Tube Of Circular Cross-section Research Articles

Transient mixed convection flow of nanofluids in a vertical tube

Purpose – In the present work, a theoretical model based on the full Navier-Stokes and energy equations for transient mixed convection in a vertical tube is extended to nanofluids with nanoparticle volume fraction up to 5 percent to ensure a Newtonian fluid behaviour. The paper aims to discuss these issues. Design/methodology/approach – The nanofluids considered, alumina/water and CuO/water, flow inside a vertical tube of circular cross-section, which is subjected to convective heat exchange at the outer surface. The transient regime is caused by a sudden change of nanofluid temperature at the tube inlet. The range of the Richardson number (1.6=Ri=2.5) investigated in this study corresponds to classic cases of mixed convection flow. Findings – Results have shown a significant reduction in the size of the recirculation zone near the wall when the particle volume fraction increases. This may be attributed to the viscosity increase with the volume fraction. Moreover, the flow structure clearly changes when the convective heat transfer coefficient is modified. A decrease of the wall temperature along the tube was found when increasing the convective heat transfer coefficient imposed at the tube external surface. Research limitations/implications – The problem formulation in 2D axisymmetric geometry includes the continuity, the Navier-Stokes and energy equations and is based on the stream function and vorticity; the numerical solution of equations is carried out using a finite difference method. Practical implications – From an economic point of view, this research paper is innovative in the sense that it considers nanofluids as a new and more efficient way to transfer heat. This paper could find applications for heat exchange purposes of compact systems with high thermal loads. Originality/value – Across the world, a still growing number of research teams are investigating nanofluids and their properties. Investigations concern several aspects such as the preparation of the nanofluids, as well as the applications of these nanofluids for convective heat transfer purposes. The dynamical study will consist in the instantaneous and spatial characterization of the dynamic flow field for different nanoparticle volume fractions.

End effects in rarefied gas outflow into vacuum through a long tube

The problem of steady outflow of rarefied gas from a reservoir into vacuum through a long cylindrical tube of circular cross-section at a constant temperature is considered on the basis of the kinetic S-model. The kinetic equation is solved numerically using a conservative second-order method. The basic calculated quantity is the gas flow rate through the channel; the flowfields are also studied. The solutions obtained are compared with the known results.

Secondary magnetic field harmonics dependence on vacuum beam chamber geometry

The harmonic magnetic field properties due to eddy currents have been studied with respect to the geometry of the vacuum beam chamber. We derived a generalized formula enabling the precise prediction of any field harmonics generated by eddy currents in beam tubes with different cross-sectional geometries. Applying our model to study the properties of field harmonics in beam tubes with linear dipole magnetic field ramping clearly proved that the circular cross section tube generates only a dipole field from eddy currents. The elliptic tube showed noticeable magnitudes of sextupole and dipole fields. We demonstrate theoretically that it is feasible to suppress the generation of the sextupole field component by appropriately varying the tube wall thickness as a function of angle around the tube circumference. This result indicates that it is possible to design an elliptical-shaped beam tube that generates a dipole field component with zero magnitude of sextupole. In a rectangular-shaped beam tube, one of the selected harmonic fields can be prevented if an appropriate wall thickness ratio between the horizontal and vertical tube walls is properly chosen. Our generalized formalism can be used for optimization of arbitrarily complex-shaped beam tubes, with respect to suppression of detrimental field harmonics.

A one-dimensional model for unsteady axisymmetric swirling motion of a viscous fluid in a variable radius straight circular tube

A one-dimensional model for the flow of a viscous fluid with axisymmetric swirling motion is derived in the particular case of a straight tube of variable circular cross-section. The model is obtained by integrating the Navier–Stokes equations over cross section the tube, taking a velocity field approximation provided by the Cosserat theory. This procedure yields a one-dimensional system, depending only on time and a single spatial variable. The velocity field approximation satisfies exactly both the incompressibility condition and the kinematic boundary condition. From this reduced system, we derive unsteady equations for the wall shear stress and mean pressure gradient depending on the volume flow rate, the Womersley number, the Rossby number and the swirling scalar function over a finite section of the tube geometry. Moreover, we obtain the corresponding partial differential equation for the scalar swirling function.

Time-dependent experimental analysis of a thermal transpiration rarefied gas flow

Thermal transpiration is the macroscopic movement induced in a rarefied gas by a temperature gradient. The gas moves from the lower to the higher temperature zone. An original method is proposed here to measure the stationary mass flow rate of gas created by thermal transpiration in a micro-tube heated at its outlet. In addition, by means of a time-dependent study, parameters such as the pressure variation, the pressure variation speed, and the characteristic time of the system are analyzed. The experimental system is composed of a glass tube of circular cross section and two reservoirs positioned one at the inlet and one at the outlet of the capillary. The reservoirs are connected to two fast response time capacitance diaphragm gauges. By monitoring the pressure variation with time inside both reservoirs, it is possible to measure the macroscopic movement of the gas along the tube. Three gases, nitrogen, argon, and helium, are studied and three temperature differences ΔT = 37, 53.5, and 71 K are applied to the tube. The analyzed gas rarefaction conditions vary from near free molecular to slip regime. Finally, Poiseuille counter flows consistent with the experimental zero flow conditions of the thermal transpiration process are proved to be possible.

High-shear drag reduction of surfactant solutions

Drag reducing surfactants produce micellar structures. Over critical shear stress, the break-up rate of these structures tends to be faster than the formation rate. An excess of counterions raises the critical shear stress, and enhances surfactant drag reduction up to higher shear in turbulent flow. We investigated this high-shear drag reduction for small circular cross-section tubes. The enhancement became saturated when the counterion to surfactant molar concentration ratio was higher than ten. This saturated condition allowed a 30% drag reduction for a wall shear stress of 103Pa (approximately 106s−1 shear rate) for a 1500ppm surfactant solution. The frictional drag of the tube flow was dependent on a Reynolds number at low shear and obeyed the friction formulas of a laminar pipe flow and a limiting drag reduction flow. Over critical shear stress, high shear decreased the drag reduction because of break-up of drag reducing micellar structures. This disappearance of the drag reduction was found to be a function of wall shear stress and independent of the tube diameters under saturated counterion-effect conditions. A phenomenological formula was introduced to describe the flow friction for these saturated counterion-effect conditions.

Determination of bone properties from Lamb type of waves

Our domain of interest is cortical bone characterization, i.e. determination of structural and material properties by means of ultrasound waves. The frequency spectrum of Lamb type of waves are the input data of the method of parameters identification, based on a least mean square algorithm. Specificities of clinical measurements of long bones require first to address the issue of measurements with limited access [Minonzio & al., J. Acoust. Soc. Am., 2010] and second, to develop methods for a combined determination of structural and elastic properties. The first physical model was the free plate under plane strain assumption for transverse isotropic materials with 4 elastic parameters and one structural (thickness). Experiments on flat plates and tubes of circular cross section are used as test case and the method was validated by comparison to independent techniques dedicated to material properties [Bernard S. & al. 2012]. The same method was applied on in vitro experiments on cortical bone samples. Here, the bone thickness was measured independently by X ray technique. This study opens the perspective of updating the physical model to determine additional relevant parameters, the tube diameter and/or the soft tissue properties in case of in vivo measurements.

The development and deceleration of the flow of an elastoplastic medium in a cylindrical tube*

An exact solution of the quasistatic problem of elastoplastic theory of the development of the flow of an incompressible medium in a cylindrical tube of circular cross section due to an increase in the pressure drop with time, and on the subsequent flow when there is a constant pressure and a deceleration due to its slow reduction. The conditions for the occurrence and regularity of the advancement of the elastoplastic boundaries for different types of loading are indicated.

Second Law Analysis of Al2O3-Water Nanofluid Turbulent Forced Convection in a Circular Cross Section Tube with Constant Wall Temperature

The present paper proposes an analysis based on the second principle of thermodynamics applied to a water-Al2O3 nanofluid. The nanofluid flows inside a circular section tube subjected to constant wall temperature. The aim of the investigation is to understand, by means of an analytical model, how entropy generation within the tube varies if inlet conditions, particles concentration, and dimensions are changed. To gather these information is of fundamental importance, in order to optimize the nanofluid flow. The results show that according to the inlet condition, there is a substantial variation of the entropy generation, particularly when Reynolds number is kept constant there is an increase of entropy generation, whereas when mass flow rate or velocity are taken constant, entropy generation decreases.

Flow in Circular Cross Section Tube Using the Partial Surface Wetting Conditions

Clanek pojednava o možnosti numerickeho modelovani prouděni kapaliny ve vodorovnem potrubi kruhoveho průřezu s využitim okrajove podminky zahrnujici vliv castecne smacivosti stěn. Pro numericke modelovani bylo využito programu ANSYS Fluent. Okrajova podminka zahrnujici vliv castecne smacivosti stěn vychazi z teorie, kde smacivost povrchu stěny urcuje adhesni soucinitel k, který definoval prof. Pochylý [1-5]. Pro tuto podminku byla vytvořena uživatelsky definovana funkce (UDF), ktera byla ověřena ve 2D i 3D geometrii. Výsledky z numerickeho modelovani byly ověřeny dle teoretických předpokladů. Dale clanek představuje výsledky z provedeneho fyzikalniho experimentu měřeni tlakových ztrat v potrubi kruhoveho průřezu z různých materialů při laminarnim prouděni. Nasledně jsou výsledky experimentu srovnany s teorii smacivých i castecně smacivých stěn s cilem stanovit adhezni soucinitel k pro použite materialy potrubi.

The influence of core geometry on the crystallography of silicon optical fiber

Crystalline semiconductor core optical fibers have received growing attention as greater understanding of the underlying materials science, coupled with advances in fiber processing and fabrication, have expanded the quality and portfolio of available materials. In a continued effort to better understand the nature of the crystal formation this work studies the role of the cross-sectional geometry on the resultant core crystallography with respect to the fiber axis. More specifically, a molten-core approach was used to fabricate silicon optical fibers clad in silica tubes of either circular or square inner cross-sections. In both geometric cases, the silicon core was found to possess regions of single crystallinity where specific crystal orientations persisted along a fiber length of about 4–5mm prior to transitioning through polycrystalline regions. However, the rotation and tilting angular combination needed to align a given crystallographic axis with the fiber axis was more constant over the single crystalline region in the case of the square-core fiber while more significant variations were observed in the round-core case. This work begins to elucidate some of the microstructural features, not present in conventional glass optical fibers, that could be important for future low-loss single crystalline semiconductor optical fibers.

ТеплоАеродинАмічнА ефективністЬ гвинтоподібниХ труБ З рівнорозвиненоЮ поверхнеЮ

Виконані дослідження процесів теплообміну та гідродинаміки всередині труб з рівнорозвиненою поверхнею, розроблених в НТУУ «КПІ». Приведений аналіз теплоаеродинамічної ефективності таких труб, її оцінка порівняно з гладкими трубами круглого поперечного перетину. Показані переваги теплообмінних поверхонь, виготовлених з гвинтоподібних труб

Thermally induced two-phase oscillating flow inside a capillary tube

This paper deals with thermally induced meniscus oscillations in a two-phase system consisting of a liquid plug and a vapor bubble in a capillary tube of circular cross-section. This system represents the simplest version of a heat transfer device called “pulsating heat pipe” (PHP). Our purpose is to gain fundamental understanding of the physical processes that cause self-sustained thermally driven oscillations. A visualization experiment is performed and the oscillations of the liquid–vapor meniscus and the vapor pressure are observed. We propose next a theoretical model. It differs from existing models by the account of the two-phase equilibrium that occurs locally at the vapor–liquid interface and by introduction of the time varying wetting films through which the major part of the heat and mass transfer occurs. Results from the proposed model show a good agreement with the experiment.

A nonlocal model of spatially nonuniform positive column of DC discharge

A self-consistent kinetic model is given of the positive column of low-pressure low-current dc discharge in neon in a tube of circular cross section, which is nonuniform in both the radial and longitudinal directions. The model is based on the solution of kinetic equation for the electron distribution function in nonlocal approximation, of drift equation for ions, of diffusion equation for metastable ions, and of Poisson equation for electric field. Calculations are performed for the case where the longitudinal nonuniformity of discharge is caused by a jumpwise variation of the discharge tube radius.

In vivo imaging of the cyclic changes in cross‐sectional shape of the ventricular segment of pulsating embryonic chick hearts at stages 14 to 17: A contribution to the understanding of the ontogenesis of cardiac pumping function

The cardiac cycle-related deformations of tubular embryonic hearts were traditionally described as concentric narrowing and widening of a tube of circular cross-section. Using optical coherence tomography (OCT), we have recently shown that, during the cardiac cycle, only the myocardial tube undergoes concentric narrowing and widening while the endocardial tube undergoes eccentric narrowing and widening, having an elliptic cross-section at end-diastole and a slit-shaped cross-section at end-systole. Due to technical limitations, these analyses were confined to early stages of ventricular development (chick embryos, stages 10-13). Using a modified OCT-system, we now document, for the first time, the cyclic changes in cross-sectional shape of beating embryonic ventricles at stages 14 to 17. We show that during these stages (1) a large area of diminished cardiac jelly appears at the outer curvature of the ventricular region associated with formation of endocardial pouches; (2) the ventricular endocardial lumen acquires a bell-shaped cross-section at end-diastole and becomes compressed like a fireplace bellows during systole; (3) the contracting portions of the embryonic ventricles display stretching along its baso-apical axis at end-systole. The functional significance of our data is discussed with respect to early cardiac pumping function.

Experimental and numerical investigation of non-predictive phase-control strategies for a point-absorbing wave energy converter

Phase control may substantially increase the power absorption in point-absorber wave energy converters. This study deals with validation of dynamic models and latching control algorithms for an oscillating water column (OWC) inside a fixed vertical tube of small circular cross-section by small-scale testing. The paper describes experimental and numerical results for the system's dynamics, using simple and practical latching control techniques that do not require the prediction of waves or wave forces, and which will be relevant to any type of point-absorbing devices. In the experimental set-up, the upper end of the tube was equipped with an outlet duct and a shut-off valve, which could be controlled to give a latching of the inner free surface movement. The pressure drop through the open valve is used as a simplified measure of the energy extraction. The control was realized by using the real-time measurement signals for the inner and outer surface displacement. A mathematical model of the system was established and applied in numerical simulation. In the case the OWC's diameter is much smaller than the wavelength and the wave amplitude much smaller than the draft, the free surface movement inside the tube can be described as an oscillating weightless piston. For this hydrodynamic problem an analytical solution is known. In addition, the mathematical model includes the effects of viscous flow losses, the air compressibility inside the chamber and the pressure drop across the valve. Experimental results were used to calibrate some of the model parameters, and the total model was formulated as a coupled system of six non-linear, first-order differential equations. Time-domain integration was used to simulate the system in order to test the control strategies and compare with experimental results.

Treatment of losses of ultrafine aerosol particles in long sampling tubes during ambient measurements

Long sampling tubes are often required for particle measurements in street canyons. This may lead to significant losses of the number of ultrafine (those below 100 nm) particles within the sampling tubes. Inappropriate treatment of these losses may significantly change the measured particle number distributions (PND), because most of the ambient particles, by number, exist in the ultrafine size range. Based on the Reynolds number (Re) in the sampling tubes, most studies treat the particle losses using the Gormley and Kennedy laminar flow model (Gormley, P.G., Kennedy, M., 1949. Diffusion from a stream following through a cylinderical tube. Proceedings of Royal Irish Academy 52, 163–169.) or the Wells and Chamberlain turbulent flow model (Wells, A.C., Chamberlain, A.C., 1967. Transport of small particles to vertical surfaces. British Journal of Applied Physics 18, 1793–1799.). Our experiments used a particle spectrometer with various lengths (1.00, 5.47, 5.55, 8.90 and 13.40 m) of sampling tube to measure the PNDs in the 5–2738 nm range. Experiments were performed under different operating conditions to measure the particle losses through silicone rubber tubes of circular cross-section (7.85 mm internal diameter). Sources of particles included emissions from an idling diesel engine car in a street canyon, emissions from a burning candle and those from the generation of salt aerosols using a nebuliser in the laboratory. Results showed that losses for particles below ≈20 nm were important and were largest for the smallest size range (5–10 nm), but were modest for particles above ≈20 nm. In our experiments the laminar flow model did not reflect the observations for small Re. This may be due to the sampling tubes not being kept straight or other complications. In situ calibration or comparison appears to be required.

Experiments on rarefied gas flows through tubes

Experiments were carried out in order to evaluate the conductance of tubes of circular cross section as a function of the Knudsen number. The range of the rarefaction level spans from the free molecular flow to the continuum regime. A different experimental approach was followed with respect to previous researches in that the mass flow rate was assigned and the corresponding pressure drop was measured. Single tubes and a bundle of capillaries were adopted. The results are compared with the existing experimental data and analytical models. An analysis of the sensitivity of the conductance to the error in measurements, in free molecular flow, was also dealt with.

Melting of ice slurry under forced convection conditions in tubes

The paper presents the results of experimental research on heat transfer of ice slurry during its flow through tubes of circular, rectangular and slit cross-sections. Moreover, the work discusses the influence of solid particles, type of motion and cross-section on the changes in the heat transfer coefficient. The analysis presented in the paper allows for an identification of the criterial relations used to calculate the Nusselt number for laminar and turbulent flow, taking into account elements such as phase change, which accompanies the heat transfer process. Ice slurry flow is treated as a generalized flow of a non-Newtonian fluid.

High‐resolution in vivo imaging of the cross‐sectional deformations of contracting embryonic heart loops using optical coherence tomography

The embryonic heart tube consists of an outer myocardial tube, a middle layer of cardiac jelly, and an inner endocardial tube. It is said that tubular hearts pump the blood by peristaltoid contractions. The traditional concept of cardiac peristalsis sees the cyclic deformations of pulsating heart tubes as concentric narrowing and widening of tubes of circular cross-section. We have visualized the cross-sectional deformations of contracting embryonic hearts in chick embryos (HH-stages 9-17) using real-time high-resolution optical coherence tomography. Cardiac contractions are detected from HH-stage 10 onward. During the cardiac cycle, the myocardial tube undergoes concentric narrowing and widening while the endocardial tube undergoes eccentric narrowing and widening, having an elliptic cross-section at end-diastole and a slit-shaped cross-section at end-systole. The eccentric deformation of the endocardial tube is the consequence of an uneven distribution of the cardiac jelly. Our data show that the cyclic deformations of pulsating embryonic heart tubes run other than originally thought. There is evidence that heart tubes of elliptic cross-section might pump blood with a higher mechanical efficiency than those of circular-cross section. The uneven distribution of cardiac jelly seems to prefigure the future AV and cono-truncal endocardial cushions.