Dana Grecov

Agglomeration of Bitumen-Coated Coke Particles in Fluid Cokers

A simplified mathematical model is proposed to determine the agglomeration tendency of bitumen-coated coke particles in fluid cokers. The model calculates a theoretical critical velocity that depends on key parameters such as the particle size, bitumen viscosity, and bitumen thickness; it also accounts for the temperature- and reaction-dependent variations of the bitumen thickness and viscosity. A peak theoretical critical velocity at the intermediate reaction times for all coking temperatures is predicted. By comparing this peak critical velocity with the estimated inter-particle collision velocities within an industrial-scale reactor, the agglomeration tendency of coke particles is determined within fluid cokers. The results show that at low temperature regions (T=400 °C), there is no agglomeration tendency; however, at high coking temperatures (T=503 and 530 °C), substantial agglomeration tendency is expected. It is also found that the number of coke particles constituting an agglomerate could be as high as a few hundreds.

Steady state and transient rheological behavior of mesophase pitch, Part II: Theory

This paper presents a comprehensive nonlinear numerical analysis of the flow modeling of mesophase pitches performed using a previously formulated mesoscopic viscoelastic rheological theory (Singh and Rey (2000)) that takes into account short-range order elasticity, long range elasticity, and flow-induced texture transformations. A complete extra stress tensor equation is developed from first principles for liquid crystal materials under nonhomogeneous arbitrary flow. This mesoscopic viscoelastic model has been adapted to describe the rheology of flow-aligning thermotropic discotic nematic liquid crystals as models of mesophase pitches. Predictions for simple shear flow (under nonhomogeneous conditions) for the shear viscosity, first normal stress differences, and transient shear stress are presented. The accuracy of the numerical results is established by a thorough validation procedure based on [Cato et al. (2004)], which is the companion paper, and permit to validate this mesoscopic viscoelastic theory...

Impact of liquid crystal additives on a canola oil-based bio-lubricant

Vegetable oils can offer important environmental advantages with respect to biodegradability and renewability, as well as good performances in different applications. In this work, the rheological and tribological properties of an industrial canola oil-based bio-lubricant were investigated. They were compared with the properties of a mineral hydraulic lubricant used for similar applications. The tribological performance of the lubricants was investigated using a four-ball test and a pin-on-disk setup, and the rheological behavior was studied using oscillatory rheometry. The tested bio-hydraulic oil exhibited excellent lubricity. Liquid crystal additives affected the viscoelasticity differently. Moreover, the added ionic liquid crystal improved the wear resistance of the bio-hydraulic oil, but the original lubricant preserved the lowest coefficient of friction.

Estimation of viscosity coefficients and rheological functions of nanocrystalline cellulose aqueous suspensions

This paper presents a methodology to calculate different rheological functions and viscosity coefficients for lyotropic liquid crystals (LCs) using analytical calculations and experimental rheological data. By implementing Doi and Larson models for lyotropic nematic liquid crystals, the Leslie viscosity coefficients for nanocrystalline cellulose (NCC) aqueous suspensions have been calculated as a function of concentration of molecules per unit volume. The Landau viscosity coefficients from the symmetric viscous stress tensor have been calculated based on the mapping between Landau-de Gennes theory and the Leslie–Ericksen theory. Various parameters and dimensionless numbers from the Landau-de Gennes theory were calculated and related to experimental data. The validation was done using numerical simulations of the Landau-de Gennes equations for transient simple shear flow between parallel plates of 7 wt NCC aqueous suspensions and experimental rheological data. Apparent viscosity and shear stress from numerical simulations have been compared with the experimental results for the same concentration and found a good agreement. A cholesteric pattern was observed for low shear rates and flow-aligning regime for higher shear rates.

Shear flow induced microstructure of a synthetic mesophase pitch

Carbon fibers and composites derived from mesophase pitch exhibit ultrahigh stiffness and thermal conductivity due to a high degree of graphitic content, which is generated by the liquid crystalline state of the precursor and the molecular orientation that is developed during melt processing steps. To understand the flow and its effect on microstructure, this paper presents an integrated experimental and modeling approach for a synthetic discotic mesophase pitch (AR-HP). Careful control of shear rate and strain was exercised throughout the rheological studies. Wide-angle x-ray diffraction studies were conducted on carefully solidified rheological specimens to obtain azimuthal profiles for layer plane orientation. Cross-polarized microscopy was conducted in the reflected mode using a first-order red plate to examine the orientation in three orthogonal planes. Under the influence of steady shear, the microstructure of mesophase pitch became flow-aligned, which is similar to the fibrous structure reported in...

Numerical Simulation of an Evaporative Spray in a Gas-Solid Crossflow

Evaporative liquid feeds are often introduced into gas-solid systems through horizontal nozzles in many industry applications. The interactions of gas, particles and evaporative spray play key roles in affecting process efficiency and product quality. Good understanding of the fundamental hydrodynamics of this phenomenon is essential to the design improvement and process optimization. In this work, an Eulerian multi-fluid model for evaporative spray in gas-solid flows is proposed and a three-dimensional numerical simulation of a gas-liquid nitrogen jet in a uniform gas-solid crossflow is performed. In this model, the droplet evaporation and interphase heat transfer are modeled with appropriate semi-empirical correlations. Detailed numerical results of the flow field are obtained and the predicted temperature distribution is compared with experimental measurements in the literature. The evaporative spray jet is investigated under different particles loadings and the effect of spray evaporation on the flow behavior is discussed. Finally, parametric studies are conducted to evaluate the effects of droplet size, jet velocity on the jet behavior.

Flow modelling and rheological characterization of nematic liquid crystals between concentric cylinders

Liquid crystals (LCs) have good load-bearing properties and the ability to lower friction, wear and temperature between rubbing surfaces. The steady flow of thermotropic flow-aligning nematic liquid crystalline materials was studied numerically between two concentric cylinders with small gap sizes. The rheological behaviour and flow characterisation of LCs for different inner cylinder rotational velocities and anchoring angles at the boundaries were also investigated. The Leslie–Ericksen theory was implemented to model the LC microstructures. Continuity and momentum equations were simultaneously solved with the microstructure equation. Considering the nature of the governing equations for LCs, the relaxation method was selected to solve this set of nonlinear ordinary differential equations (ODEs). The orientation angle distribution and apparent viscosity were presented as a function of the rotational velocity and shear rate. Rheological characterisation of N-(4-Methoxybenzylidene)-4-butylaniline (MBBA) was performed using a rotary rheometer and the results were compared with those of numerical simulations. Furthermore, it was shown that the preferred orientation of the molecules of liquid crystalline materials in vicinity of solid surfaces gives them an advantage over isotropic Newtonian fluids by reducing the amount of resistance torque on the inner cylinder.

Hemodynamic assessments of the ascending thoracic aortic aneurysm using fluid-structure interaction approach

Current assessment and management of ascending thoracic aortic aneurysm (ATAA) rely heavily on the diameter of the ATAA and blood pressure rather than biomechanical and hemodynamic parameters such as arterial wall deformation or wall shear stress. The objective of the current study was to develop an accurate computational method for modeling the mechanical responses of the ATAA to provide additional information in patient evaluations. Fully coupled fluid structure interaction simulations were conducted using data from cases with ATAA with measured geometrical parameters in order to evaluate and analyze the change in biomechanical responses under normotensive and hypertensive conditions. Anisotropic hyperelastic material property estimates were applied to the ATAA data which represented three different geometrical configurations of ATAAs. The resulting analysis showed significant variations in maximum wall shear stress despite minimal differences in flow velocity between two blood pressure conditions. Additionally, the three different ATAA conditions identified different aortic expansions that were not uniform under pulsatile pressure. The elevated wall stress with hypertension was also geometry-dependent. The developed models suggest that ATTA cases have unique characteristic in biomechanical and hemodynamic evaluations that can be useful in risk management.

Non-Newtonian behavior in canola-oil-based bio-hydraulic oil

The use of vegetable oils can offer important environmental advantages with respect to biodegradability and renewability, along with good performance in a range of different applications. Unlike petroleum-based lubricants, which have been studied and developed over a century, knowledge related to vegetable-oil-based lubricants is limited. In this work, the rheological properties of industrial canola-oil-based bio-lubricants were investigated using a rotary rheometer. The bio-hydraulic oil exhibited constant viscosity at both moderate and high shear rates, as well as shear thinning at low shear rates and temperatures less than 30 ℃. Frequency sweep tests revealed significant viscoelasticity in the bio-hydraulic oil, which developed over time. Time dependence and structure recovery effects were also investigated. These experiments reveal some characteristic liquid crystal fingerprints. To the best of our knowledge, this study is the most extended rheological characterization of low-viscosity vegetable-oil-based lubricants.

Yield Stress Analysis of a Fumed Silica Lubricating Grease

Abstract Yield stress is an important property for identifying the suitability of a grease for a lubrication system. There is no consensus on the definition of the yield stress or its measurement method. Although fumed silica can be a suitable thickening agent, studying fumed silica–based lubricating greases has not received much attention. In this work, the authors performed different rheological tests on fumed silica–based lubricating grease to study its yielding behavior. The results revealed that the yield points obtained by most of these methods are roughly similar. The slight differences between these results have been explained as well. The microstructure of this grease is visualized using scanning electron microscopy (SEM) in cryo and non-cryo modes, demonstrating the fractal structure of the thickener and its corresponding structural hierarchy. Steady-state flow curves and microstructural analysis revealed some of the differences between this grease and other commercial greases.