor for catalyst trapping in comparison with a conventional continuous reactor (continuous flow tubular reactor). They also show that it is not possible to reach a stable operation and a permanent regime without catalyst makeup that compensate the leaching. Then, an experimental setup was built to verify, to validate the simulations results, and to demonstrate, at the pilot scale, the feasibility of the reverse flow technology to separate and to trap the adsorbate inside the adsorber. And similarly to the reverse flow reactor for heat trapping, the reverse flow adsorber reactor for catalyst trapping and separation can have an asymptotic behavior under certain operating conditions, and approaches the operation of a counter current adsorber reactor. By modeling and numerical calculation, it has been determined the operating conditions, at which, the reverse flow and the counter current adsorber reactors are equivalents. The counter current asymptotic model allow a rapid reverse flow reactor computing and pre-design