No description available.

Mixing in stirred tanks influences conversion of reactants for fast reactions, and the efficiency of a mixing process can be determined from the power consumption and mixing time, which are the two parameters that define homogenization energy. In this study, the computational fluid dynamics (CFD) and laser Doppler velocimetry (LDV) techniques were employed to study the effect of the Rushton turbine bottom clearance on the flow field, mixing time and power consumption in a stirred tank. Experimental and simulation studies were conducted in a tank with and without a draft tube where a conductivity meter and decolourization methods were employed in validating the mixing time simulation results. A good agreement between the experimental and simulation results for the flow field and mixing time was obtained. The results showed a reduction in mixing time and power consumption at a low impeller clearance, with reference to the standard clearance, and a further reduction of the same parameters was obtained for a system fitted with a draft tube. At the low clearance, there was an increase in mixing efficiency by 46%, for a system without draft tube and 61% for that with the draft tube.

Particle distribution and settling in suspensions with non-Newtonian liquids agi- tated with a Rushton turbine in a lab-scale tank have been studied. The rheology of the non-Newtonian liquids can be described by the power-law, Bingham and Herschel-Bulkley fluid models. The dynamics of the dispersed phase settling particles (size 0.65mm) is modeled by a Lagrangian tracking approach while the liquid phase is resolved by the lattice-Boltzmann method. Qualitative insight emerging from exploration of shear-thinning/thickening, New- tonian, yield-stress fluid models at Reynolds number, Re=6 103 , 8.5 103 and 1.25 104 indicate that bottom particle concentration is highest in power-law liq- uids than in Newtonian; while yield stress fluids had more uniform particle con- centration and least bottom concentration. Also, turbulent kinetic energy and vis- cous dissipation are highest in the Newtonian liquid. Extra viscous diffusion due to fluctuating non-Newtonian viscosity in the turbulent kinetic energy equation attributes to these differences. / Chemical Engineering

No description available.

Sensitivity analysis was performed on a soot model using a partially stirred reactor (PaSR) in order to determine the effects of mixing model parameters on soot scalar values. The sensitivities of the mixture fraction zeta and progress variable C to the mixing model constant C_phi were calculated; these values were used to compute the sensitivity of water mass fraction Y_H2O to C_phi and several soot quantities to soot moments. Results were validated by evaluating the mean mixture fraction sensitivity and a long simulation time case. From the baseline case, it was noted that soot moment sensitivities tended to peak on the rich side of the stoichiometric mixture fraction zeta_st. Timestep, number of notional particles, mixing timescale tau_mix, and residence time tau_res were varied independently. Choices for timestep and notional particle count were shown to be sufficient to capture relevant scalar profiles, and did not greatly affect sensitivity calculations. Altering tau_mix or tau_res was shown to affect sensitivity to mixing, and it was concluded that the soot model is more heavily influenced by the chemistry than mixing. / text

The hydrodynamics of an agitated tank have been studied byphase-Doppler anemometry. The focus is on the impeller anddischarge region of a 45o pitched blade turbine (PBT). Thestudy includes agitation of pure water as well as of a dilutesuspension of process particles. A three-dimensionalphase-Doppler anemometer is used to measure local,instantaneous, three-dimensional velocities of the fluid and ofthe suspended particles. A shaft encoding technique is used toresolve the turbulent fluctuations from the periodic velocityfluctuation due to the impeller blades, and to provide moredetailed information about the variations relative to theimpeller blade. Velocity bias is corrected for by the total3-dimensional velocity. The mean flow field, the fluctuating velocities, and thecomplete Reynolds stress tensor, are reported for the liquidphase flow. The periodic fluctuations in the flow that aregenerated by the impeller blades are eliminated in theexamination of the turbulence. The anisotropy of the turbulenceis assessed by the invariants of the anisotropy tensor. Thetrailing vortex structure is demonstrated to be associated withhigh kinetic energy and strong anisotropy of the turbulence.The vortex is still observable 130-140 degrees behind theblade. It gradually moves down from the impeller blade but thelocation in radial direction remains essentially unchanged. Theinfluence of the periodic fluctuations is examined and it isshown that the turbulence appears more isotropic when theperiodic fluctuations are not eliminated. The solid particle concentration is low below the impellerand is high above the impeller tip. The particles diverge fromthe liquid flow mean direction, especially below the agitatorclose to the tip where the strongest turbulence is found.Periodic fluctuations in the particle concentration relate tothe variations found in the angle-resolved mean velocity andfluctuating velocity. The ratio of the maximum to the minimumconcentration is about 2.0 in the present study. The baffles influence on the conditions in the impellerregion, and this influence can be observed on the fluid meanvelocity field, the angle-resolved velocities, the kineticenergy, and on the behavior of larger process particles. In theimpeller region the highest kinetic energies are about 15%higher upstream of the baffle than at the middle plane betweenthe baffles. The highest energy level in the middle plane isactually the lowest value and is therefore not representativewhen rotation symmetry is assumed. Local energy dissipation rates have been investigated, andthe integration of the local energy dissipation rates overdifferent control volumes has been compared with macroscopicenergy balance calculations. The discrepancy is significant.Different reasons have been analyzed and recommendations forfurther investigation are given. I n the outflow region there is a significant variation alsoin the direction of the instantaneous velocity, which may leadto direction bias in the case of non-spherical measurementvolume. In order to account for this direction bias, amathematical model is developed to estimate the projected areaof the measurement volume in LDA or PDA. It is shown that theprojected area variation can lead to a significant directionbias in determination of time averaged values and localparticle concentration in a highly turbulent stirred tank flow.This bias is however negligible for an orthogonal optical setup, as is used in the present study. <b>Keywords:</b>Hydrodynamics, phase-Doppler anemometer,suspension, pitched-blade turbine, anisotropy, turbulence,Reynolds stresses, trailing vortex, kinetic energy, stirredtank

<p>The hydrodynamics of an agitated tank have been studied byphase-Doppler anemometry. The focus is on the impeller anddischarge region of a 45o pitched blade turbine (PBT). Thestudy includes agitation of pure water as well as of a dilutesuspension of process particles. A three-dimensionalphase-Doppler anemometer is used to measure local,instantaneous, three-dimensional velocities of the fluid and ofthe suspended particles. A shaft encoding technique is used toresolve the turbulent fluctuations from the periodic velocityfluctuation due to the impeller blades, and to provide moredetailed information about the variations relative to theimpeller blade. Velocity bias is corrected for by the total3-dimensional velocity.</p><p>The mean flow field, the fluctuating velocities, and thecomplete Reynolds stress tensor, are reported for the liquidphase flow. The periodic fluctuations in the flow that aregenerated by the impeller blades are eliminated in theexamination of the turbulence. The anisotropy of the turbulenceis assessed by the invariants of the anisotropy tensor. Thetrailing vortex structure is demonstrated to be associated withhigh kinetic energy and strong anisotropy of the turbulence.The vortex is still observable 130-140 degrees behind theblade. It gradually moves down from the impeller blade but thelocation in radial direction remains essentially unchanged. Theinfluence of the periodic fluctuations is examined and it isshown that the turbulence appears more isotropic when theperiodic fluctuations are not eliminated.</p><p>The solid particle concentration is low below the impellerand is high above the impeller tip. The particles diverge fromthe liquid flow mean direction, especially below the agitatorclose to the tip where the strongest turbulence is found.Periodic fluctuations in the particle concentration relate tothe variations found in the angle-resolved mean velocity andfluctuating velocity. The ratio of the maximum to the minimumconcentration is about 2.0 in the present study.</p><p>The baffles influence on the conditions in the impellerregion, and this influence can be observed on the fluid meanvelocity field, the angle-resolved velocities, the kineticenergy, and on the behavior of larger process particles. In theimpeller region the highest kinetic energies are about 15%higher upstream of the baffle than at the middle plane betweenthe baffles. The highest energy level in the middle plane isactually the lowest value and is therefore not representativewhen rotation symmetry is assumed.</p><p>Local energy dissipation rates have been investigated, andthe integration of the local energy dissipation rates overdifferent control volumes has been compared with macroscopicenergy balance calculations. The discrepancy is significant.Different reasons have been analyzed and recommendations forfurther investigation are given. I</p><p>n the outflow region there is a significant variation alsoin the direction of the instantaneous velocity, which may leadto direction bias in the case of non-spherical measurementvolume. In order to account for this direction bias, amathematical model is developed to estimate the projected areaof the measurement volume in LDA or PDA. It is shown that theprojected area variation can lead to a significant directionbias in determination of time averaged values and localparticle concentration in a highly turbulent stirred tank flow.This bias is however negligible for an orthogonal optical setup, as is used in the present study.</p><p><b>Keywords:</b>Hydrodynamics, phase-Doppler anemometer,suspension, pitched-blade turbine, anisotropy, turbulence,Reynolds stresses, trailing vortex, kinetic energy, stirredtank</p>

ABSTRACT Stirred yogurt is a cultured dairy product produced by mixed cultures of lactic acid bacteria. It is a semi-viscous liquid whose rheological properties are major quality determinants. These are influenced by several manufacturing factors such as heat treatment of the yogurt milk. Improvement of the properties of stirred yogurt made under different manufacturing conditions was the key focus in this research. The research included an investigation of the key factors involved in the development of nodulation which is an unsightly defect, as consumers expect stirred yogurt to have a smooth consistency. This research consisted of three main experimental parts: 1. Effect of different heat treatments on the properties of stirred yogurt; 2. Factors involved in nodulation in stirred yogurt; and 3. The physical and chemical nature of nodules. The types and levels of heat treatment have a major influence on the properties of stirred yogurt. In this study, yogurt milk was treated at pasteurization and UHT conditions. The milk heated at pasteurization, 80-95oC, and UHT, 130-145oC, conditions had similar levels of whey protein denaturation, approximately 85-99%, while the milk treated at low temperature for a long time, 65oC for 4 hr, had a significantly lower level of denaturation (~55%). Yogurt made from milk pasteurized at 85oC for 30 min and 92oC for 7 min and UHT treated milk at 130-145oC for 5 s had similar hardness and viscosity while yogurt made from milk treated at 65oC for 4 hr had significantly lower hardness and viscosity than corresponding yogurts from high heat treatments. In addition, the water-holding capacity and syneresis of yogurts made from either pasteurized or UHT milk were not significantly different. Yogurt (made from either pasteurized or UHT-treated milk) enriched with non-dairy ingredients, gelatin, inulin and sugar, showed higher hardness, viscosity, water-holding capacity than yogurt made with only dairy ingredients. These yogurts also showed no syneresis. This can be largely attributed to the gelatin which improves the texture, binds additional water and prevents syneresis. The formation of nodules in yogurt has been reported to be influenced by several factors. In this study, the heat treatment of the yogurt milk, the types and levels of sugar added, and the type of starter cultures were found to significantly influence the level of nodulation in the stirred yogurts. Severe pasteurization heat treatments, at temperatures < 100ºC, caused more nodules than mild heat treatments at temperatures in this range. The type of heat treatment was also important, with yogurt made from UHT-treated milk showing much less nodulation than yogurt made from pasteurized. This was observed when the levels of whey protein denaturation in the pasteurized and UHT milk were similar, indicating that the extent of whey protein denaturation alone is not a major factor in the development of nodules. Sucrose added either before or after heat treatment of yogurt milk also affected the extent of nodulation. Increasing the amount of added sucrose from 0 to 6.5% caused correspondingly higher numbers of nodules. In addition, there was a positive synergistic effect between heat treatment and sugar addition on nodulation; the highest numbers of nodules appeared when the yogurt milk was severely heated and sugar was added at the highest level, 6.5%. Addition of lactose or fructose instead of sucrose did not promote nodule formation while glucose caused nodulation in a similar manner to sucrose. The use of different starter cultures affected the numbers of nodules; yogurts made with the culture ABT 10 had much less nodulation than those made with ABT5 and ABT6, even under heating conditions and sugar addition conducive to nodule development. The amount of exopolysaccharide (EPS) produced by the starter cultures, ABT5 and ABT10, did not correlate with the extent of nodulation in the yogurt. Disturbance during yogurt fermentation by changing the temperature (from 37 to 42 or 45 to 42oC) when the gel was forming increased the numbers of nodules while refrigerated storage of yogurt and altering the pH during heat treatment (from pH 6.46 to 6.90) did not affect the number of nodules. Ultrasonication of the yogurt milk caused a reduction in the numbers of nodules in the yogurt. This was attributed to disruption of clumps of starter culture bacteria, thus preventing excessive localised build-up of acid around the bacterial clumps which could form the nucleus of nodules. The major conclusions from this study are that heat treatment of the yogurt milk and the level and type of added sugar are important factors affecting the extent of nodulation of stirred yogurt. Other factors such as the type of starter culture bacteria and their degree of clumping are also significant. Optimisation of these factors would allow yogurt manufacturers to minimise nodulation in stirred yogurt.

A better understanding and control of internal combustion engine pollutants require more insightful investigation of gasoline oxidation chemistry. An oxidation study has been done on n-heptane, iso-octane, their binary mixtures (Primary Reference Fuel, (PRF)), and nine hydrocarbon mixtures which represent the second generation of gasoline surrogates (multi-component surrogates). This study aims to develop a better understanding of the combustion reaction by studying the oxidation reaction of different fuels inside a jet-stirred reactor and numerically simulating the reaction using different models under the following conditions: pressure 1 bar, temperature 500-1050K, residence time 1.0 and 2.0s, and two fuel-to-oxygen ratios (ϕ=0.5 and 1.0). Intermediate and product species mole fractions versus temperature profiles were experimentally measured using a GC (gas chromatograph). The experiment was performed within the high and low-temperature regions, where the high-temperature oxidation showed similar behavior for different compositions but the low-temperature oxidation showed significant dependence on the composition of the surrogates. Additionally, the effect of octane number on oxidation chemistry has been investigated and it was found that the low octane number surrogates were more reactive than high octane number surrogates during the low temperature regime. Furthermore, Kinetic analysis was conducted to provide insightful understanding of different factors of fuel reactivity. In addition, the pyrolysis of two TPRF, (Toluene primary reference fuels) mixtures (TPRF70 and TPRF97.5), representing low octane (research octane number 70) and high octane (research octane number 97.5) gasoline, was also studied in jet-stirred reactor coupled with gas chromatography (GC) analysis to investigate the formation of soot and polycyclic aromatic hydrocarbons (PAH) formation.

In the first part of this work, instability is correctly predicted for a Rijke tube with a new two-term acoustic forcing term derived from a one-dimensional flame dynamics model. The new two-term acoustic forcing term, which is comprised of the summation of chemical heat release rate and heat transfer due to convection, correctly predicts instability where older models of acoustic forcing based solely on chemical heat release rate incorrectly predicted stability. This stability analysis correctly predicts the inlet conditions of the instability in addition to the frequency of instability. In the second part of this work, networks of dynamic well-stirred reactors are used to model qualitative behavior observed in turbulent combustion. First a model of dynamic well-stirred reactor is derived, and then several reactors are coupled together by recirculation. The dynamics of the various models are computed and assessed. The models exhibit interesting behavior that has been viewed experimentally including hysteresis and peaking in the dynamic response. / Master of Science