Axial Flow Velocity Research Articles

Vibration analysis of a fluid conveying sandwich cylindrical shell with a soft core

The free vibrations of a circular cylindrical sandwich shell with a flexible core containing a flowing fluid are investigated. Within the framework of the higher-order sandwich panel theory which assumes a variable displacement through the thickness, the core flexibility is taken into account. Using Hamilton’s extended principle, for an inviscid, non-rotational, laminar and incompressible flow, via the potential flow theory, the governing equations of the coupled structure and fluid system are derived. The eigenvalue problem’s equations are found by implementing a modal expansion method which has been validated by the existing results and is found to be accurate. The effect of the axial flow velocity, circumferential wave number, core to total thickness ratio and total thickness to mean radius ratio on the natural frequencies are investigated. It is found that taking into account the core's flexibility results in lower frequency values. The study shows that, unlike in dry shells, increasing the ratio of core thickness to total thickness reduces the natural frequencies monotonically in wet shells. For a constant total shell thickness, the critical velocity of the fluid decreases with increases in core thickness.

Sensitivity of Axial Velocity at the Air Gap Entrance to Flow Rate Distribution at Stator Radial Ventilation Ducts of Air-Cooled Turbo-Generator with Single-Channel Ventilation

In the design and calculation of a 330 MW water-water-air cooling turbo-generator, it was found that the flow direction of the fluid in the local stator radial ventilation duct is opposite to the design direction. In order to study what physical quantities are associated with the formation of this unusual fluid flow phenomenon, in this paper, a 100 MW air-cooled turbo-generator with the same ventilation structure as the abovementioned models is selected as the research object. The distribution law and pressure of the fluid in the stator radial ventilation duct and axial flow velocity at the air gap entrance are obtained by the test method. After the calculation method is proved correct by experimental results, this calculation method is used to calculate the flow velocity distribution of the outlets of multiple radial ventilation ducts at various flow velocities at air gap inlets. The relationship between the flow distribution law of the stator ventilation ducts and the inlet velocity of the air gap is studied. The phenomenon of backflow of fluid in the radial ventilation duct of the stator is found, and then the influence of backflow on the temperature distribution of stator core and winding is studied. It is found that the flow phenomenon can cause local overheating of the stator core.

Slip analysis with thermally developed peristaltic motion of nanoparticles under the influence of variable viscosity in vertical configuration

The aim of this paper is to present the analytical investigation on the peristaltic propulsion of an incompressible nanomaterial fluid passing through a two-dimensional symmetric channel. The stream flow comprises temperature dependent viscosity under the influence of mixed convection, heat transfer, temperature jump and velocity slip effects. Keeping pure water as the base fluid and using Silicon dioxide, Silver and Copper oxide as the selected nanoparticles, examination is carried out for the peristaltic transportation which has countless industrial appliances. Under the assumption of long wavelength and low Reynolds number, physical parameter effects on the exact solution established under physical boundary conditions for axial velocity, temperature profile, pumping characteristics and the trapping phenomenon are developed graphically and addressed in brief. Acquired solutions are employed to describe the influence of various emerging parameters on significant occurrences related to peristaltic propagation. The results we came across are that the axial flow velocity shows converse variations in the central part and around the walls of the channel, whereas heat transfer and thermal slip parameter increase shows rise in fluid temperature. Moreover, bolus trapping for increasing velocity slip brings improvement in both size and number for all nanoliquid particles CuO, SiO2, Ag and base fluid, whereas for rising thermal slip $ \gamma$ value, a decline in bolus size for all nonofluids occurs. However, pure water shows increment in size and number. A relative study is also reconnoitered for CuO, SiO2, Ag nanofluids and pure water.

Theoretical investigations on flow-induced vibration of fuel rods with spacer grids in axial flow

Fuel rods, one of the most important components in the nuclear reactor core, are usually supported by spacer grids along the axial direction. Since those fuel rods are long and slender, flow-induced vibration (FIV) will be induced and cause abrasion of the cladding in the axial flow of high-velocity coolant. A theoretical study applying modal analysis method and power spectral analysis method is carried out to analyze the vibration characteristics and FIV root-mean-square (RMS) response of fuel rods. In this study, a fuel rod with spacer grids is modeled as one dimensional multi-span beam and the fluid force along the fuel rod is considered as random impulsive load. The influences of span length, the axial flow velocity and the correlation length coefficient of axial flow on FIV response displacement of fuel rods are studied. The sensitivity analysis results reveal that the FIV of fuel rods has strong negative correlation with span length, while it is positive by related to the axial flow velocity and correlation length coefficient.

The Effects of Traction Velocity on the Gas-assisted Extrusion Forming and Traditional Extrusion Forming of Plastic Microtubes

The effects of traction velocity on gas-assisted extrusion (GAE) and traditional extrusion (TE) of plastic micro-tubes (PMTs) were numerically studied in this paper. Under same 3D geometric model, boundary conditions setting and constitutive parameters, the extrudate profile change and swell ratios of GAE and TE were all obtained. The radial and axial flow velocities, shear stress and first normal stress difference distributions of melt under different traction velocities were also gotten and compared. Results show that with the increase of traction velocity, the swell ratio increases first and then decreases. When the traction velocity is less than the axial velocity of melt, the radial velocity of melt outside of die increases, which demonstrates the generation of radial swell. When traction velocity is larger than that of the melt, the negative radial velocity of melt generates, which demonstrates the generation of shrinkage phenomenon. In addition, the changes of shear stress values and first normal stress difference values are not large, and the sudden increase of stresses always generates at the outlet die for the TE. However, the stresses of melt are well removed at the outlet of die, which demonstrates that extrudate swell and melt fracture problems are well eliminated by the GAE during the extruded PMTs dragged by traction velocity.

The effect of inner assisted gas length on the gas-assisted extrusion forming of plastic micro-tube

The effect of inner assisted gas length on the gas-assisted extrusion forming of plastic micro-tube was numerically studied in this paper. The 2D geometric models with different lengths of inner assisted gas were established. Under the same boundary conditions and material parameters, the profile changes of extruded plastic micro-tubes were obtained. To ascertain the reasons of profile changes induced by different lengths of inner assisted gas, the radial and axial flow velocities, shear stress and the first normal stress differences of melt were all obtained. Numerical results show that the diameter swell or shrinkage phenomenon occurs when the inner assisted gas length mismatches the outer assisted gas length. The deformations are generated by the unbalance axial flow velocity of melt induced by the mismatch of two assisted gases lengths, especially at the outlet of assisted gases. At the same time, the shear stress focus and first normal stress difference focus of melt are also existed at the outlet of assisted gases. In practice, the lengths of two assisted gases should be equal in the gas-assisted extrusion of plastic micro-tube.

Stream broadening due to fluid shear across the wider transverse dimension of a free-flow zone electrophoresis channel.

While the pressure-gradient applied along the length of a free-flow zone electrophoresis (FFZE) chamber is known to produce a parabolic flow profile for the carrier electrolyte across the narrower channel dimension (typically the channel depth), additional fluid shear can arise across the channel width due to a variety of reasons. Most commonly, any variation in the pressure-drop or channel depth across this wider dimension can lead to a gradient in the liquid flow velocity along it, significantly altering the stream broadening and, thereby, the separation performance of the assay. This article assesses the effect of such fluid shear on stream broadening during the FFZE process by describing a mathematical framework for solving the relevant advection-diffusion equation based on the method-of-moments approach. A closed-form expression for the leading order term describing the additional contribution to the spatial stream variance has been derived considering a small linear gradient in the liquid velocity across the wider transverse dimension of the FFZE chamber. The noted analysis predicts this contribution to be governed by two Péclet numbers that are evaluated based on the axial pressure-driven flow and transverse electrophoretic solute velocities. More importantly, this contribution is shown to vary quadratically with the axial distance traversed by the analyte stream as opposed to the classical linear variation known for all other stream broadening contributions in FFZE systems. The results from the analytic theory have been validated with Monte Carlo simulations, which also establish a time and length scale over which the noted analytical results are applicable.

Experimental study on flammability limits of electrolyte solvents in lithium-ion batteries using a wick combustion method

To quantify the flammability limits of organic electrolyte solvents used in lithium-ion batteries, a unique wick combustion system was developed in conjunction with limiting oxygen concentration (LOC) of candle-like flame, named wick-LOC method. By controlling the oxygen-nitrogen ratio of external flow of the wick diffusion flame, the flammability limits (LOC) of electrolyte solvents were determined experimentally. To provide reproducible results under specified conditions, the effects of axial flow velocity, exposed wick length and elapsed time after ignition on the wick-LOC were studied, and the proper experimental conditions were selected for further applications. To validate the reliability of wick-LOC in flammability evaluation, correlation analyses to other flammability properties (flash point, auto-ignition temperature, the heat of combustion and other types of LOC) were conducted. The wick-LOC method was then applied to quantify the flammability of mixed solvents. The linear changes of wick-LOC with mixing ratios were found in the mixture of linear and cyclic carbonates, while the non-linear trends were found in carbonate-ether mixed solvents. To evaluate the flame-retardant effectiveness of organophosphorus compounds (OPCs) as additives in electrolyte solvents, a series of tests were conducted. Results showed that small amounts of OPCs had significant flame-retardant effects, but the efficiency decreased with the higher OPC additions. The effectiveness of four OPCs was distinguished as well. The results of this work provided valuable information about the flammability limits of single and mixed electrolyte solvents, and it may be useful for designing electrolyte balanced in both performance and safety.

Fluid Dynamic Aspects of Protostellar Disks Formation

Abstract An analysis is presented of fluid dynamic aspects of the interaction between a disk and the infall of material from a surrounding non-magnetized rotating molecular cloud. The study put emphasis on the accretion shock formed in the vicinity of the disk and the post-shock flow field. This post-shock flow is considered parallel to the disk symmetry plane involving a super-Keplerian azimuthal velocity field v ∼ 1/r (potential vortex). The relative rotation between fluid and the nearly Keplerian rotating thin disk induces along its symmetry plane a boundary layer capable of transporting large quantities of fluid inward. This radial flow results in an axial flow i.e., a mass ejection from the boundary layer with axial jet flow velocities of up to about 300 km s−1.

Effects of Straw and Biogas Co-firing on the Combustion Characteristics

In this study, effects of straw co-firing with a renewable fuel – low heating value biogas – on the combustion characteristics, heat output from the device and composition of flue gas, are studied and analyzed with the aim to achieve more efficient use of straw for energy production. The study combines experimental research and mathematical modelling of the processes developing at co-firing of straw pellets with a low heating value biogas, which consists of 60 % methane and 40 % CO2. The complex research of the effects of straw co-firing on the gasification and combustion characteristics suggests that the increase of the biogas supply from 0 up to 1.35 l/min (additional heat input up to 0.4 kW) stabilizes the after-flame stage of the combustion of straw pellets, thus completing the burnout of volatiles, increasing the average value of the axial flow velocity, the flame temperature and the heat output from the device by about 25 – 30 %. The mathematical modelling of the straw co-firing with biogas is provided with account of variations of combustible volatiles by increasing the biogas supply into the experimental setup. A mathematical model of the combustion dynamics has been developed using MATLAB. A system of dimensionless parabolic type partial differential equations was used to describe the formation of the 2D compressible reacting swirling flow, the variations of the mass fraction of volatiles, flame species and flame temperature at thermo-chemical conversion of volatiles. Results of the numerical simulation were analyzed considering the processes developing downstream the combustor.

Microfluidic and computational study of structural properties and resistance to flow of blood clots under arterial shear

The ability of a blood clot to modulate blood flow is determined by the clot’s resistance, which depends on its structural features. For a flow with arterial shear, we investigated the characteristic patterns relating to clot shape, size, and composition on the one hand, and its viscous resistance, intraclot axial flow velocity, and shear distributions on the other. We used microfluidic technology to measure the kinetics of platelet, thrombin, and fibrin accumulation at a thrombogenic surface coated with collagen and tissue factor (TF), the key clot-formation trigger. We subsequently utilized the obtained data to perform additional calibration and validation of a detailed computational fluid dynamics model of spatial clot growth under flow. We then ran model simulations to gain insights into the resistance of clots formed under our experimental conditions. We found that increased thrombogenic surface length and TF surface density enhanced the bulk thrombin and fibrin generation in a nonadditive, synergistic way. The height of the platelet deposition domain—and, therefore, clot occlusivity—was rather robust to thrombogenic surface length and TF density variations, but consistently increased with time. Clot viscous resistance was non-uniform and tended to be higher in the fibrin-rich, inner “core” region of the clot. Interestingly, despite intraclot structure and viscous resistance variations, intraclot flow velocity variations were minor compared to the abrupt decrease in flow velocity around the platelet deposition region. Our results shed new light on the connection between the structure of clots under arterial shear and spatiotemporal variations in their resistance to flow.

Experimental and numerical studies for the gas-assisted extrusion forming of polypropylene micro-tube

In this study, to eliminate the extrudate swell, melt fracture and extrudate distortion problems of polypropylene (PP) micro-tube, the experiments of gas-assisted extrusion forming was firstly carried out. Meanwhile, a set of modified gas-assisted extrusion system was set-up, and a kind of novel gas-assisted die was also designed and manufactured. Under the role of gas-assisted extrusion system, PP micro-tubes were successfully prepared. Moreover, the macroscopic and microscopic qualities of PP micro-tubes for gas-assisted extrusion were obtained and compared with those of the traditional extrusion. Experimental results show that gas-assisted extrusion not only overcomes extrudate swell, but also greatly improves the smoothness and transparency of PP micro-tube. In addition, to ascertain the mechanisms of gas-assisted extrusion of PP micro-tube, numerical simulations were also performed. The swell ratios, flow velocities, pressure and stresses distributions of melt for gas-assisted extrusion and traditional extrusion were analyzed and compared. Moreover, the effects of inner cavity’s gases pressures and traction force on the gas-assisted extrusion of PP micro-tube were numerically investigated and discussed with the traditional extrusion. Numerical results show that the swell ratio of PP micro-tube increases with the inlet volume flow rate for traditional extrusion, but it is nearly equal to 0 for the gas-assisted extrusion. Moreover, due to the assistance of gas-layers, not only the axial flow velocity of melt is uniformed and radial flow velocity is removed, but also the pressure, shear stress and first normal stress difference of melt are all greatly reduced, especially at the outlet of die. These weakened pressure and stresses diminish the elastic energy storage and tensile stress of melt, and finally result in the elimination of extrudate swell, melt fracture and extrudate distortion. With the increase of inner cavity’s pressure, the inner and outer radiuses of the gas-assisted extrusion of PP micro-tube all increase, but the wall thickness decreases because the swell ratios of inner radius are all larger than that of the outer radius. In addition, under the same traction force, the sizes of gas-assisted extrusion of PP micro-tube are smaller than that of the traditional extrusion, although the sizes for the both extrusions of PP micro-tube all exponentially decrease with the increase of traction force. Moreover, the extrudate radial swell and melt fracture problems for the traditional extrusion are always existed at the outlet of die although the radial swell of melt decreases with the increase of traction force. However, the extrudate swell and melt fracture problems are greatly removed by the gas-assisted extrusion.

Inter-visit measurement variability of conjunctival vasculature and circulation in habitual contact lens wearers and non-lens wearers

BackgroundThe inter-visit variation of measuring bulbar conjunctival microvasculature and microcirculation needs to be considered when the results from multiple visits are interpreted. This study examined the inter-visit variability of measuring conjunctival microvasculature and microcirculation in habitual contact lens (HCL) wearers and non-contact lens (NCL) wearers.MethodsTwenty-eight subjects were recruited including 13 HCL wearers (10 females and 3 males; mean age ± standard deviation, 25.8 ± 4.6 years) who had worn contact lenses on a daily basis for at least 3 years and 15 NCL wearers (10 females and 5 males, age 25.5 ± 4.0 years) were recruited. The temporal bulbar conjunctiva was imaged using a functional slit-lamp bio-microscope (FSLB) imaging system. FSLB imaging was performed in the morning when the HCL wearers did not wear their lenses. The measurements included conjunctival vessel diameter, vessel density, blood flow velocity and flow volume. In addition, conjunctival microvasculature was analyzed using monofractal (Dbox, representing vessel density) and multifractal (D0 representing vessel complexity) analyses. The repeated measurement was conducted at least one week after the first visit and both eyes of each participant were imaged. The coefficient of variation (CV) was calculated as the standard deviation of the differences between test and re-test then divided by the mean of the measurements. The intra-class correlation coefficient (ICC) was also calculated.ResultsNo significant differences of all vascular measurements in both the right and left eyes were found between two groups (P > 0.05). Between two measurements on two different visits, the CV was from 2.4% (vessel density D0) to 63.5% (blood flow volume Q) in HCL wearers and from 3.4% (D0) to 40.6% (blood flow volume) in NCL wearers. The ICC was from 0.60 (vessel diameter) to 0.81 (axial blood flow velocity VA) in HCL wearers and from 0.44 (Q) to 0.68 (cross-sectional blood flow velocity VS) in NCL wearers.ConclusionsThe measurement variability of the vessel density of the bulbar conjunctiva appeared to have the smallest inter-visit variation. The measurement variability of the vasculature and circulation in HCL wearers were similar to that in NCL wearers.

A Rayleigh--Ritz method for Navier--Stokes flow through curved ducts

We present a Rayleigh–Ritz method for the approximation of fluid flow in a curved duct, including the secondary cross-flow, which is well known to develop for nonzero Dean numbers. Having a straightforward method to estimate the cross-flow for ducts with a variety of cross-sectional shapes is important for many applications. One particular example is in microfluidics where curved ducts with low aspect ratio are common, and there is an increasing interest in nonrectangular duct shapes for the purpose of size-based cell separation. We describe functionals which are minimized by the axial flow velocity and cross-flow stream function which solve an expansion of the Navier–Stokes model of the flow. A Rayleigh–Ritz method is then obtained by computing the coefficients of an appropriate polynomial basis, taking into account the duct shape, such that the corresponding functionals are stationary. Whilst the method itself is quite general, we describe an implementation for a particular family of duct shapes in which the top and bottom walls are described by a polynomial with respect to the lateral coordinate. Solutions for a rectangular duct and two nonstandard duct shapes are examined in detail. A comparison with solutions obtained using a finite-element method demonstrates the rate of convergence with respect to the size of the basis. An implementation for circular cross-sections is also described, and results are found to be consistent with previous studies. doi:10.1017/S1446181118000287

Study of the internal flow structure of an ultra-small axial flow hydraulic turbine

The ultra-small axial flow hydraulic turbine is a portable, palm-sized hydraulic turbine that can be applied to low head applications. By redesigning the original runner of this turbine using a method which uniformized the axial flow velocity, we succeeded in improving the performance of the runner. However, the mechanism of the improved performance of the optimized runner was not sufficiently clear; furthermore, the internal flow of the ultra-small axial hydraulic turbine had hardly been elucidated. This study aims to elucidate the internal flow structure of the ultra-small axial flow hydraulic turbine and to clarify the mechanism behind the improved performance of the optimized runner. To this end, we investigated in detail the internal flow structure of the original and optimized runners using PIV measurements and numerical analysis, focusing on the vortex structures. As a result, it was found that losses due to the tip leakage vortex and vortices near the hub at the blade outlet were smaller for the optimized runner than for the original runner. In particular, it was found that the turbine efficiency was greatly improved because vortices due to the swirling flow downstream of the runner were suppressed, and losses resulting from these vortices were thus reduced.

Numerical study on the flow in balancing hole

In order to investigate the value of discharge coefficient of balancing hole Cd for wide range of axial flow velocity v in the hole and rotational speed u, series of CFD simulations are made. Some of the result agrees well with the existing experimental data, but the difference is about 20% in some cases with high Rev. Also, for wide area of Rev, the effect of rotating speed u becomes significant. So it is considered that the parameter v/u is a dominant indicator of Cd. Furthermore, for very low range of v/u, we found a rare flow situation with S-shaped streamlines. In this flow, pumping of disk is so strong and Cd may differ largely.

Computational study of thermal influence in axial annular flow of a reactive third grade fluid with non-linear viscosity

This study numerically investigates the mass and heat transfer in a pressure induced flow of a reactive third-grade fluid with Reynolds’ viscosity model through a fixed cylindrical annulus, neglecting material consumption. The coupled nonlinear system of equations governing the flow are obtained in cylindrical coordinates and further cast into dimensionless forms via standard transformation. With graphical illustrations, the influence of salient physical parameters on the axial temperature and flow velocity are studied. Critical conditions are discussed using shooting method. The numerical solutions show very good agreement with those previously obtained from two-term series expansion as a special case. The effects of emerging parameters on the thermal critical characteristics are also discussed in detail. The results obtained are validated with relevant published articles in the literature. The quantitative effects of a state variable and several dimensionless parameters such as pressure gradient, numerical exponent of viscosity, an exponent of the pre-exponential factor, cylinder ratio, viscous heating parameter, and the material parameter accounting for the non-Newtonian nature of the fluid, on the activation energy, Frank-Kamenetskii parameter and Maximum inner cylinder temperature are explored.

A RAYLEIGH–RITZ METHOD FOR NAVIER–STOKES FLOW THROUGH CURVED DUCTS

We present a Rayleigh–Ritz method for the approximation of fluid flow in a curved duct, including the secondary cross-flow, which is well known to develop for nonzero Dean numbers. Having a straightforward method to estimate the cross-flow for ducts with a variety of cross-sectional shapes is important for many applications. One particular example is in microfluidics where curved ducts with low aspect ratio are common, and there is an increasing interest in nonrectangular duct shapes for the purpose of size-based cell separation. We describe functionals which are minimized by the axial flow velocity and cross-flow stream function which solve an expansion of the Navier–Stokes model of the flow. A Rayleigh–Ritz method is then obtained by computing the coefficients of an appropriate polynomial basis, taking into account the duct shape, such that the corresponding functionals are stationary. Whilst the method itself is quite general, we describe an implementation for a particular family of duct shapes in which the top and bottom walls are described by a polynomial with respect to the lateral coordinate. Solutions for a rectangular duct and two nonstandard duct shapes are examined in detail. A comparison with solutions obtained using a finite-element method demonstrates the rate of convergence with respect to the size of the basis. An implementation for circular cross-sections is also described, and results are found to be consistent with previous studies.

The Fluid Force on a Whirling Columnar Rotor

In the operation of hydro turbines and pump, self-excited vibration can occur due to fluid force which is generated as a response to the vibration and is called rotordynamic fluid force. The tangential component of rotordynamic fluid force has large impact on the stability of rotors. Characteristics of the tangential fluid force acting on a whirling columnar rotor which is modelled on the front shroud of a Francis turbine during self-excited vibration were investigated in computation in the present study. The angular velocity of the whirling motion of the rotor and the tangential force could be normalized by the axial velocity of axial leakage flow through the clearance between the rotor and the casing and the square of it, respectively. In the range of large whirling velocity, the tangential force was caused by a pressure distribution in the case without axial leakage flow. In the range of small whirling velocity, it is known that the tangential force is generated by the inertia of axial leakage flow. In the range of middle whirling velocity, the tangential force was found to be caused by both effects of whirling motion of the rotor in static fluid and the inertia of axial leakage flow. The pressure distribution without axial leakage flow was due to pressure loss in the wake of the rotor in addition to effects of shear stress and discharge from /suction into the clearance.

Flow Analysis through Collector Well Laterals: A Case Study from Sonoma County Water Agency, California

The Sonoma County Water Agency (SWCA) uses six radial collector wells along the Russian River west of Santa Rosa, to provide water for several municipalities and water districts in north-western California. Three collector wells (1, 2, and 6) are located in the Wohler area, and three collector wells (3, 4, and 5) are located in the Mirabel area. The objective of this paper is to highlight the performance of the three collector wells located in the Mirabel area since their construction. The 2015 investigation showed a lower performance of Collectors 3 and 4 compared to their original performances after construction in 1975, while the performance of Collector 5 was relatively stable since 1982. The potential change in capacity could be due to the increase in encrustation observed during the visual inspection of laterals in all three collector wells. Overall, the three collectors are still within the optimal design parameters (screen entrance velocity < 0.305 m min−1 and axial flow velocity of lateral screens < 1.524 m s−1).