Mark Martinez


Research Interests

Fluid mechanics
Complex fluids
Flow visualization

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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2019)
Characterising the non-ideal behaviour of a Carbopol gel flowing in thin conduits (2020)

This thesis examines three problems related to the slow flow of a viscoplastic fluidin a thin conduit in order to characterise unique effects of its complex rheology.The methodology is experimental and we use Carbopol exclusively as a modelyield stress fluid. The first part of this thesis is dedicated to exploring the flow behaviournear obstructions in a thin slot. We find that the fore-aft symmetry whichis expected theoretically to be broken. The asymmetry is robust, as demonstratedby varying the shape and number of the obstacles, the surfaces of the cell walls,and the steadiness of the flow rate. The results suggest that a rheological hysteresisnear the yield point may be the cause of the asymmetry. The second part of thisthesis demonstrates wall-slip behaviour in a fully developed Poiseuille flow. Thesimultaneous velocity and in-situ viscometry measurements are exploited to determinethe kinematics and dynamics of the flow in a glass capillary. The slip velocityis related to the wall-shear stress, using a power-law scaling, with an exponent independentof the microstructure of the fluid and viscosity of the solvent. The thirdpart of this thesis is dedicated to investigating the multi-layered flow of miscibleliquids in a thin, horizontal channel with chemical reaction at the interface. Thefluid layers are vertically stratified in the thickness direction and chemical reactionleads to gelation at the interface. A systematic study is performed to examine thehydrodynamic stability of the flow as well as the growth rate of the gel. Our observationssuggest that the stability of flow configuration is sensitive to the slightinclination angle in case of fully-reacted flow, and confirm that the gel layer growsdiffusively along the length of the cell.

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On the development of a method for continuous fractionation of non-Brownian particles in a viscoplastic fluid (2020)

This study focuses on the mechanical fractionation of non-Brownian particles in layered yield stress fluids. We extend a novel principle outlined in a batchwise technique- which works based upon the difference in starting criteria for motion in a weak gel- and create a continuous separation in an annular gap undergoing the spiral Poiseuille flow. Experimental equipment is designed, constructed and operated to evaluate -continuously- the aforementioned fractionation idea. This work is presented in three different, yet complementary studies. In the first study, we performed a series of batchwise tests, in a centrifuge, to develop criteria for motion of the individual classes of particles of both monodisperse and bidisperse suspensions in layered fluids, and to determine the stability of this multilayer fluid undergoing centrifugation. We also examined the usefulness of this separation technique on three different suspensions related to the bio-product industry. In the second part, the design challenges of the continuous device were elaborated and the essential design elements were addressed. Next, the fully developed flow field inside the fractionator was analyzed and shown that not all the operating conditions result in stable operation. We found that there is a subset of all potential operating conditions in which this methodology will work and this critically depends upon rheology and radii of the multilayer fluid. We summarized our findings into a number of qualitative "rules of thumb" to run the device. In the last part of the work, we extended the work to a continuous methodology and demonstrated particle fractionation using both ideal and industrial particle suspensions. To benchmark our calibration curves, we examined and measured the critical force to initiate the motion for (monodisperse) spherical and fibre-shape particle suspensions and found that this critical force presents a similar trend to the batchwise test but at a lower threshold. A similar finding was found in the second test where we examined the separation of MFC. We argue that this is an anticipated result as the two geometries are in different stress states. Despite this, we were able to achieve a separation at the same trend as the batchwise methodology.

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On low consistency refining of mechanical pulps (2019)

The aim of this thesis is to develop comprehensive knowledge to fill the gaps in the understanding of three key aspects of low consistency refining of mechanical pulps.Firstly, the fibre shortening mechanisms are formally studied by using a comminution model. Fibre length distribution data from before and after refining with a variety of pulp types, net-powers, feed flow rates, angular velocities and plate geometries was analyzed. Fibres' cutting rate and cutting location were found to be highly correlated with refiner gap. Plate geometry was also demonstrated to have a role in the fibre cutting location. Secondly, the relationship between net-power and gap was described using a correlation built entirely from pilot-scale refining data. Results showed that a properly defined dimensionless net-power number is crucial to compare different refiner sizes under the same grounds. The developed correlation was compared to industrial-scale data showing that the correlation is well suited for predictions. Key assumptions of the correlation were validated using bar-force sensor measurements data. Finally, the framework developed in the first two parts of this thesis were used together with pressure screening models available in literature to theoretically analyze refining systems typically found in TMP lines. Fibre length was used to assess each system performance in terms of refiner gap, reject ratio and refiner power. Moreover, the impact of some design aspects such as refiner size, recirculation and split-ratios was also described.

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Understanding radionuclide production in cyclotron target systems (2019)

Diagnostic scans in nuclear medicine often make use of radionuclides produced on-demand using fluid target systems on medical cyclotrons. The prevailing theoretical model of radionuclide production applies adequately to solid target systems but overlooks a host of behaviour unique to fluid targets and poorly describes the radioactivity recovered from them. The design of more efficient and reliable target systems for nuclear medicine demands an understanding of these distinct phenomena. This thesis augments the foundational work of Krasnov [1974] in describing radionuclide production by providing theoretical models and experimental investigations elucidating fluid effects in target systems. Examining gas target systems, a new production model is created which accounts for radioactivity adsorbing to the walls of the target body. The equations of this model are simplified by solving for leading-order terms and used to describe a variety of data sets. This adsorption model is expanded to incorporate in-target chemical kinetics by examining ¹¹C production. The expanded model is simplified by neglecting contributions from short lived species and the results are generalized into an augmented production equation that is able to describe experimental data with great accuracy. The model provides expressions for the initial production rate and saturation activity in these target systems and indicates that inefficiencies in [¹¹C]CH₄ production result primarily from adsorption of precursor radioactive species. These new models show that adsorption is the primary driver of low activity yields and paves the way for new target designs that limit adsorption, favour chemical conversion and induce desorption for better radioactivity recovery. In liquid solution targets the formation of radiolytic gas products is experimentally shown to be sensitive to initial solution composition. Radiolysis is reduced by over 60% by introducing nitrite to low pH solutions prior to irradiation. Finally, foil degradation in solution targets is studied with experiments indicating that electrochemical etching is responsible for premature target failure. This work lays the foundation for a fundamental understanding of radionuclide production in fluid target systems and is a necessary step in making radionuclides more easily available for nuclear medicine.

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Mathematical modeling of dynamical response of medical cyclotron targets (2018)

The focus of this thesis is to understand the dynamical behavior of fluid targetsunder proton beam bombardment. Although the reactor-based beam-matter interactionhas different applications, the motivation of this work stems from nuclearmedicine, i.e. maximizing the yield of radioisotopes for medical diagnoses. Thiscan be improved through increases in heat transfer rates to the cooling fluids. Todo so, there is a need to understand the transport mechanisms in the targets duringbombardment, an area which is relatively unexplored. In this thesis, we build uponthe work of Wojciechowski et al. [64] and Peeples et al. [42–45] and developa system of equations based on the conservation laws. The complexity of themodel is reduced to something which is tractable through volume-averaging technique.Our modeling effort is presented in four different yet complementary studies.In the first and second studies, the model is validated against the pressure-rise of agas target and is expanded to describe the final response of a liquid target at steadystate.Excellent agreement is achieved in all cases. In the third study, transientbehavior of a liquid target is examined. In what we consider unique findings, selfsustainedoscillatory behaviors were observed and captured by the model. In thelast study, we seek the further limits of the model through a linear stability analysis.The existence, uniqueness, and stability of steady-state points are challenged andthe conditions leading to an oscillatory behavior are identified. Finally, this workis the necessary first step in a larger research program to design cyclotron targets.

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Understanding hemicellulose and silica removal from bamboo (2017)

In this work, the hydrothermal pretreatment under both acidic and alkaline conditions were conducted to study hemicellulose and silica removal from bamboo. In the first part of this work, evolution of proton concentration was examined during both auto- and dilute-acid hydrolysis of hemicellulose from green bamboo. An approximate mathematical model (toy model) to describe the proton concentration based upon conservation of mass and charge during deacetylation and ash neutralization coupled with a number of competing equilibria, was derived. This model was qualitatively compared to experiments where pH was measured as a function of time, temperature, and initial acid level. The toy model predicts the existence of a steady state proton concentration dictated by equilibrium constants, initial acetyl groups, and initial added acid. At room temperature, it was found that pH remains essentially constant both at low initial pH and autohydrolysis conditions. At elevated temperatures, one case of non-monotonic behaviour in which the pH initially increased, and then decreased at longer times, was found.As silica in bamboo creates processing problems, in the second part of this work, alkaline pretreatment of pure amorphous silica particles, bamboo powder and bamboo chips was carried out to study the underlying mechanism for silica and hemicellulose extraction. Response surface methodology was also used to optimize the treatment conditions that could completely extract silica and partially extract hemicellulose from bamboo chips prior to processing. Alkaline pretreatment resulted in significant improvement in the delignification of treated bamboo chips during subsequent kraft pulping, offering an option to reduce the effective alkali charge or the H-factor. The pre-extracted silica and hemicellulose in the liquor were recovered through a sequential procedure of CO2 and ethanol precipitation. Moreover, the feasibility of adopting alkaline pretreatment to the typical kraft pulping process was tested. Results demonstrated that all silica, and up to 50% of hemicellulose in raw feedstocks, could be extracted without degrading cellulose and lignin. Approximately 96% of extracted silica in the APEL could be recovered as a high purity (>99.8%) silica nanoparticles. These results demonstrated that the proposed modification may benefit kraft pulping and fit well into the proposed biorefinery concept.

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Energy and Paper Recycling: An Investigation of Repulping (2016)

Fibers from waste paper are recovered through repulping. Repulping is accomplished using machines called repulpers. Repulpers are large machines which use a high-speed rotor mounted in a vat to disintegrate waste paper for the recovery of fibers. Repulpers consume a significant amount of energy to recover fibers from waste paper. For the work in this thesis, a 0.25m³ laboratory repulper was built for the purpose of determining which variables affect the specific energy (energy/mass) required for repulping. Scale replicas of 3 commercial repulper rotors were constructed to test the effect of rotor geometry on repulping specific energy. It was found that the flake content as a function of specific energy follows the form dF⁄dE=-λF where F is flake content, E is specific energy, and λ is a rate constant. The rate constant λ varies with pulp type, temperature, consistency, repulper volume, and rotor design. It was found that a given material at a given temperature and consistency requires a unique quantity of energy to be repulped independent of the rate of energy addition. An analytical model for repulping linking pulp material properties, consistency, temperature, and rotor and vat geometry is provided which allows for the accurate prediction of the time and energy required for repulping in both the 0.25m³ laboratory scale repulper and a 15m³ industrial repulper. The model assumes that all work to deflake is done by the repulper rotor in the rotor swept volume by turbulence and that no deflaking occurs in the rest of the vat. CFD simulations of the flow field produced by each rotor and high-speed film of each rotor indicate that the rotors tested in this thesis all produce strong trailing vortices akin to those produced by common mixing impellers like the Rushton turbine. Uniform mixing is important for efficient repulping. Solid body motion of the suspension in the repulper makes for poor repulping energy efficiency. Repulping time and energy savings can be accomplished by increasing the suspension consistency and the rotor swept-volume/vat volume ratio by either increasing rotor size or reducing vat volume all while ensuring complete mixing and circulation in the vat.

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Observation of transition from laminar plug to well-mixed flow of fibre suspensions in Hagen-Poiseuille flow (2016)

The focus of the present work is an experimental study of the transition to turbulent flow of papermaking fibres in a cylindrical pipe. The suspensions used in this study possess a yield stress. With this class of fluid the radial profile in fully developed slow flow is characterized by an unyielded or plug zone. With increasing flow rates the size of the plug diminishes. One of the open remaining questions with these suspensions is the role of the plug during transition. In this work we characterize the size of the plug using ultrasound Doppler velocimetry as a function of flowrate for dilute, i.e. less than 2% consistency papermaking suspensions in a 50mm diameter pipe. The plug size was determined through analysis of local spatial and temporal variations in velocity, strain-rate, and the fluctuating component of velocity. With these we were able to estimate the yield stress of the suspension through knowledge of the applied pressure gradient and find the yield stress to be in the range of 2-60 Pa, depending upon consistency, fibre type and manufacturing methodology. The yield stress measurements were benchmarked against measurement methodologies reported in the literature. During flow, we observe complex behavior with the plug in which we found that with increasing velocity the plug diminishes through a densification mechanism in response to increasing frictional pressure drop. At higher Re, it diminishes through an erosion type mechanism. We estimate the critical Reynolds number for the disappearance of the plug to be Rec 10⁵. In perhaps the most unique measurements in this work we find that drag reduction begins when rp/R
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On the Fluid Mechanics of Viscoplastic Particle Suspension Fractionation: Understanding Multilayer Spiral Poiseuille Flow in an Annulus (2016)

The focus of the present work is the study of laminar spiral multi-layer viscoplastic flow in annular geometries. The motivation for the present work stems from an interest in utilizing such flow in fractionation of particle suspensions. The work is presented in four studies. In the first study we solve the fully developed condition of the flow analytically. This solution is considered a reliable reference to validate the remaining numerical studies. Also it provides a means to test the stability of the flow. The second study is related to the fractionation of particle suspensions utilizing the solution demonstrated in the first study. We develop fractionation curves of particles of different sizes in fluids with different rheology. We develop a code to simulate thousands of flow cases (a flow case has a unique combination of streams flowrates) of known fluids properties. We predict the fractionation operating window (some range of streams flowrates) needed for successful fractionation. In the third study, we examine the flow in the full annulus geometry including the entrance region of the flow. Here, we estimate the flow entrance length in order to design the length of the mixing zone of the two streams in the continuous fractionation device. We study the effect of Kelvin-Helmholtz and density current instabilities on the flow. In the fourth study, we attempt to design the continuous fractionation device in which we use the results of the first three studies together with the analysis of the constraints imposed by the physical construction of the device. We explore the flow behavior in the exit region of the device and suggest some guidelines to achieve successful fractionation accordingly. Results of this work show that spiral multi-layer Poisuille viscoplastic flow can be stable and the range of stability expands with increasing fluids yield stress. Gravity current instability is evident in density unstable cases and the flow can be stabilized by increasing the fluids yield stress. Kelvin Helmholtz instability was not found on conditions tested. Viscoplastic flow entrance length was found to be shorter than the equivalent Newtonian one for the same range of Reynolds number.

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Modelling Latency Removal of Mechanical Pulping Processes (2014)

Latency removal is an essential step in the mechanical pulping process. It occurs in a continuous stirred-tank reactor (CSTR) and non-ideal mixing lowers the performance. In order to optimize the latency removal process and reduce the energy consumption in the operation, a kinetic study was carried out.In this work, latency removal was studied at both the individual fibre and the pulp suspension frames of reference. In the first study, the removal of latency of individual TMP fibres was studied using optical microscopy. The fibre deflection under the influence of the heat and water absorption was measured as a function of time. At the pulp suspension level, latency removal was characterized by the change of different pulp properties and the dependency of each property on treatment conditions was determined. Kinetic models of latency removal for secondary refiner TMP and BCTMP pulps were developed, which were based on the rate of latency elimination characterized by freeness. The kinetic study reveals that a potential energy reduction in industrial operation of latency removal can be achieved by properly increasing the power intensity to get better mixing. These results were then complemented in a third study of a more direct measure of latency, i.e. curl index. The change in curl index of TMP pulp was examined and its dependence on temperature and other treatment conditions was determined. The development of tensile and tear strengths of TMP pulp was explored in terms of different treatment conditions and the results were analyzed in terms of fibre straightening and fibre deflocculation. Linear correlations between strength properties, curl index and freeness have been found. In the final portion of the work an industrial case study was performed, where the latency removal of primary BCTMP pulp was examined for the purpose of optimizing an industrial latency removal process. The result of the laboratory test and the onsite measurement in the mill shows latency removal of primary BCTMP pulp is a much faster process in comparison with the secondary BCTMP pulp, and the latency removal process in the pulp mill can be optimized using an existing smaller sized mixing chest.

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The Effects of Turbulent Drag Reducing Additives on Hydrocyclone Operation: An Evaluation of the Flow Behaviour and Particle Mechanics (2014)

The effects of adding drag reducing additives to a pulp processing hydrocyclone were experimentally investigated. This work was found to contribute towards improving particle separation efficiencies and reducing energy consumption. To effectively evaluate the performance improvements, the flow field was initially measured using laser Doppler velocimetry. In the presence of a particulate phase, the motions of variously sized particles were measured using a three dimensional, dual camera set-up. Quantification of the drag reducing potential of the various polymer solutions and fibre suspensions studied was experimentally determined using an integral analysis for a fixed control volume.The addition of drag reducing polymer additives was found to fundamentally change the hydrocyclone flow field from what is classically observed with water alone. For the conditions studied in this work, the effectiveness of a hydrocyclone towards removing contaminants would likely be reduced, as a particles separation zone was limited. The addition of polymer additives to a hydrocyclone was found to increase the size of particles susceptible to overflow removal. It was found that particles of density 1280 kg/m³ and diameter 500 - 600 microns displayed inwards motion in a 0.03% APAM solution, where purely outwards motion was measured for identically sized particles suspended in water. The flow field, however, indicated that overflow removal is limited to only a small region near the vortex finder.Polymer additives were found to be effective in reducing energy consumption in a hydrocyclone. Maximum drag reduction was found to occur at a reject ratio of 50% for polymer solutions, independent of inlet velocity. The energy savings potential for polymer additives in a pulp processing hydrocyclone, however, was found to be limited to the inlet velocity. Most in process hydrocyclones operate well above the minimum inlet velocities measured for rejects ratios of 25% and 50%, suggesting that additional energy savings would likely occur. The phenomenological degradation of the polymer agents investigated in this work suggests that the practical use of these additives would be difficult. This was found to be most significant with cellulose fibre suspensions containing cationic polyacrylamide (CPAM), as polymer adsorption resulted in rapid polymer degradation.

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Understanding of no-load power in low consistency refining (2014)

Low Consistency (LC) refining is the primary means of improving the strength and smoothness of paper by imparting energy to fibres through repeated fibre-bar interactions. The useful part of the energy modifies the morphology of the fibres and the remaining, no-load power, mainly overcomes the hydraulic, pumping and mechanical losses in the refiner. This thesis is aimed to explore the no-load power in LC refining both experimentally and computationally. The contribution of this thesis comes in three parts.Firstly, the effect of consistency, operational and plate design parameters on noload power was experimentally determined on two pilot scale LC refiners with different plate diameters. The obtained data were used to provide a statistical model for prediction of no-load power. To study the effect of diameter and groove depth, the no-load power consumption of some mills corresponding to their operating conditions, and the specifications of the relevant refiner discs were collected. Based on this model, no-load power is described in terms of two main components, hydraulic and pumpingpowers, and an empirical equation is proposed.Secondly, we numerically examined the two-dimensional flow of a Newtonian fluid in the gap formed between two opposing cavities which represent the cross-sectional flow in LC refiner. A large number of unsteady simulations were conducted to characterize the effect of gap size on the flow field over the range of velocities. Then, weexamined material transport between the cavities by introducing a passive scalar to represent the motion of tracer particles. Over the range of parameters studied, we identify two characteristic flow fields, defined as either steady or unsteady. We also find that particles are transported to the region near the leading edges of the bars only under the conditions of unsteady flow.Thirdly, we extended the numerical study by characterizing the effect of cavity depth on the flow field over the range of velocities. We find that the aspect ratio of the cavity dictates three characteristic flow fields based on the number of vortices formed within cavity and we propose criteria for cavity aspect ratio in terms of therefiner application.

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From displacement to mixing in a slightly inclined duct (2012)

This thesis studies buoyant displacement flows with two miscible fluids in pipes and 2D channels that are inclined at angles (β) close to horizontal. Detailed experimental, analytical and computational approaches are employed in an integrated fashion. The displacements are at low Atwood numbers and high Péclet numbers, so that miscibility effects are mostly observable after instability and via dispersive mixing.For iso-viscous Newtonian displacements, studying the front velocity variation as a function of the imposed flow velocity allows us to identify 3 distinct flow regimes: an exchange flow dominated regime characterized by Kelvin-Helmholtz-like instabilities, a laminarised viscous displacement regime with the front velocity linearly increasing with the mean imposed flow rate, and a fully mixed displacement regime. The transition between the first and the second regimes is found to be marked by a stationary layer of displaced fluid. In the stationary layer the displaced fluid moves in counter-current motion with zero net volumetric flux. Different lubrication/thin-film models have been used to predict the flow behaviour. We also succeed in characterising displacements as viscous or inertial, according to the absence/presence of interfacial instability and mixing. This dual characterisation allows us to define 5-6 distinct flow regimes, which we show collapse onto regions in the two-dimensional (Fr, Re cosβ/Fr)-plane. Here Fr is the densimetric Froude number and Re the Reynolds number. In each regime we have been able to offer a leading order approximation to the leading front velocity. A weighted residual method has also been used to include the effect of inertia within the lubrication modelling approach, which allows us to predict long-wave instabilities.We have extended the study to include the effects of moderate viscosity ratio and shear-thinning fluids. We see many qualitative similarities with the iso-viscous studies. Predictive models are proposed (and compared with experiments and simulations) for the viscous and inertial regimes.Having a significant yield stress in the displaced fluid leads to completely new phenomena. We identify two distinct flow regimes: a central-type displacement regime and a slump-type regime for higher density differences. In both regimes, the displaced fluid can remain completely static in residual wall layers.

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Characterizing the permeability and dispersion of flows through compressible wood-chip beds (2011)

During Kraft pulping, cellulose fibres are liberated from a lignin-matrix, found in a wood chip, by reaction at elevated temperature and pressure using an alkaline solution. Poor lignin removal is a deleterious effect that leads to downstream operational difficulties and decreased product quality. A number of research groups speculate that this is caused by the uneven distribution of the alkaline solution through the wood chip bed during reaction. As a result, the goal of this thesis is to characterize the ease by which fluid flows, and disperses, through wood chip beds. One of the open remaining scientific questions is understanding the effect of bed compressibility on the resulting flow patterns.In the first portion of this work we present a methodology to characterize the permeability of a compressible bed of wood chips under mechanical load. We show that under the limiting condition of when the mechanical load is large in comparison to hydraulic pressure the equations of motion can be linearized and solved to produce an expression approximating the variation in porosity along the length of the bed. We show how this may be used, in conjunction with multiple linear regression, to estimate permeability of the bed. The usefulness of these estimates was then tested by predicting the pressure drop versus flow relationship for conditions outside the range of the linearized solution. Good agreement was obtained.In the second portion of this work we present a methodology to characterize the axial dispersion of a solute during steady-flow through a compressible bed of wood chips under mechanical load. We use a non-invasive imaging technique, namely electrical resistance tomography (ERT), to visualize the uniaxial displacement of a salt solution. Here we demonstrate that under two limiting cases the porosity of the porous bed varies slowly in the flow-direction and to the lowest order can be considered a constant. This simplified the optimization routine we used to match the experimental data to the numerical results of the advection-diffusion equation. Using this, a methodology to estimate the axial dispersion is given by a minimization scheme.

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Development of a methodology to optimize low consistency refining of mechanical pulp (2011)

In this dissertation we present a novel two-stage procedure to relate low consistency (LC) refiner operating conditions to changes in fibre morphology. To do so, a large database of operating conditions and resulting pulp properties were collected over a range of both pilot and industrial LC refiners operating with mechanical pulps. In total eight different Andritz TwinFlo™ were sampled over a three year period in both North America and Scandinavia. The two-stage methodology is based upon a classical dimensional analysis in which a reduced parameter space is related to each other through the use of statistical modelling. In the first stage we demonstrate a relationship between net power and operating parameters such as gap, rotational speed, diameter, plate pattern and consistency of the fibre suspension. For all refiners tested the model indicates that the net power increases nearly linearly with the inverse of gap size. In this portion of the analysis we found statistically significant relationships between operating conditions and suspension properties such as change in fibre length and Canadian Standard Freeness, an industrial standard related to pulp dewatering. In the second stage of this methodology, we build upon the work of Forgacs [1] and demonstrate that most paper properties, e.g. the mechanical strength, are related primarily to fibre length and freeness; over 80% of all variation in the data can be attributed to these two parameters. With this novel framework, in conjunction with the statistical models, we demonstrate that an optimum operating condition exist to maximize strength, and demonstrate the sensitivity of this relationship using a number of different type pulps. In the second portion of the thesis, we further develop a novel mechanical pulping process in which multiple stages of LC refining replace the second stage HC refining in a conventional TMP process. This work is motivated from the need to reduce electrical energy consumption to produce mechanical pulp. Using the two-stage methodology developed in the first portion of the thesis, we demonstrate under pilot plant conditions an energy savings of over 20% in comparison to a conventional TMP process to generate mechanical pulp of equal quality.

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Fractionation of Particle Suspensions in a Viscoplastic Fluid: Towards a Novel Process (2011)

The focus of this thesis is the separation or sorting of particle suspensions in a yield stress or viscoplastic fluid. Although the process is applicable to most industrial suspensions, the motivation of the work stems from pulp and paper industry,i.e. papermaking and microfibrillated cellulose (MFC) suspensions. The work is presented in four different yet complementary studies.In the first study, the concept of particle fractionation in a viscoplastic fluid is introduced. Here this novel principle is demonstrated, batch wise, by measuringthe difference in centrifugal force required to initiate motion of an initially stable particle suspension in a gel. The criteria for motion is delineated as the ratio of the centrifugal force to yield stress as a function of particle size and orientation. Demonstration experiments are given to illustrate that the separation process is very efficient. In the second and third studies we demonstrate the principle on two industrial suspensions, i.e. a SBK (semi-bleached kraft) papermaking fibre and MFC. With papermaking fibres, it is shown that efficient separation, based upon cell wall thickness can be achieved. With MFC, it is shown that the process is more efficient than traditional separation techniques, i.e hydrocyclone and pressure screen. In the final study, we speculate regarding the conditions required to make a continuous process based upon the batch testings. Here, it is identified that a spiral Poiseuille flow would be sufficient to achieve separation. The questions addressed in this study are what is the size of the unyielded region for this flow field and what is the bound for transition to turbulent flow. It was found that the magnitude of the swirling flow does not affect the size of the plug and the axialvelocity is decoupled from the rotational rate. In addition, the yielded region is always formed in the middle of the annular gap. To address the flow state, a linear stability analysis was performed using the method of normal modes. The flow was found to be linearly stable for all conditions tested.

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Observation of laminar-turbulent transition of a yield stress fluid in Hagen-Poiseuille flow (2009)

The main focus of this work is to investigate experimentally the transition to turbulence of a yield stress shear thinning fluid in Hagen-Poiseuille flow. By combining direct high speed imaging of the flow structures with Laser Doppler Velocimetry (LDV), we provide a systematic description of the different flow regimes from laminar to fully turbulent. Each flow regime is characterized by measurements of the radial velocity, velocity fluctuations, and turbulence intensity profiles. In addition we estimate the autocorrelation, the probability distribution, and the structure functions in an attempt to further characterize transition. For all cases tested, our results indicate that transition occurs only when the Reynolds stresses of the flow equals or exceeds the yield stress of the fluid, i.e. the plug is broken before transition commences. Once in transition and when turbulent, the behavior of the yield stress fluid is somewhat similar to a (simpler) shear thinning fluid. We have also observed the shape of slugs during transition and find that their leading edges to be highly elongated and located off the central axis of the pipe, for the non-Newtonian fluids examined. Finally we present a new phenomenological approach for quantifying laminar-turbulent transition in pipe flow. This criterion is based on averaging a local Reynolds number to give ReG. Our localised parameter shows strong radial variations that are maximal at approximately the radial positions where puffs first appear during the first stages of turbulent transition.

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The Orientation of Semi-Dilute Rigid Fibre Suspensions in a Linearly Contracting Channel (2008)

No abstract available.

Master's Student Supervision (2010 - 2018)
Measurement of the changes to the liquid transport properties of wood due to compression (2017)

Compression of wood chips coupled with chemical treatment in a liquid bath has beenshown to be an effective method to reduce the energy required to mechanically produce wood pulp. Though the utility of such compression/chip impregnation processes has become well known through implementation at pilot and mill scales, a fundamental understanding of the operational mechanism of these processes remains to be obtained. Towards this end, an experimental and statistical study has been conducted. In this work, better understanding the mechanism of action and measuring the effectiveness of a given compression/impregnation process has come about via developing two complementary methods to characterize arbitrarily compressed woodchips. The first method, applied to chips compressed at mill and pilot-scale, involvesextracting a pore size distribution from a wood sample by use of a combination ofmicroscope imaging, digital image analysis, and model fitting to the data obtained fromimage analysis. The second method, applied to wooden blocks compressed at lab-scale, involves directly measuring liquid uptake of a wooden block from a reservoir.Results from image analysis suggest that compression changes the structure ofwood in a way which hinders liquid uptake. This observation, taken with existingheuristic relationships between compression and liquid uptake ability of wood chips,implies that the effectiveness of a wood chip compression/impregnation process relieson the alteration of chip properties on a length scale larger than that captured by theimage analysis (i.e., that of individual fibres). Direct measurement of liquid uptakeof compressed wood blocks at lab-scale resulted in the emergence of a stochasticrelationship between compression conditions (compression rate and total strain appliedto a block) and the changes imparted to wood’s micro-structure. Taken in combination, the results of the experimental program conducted suggest that the effectiveness of wood chip compression/impregnation processes lies within the ability of compression to modify the structure of an aggregate of wood chips rather than individual wood chips on a fibre scale.

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Xylan removal by xylanase for the production of dissolving pulp from bamboo (2017)

With α-cellulose content and fiber characteristics similar to those of wood, bamboo is an attractive alternative feedstock for the production of dissolving grade pulp. A high level of hemicellulose in bamboo will lead to substantial complications in downstream processing of dissolving pulps into cellulose derivatives such as viscose, acetates, ethers etc. Xylanase treatment is an environment-friendly method that enables the selective removal of xylan (the major hemicellulose in bamboo) without detrimental effects on cellulose. In this study, we investigated a combination of mechanical refining with xylanase treatment for incorporation into a pre-hydrolysis kraft-based bamboo dissolving pulp production process. Laboratory PFI refining and xylanase treatment were combined to improve the xylan removal efficiency. Refining at 9000 revolutions increased the efficiency of subsequent enzymatic treatment resulting in a 44% removal of beta- plus gamma-cellulose with only 3 h of xylanase treatment. The alpha-cellulose content of bleached pulp prepared following combined refining-xylanase treatments was 93.37% (w/w) while the xylan content was only 2.38%. The properties of refined fibers prior to xylanase treatment, such as freeness, water retention value, fiber size and Scanning Electron Microscopy (SEM) images were investigated to further understand the underlying mechanism of the effect of refining on enzymatic treatment. The brightness, reactivity and viscosity of bleached bamboo dissolving pulp after ECF bleaching (D-EP-D) sequence were also evaluated. These results demonstrated the feasibility of combining refining and xylanase treatment to produce high quality bamboo dissolving pulp.

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Automated Fibre Segmentation in Micro CT Images of Paper (2014)

Characterizing the structure of paper products made from Northern Bleached Softwood Kraft (NBSK) pulp after Low Consistency refining is a vital step to understanding the strengthening mechanisms of NBSK fibres as well as the effect of refining on the fibre morphology. X-ray micro tomographic (µCT) imaging coupled with advanced image analysis enables the characterization of the internal structure of paper at a very high resolution in 3D. In this work, a novel algorithm has been developed to isolate individual papermaking fibres in µCT images of paper handsheets as a first step to characterize the paper structure. The three step fibre segmentation algorithm segments the papermaking fibres by (i) tracking the hollow inside the fibres via a modified connected component methodology, (ii) extracting the fibre walls using a distance transform and (iii) labeling the fibres through collapsed sections by a final refinement step. Further, post processing algorithms have been developed to calculate the length, coarseness and relative bonded area of the fibres thus segmented. The algorithms have been validated by segmenting hollow aluminum tubes in test geometry similar in complexity to the paper structure. The capabilities and limitations of the algorithms have been evaluated by segmenting 2484 papermaking fibres within a 1mm x 1mm handsheet sample manufactured from NBSK pulp. The Fibre Segmentation algorithm is the first ever reported method for the automated and robust segmentation of the tortuous 3D morphology of papermaking fibres within 3D images of paper handsheets. The segmented structure thus obtained provides the capabilities to calculate several important properties of paper. The fibre segmentation algorithm is a unique and powerful tool to analyze the paper structure and provide novel insights into the papermaking process.

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Measuring uniformity in kraft digesters using flow-following sensors (2010)

Measurements of pulp variability and temperature distributions within kraft digesters have been the subject of intense interest for many years. An extensive survey carried out on the previous approaches to measure digester variability has shown that data directly taken from within the digester, during the kraft cook is scarce. Moreover, the increased size of modern digesters is believed to reduce the flow uniformity within them, increasing the risk of pulp variability and highlighting the need to collect data from within the digesters themselves. This has motivated the development of a new method for measuring digester variability: the “SmartChip”. This device is a flow-following sensor package that measures and records the temperature directly within the digester during the kraft cook.This research proposes a theoretical model to describe the heat transfer mechanisms occurring within the digester and then uses the data captured by the SmartChip to measure the digester variability. Preliminary test trials were conducted to ensure that the SmartChip could withstand the harsh environment of the kraft cook. Subsequently, multiple SmartChips were deployed in a single cook to provide insight on the temperature variability and heat transfer mechanisms occurring within the digester and these experimental results were then be compared with the predictions of the proposed model. Moreover, by taking pulp samples in the vicinity of the SmartChip sensors, the relation between nonuniformity and temperature gradients is established. The SmartChips have been tested in two laboratory batch digesters and as expected, temperature was already well-controlled within them and little variability was observed in these small-scale devices. Moreover, a good agreement is found between the model predictions and the experimental results and the model is experimentally validated, showing that the heat transfer through the digester contents is by pure advection with almost negligible thermal energy required for the heating of wood chips. These findings further suggest that the SmartChip works well under the harsh conditions of the kraft cook and future work is warranted to develop the instrument for its use at the industrial scale, where the temperature fluctuations along the digesters are more noticeable.

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