Constant Axial Flow Research Articles

Xylem conductance of sweet cherry pedicels

Flow rates were quantified on detached pedicels and conductances calculated. Flow rates and conductances were independent of time and temperature, increased with the pressure applied and slightly decreased during development. Water uptake is thought to contribute to cracking of sweet cherry fruit. Water uptake/loss occurs not only through the fruit surface but also through the xylem and phloem of the pedicel. The objectives here were: (1) to quantify the hydraulic conductance of the pedicel xylem and (2) to identify the factors affecting pedicel xylem conductance. Using a modified pressure probe, water was forced through excised pedicels. The rates of axial flow were quantified gravimetrically by collecting water at the pedicel’s distal end in a pre-weighed Eppendorf tube. Applying pressure to the proximal end of a pedicel induced a constant axial flow of water through it. The introduction of air embolisms to the xylem markedly decreased axial conductance from 1.3 ± 0.1 × 10−10 m3 MPa−1 s−1 to near zero. Steam girdling had no effect on axial conductance indicating the water flows to be through the xylem, not significantly through the phloem in our ex situ arrangement. Compared with a petiole, the pedicel conductance was 50-fold lower. A cross-section of a pedicel near its distal end revealed about 10 vascular bundles that diverged from a complete ring closer to the proximal end. Conductance estimates from the numbers and sizes of xylem vessels using Hagen–Poiseuille’s law exceed those determined experimentally by about three to fourfold. Conductance of pedicels decreased slightly in developing fruit and then remained about constant until maturity. Surprisingly, there was no significant effect of temperature on conductance. The conductance estimates may be used to model xylem water flows through the pedicel.

Hot zones formation during regeneration of diesel particulate filters

AbstractA diesel particulate filter (DPF) is used to remove particulate matter (PM) from the diesel engine exhaust. The accumulated PM is periodically removed by combustion, which sometimes leads to excessive temperature excursions that melt the ceramic filter. This behavior cannot be explained by operation under stationary feed conditions. We propose that these temperature excursions are a dynamic effect following a rapid change in the driving mode while the DPF is being regenerated. Specifically, a rapid decrease in the exhaust temperature can lead to a counterintuitive large transient temperature rise above that which would exist under a higher stationary feed temperature. This unexpected behavior is similar to the well‐known wrong‐way behavior in packed‐bed reactors, even though the axial‐dependent flow through the filter in a DPF is rather different from the constant axial flow through a packed bed. We present simulations that provide insight about the dependence of the amplitude of this wrong‐way temperature rise on the filtration velocity, the PM loading, dimensions of the DPF, and the amplitude of the rapid temperature decrease and when it occurs after the start of the regeneration. The insight provided by these simulations will help develop operation and control protocols that circumvent or at least decrease the probability of the occurrence of the destructive melting of the DPF. © 2010 American Institute of Chemical Engineers AIChE J, 2011

Numerical Study of Swirl Properties of Rotating Conical Channel with Axial Flow

This work is concerned with a numerical study of the laminar flow of a viscous fluid between two coaxial cones, with the inner one rotating on which a constant axial flow is imposed. The two cones have the same apex angle value, so that the gap width between them is constant. Two configurations of the same volume are studied: (i) divergent flow passage (Div) and (ii) convergent flow passage (Cov), which represents geometrically the reverse case of (i). Analysis with the aid of the boundary layer assumptions is carried out by the use of an implicit finite difference method. A coordinate transformation is applied to the governing equations in order to remove the explicit effect of the apex angle from the calculation process for both cases, so that the boundary conditions can be simplified. The swirl characteristics are studied in both configurations with regard to the same inlet flux and the same rotation speed of the inner cone. The competition between centrifugal and axially-transporting effects in both cases is discussed. As a result, it is found that the swirl increases in Div from the entrance for a fixed value of the apex angle, while the swirl evolution in Cov depends strongly on the apex angle value.

Instabilities of the Belousov-Zhabotinsky Reaction in a Taylor Vortex Flow Reactor with Constant Axial Flow.

Instabilities of the Belousov-Zhabotinsky reaction were investigated in a Taylor vortex flow reactor with constant axial flow regarded as a one-dimensional open reaction-diffusion system. The effective axial diffusion coefficient was used as one of the controlling parameters, and changed by varying the rotating speed of the inner cylinder. Effluent from an isothermal continuous stirred tank reactor was supplied as the feed into the test reactor so as to clearly specify the inlet condition of concentration oscillation. The feed rate was changed as another controlling parameter. Irregular disturbances appeared at low-concentrations of Ce4+ when the inlet condition was singly periodic oscillation, whereas no remarkable irregular oscillation could be found when the inlet condition was dynamical equilibrium. The spatio-temporal oscillation patterns were successfully described qualitatively by a numerical simulation based on the ‘Coupled Map Lattice’ method using an Oregonator reaction model. The numerical results also suggested that complex spatial and temporal behavior occurred under the inlet condition of singly periodic oscillation.

Heat/mass transfer in taylor vortex flow with constant axial flow rates

An experimental investigation was done by the use of electrochemical technique in order to obtain the interrelation between the axial movement of Taylor vortices and the periodically varying rates of heat/mass transfer on the internal surface of the outer cylinder. When the Reynolds number of the axial flow is raised gradually at a fixed value of the Taylor number of the rotating flow, the regular sinusoidal variation of Sherwood numbers is distorted by the axial flow added, and then its mean value and amplitude are greatly reduced. The axial flow causes the damping effect not only on the initial formation of Taylor vortices but also on the heat/mass transfer mechanism involved in the momentum vortex boundary layer. It has been found that application of the heat-transfer data obtained for zero axial flow rate in order to design this type of heat exchanger with small, constant axial flow leads to dangerous 30–50% underestimation of the surface area of heat transfer in the experimental range.

IDEAL PLUG-FLOW PROPERTIES OF TAYLOR VORTEX FLOW

Mixing properties of Taylor vortex flow with small constant axial flow rates were investigated in an annulus between two concentric rotating cylinders by injecting a salt-solution tracer or by suspending small soluble particles. Longitudinal intermixing of fluid elements between the neighboring vortices was examined by a method of two-point measurement. Circumferential mixing in each cellular vortex was also evaluated by comparing two successive time variations of concentration at a certain point in the case of no axial flow. The modes of flow were mapped in terms of Taylor number and Reynolds number. This vortex flow has been found to be an ideal plug-flow for the range of 51.4<Ta<640 and 0<Re<90. Each cellular vortex marches through the annulus in single file without intermixing over the cell boundary, whereas the toroidal motion of fluid elements causes highly effective radial mixing in the vortex. When Taylor number exceeds 640, the plug flow cannot be maintained owing to the occurrence of longitudinal intermixing over the cell boundary, which is attributable to the transition from singly to doubly periodic flow.