published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

This thesis examines the transient response of a packed bed heat regenerator when heated from an initial uniform bed temperature. Very large (1700 K) temperature differences were studied as well as the effect of simultaneous chemical reaction in the gas phase. First the effects of temperature on physical and transport properties were studied in detail in the absence of a reaction. Models with compressible flow were compared with conventional models with constant properties and incompressible flow. Several measures of the regenerator's response to a step change in inlet gas temperature were calculated to characterize the spread of the temperature front. Variances of the spatial derivative of the gas temperature profile and the time derivative of the product gas temperature were used to evaluate thermal efficiency. The effects of an exothermic homogeneous gas phase reaction in the regenerator process were also studied. Several simple kinetic schemes and inlet conditions were simulated and the profiles of reaction rate and conversion as well as temperature were analyzed. / Graduation date: 1993

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Hydrocarbon reserves are generally produced through wells drilled into reservoir pay zones. During production, gas liberation from the oil phase occurs due to pressure decline in the wellbore. Thus, we expect multiphase flow in some sections of the wellbore. As a multi-phase/multi-component gas-oil mixture flows from the reservoir to the surface, pressure, temperature, composition, and liquid holdup distributions are interrelated. Modeling these multiphase flow parameters is important to design production strategies such as artificial lift procedures. A wellbore fluid flow model can also be used for pressure transient test analysis and interpretation. Considering heat exchange in the wellbore is important to compute fluid flow parameters accurately. Modeling multiphase fluid flow in the wellbore becomes more complicated due to heat transfer between the wellbore fluids and the surrounding formations. Due to mass, momentum, and energy exchange between the wellbore and the reservoir, the wellbore model should be coupled with a numerical reservoir model to simulate fluid flow accurately. This model should be non-isothermal to consider the effect of temperature. Our research shows that, in some cases, ignoring compositional effects may lead to errors in pressure profile prediction for the wellbore. Nearly all multiphase wellbore simulations are currently performed using the "black oil" approach. The primary objective of this study was to develop a non-isothermal wellbore simulator to model transient fluid flow and temperature and couple the model to a reservoir simulator called General Purpose Adaptive Simulator (GPAS). The coupled wellbore/reservoir simulator can be applied to steady state problems, such as production from, or injection to a reservoir as well as during transient phenomena such as well tests to accurately model wellbore effects. Fluid flow in the wellbore may be modeled either using the blackoil approach or the compositional approach, as required by the complexity of the fluids. The simulation results of the new model were compared with field data for pressure gradients and temperature distribution obtained from wireline conveyed pressure recorder and acoustic fluid level measurements for a gas/oil producer well during a buildup test. The model results are in good agreement with the field data. Our simulator gave us further insights into the wellbore dynamics that occur during transient problems such as phase segregation and counter-current multiphase flow. We show that neglecting these multiphase flow dynamics would lead to unreliable results in well testing analysis.

A comprehensive study of heat transfer and pressure drop of refrigerant R410A during condensation and supercritical cooling at near-critical pressures was conducted. Investigations were carried out at five nominal pressures: 0.8, 0.9, 1.0, 1.1 and 1.2xpcrit. The refrigerant was tested in commercially available horizontal smooth tubes of 6.2 and 9.4 mm I.D. Heat transfer coefficients were measured using a thermal amplification technique that measures heat duty accurately while also providing refrigerant heat transfer coefficients with low uncertainties. For condensation tests, local heat transfer coefficients and pressure drops were measured for the mass flux range 200 G 800 kg/m2-s in small quality increments over entire vapor-liquid region. For supercritical tests, local heat transfer coefficients and pressure drops were measured for the same mass flux range as in the condensation tests for temperatures ranging from 30 110oC. Condensation heat transfer coefficients and pressure drops increased with quality and mass flux. The effect of reduced pressure on heat transfer is not very significant, while this effect is more pronounced on the pressure gradient. The flow regime transition criteria of Coleman and Garimella (2003) were used to initially designate the prevailing flow regimes for a given combination of mass flux and quality. The condensation data collected in the present study were primarily in the wavy and annular flow regimes. During supercritical cooling, the sharp variations in thermophysical properties in the vicinity of the critical temperature resulted in sharp peaks in the heat transfer coefficients and sudden jumps in the pressure drop. Based on the characteristics of the specific work of thermal expansion (contraction), the data from the supercritical tests were grouped into three regimes: liquid-like, pseudo-critical transition and gas-like regimes. Flow regime-based heat transfer and pressure drop models were developed for both condensation and supercritical cooling. For condensation, the overall heat transfer model predicts 98% of the data within 15% while the overall pressure drop model predicts 87% of the data within 15%. For supercritical cooling, the heat transfer model predicted 88% of the data within 25% while the pressure gradient model predicts 84% of the data within 25%.

A study of heat transfer and pressure drop in zero ozone-depletion-potential (ODP) ‎refrigerant blends in small diameter tubes was conducted. The azeotropic refrigerant ‎blend R410A (equal parts of R32 and R125 by mass) has zero ODP and has properties ‎similar to R22, and is therefore of interest for vapor compression cycles in high-‎temperature-lift space-conditioning and water heating applications. Smaller tubes lead to ‎higher heat transfer coefficients and are better suited for high operating pressures.‎ Heat transfer coefficients and pressure drops for R410A were determined experimentally ‎during condensation across the entire vapor-liquid dome at 0.8, 0.9xPcritical and gas ‎cooling at 1.0, 1.1, 1.2xPcritical in three different round tubes (D = 3.05, 1.52, 0.76 mm) ‎over a mass flux range of 200 < G < 800 kg/m2-s. A thermal amplification technique was ‎used to accurately determine the heat duty for condensation in small quality increments ‎or supercritical cooling across small temperature changes while ensuring low ‎uncertainties in the refrigerant heat transfer coefficients. ‎ The data from this study were used in conjunction with data obtained under similar ‎operating conditions for refrigerants R404A and R410A in tubes of diameter 6.22 and ‎‎9.40 mm to develop models to predict heat transfer and pressure drop in tubes with ‎diameters ranging from 0.76 to 9.40 mm during condensation. Similarly, in the ‎supercritical states, heat transfer and pressure drop models were developed to account for ‎the sharp variations in the thermophysical properties near the critical point.‎ The physical understanding and models resulting from this investigation provide the ‎information necessary for designing and optimizing new components that utilize R410A ‎for air-conditioning and heat pumping applications.‎

La cuisson par contact direct est un mode de préparation des aliments très courant à travers le monde, cependant peu d’études s’intéressent à cette problématique à l’échelle domestique. Ces travaux tentent d’apporter une contribution à l’étude des phénomènes mis en jeu durant cette opération. Ce manuscrit débute par une revue de différents facteurs impliqués lors de la cuisson : les consommations d’énergie, les types d’appareils utilisés, les phénomènes physico-chimiques intervenant dans le produit ou les problématiques de modélisation y sont abordés. Il s’en dégage qu’une donnée essentielle est la connaissance du flux de chaleur transmis au produit. Une méthode d’estimation de ce flux basée sur les techniques inverses est développée. Celle-ci a contribué à concevoir un prototype instrumenté permettant de suivre les cinétiques des températures dans la plaque chauffante et dans un élastomère simulant un produit alimentaire. Les résultats obtenus montrent que la méthode permet d’estimer le flux de chaleur transmis avec une bonne précision. Dans un second temps, une étude expérimentale de la cuisson d’une pâte céréalière d’environ 8mm d’épaisseur est présentée. Après avoir caractérisé les propriétés thermophysiques et hydriques, le prototype est utilisé pour suivre les évolutions de différents paramètres comme les températures, le flux de chaleur, la masse en dynamique, et les teneurs en eau. La répétabilité et la variabilité des résultats suivant la température initiale de la plaque sont menés. Ensuite, un modèle 1D simulant les transferts de matière et d’énergie, est utilisé pour étudier les différents facteurs intervenant lors de la cuisson. Un second modèle 2D est réalisé permettant de tester les consommations d’énergie lors d’une opération de cuisson en cadence suivant différents scénarios de conception du prototype. / Direct contact cooking is a very common way of preparing foods throughout the world, but few studies are interested in this issue at the domestic scale. This work attempts to contribute to the study of the phenomena involved during this operation. This thesis begins with a review of the various factors involved in the cooking process: energy consumption, types of appliances used, physico-chemical phenomena implied in the product or modeling problems are discussed. It emerges from this that an essential fact is the knowledge of the heat flux transmitted to the product. A method for estimating this flux based on inverse techniques is developed. This has contributed to design an instrumented prototype allowing to follow the kinetics of the temperatures in the heating plate and in an elastomer simulating a food product. The results obtained show that the method makes it possible to estimate the heat flux transmitted with a good accuracy. In a second step, an experimental study of cooking of a 8 mm thick cereal batter is presented. After having characterized the thermophysical and hydric properties, the prototype is used to monitor changes in various parameters such as temperatures, heat flux, mass in dynamics, and water contents. The repeatability and the variability of the results according to the initial temperature of the plate are carried out. Then, a 1D model simulating mass and heat transfers is used to study the different factors involved in cooking. A second 2D model is realized to test the energy consumption during a cooking operation in cadence according to different prototype design scenarios.

In-situ vapor extraction is performed by applying a pressure differential across a hydrophobic porous membrane that forms a wall of the channel as a means of reducing the local quality of flow boiling within the channel. As the local quality is reduced, the heat transfer capability can be improve while large pressure drops and flow instability can be mitigated. The present study investigates the potential of vapor extraction, by examining the characteristics and mechanisms of extraction. The physics based models for transition among extraction regimes are developed which can be used as a basis for a regime-based vapor extraction rate model. The effects of vapor extraction on flow boiling in a microscale fractal-like branching network and diverging channels are studied by using a one-dimensional numerical model based on conservation of mass and energy, along with heat transfer and pressure drop correlations. The results show the improvement in reduced pressure drop and enhanced flow stability, and show the potential of heat transfer enhancement. / Graduation date: 2013 / Access restricted to the OSU Community at author's request from March 25, 2013 - March 25, 2014