MSCAGR / Department of Agricultural Economics and Agribusiness / The study was conducted in the Mutale Local Municipality, Vhembe District of South Africa on a proportionally randomly selected sample of 153 smallholder farmers after clustering them into agricultural zones and commodity groupings (vegetables under irrigation, dryland maize and citrus fruit farming). Data were collected through a structured qualitative and quantitative questionnaire that was administered face-to-face to respondents and captured into the SPSS Version 24 computer program. The same program was used to analyse data through cross tabulations and logistic regression modelling. In particular, the study focussed on the impact of socio-economic characteristics, challenges that farmers face and views of extension officers on transforming subsistence farmers towards commercialization. The most critical findings of the study were dominance of women, lower youth participation, poor training and educational achievements, non-membership to agricultural organizations, low income levels and dependence on social grants and lack of credit as factors that could impact on farmers’ transformation process. Farmers’ challenges that could impact on transformation were identified as lack of production inputs, water, access to market and supportive infrastructure such as mechanization. However, the views of extension officers regarding transformation centred mostly around insufficient land holdings, climate change and financial support. In contrast to farmers, extension officers viewed market access as a minor challenge. The study recommended for development of strategies that could increase youth participation in farming such as start-up credit, reduction of dependence on social grants by adopting strategies that could increase productivity and thus income, exposure to funding opportunities through training and increased involvement of institutions of higher learning into smallholder farming activities.

Une modélisation de la combustion turbulente est proposée avec une chimie détaillée et des PDFs, afin de quantifier les concentrations des espèces chimiques (dont le CO). Le modèle PCM-FTC, Presumed Conditional Moment – Flame Tabulated Chimistry, est basé sur des méthodes de tabulations de flammes laminaires prémélangées et de diffusion pour prédire les phénomènes de prémélange partiel présents dans les foyers aéronautiques. La méthode de tabulation est basée sur une analyse de la structure chimique des flammes laminaires prémlélangées et de diffusion. Les hypothèses formulées dans cette modélisation sont testées et validées. Le modèle est alors implanté dans un logiciel de simulation des écoulements turbulents. Trois configurations sont retenues pour quantifier le pouvoir prédicteur de ce type d'approche : les flammes jets turbulentes de Sandia (D et F) et les flammes suspendues de méthane/air de Stanford. Les comparaisons numériques et expérimentales ont permis de valider la méthode.

As intra-industry trade increases in U.S. agricultural and food processing industries, the historical agricultural trade surplus is tightening. In efforts to maintain the trade surplus a focus has shifted towards the promotion of agricultural and processed food exports among small and medium sized firms. This study intends to identify and evaluate the potential for exports among small to medium sized agricultural and food processing firms in Kentucky through a collection of survey data. The objectives of this thesis are to identify the state’s product marketing opportunities and product specifications for international exports while identifying transaction requirements for potential exports. An analysis of the constraints and challenges faced by firms in the decision to export reveals rational behavior Binary logistic regression analysis is used to identify the impact of firm characteristics, perceived marketing conditions and information constraints, and financial aspects on a firm’s decision to export. A second logit regression analyzes the impact on a non-exporting firm’s interest in international marketing opportunities. The lack of international market information, financial constraints, and risk are found to be significant factors in the decision to export and interest in foreign marketing.

Les responsabilités environnementales font que les constructeurs automobiles visent à acquérir des connaissances approfondies sur les phénomènes physico-chimiques des chambres de combustion des moteurs pour la compréhension et le contrôle des émissions polluantes. En dépit des performances remarquables dans le domaine du calcul numérique intensif de très haute performance, les simulations numériques des chambres de combustion des moteurs à combustion interne ne permettent pas encore une description détaillée des processus chimiques. L'objectif de ces travaux est d'améliorer les modèles de combustion turbulente basés sur la Simulation aux Grandes Echelles en développant une méthode de tabulation de la cinétique chimique représentative de la combustion dans un cycle moteur. Une attention toute particulière est portée sur la capacité de la méthode de tabulation à reproduire la formation des espèces polluantes lors des phases de compression, d'auto-allumage et de détente d'un moteur à combustion interne. Cette méthode suppose que la chimie dans une chambre de combustion interne peut être approchée par une tabulation des résultats issus de calculs de réacteurs isochores 0-D, les coordonnées de la table étant: la variable de progrès, l'énergie et la masse volumique. La capacité prédictive de la technique de tabulation a été testée par des simulations d'auto-allumage en volume variable. La méthode de tabulation "Tabulated Thermo Chemistry" (TTC) a été initialement développée pour coupler la cinétique chimique à des solveurs Navier-Stokes compressibles dans des situations d'écoulement à faible nombre de Mach. Dans ce travail, la méthode TTC a été adaptée pour la combustion à pression variable. Une version TTC dédiée à la combustion dans les moteurs est donc implantée dans un code instationnaire LES compressible. La méthode de couplage a été testée avec succès tant sur des configurations simplifiées que sur une Machine à Compression Rapide.

Les responsabilités environnementales font que les constructeurs automobiles visent à acquérir des connaissances approfondies sur les phénomènes physico-chimiques des chambres de combustion des moteurs pour la compréhension et le contrôle des émissions polluantes. En dépit des performances remarquables dans le domaine du calcul numérique intensif de très haute performance, les simulations numériques des chambres de combustion des moteurs à combustion interne ne permettent pas encore une description détaillée des processus chimiques. L’objectif de ces travaux est d’améliorer les modèles de combustion turbulente basés sur la Simulation aux Grandes Echelles en développant une méthode de tabulation de la cinétique chimique représentative de la combustion dans un cycle moteur. Une attention toute particulière est portée sur la capacité de la méthode de tabulation à reproduire la formation des espèces polluantes lors des phases de compression, d’auto-allumage et de détente d’un moteur à combustion interne. Cette méthode suppose que la chimie dans une chambre de combustion interne peut être approchée par une tabulation des résultats issus de calculs de réacteurs isochores 0-D, les coordonnées de la table étant: la variable de progrès, l’énergie et la masse volumique. La capacité prédictive de la technique de tabulation a été testée par des simulations d’auto-allumage en volume variable. La méthode de tabulation "Tabulated Thermo Chemistry" (TTC) a été initialement développée pour coupler la cinétique chimique à des solveurs Navier-Stokes compressibles dans des situations d’écoulement à faible nombre de Mach. Dans ce travail, la méthode TTC a été adaptée pour la combustion à pression variable. Une version TTC dédiée à la combustion dans les moteurs est donc implantée dans un code instationnaire LES compressible. La méthode de couplage a été testée avec succès tant sur des configurations simplifiées que sur une Machine à Compression Rapide. / Due to environmental concerns, automotive manufacturers aim at acquiring knowledge of physical and chemical phenomena inside the combustion chamber to understand and control pollutant emissions. Despite the outstanding performances in the domain of high performance intensive numerical calculation, the numerical simulations of the combustion chambers of the internal combustion engines do not allow a detailed description of the chemical processes. The present study aims to improve the turbulent combustion models based on Large Eddy Simulation approach by developing an efficient cost cutting tabulation method to fit chemistry in engine combustion modeling. A particular attention is paid to the capacity of the tabulation method to reproduce the pollutant species formation during the compression stroke, the reaction phase, and the power stroke of an ICE. This method assumes that IC engine chemistry can be mapped by a collection of 0-D reactor computation data, using for coordinates: the progress variable, the energy and the density. In a first step, the chemical prediction capability of this technique is validated on an auto-igniting variable volume simulation. The Tabulated Thermo Chemistry (TTC) method was initially developed to couple the chemical kinetics with compressible Navier-Stokes solvers for low Mach number flows. In this work, the TTC method was adapted in the framework of variable pressure combustion. A TTC version dedicated to engine combustion is thus coupled in an unsteady compressible LES code. The method of coupling was successfully tested both on simplified configurations and on a Rapid Compression Machine.

Cette thèse se situe dans le cadre du projet CANOE, piloté par GDF SUEZ et l'ADEME, qui vise à étudier la faisabilité et la viabilité du régime de combustion sans flamme dans les chaudières industrielles. Il est maintenant établi que le préchauffage des réactifs permet d'améliorer le rendement thermique et de diminuer la consommation de combustible d'une configuration. Pour contourner la formation d'oxydes d'azote résultant de l'augmentation de température des réactifs, ces derniers peuvent être massivement dilués par des produits de combustion. Cela permet d'éviter la formation de points chauds et d'homogénéiser les gradients de température: c'est le principe de la combustion sans flamme. L'objectif de cette thèse est de développer un modèle de combustion turbulente adapté à ce type de régime. La cinétique chimique complexe et le contrôle des pertes thermiques est d'une importance capitale dans l'établissement et la stabilisation du processus de combustion sans flamme. Ici, ces effets sont considérés dans une approche de tabulation de la cinétique chimique de type FPV. Pour discriminer les effets associés aux évolutions suivant les paramètres de contrôle de nos bases de données, on effectue une analyse des réponses de flammes laminaires à différents niveaux de dilution et de pertes thermiques. De plus, nous évaluons l'importance de l'utilisation de tabulations de dimensions élevées, et les capacités prédictives des méthodes de tabulation développées sont mises en exergue. Puis, des simulations aux grandes échelles de la turbulence de configurations adiabatique et à parois refroidies sont effectuées. On compare des tabulations de nombre de degrés de libertés variés aux données expérimentales. Les résultats obtenus sont en très bon accord avec ces dernières pour les tabulations les plus complexes, alors que des limitations significatives apparaissent pour des tabulations de dimensions inférieures. Les simulations proposées indiquent la capacité de nos modèles à reproduire des structures de flammes réalistes. / This thesis is within a framework of the CANOE project, under the responsibility of GDF SUEZ and the ADEME, and aims at studying the feasibility of the flameless combustion regime in industrial boilers. It is now well established that reactants preheating allows an improvement on thermal efficiency as well as fuel savings. To avoid an increase in nitrogen oxides emissions arising from reactants temperature augmentation, massive dilution of the reactants by burnt gases can be used. While doing so, hot spots are averted and temperature gradients are smoothed: it’s the principle of flameless combustion. Even though this combustion regime is a subject of increasing interest to the industry, its mechanisms are not yet fully understood. The objective of this work is to develop and validate a new turbulent combustion model adapted to this kind of regimes. Complex chemistry as well as heat losses control is of paramount importance in the establishment and stabilization of the flameless combustion process. Here, these effects are taken into account in an FPV-type chemistry tabulation approach. To discriminate the effects associated with evolutions along the database control parameters, we analyze laminar flame responses to different levels of dilution and heat losses. Moreover, we assess the importance of high-order tabulations, and predictive capabilities of our approaches are highlighted. Then, large-eddy simulations of laboratory scale experiments (an adiabatic and a cooled wall configuration) are carried out. To this end, we compare databases featuring different number of dimensions to experimental data on temperature and species distributions available in the literature. Simulation results are in very good agreement with experimental data for complex tabulations, while discrepancies arise for lower order tabulations. Simulation results show that flameless combustion features a wide variety of flame structures, and that our models are able to tackle realistic flame structures.

The main aim of my thesis is to compare results of different groups of students who took part in the statistic course on VŠE. The thesis is dividend into two parts. The first part is focused on theoretical solution of used data comparison. The second part is practical and it is focused on results comparison from different angles with descriptive statistics and some statisistics tests. The results from partial comparison of data from different points of view are summarized at the end of the thesis.

Two new models to calculate the species instantaneous and filtered reaction rates for multi-step, multi-species chemical kinetics mechanisms are developed based on the artificial neural networks (ANN) approach. The proposed methodologies depend on training the ANNs off-line on a thermo-chemical database representative of the actual composition and turbulence level of interest. The thermo-chemical database is constructed by stand-alone linear eddy mixing (LEM) model simulations under both premixed and non-premixed conditions, where the unsteady interaction of turbulence with chemical kinetics is included as a part of the training database. In this approach, the information regarding the actual geometry of interest is not needed within the LEM computations. The developed models are validated extensively on the large eddy simulations (LES) of (i) premixed laminar-flame-vortex-turbulence interaction, (ii) temporally mixing non-premixed flame with extinction-reignition characteristics, and (iii) stagnation point reverse flow combustor, which utilizes exhaust gas re-circulation technique. Results in general are satisfactory, and it is shown that the ANN provides considerable amount of memory saving and speed-up with reasonable and reliable accuracy. The speed-up is strongly affected by the stiffness of the reduced mechanism used for the computations, whereas the memory saving is considerable regardless.

Large-scale processes that are modeled using differential algebraic equations based on mass and energy balance calculations at times require excessive computation time to simulate. Depending on the complexity of the model, these simulations may require many iterations to converge and in some cases they may not converge at all. Application of a storage and retrieval technique, named in situ adaptive tabulation or ISAT is proposed for faster convergence of process simulation models. Comparison with neural networks is performed, and better performance using ISAT for extrapolation is shown. In particular, the requirement of real-time dynamic simulation is discussed for operating training simulators (OTS). Integration of ISAT to a process simulator (CHEMCAD®) using the input-output data only is shown. A regression technique based on partial least squares (PLS) is suggested to approximate the sensitivity without accessing the first-principles model. Different record distribution strategies to build an ISAT database are proposed and better performance using the suggested techniques is shown for different case studies. A modified ISAT algorithm (mISAT) is described to improve the retrieval rate, and its performance is compared with the original approach in a case study. State estimation is a key requirement of many process control and monitoring strategies. Different nonlinear state estimation techniques studied in the past are discussed with their relative advantages/disadvantages. A robust state estimation technique like moving horizon estimation (MHE) has a trade-off between accuracy of state estimates and the computational cost. Implementation of MHE based ISAT is shown for faster state estimation, with an accuracy same as that of MHE. Flowsheet optimization aims to optimize an objective or cost function by changing various independent process variables, subject to design and model constraints. Depending on the nonlinearity of the process units, an optimization routine can make a number of calls for flowsheet (simulation) convergence, thereby making the computation time prohibitive. Storage and retrieval of the simulation trajectories can speed-up process optimization, which is shown using a CHEMCAD® flowsheet. Online integration of an ISAT database to solve the simulation problem along with an outer-loop consisting of the optimization routine is shown using the sequential-modular approach. / text

[pt] O desenvolvimento de modelos computacionais para simulação de escoamentos reativos operando em regime de turbulencia requer a soluçao das equações diferenciais parciais que representam os balanços de massa, quantidade de movimento linear, espécies químicas e energia. Além disso, as reações químicas do modelo necessitam de um mecanismo cinético detalhado para descrição dos fenomenos físico-químicos associados. Um dos maiores desafios encontrados é a rigidez da simulação numérica desses modelos e a natureza não linear do termo de produção das espécies químicas. Esta dissertação apresenta uma revisão das principais técnicas disponíveis na literatura para o desenvolvimento de modelos reduzidos de cinética química, em particular para a combustão, bem como de técnicas para solução eficiente dos modelos de escoamentos reativos. Após uma apresentação da formulação matemática associada, a metodologia denominada tabulação adaptativa in situ (ISAT) é implementada e avaliada quanto a sua acurácia, eficiencia e uso de memória na simulação de alguns modelos de reator homogeneo agitado. Avalia-se a combustão de misturas de monóxido de carbono/oxigenio e metano/ar cujos mecanismos cinéticos tem 4 e 53 espécies, 3 and 325 reações respectivamente. Os resultados destassimulações indicam que a presente implementação da técnica ISAT tem erro relativo global inferior a 1%. Além disso, a técnica ISAT propiciou ganhos, em termos de tempo computacional, de at´e 80% quando comparado a simulação direta da cinética detalhada. Entretanto, em termos de utilização da memória, a implementação desenvolvida da técnica ISAT se mostrou excessivamente exigente. / [en] The development of computational models for the numerical simulation of chemically reacting flows operating in the turbulent regime requires the solution of partial differential equations that represent the balance of mass, linear momentum, chemical species and energy. Moreover, the chemical reactions of the model may require a detailed reaction mechanism for the description of the physicochemical phenomena involved. One of the biggest challenges is the stiffness of the numerical simulation of these models and the nonlinear nature of species rate of reaction. This dissertation presents an overview of the main techniques available in the literature for the development of reduced models of chemical kinetics, particularly for the combustion, as well as the techniques for efficient computation of the chemically reacting flows models. After a presentation of the associated mathematical formulation, the methodology dubbed in situ adaptive tabulation (ISAT) is implemented and its accuracy, efficiency and memory usage are evaluated in the simulation of homogeneous stirred reactor models. The combustion of carbon monoxide with oxygen and methane with air mixtures is considered, which detailed reaction mechanisms involve 4 and 53 species, 3 and 325 reactions respectively. The results of these simulations indicate that the development implementation of the ISAT technique has a absolute global error of less than 1%. Moreover, the ISAT technique provided gains, in terms of computational time, of up to 80% when compared to the direct integration of the full chemical kinetics. However, in terms of memory usage the present implementation of ISAT technique was found to be excessively demanding.