Discharge Regimes of Polydisperse Powders through Conical Hoppers under Positive Flow Conditions

Discharge characteristics of polydisperse powders through a conical hopper were experimentally investigated by observing the solids discharge regimes under different positive pressure differences (i.e., ΔP > 0) and also measuring the corresponding solids discharge rates. The observations and measurements were systematically conducted at least four diameters of hopper orifice (2.5 mm ≤ D_o ≤ 30.0 mm) under five positive ΔP values (0 Pa ≤ ΔP ≤ 1405 Pa) for the test powders: river sand, zeolite, fertilizer, and sawdust. Effects of positive ΔP on both discharge regime and solids discharge rate are first predicted for different orifice diameters and test powders in this study. One of the novel results on the discharge regime is that increase of positive ΔP value up to a critical value conserves or successively stabilizes the discharge regime observed when ΔP = 0 (i.e., for gravity flow condition) through the same hopper. However, the additional increase of positive ΔP over this critical value destabilizes the discharge regime, as in conformity with the effect of increase in negative ΔP cited in the literature.     

A simplified correlation for fixed bed pressure drop

An experimental study was conducted on the pressure drop characteristics of a variety of vertical packed beds in turbulent flow of air. The materials of different particle diameter, D_p, with a range of sphericity Φ, 0.55 ≤ Φ ≤ 1.00 were used in random loose packing to produce beds of different lengths, L, with a range of porosity, ε, 0.36 ≤ ε ≤ 0.56. In the covered test cases the cross-sectional velocity distribution at the exit plane of the packed beds and the pressure drop ΔP_Bed were measured in a particle Reynolds number range of Re_p, 675 ≤ Re_p ≤ 7772. The particular emphasis of the study was given to determine the influence of ε, Φ, D_p, L, Re_p on ΔP_Bed. In this respect the measurements of ΔP_Bed were compared with the well-known Ergun's Equation and the data were expressed in terms of correlations through introduced dimensionless parameters of pressure coefficient, ΔP^? and exit Reynolds number Re_exit. The proposed correlations of ΔP^? = ΔP^?(εRe_pD_p / L) and Re_exit = Re_exit(Re_pD_p / L) are found to be appropriate for the determination of ΔP_Bed and mean exit velocity, U, respectively with an acceptable fit of experimental data in an error margin less than ± 20%. The methodology is presented in this paper as an alternative approach to the available literature on packed beds.     

An approach for transition correlation of laminar pulsatile pipe flows via frictional field characteristics

Transition of laminar pulsatile pipe flow into turbulence is one of the current research topics in flow dynamics. Despite the existence of a considerable number of theoretical and experimental studies, the physical mechanism of transition is not well defined. Furthermore, there is almost no information on the start and the end of the transition in terms of pulsatile flow parameters.

In this paper, an approach which consists of attempts to correlate the governing flow parameters is presented to reveal the transition process with particular emphasis on the frictional field. The experimental data collected in slightly compressible, Newtonian, one-dimensional laminar pulsatile pipe flow without a flow reversal were compiled for this purpose. The common oscillation parameters, dimensionless frequency parameter and velocity amplitude ratio A₁ were the main variables of the experimental study covering the ranges of and 0.05≤A₁≤0.8. The time history of local static pressure gradient and axial velocity field were accumulated and the data were expressed through pulsatile flow, instantaneous λ_u(t) and time-averaged λ_u,ta friction factors using momentum-integral equation. A reference friction factor ratio λ_R, whose definition was based on the concept of steady flow friction was introduced. The start and the end of transition were predicted through the functional relationships of λ_R with time-averaged and oscillating Reynolds numbers, Re_ta and Re_os by means of a trial–error procedure. The proposed correlations and determined approximate critical limits of transition are only valid in the corresponding ranges of 2000≤Re_ta≤60 000, 620≤Re_os≤18 800 and the analysis is open to discussion.     

Cost–benefit analysis of the mechanisms that enable migrating cells to sustain motility upon changes in matrix environments

Cells can move through extracellular environments with varying geometries and adhesive properties. Adaptation to these differences is achieved by switching between different modes of motility, including lamellipod-driven and blebbing motility. Further, cells can modulate their level of adhesion to the extracellular matrix (ECM) depending on both the level of force applied to the adhesions and cell intrinsic biochemical properties. We have constructed a computational model of cell motility to investigate how motile cells transition between extracellular environments with varying surface continuity, confinement and adhesion. Changes in migration strategy are an emergent property of cells as the ECM geometry and adhesion changes. The transition into confined environments with discontinuous ECM fibres is sufficient to induce shifts from lamellipod-based to blebbing motility, while changes in confinement alone within a continuous geometry are not. The geometry of the ECM facilitates plasticity, by inducing shifts where the cell has high marginal gain from a mode change, and conserving persistency where the cell can continue movement regardless of the motility mode. This regulation of cell motility is independent of global changes in cytoskeletal properties, but requires locally higher linkage between the actin network and the plasma membrane at the cell rear, and changes in internal cell pressure. In addition to matrix geometry, we consider how cells might transition between ECM of different adhesiveness. We find that this requires positive feedback between the forces cells apply on the adhesion points, and the strength of the cell–ECM adhesions on those sites. This positive feedback leads to the emergence of a small number of highly adhesive cores, similar to focal adhesions. While the range of ECM adhesion levels the cell can invade is expanded with this feedback mechanism; the velocities are lowered for conditions where the positive feedback is not vital. Thus, plasticity of cell motility sacrifices the benefits of specialization, for robustness.     

Dye-Surfactant interaction in the premicellar region

The interaction of safranin-O (SO), a cationic dye, with anionic surfactants [sodium dodecylsulfate (SDS) and sodium dodecylsulfonate (SDSo]) having the same hydrophobic group but different polar groups was studied spectrophotometrically in the premicellar region far below the critical micelle concentration in aqueous media using the method of continuous variations, also called Job's Method. This method was applied to determine the equilibrium complex formation constant (K) as well as the molecular complex formation ratio. Both SDS and SDSo formed stable associations with SO, and the molecular complex formation ratio was determined as 1:1. The data obtained from Job's Method indicated that the equilibrium complex formation constant of SDS is higher than that of SDSo. We also studied the influence of cosolvents such as methanol, dimethylformamide (DMFA), and dioxan (DX) on molecular complex formation between SDS and SO, and observed that the presence of cosolvents inhibited molecular complex formation. This inhibitory effect of cosolvents on molecular complex formation followed the order of methanol>DMFA>DX.     

Determination of laminar and turbulent flow ranges through vertical packed beds in terms of particle friction factors

Despite the presence of a variety of studies dealing with the magnitude of particle Reynolds number, Re_p defining transition from laminar to turbulent regime for flow through packed beds, the manner is still one of the unknowns. An approach based on the experimental data concerning upward airflow through fixed cylindrical packed beds is presented in this paper. The utilized packed beds had the following ranges of; sphericity, Φ, 0.55 ? Φ ? 1.00, packing material diameter to bed length ratio, D_p/L, 0.04 ? D_p/L ? 0.72, and bed porosity, ε, 0.36 ? ε ? 0.56. The test cases covered the ranges of particle Reynolds number, Re_p 708 ? Re_p ? 7772 and particle Froude number; Fr_p 2.86 ? Fr_p ? 10.39. The measurements of pressure drop through packed bed; ΔP_Bed and superficial mean exit velocity; U are used to determine bed frictional effects in reference to the available literature on particle friction factors, f_p. The magnitude of Re_p defining transition is assumed to be 2000 with particular emphasis to the flow dynamics. The definitions of Bird et al. [R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena, John Wiley and Sons, NY, 1960] are used to calculate f_p. The calculated f_p for the covered test cases are given as a function of pressure coefficient, ΔP* and Re_p, Fr_p, Φ, ε, D_p/L in the approximate ranges of laminar and turbulent flow for Re_p < 2000 and Re_p > 2000, respectively. The proposed separate equations of f_p = f_p(ΔP*, Re_p, Fr_p, Φ, ε, D_p/L) are satisfied for laminar and turbulent flows with corresponding average error margins of ±7.6% and ±18%. Furthermore ranges of transitional and fully rough flow through packed beds are estimated as 2000 ? Re_p ? 4000 and Re_p > 5000 with an analogy to the well-known Moody Chart in pipe flows.     

Controlled release of doxycycline within core/shell poly(ε-caprolactone)/poly(ethylene oxide) fibers via coaxial electrospinning

The development of fibers with desired drug release properties has gained a high research interest for water-soluble drugs with controlled drug delivery systems obtained by coaxial electrospinning technique. The objective of this study is to achieve the controlled-release of doxycycline hyclate (DOXH) from the fabricated electrospun fibers. In this case, three different electrospun core/shell fibers have been successfully fabricated using this technique and the model drug, DOXH, has been entrapped in the core layers. The results of the structural properties and in vitro release studies have been compared with electrospun monostructural fibers fabricated by conventional electrospinning technique. Scanning electron microscopy and transmission electron microscopy images have proved that the fabricated electrospun fibers have core/shell structures. Fourier transform infrared spectroscopy has shown convenient interaction and compatibility between polymers and the model drug. X-ray diffraction analysis has revealed that all the encapsulated DOXH are transferred into amorphous physical state and lost its crystalline state in the fibers. Moreover, drug release studies have demonstrated that the electrospun core/shell fibers show a better-controlled release than the monostructural fibers. It can be concluded that the fibers obtained by blending hydrophilic and hydrophobic polymers such as poly(ε-caprolactone) and poly(ethylene oxide) in both shell and core sides are promising candidate for controlled drug release.