Flow Entrance Research Articles

Jet dispersion and deposition of charged particles in confined chambers

Dispersion and surface deposition of charged particles by gas–solids jets in confined chambers are constantly encountered in many industrial applications such as in electrostatic precipitation and dry powder coating processes. Understanding and control of flow patterns and trajectories of charged particles are important to the optimal design and operation of such devices. In this study, modeling of flow fields and particle trajectories of dilute gas–solid two-phase flows with charged particles in confined chambers is performed. The dilute gas–solid two-phase flows are simulated by use of a hybrid Eulerian–Lagrangian approach with the one-way coupling between the gaseous phase and particle phase. The space charge distribution is included as a source term in equations of motion or Lagrangian equation of charged particles, which in turn depends on the particle trajectories that determine the space charge distribution. Our modeling predictions suggested that the electrostatic charge plays a significant role in particle radial dispersion. Effect of voltage has limited influence on particle trajectories however it can have a big impact on the residence time. Cone angle has a significant effect on the structure of flow field. For cone with a larger cone angle (typically over 15°), there will be a flow separation along the side wall near the flow entrance region. By comparing with the conical chamber, the cylindrical chamber has a big vortex and three smaller vortexes in the lower part of the chamber, which would complicate the particle dispersion with or without the coupling of charging.

A numerical investigation of developing flow and heat transfer in a curved pipe

PurposeThis article aims to study numerically three dimensional developing incompressible flow and heat transfer in a fixed curved pipe.Design/methodology/approachA projection algorithm based on the second order finite difference method is used for discretizing governing equations written in the toroidal coordinate system.FindingsThe effects of curvature and governing non‐dimensional parameters consisting of Reynolds, Prandtl, and Dean numbers on the flow field, entrance length, and heat transfer are studied in detail. The numerical results indicate that the entrance length depends only on the Reynolds number for the curvature ratios greater than 1/7 and therefore, Dean number is not a pertinent parameter in this range.Research limitations/implicationsFor heat transfer analysis, two different thermal boundary conditions, i.e. constant wall temperature and constant heat flux at the wall are implemented. The results are calculated for the Dean numbers in the range of 76‐522 and for the two prandtl numbers of 0.5 and 1.Practical implicationsThe results can be used in designing heat exchangers, piping systems, and cooling of gas turbine blades.Originality/valueThe numerical results obtained here concentrate on the detailed investigation of flow and temperature field at the entrance region by a quantitative analysis of hydrodynamic and thermal entrance length. The effects of different thermal boundary conditions and different inlet profiles on the flow and temperature fields are studied in the circular curved pipe for the first time.

Three-dimensional laminar slip-flow and heat transfer in a rectangular microchannel with constant wall temperature

Three-dimensional laminar slip-flow and heat transfer in rectangular microchannels having constant temperature walls are studied numerically using the finite-volume method for thermally and simultaneously developing flows. The Navier–Stokes and energy equations are solved with velocity slip and temperature jump at the wall. A modified convection–diffusion coefficient at the wall–fluid interface is defined to incorporate the temperature-jump boundary condition. Validity of the numerical simulation procedure is established and the effect of rarefaction on hydrodynamicaly developing flow field, pressure gradient and entrance length is analyzed. A correlation for the fully developed friction factor is presented as a function of Knudsen number ( Kn) and aspect ratio ( α). The influence of rarefaction on the Nusselt ( Nu) number is investigated for thermally and simultaneously developing flows. The effect of velocity slip is found to increase the Nu number, while the temperature-jump tends to decrease it, and the combined effect could result in an increase or a decrease in the Nu number. In the fully developed region, there could be high as 15% increase or low as 50% decrease in Nu number is plausible for the range of parameters considered in this work.

Simultaneous measurements of velocity and stress distributions in polyisobutylenes using laser-Doppler velocimetry and flow induced birefringence

An experimental device was set up for the synchronous measurement of velocities and stresses in polyisobutylenes using laser-Doppler velocimetry (LDV) and the two-colour flow-induced birefringence method (FIB). The materials investigated are three low molecular polyisobutylenes. Velocity (LDV) and stress (FIB) measurements are performed in the flow entrance region and inside a slit die with a contraction ratio of 1:10. The behaviour of the polyisobutylenes is Newtonian under the flow conditions applied. Therefore, the stresses inside the fluids can be calculated and compared to the stresses experimentally determined. A good agreement in shear and elongational flows was found between the calculated (LDV) and directly measured stresses (FIB). This result demonstrates the applicability of the experimental setup as an optical rheometer that can preferentially be used to measure elongational properties of low viscous fluids.

Hydrodynamics of Gas−Liquid Slug Flow along Vertical Pipes in the Laminar RegimeExperimental and Simulation Study

An experimental and simulation study on free-bubbling vertical slug flow in the laminar regime was developed. A non-intrusive image analysis technique and a developed simulation code (SFS, slug flow simulator) were used for the purpose. A single correlation was obtained for the prediction of the bubble velocity as a function of the length of the liquid slug ahead of the bubble. Strong bubble interaction was found for liquid slugs shorter than 2D, with weak and decreasing interaction persisting for longer liquid slugs. Coalescence, though sparse, was found to occur along the whole column length (6.5 m). These observations differ from the findings regarding turbulent regime (bubble interaction for liquid slugs shorter than 8−10D and coalescence mainly in the lower part of the column). A slug flow entrance length of 70−100D was obtained for inlet slug length distributions centered on 2−4D, for the ranges of superficial gas and liquid velocities studied (0.05−0.20 m/s). Different inlet slug length distributions (for instance centered on 2D and 5D) were found not to evolve to a single flow pattern within the 6.5 m length of the column. General expressions were proposed to predict the evolution of the mode and standard deviation of bubble velocity, bubble length, and liquid slug length distributions as a function of the vertical column coordinate and superficial gas and liquid velocities. Bubble coalescence was found to govern the evolution of the liquid slug length along the column. Gas expansion and bubble coalescence were found to play important roles in the evolution of bubble length and velocity.

Test of flow field on the annular meridian plane in a tubular membrane separator with rotary tangential flow

AbstractEnhancement of membrane microfiltration by rotary tangential flow is a new technique, which is based on the hydrocyclone mechanism. It improved the structure of the general membrane separator and the form of the liquid suspension flowing into the separator, so as to increase membrane fluxes and decrease membrane fouling. In our research, a tubular membrane separator with rotary tangential flow was designed for the first time. The flow field characteristics of polypropylene tubular membrane microfiltration in this tubular separator were studied systematically by means of the Particle Image Velocimetry (PIV) test. Streamlines and velocity distributions of the meridian plane of the separator under different operating parameters were obtained. The velocity distribution characteristics of rotary circular tangential flow were analyzed quantitatively with the following conclusions being obtained: In the non‐vortex area, no matter how the operating parameters (flux, entry pressure) change, the velocity near the rotary tangential flow entrance is higher than the velocity far from the entrance at the same radial coordinates. In the vortex area, generally the flow velocity of the inner vortex is lower than that of the outer vortex. At the vortex center, the velocity is the lowest, the radial velocity being generally equal to zero. In the vortex zone, the radial velocity is less than the axial velocity. Under test conditions, the radial velocity and the axial velocity of the vortexes' borders are 1–2 times the average axial velocity in the annular gap of the membrane module. The maximum radial velocity and axial velocity of Taylor vortexes are 2–5 times the average axial velocity in the annular gap of the membrane module. In the vortexes that formed on the meridian plane, it was found that mass transfer occurred between the inner and outer parts of the fluid. Much fluid moved from the outer vortexes into the inner ones, which was able to prevent particles blocking the membrane tube. Copyright © 2004 Society of Chemical Industry

Analysis of heat and mass transfer in porous sorbents used in rotary regenerators

A new dimensionless formulation, that locally accounts for heat conduction and mass diffusion in solid sorbent materials occurring in either enthalpy exchangers or desiccant wheels has been developed. Governing equations were fully normalized using classical dimensionless groups for heat and mass transfer. The model was validated using previously published results, including experimental data. The proposed equations are easily adaptable to incorporate additional features such as the influence of a supporting structure or the effects of flow entrance regions. Finally, results of a test-case are presented, indicating a possible optimization to wheel construction and compactness by reducing the felt thickness.

Flow Behavior in the Dump Diffuser Combustor with a Split Flow. Effects of Inclined Wall in a Sudden Expansion Region.

The cold flow experiments are made in a two-dimensional dump diffuser with a split flow entrance at flame dome head. Flow behavior over a flame dome head and its reattachment to the inclined wall are presented, when a symmetric flow is provided at the diffuser inlet. A Laser-Doppler Velocimeter is used for the velocity measurements. The effects of four types of inclined wall in a sudden expansion region on the diffuser performance are clarified. Throat area between inclined wall and flame dome head is the dominant parameter affecting the total pressure loss.

Flow properties and entrance corrections of polymer melts by a mobile barrel capillary rheometer

This paper investigates the flow properties and entrance corrections of a polyethylene melt in a capillary rheometer that features the possibilities of moving either the piston or the barrel, the latter mode being novel in operation. The paper mainly aims to investigate the variations in the flow properties and the entrance corrections due to the mode of operation. It is found that the flow properties and entrance corrections are dependent on the mode of operation and the die sizes used in the capillary rheometer, this being thought to be associated with the flows occurring in the capillary rheometer.

Changes in the Pulmonary Circulation after Total Cavopulmonary Anastomosis

Background:The hemodynamic superiority of total cavopulmonary anastomosis (TCPA) over atriopulmonary connection as a modification of Fontan-type operation, began to be acknowledged with more applications to clinical cases. The need of adequate resolutions for the investigation and improvement of residual hemodynamic derangements including abnormal distribution of the pulmonary blood flow, is emerging. Methods:We studied 20 patients (M:F=12:8, age:67.9±41.5 months) who have had the TCPA and were followed-up by cardiac catheterization, angiography and lung perfusion scan 24.5±15.7 months after the operation. Pulmonary arterial growth and hemodynamic influences including the pulmonary blood flow distribution were investigated to verify the appropriateness of the conventional TCPA method and to aid in the determination of the consequences and prognosis of the operation. Results:The mean pulmonary arterial pressure before and after the operation was 15.1±3.2 mmHg and 13.9±4.8 mmHg respectively without significant difference and there was no significant postoperative changes in the cross-sectional area index of pulmonary artery in regard to the variability of body surface area. The pulmonary blood flow was distributed with a greater amount in the ipsilateral side of IVC flow entrance (IVCipsi) than the contralateral side (IVCcontra), with an ipsilateral to contralateral perfusion ratio (i/cPR) of 1.24±0.42. Comparing the subgroups by the type of superior vena caval inflow, unilateral superior cavopulmonary anastomosis (SCPA) group showed significantly higher i/cPR (1.47±0.33) than the bilateral SCPA group (1.07±0.21). Comparing the subgroups by the type of IVC inflow, the i/cPR of the intraatrial tunnel group was higher than the hemiazygous continuation group, but there was no statistical significance. Conclusion:TCPA does not influence the growth of pulmonary artery, and the type of cavopulmonary anastomosis and the bilaterality of superior vena cava may have major influences on the distribution of the pulmonary blood flow. The details of surgical methods should be evaluated case by case in respect to the associated anomalies in order to achieve adequate postoperative pulmonary blood flow distribution. (Korean Circulation J 2000;30(1):90-102)

Effects of Perpendicular Flow Entry on Convective Heat/Mass Transfer From Pin-Fin Arrays

Convective heat transfer with pin-fin arrays have been studied extensively in laboratory experiments where flow is introduced to the array uniformly over the channel span. However, the flow path in actual cooling designs is often serpentine-shaped with multiple turns, and the pin-fin array section is often located immediately downstream of a turn. The present study, using an analogous mass transfer technique based on naphthalene sublimation, investigates the effects of three different, nonaxial flow entries on array heat transfer for both an inline and a staggered arrangement of pins. The measurement acquires the mass transfer rate of each individual pin in a five row by seven column array for the Reynolds number varying from 8000 to 25,000. The mass transfer and associated flow visualization results indicate that the highly nonuniform flow distribution established at the array entrance and persisting through the entire array can have significant effects on the array heat transfer characteristics. Compared to the conventional case with axial-through flow entrance, the overall array heat transfer performance can be either enhanced or degraded, depending on the actual inlet arrangements and array configurations.

Experimental study on the pressure drop and heat transfer characteristics of tubes with internal wave-like longitudinal fins

Experiments were performed to determine the heat transfer and pressure drop characteristics in the entrance and fully developed regions of tubes with internal wave-like longitudinal fins. The test tube has a double-pipe structure, with the inner tube as an insertion. The wave-like fins are in the annulus and span its full width. Experiments were conducted for two cases: one with the inner tube blocked (no air flowing through it) and the other with the inner tube unblocked. The outer tube was electrically heated. Local and average heat transfer coefficients and friction factors were measured. The friction factor and Nusselt number correlations in the fully developed region were obtained in the Reynolds number range of 9×102 to 3.5×103. It has been found that the wave-like fins enhance heat transfer significantly with the blocked case being superior. In addition, the in-tube heat transfer process is characterized by an earlier transition from laminar to turbulent flow and Reynolds number-dependent thermal entrance length.

Robust numerical treatment of flow transitions at drainage pipe boundaries

The present report describes a comprehensive modelling framework developed to handle flow transitions with drainage pipes and at their boundaries. The proposed numerical scheme is based on a finite volume formulation. Detailed specifications of boundary conditions at pipe ends are provided, along with a decision tree allowing to seamlessly manage transitions such as flow reversal, entrance drowning, or hydraulic jump migration through the boundaries. Proof-of-concept examples and comparison with laboratory data are given to illustrate the robustness of the proposed approach.

Surface instabilities of thin liquid film flow on a rotating disk

Steady flow of a liquid jet from a nozzle onto the centre of a rotating disk is studied with a streak line method to determine the superficial velocity of the spreading liquid film. Good agreement is found with an asymptotic analysis of the unperturbed flow field. Experimentally, the liquid surface is always perturbed by surface waves which appear as regular spirals, steady in the laboratory system in the low Reynolds number range. It could be shown that wave formation is very sensitive to entrance conditions. Therefore, it is assumed that wave generation is an entrance effect which acts as periodic forcing on the forming liquid film. Wave velocities outside the entrance region are measured and proved to be in good agreement with the prediction of a linear stability theory, as long as the flow rate and entrance perturbations are small. At higher flow rates or stronger disturbances, the radial development of the wave velocities takes on the characteristics predicted by nonlinear stability theories and is in qualitative agreement with experiments performed on an inclined plane.

Robust numerical treatment of flow transitions at drainage pipe boundaries

The present report describes a comprehensive modelling framework developed to handle flow transitions within drainage pipes and at their boundaries. The proposed numerical scheme is based on a finite volume formulation. Detailed specifications of boundary conditions at pipe ends are provided, along with a decision tree allowing to seamlessly manage transitions such as flow reversal, entrance drowning, or hydraulic jump migration through the bouadaries. Proof-of-concept examples and comparison with laboratory data are given to illustrate the robustness of the proposed approach.

Thermal transport due to material and gas flow in a furnace for drawing an optical fiber

The transport processes involved in the neck-down region for optical fiber drawing are numerically investigated. In this manufacturing process, a moving glass rod is heated in a furnace containing an inert gas environment and drawn into a thin optical fiber. The conjugate problem is solved considering both radiation and convection, with focus on the latter. Two different flow configurations, involving inert gas flow in the same as well as in the opposite direction as the moving preform/fiber, are considered in this study. A coordinate transformation is used to change the complicated computational domains in the gas and the fiber to cylindrical ones. The transport in the fiber is coupled with that in the gas through the boundary conditions. The radiative thermal transport is calculated using an enclosure model developed in an earlier study. The numerical results on convective flow and transport are validated by comparing with results available in the literaturefor simpler configurations. The effects of several important parameters such as fiber draw speed, inert gas velocity, furnace dimensions, and gas properties on the flow and temperature distributions are investigated. For the aiding flow case, in which the inert gases flow in the same direction as the fiber, heat transfer to the fiber increases as the gas velocity increases. For opposing flow, a recirculating region appears in the gas, close to the moving fiber surface, causing reduction in heat transfer as compared to the aiding case. The thickness of this recirculating zone decreases with increasing inert gas velocity. Radiation is found to be the dominant mode of heat transfer in the overall heating of the preform/fiber, with nitrogen as the inert gas. However, near the edges of the furnace, radiation heat transfer is relatively small and convection becomes very important. Also, the convective transfer rate is relatively large near the flow entrance because of the large temperature difference between the gas and the fiber. However, away from the entrance, the gas heats up and the temperature difference relative to the fiber decreases, resulting in a smaller convective heat transfer rate. The relevance of the results to various aspects of the fiber-drawing process is discussed.

Practical aspects in the drawing of an optical fiber

The transport processes in the furnace for the continuous drawing of optical fibers have been studied numerically and analytically. Practical circumstances and operating conditions are considered. A peripheral gas flow configuration has been modeled, along with irises at the ends, as employed in practical furnaces. The neck-down profile of the fiber is not chosen, but has been generated on the basis of a surface force balance. The results obtained are validated by comparisons with earlier experimental results. A detailed analysis has been carried out to determine the relative contributions of different forces during the drawing process. Even though the internal viscous stress is shown to be the major contributor to the draw tension, it is found that under certain operating conditions, the force due to gravity is significant, especially at the beginning of the neck-down region. For a peripheral flow configuration, the effect of flow entrance is found to be very important in determining the necking shape. However, the effect of the iris size on the fiber temperature field is found to be negligible. It is found that for a given furnace temperature and fiber radius, there is an upper limit for draw-down speed at which a fiber can be drawn without rupture. Practical ranges of draw speeds and furnace temperature conditions are identified for the process to be feasible.

Investigation of the wall shear stress in vertical bubbly flow under different bubble size conditions

New results of experimental investigation accounted for the parametric effects of the bubble size and the two-phase flow rates on the wall shear stress are reported. The mean and the time-varying fluctuation properties of the wall shear stress are measured based on a flush-mounted hot film sensor for air-water bubbly flow in a vertical channel. The unique feature of this study is that the experiments were carried out under various fixed gas and liquid fluxes, with only the bubble size being changed at the flow entrance. This has been made by using a special bubble generator to decouple the bubble size effect from the inlet condition. The test conditions cover both the wall and core peaking void distributions of bubbly two-phase flow. It is found that the wall shear stress is strongly influenced by the internal and wall proximity structure of the flow, while both the liquid phase velocity and the wall concentrated bubbles are the dominant parameters on both the magnitude and the fluctuation intensity of the wall shear stress in the regime of bubbly flow. The present data are extensively compared with some other data sources and with the models used in predicting the wall shear stress. Due to no obvious existing wall shear stress data and models were accounted for the change in bubble size systematically, the present data thus may serve as a relatively complete comparative basis for the development of theoretical models.

Flow analysis of natural rubber in a capillary rheometer. 2: Flow patterns and entrance velocity profiles in the die

AbstractAn investigation was carried out to determine the effect of variations in test conditions, such as die geometry, for example die entry angles, on the flow patterns observed in the die of a Davenport Extrusion Rheometer. In addition, the velocity profile near the die entrance was determined for different piston displacements. The results indicate that the velocity profile generated in the capillary near the die entrance was parabolic in form. However, there appeared to be a non‐zero velocity at the capillary wall. Melt fracture was observed, this being linked with the appearance of a disturbance in the flow along the capillary, the disturbance being of a converging helical form.

Single point aerosol sampling: evaluation of mixing and probe performance in a nuclear stack.

Alternative reference methodologies have been developed for sampling of radionuclides from stacks and ducts, which differ from the methods previously required by the United States Environmental Protection Agency. These alternative reference methodologies have recently been approved by the U.S. EPA for use in lieu of the current standard techniques. The standard EPA methods are prescriptive in selection of sampling locations and in design of sampling probes whereas the alternative reference methodologies are performance driven. Tests were conducted in a stack at Los Alamos National Laboratory to demonstrate the efficacy of some aspects of the alternative reference methodologies. Coefficients of variation of velocity, tracer gas, and aerosol particle profiles were determined at three sampling locations. Results showed that numerical criteria placed upon the coefficients of variation by the alternative reference methodologies were met at sampling stations located 9 and 14 stack diameters from the flow entrance, but not at a location that was 1.5 diameters downstream from the inlet. Experiments were conducted to characterize the transmission of 10 microns aerodynamic diameter liquid aerosol particles through three types of sampling probes. The transmission ratio (ratio of aerosol concentration at the probe exit plane to the concentration in the free stream) was 107% for a 113 L min-1 (4-cfm) anisokinetic shrouded probe, but only 20% for an isokinetic probe that follows the existing EPA standard requirements. A specially designed isokinetic probe showed a transmission ratio of 63%. The shrouded probe performance would conform to the alternative reference methodologies criteria; however, the isokinetic probes would not.