Non-uniform Flow Pattern Research Articles

Effect of inserted rod and cross flow on top flooding of pipe

Flow pattern in flooding pipes can be considerably changed by inserting a rod or posing a cross flow at the top end. A non-uniform flow pattern around the circumference due to the inserted rod was observed in the present experiment. By increasing the diameter of the rod, a similar flow pattern as that in a thin rectangular channel was observed and the partial delivery flow rate could be predicted well with the correlation for the thin rectangular channel. The cross flow at the top end resulted in a non-uniform distribution of the water film around the pipe periphery. The partial delivery of water flow was decreased when the cross flow velocity was increased at a given gas velocity. The limitation mechanism for the cross flow energy of the delivery water was considered.

Effective Porosity and Flow Rate with Infiltration at Low Tensions into a Well-structured Subsoil

A combination of tension infiltrometers and dye tracers was used to investigate the extent and nature of water movement at low tensions through the well-structured subsoil of Ships clay (very-fine, mixed, thermic, Chromic Hapluderts). Dyed-water was placed in the reservoirs of tension infiltrometers and allowed to infiltrate into the soil under seven tension sequences to separate different size pores effective in transmitting water. Flow patterns were then revealed by exposing dye-stained soil after each infiltration sequence. The results showed that, when water was supplied at tensions <24 cm, the effective porosity (fraction of pores that were stained) in this subsoil was primarily macro- and mesoporosity, which constituted about 5% of the total soil porosity. Under flow at 0-cm tension, macropores =0.5 mm and mesopores from 0.06 to 0.5 mm (radius for cylindrical pores or width for planar pores) contributed about 89% and 10% of the total water flux, respectively. Micropores <0.06 mm contributed the remaining 1% of the total water flux, but constituted about 95% of the total soil porosity. Dye stain patterns showed that water flow at tensions <24 cm in this structured subsoil was primarily controlled by slickenside fissures, root channels, and vertical fissures. The nonuniform flow pattern caused water to penetrate to depths as much as 11 times deeper than expected from a Green-Ampt model that considered the total soil porosity to be active in transmitting water. By replacing the air-filled porosity in the Green-Ampt model with the active macro- plus mesoporosity, calculated wetting front depths were close to the observed maximum dye depths.

Flow maldistributions and hot spots in down-flow packed bed reactors

A two-dimensional two-phase (heterogeneous) model that predicts flow maldistributions and hot spots in an adiabatic down-flow packed bed reactor is developed and analyzed. The model considers the effects of fluid flow, changes of physical properties, heat and mass transfer by conduction and convection, interphase gradients and reactant consumption by an exothermic irreversible first-order reaction with Arrhenius kinetics. The one-dimensional model is analyzed in detail using singularity theory. Linear instability theory is used to determine the boundary of the region of parameters at which the uniform down-flow loses stability, and it is shown that flow maldistributions are possible for typical operating conditions. Bifurcation theory, numerical spectral and continuation methods are combined to determine the different types of nonuniform flow patterns and corresponding temperature and concentration fields. Unlike the pseudohomogeneous model where the maximum exit temperature cannot exceed the adiabatic temperature rise ( Pe m → ∞), the heterogeneous model can have isolated high temperature branches that exist when particle Lewis number is less than unity. It is shown that this maximum temperature on the isolated branch can be as high as β/ Le where β is the adiabatic temperature rise and Le is the particle Lewis number ( Le < 1.0). Finally, a possible mechanism for the formation of the localized hot spots observed in commercial reactors is proposed; the interphase resistances cause the existence of very high temperature isolated steady-state branches which are attained by the catalyst particles when maldistributed flows occur.

Mechanisms of Recirculating Liquid Flow on Distillation Sieve Plates

This paper describes an experimental investigation into the phenomenon of flow recirculation on distillation sieve trays. A novel dye injection technique has been applied to a 1.81 m air-water simulation column and has yielded new information concerning the nature of the boundary layer of gas-liquid biphase as it detaches from the column wall. The study has shown that recirculation is strongly influenced by inlet conditions. A critical factor is the underflow clearance between the inlet downcomer apron and the tray floor. As this clearance is increased, the size of the recirculating zones passes through a minimum, indicating the existence of two different mechanisms responsible for the nonuniform flow patterns. A significant implication of this work is that tray designers may minimize the impact of recirculating on mass transfer efficiency by appropriate choice of underflow clearance.

Separation of Uranium Isotopes by the Chemical Exchange Method

Abstract The basic chemical process and technology for producing 3-5% enriched uranium has been established, through advances which allowed increases in the electron-exchange and adsorption-desorption reaction rates, effective uranium adsorption band formation and maintenance, reduction of the mobile-phase dispersion, and reduction in the height of the separation unit, which is largely determined by the diffusion coefficient, the electron exchange reaction rate of uranium ions, and the non-uniform flow pattern in the adsorption band. Physical theory and experimental results show the attainment of a specific separation power of approximately 500 SWU/m3·yr for the process, and the possibility of an enrichment cost of $41/SWU in its commercial-scale application as calculated with depreciation terms of 15 years for equipment and 45 years for buildings and interest payments at 8% on investment capital. Inherent advantages of the process, in addition to low enrichment cost, are simple, stable operation and facilitation of the nuclear fuel cycle, with efficient separation of uranium-235 from the other uranium isotopes of spent nuclear fuel and elimination of the need for UF6 conversion.

Unsteady Flow in a Centrifugal Compressor With Different Types of Vaned Diffusers

Experiments were conducted to investigate the characteristics of self-excited flow oscillations in a high-performance centrifugal compressor system with a straight channel radial vaned diffuser. Fast response dynamic pressure transducers on the shroud wall and blade-mounted strain gages were used to identify the onset of the oscillations and their characteristics in space and time. In addition, flow characteristics near the shroud wall were visualized by an oil injection method, showing the extent of upstream directed reverse flow in the impeller range during significant unsteady flow compressor operation. Rotating nonuniform flow patterns were found in a wide range of operating speeds before the occurrence of surge. The number of lobes in the nonuniform flow patterns was dependent on the operating conditions and varied from two to four. Results of this experimental investigation were compared with those obtained from a previous investigation of the same compressor but with a cambered vane diffuser. Considerable similarity between the two configurations was found in the spatial distribution of the unsteady pressure field and in the frequencies of the fluctuations. The stability margin before the occurrence of surge and the operating regimes in which very intense pressure fluctuations were found were however different. In both cases, flow visualization techniques revealed the occurrence of reversed flow near the shroud wall of the impeller. Reverse flow extent up to the leading edge of the splitter blades systematically correlated with the occurrence of a nonuniform pressure pattern rotating with relatively high speed. Low rotational speed pressure patterns were observed when the extent of the reverse flow was up to the leading edge of the long blade. These different flow characteristics can be related to the occurrence of distinct rotating stall cell numbers. This result could be confirmed by unsteady pressure and blade vibration measurements.

Scaling‐up distillation efficiencies

AbstractThis paper describes a study of the possibility of using point efficiencies, measured in a small laboratory column, in the scale‐up of distillation efficiencies. The conditions on a large rectangular tray were closely duplicated in a small glass Oldershaw column with one sieve tray having similar hole size and percent free area. Point efficiencies on the large rectangular sieve tray were deduced from the composition profiles and compared with the point efficiencies measured in the small column. A simple relationship between the two columns was found in terms of the froth heights and liquid holdups. The results demonstrate that the slightly lower point efficiencies measured in the Oldershaw column could be used in conjunction with a mathematical mass transfer model based on the eddy diffusion concept, to relate the point efficiencies to the tray efficiences, resulting in a conservative design provided either that allowance is made for other factors, such as nonuniform flow patterns and stagnant zones, or that these are eliminated. A wide range of composition was covered using the two systems MeOH‐nPrOH and EtOH‐nPrOH operating in the froth region.

Centrifugal Fan Performance With Distorted Inflows

Efforts to quantify degradation of the performance of a centrifugal fan subjected to distorted inflows are presented. The study centered on examination of pressure rise, efficiency, and onset of stall for a backwardly-inclined, airfoil-bladed fan. Nonuniform flow patterns were generated in an adjustable countervane damper assembly installed upstream of the inlet box, and systematic families of performance curves were generated. Results of these tests show that significant degradation in efficiency and pressure rise—as much as 10 to 15 percent—may result from moderately to severely distorted inflow patterns. Onset of stall was significantly influenced by severely distorted inflows.

Improving the Air Flow Distribution in a Batch Walnut Dryer

ABSTRACT ENGLISH walnuts are usually dried on farm in batch dryers. Field measurements have shown that non-uniform air flow patterns exist in one common type of bulk dryer. These non-uniform air flows are the suspected cause of non-uniform drying that has been observed by some walnut growers. A two-dimensional finite element model has been used to evaluate modifications to improve the air flow distribution within the drying bins.

Flow, bed topography, grain size and sedimentary structure in open channel bends: A three-dimensional model

Bed topography and grain size are predicted for steady, uniform flow in circular bends by consideration of the balance of fluid, gravity and frictional forces acting on bed load particles. Uniform flow pattern is adequately described by conventional hydraulic equations, with bed shear defined as that effectively acting on bed load grains. This analysis is used as a basis to predict bed topography and grain size for steady, non-uniform flow in non-circular bends (represented by a ‘sine-generated’ curve). The non-uniform flow pattern is calculated using the method of Engelund (1974a). Equilibrium bed form, hence sedimentary structure, is found by comparison of existing flow conditions with one of the schemes describing the hydraulic stability limits of the various bed forms. The model was compared with bankfull flow observations from a channel bend on the River South Esk, Scotland. Theoretical bed topography and velocity distribution were very close to the observed data. However, bed shear stress showed only a broad agreement, probably because of the use a constant friction coefficient value. Mean grain size distribution showed good agreement, but theory did not account adequately for gravel sizes in the talweg region and on the upstream, inner part of the bar, possibly due to theoretical underestimation of effective bed shear. Bed form and sedimentary structure are predicted well using the familiar stream power-grain size scheme. The behaviour of the model under unsteady uniform flow conditions in circular bends was analyzed, and suggests that any variation of grain size and bed topography with stage is likely to be limited to deeper parts of the channel.

Hydrodynamically induced crystallization of polymers from solution

This paper primarily describes the large diversity in morphology of fibrillar polyethylene crystals precipitated from stirred solutions at elevated temperatures. Shish-kebabs crystallized at 112°C from stirred xylene solutions of linear polyethylene exhibit lamellar overgrowths having thicknesses of the order of 150 A from which it was concluded i. a. that at crystallization temperatures above 100°C only backbones are formed and that the surrounding polymer molecules deposit epitaxially on the substrate at lower temperatures. The variation in the appearance of the overgrowth and the inhomogeneity of the samples can be traced back to irregular stacking and to differences in washability of the fibrillar crystals, these differences being largely due to the non-uniform flow pattern along the various stirrer parts. Unusual lamellar shapes, interconnections, sheets between lamellae, the “veil” on the Shish-kebab noted byKeller andMachin, may well be explained byKeith, Padden andVadimskys mechanism of crystallization-induced orientation of macromolecules between the lamellae and the consequent formation of extendedchain crystals. The lamellar overgrowth appears to be arranged along the backbone according to a logarithmic normal distribution, and the average diameter of the lamellar crystals increases with the average spacing, both being approximately equal in size. The fibrillar crystals end in smooth tapered tails having lengths varying between 5 and 8 μ. These observed fibre ends agree best with the “unroll” model proposed for fibrillar crystal growth in flow fields if on the average each molecule introduces one chain fold and the chain ends are randomly assimilated by the crystal lattice. Linear polyethylene below molecular weight of 50×103 and low density polyethylene did not crystallize in fibrillar habit. whereas copolymers of ethylene and slight amounts of propylene and butylene proved capable of forming fibrillar crystals and isotactic polypropylene yielded smooth and overgrown fibres.