Flow In Straight Ducts Research Articles

Laminar fluid flow in concentric annular ducts of non-conventional cross-section applying GBI method

Fluid flow in concentric or eccentric annular ducts have been studied for decades due to large application in medical sciences and engineering areas. This paper aims to study fully developed fluid flow in straight ducts of concentric annular geometries (circular with circular core, elliptical with circular core, elliptical with elliptical core, and circular with elliptical core) using the Galerkin-based Integral method (GBI method). The choice of method was due to the fact that in the literature it is not applied in ducts of cross-sections of the annular shape with variations between circular and elliptical. Results of different hydrodynamics parameters such as velocity distribution, Hagenbach factor, Poiseuille number, and hydrodynamic entrance length, are presented and analyzed. In different cases, the predicted hydrodynamic parameters are compared with results reported in the literature and a good concordance was obtained.

A novel low-resistance duct tee emulating a river course

Duct fittings are integral parts of a duct system and play important roles in fluid transportation. Resistance within the component directly affects the energy consumed by fans and pumps. This paper proposes a novel low-resistance duct tee by emulating a natural river configuration and theoretically explains the mechanism of resistance reduction in ducts based on variations in dissipation and displacement terms in the NS equation. The novel tee can reduce resistance in straight ducts with any flow ratios and aspect ratios, with a resistance reduction rate from 20.45% to 248.21%. The novel tee can also reduce resistance in branch ducts only when the flows in straight ducts are larger than in branch ducts, and the resistance reduction rate is between 0 and 817.88% (The resistance reduction rate is the degree of resistance reduction achieved by the novel duct tee). The resistance reduction rate can be improved to above 100% when the increased amplitude of momentum dissipation is below the increased amplitude of momentum convection. The turbulence model was selected based on the full-scale experiment. The resistance reduction effect of the novel tee is validated via a full-scale experiment at the end of this paper.

Flow and heat transfer of a Giesekus fluid in plane and 3D ducts

AbstractThis study aims to investigate the Graetz problem of Newtonian and viscoelastic fluid obeying Giesekus model using ANSYS Polyflow solver. The non‐isothermal flow in straight ducts of circular and noncircular cross‐sections under the constant heat flux boundary conditions is considered. The effect of the mobility parameter (α), fluid elasticity defined by Weissenberg number (We) and Reynolds number (Re) on the flow field, secondary flows, and the fully developed and developing Nusselt number along the ducts length are investigated for all geometries. The obtained results are of great importance for practical application in the polymer industries such as polymer melt.

Numerical study on secondary flows of viscoelastic fluids in straight ducts: Origin analysis and parametric effects

Abstract Numerical simulations were conducted on viscoelastic fluid flows in straight ducts with different cross sections, for which the origin of secondary flows and influences of material parameters and flow passage geometrical configuration were numerically investigated. The Giesekus constitutive model was chosen to describe the viscoelastic fluid with the second normal stress difference N 2 , and solved by embedding UDF (User-defined Function) into the CFD (computational fluid dynamics) code FLUENT. The origin of such kind of secondary flow was theoretically studied from the perspective of the budget of vorticity energy for the first time. Sufficient and necessary condition for the existence of secondary flow was then developed in terms of N 2 , the gradient of N 2 and cross-sectional geometry ϑ (i.e., generation term E Ω ). Moreover, helicity density was considered as an excellent indicator of secondary flow pattern. Effects of material properties (including anisotropic parameter α , solvent viscosity ratio β and relaxation time λ ) and flow passage geometrical configuration (aspect ratio of cross sections r , the number of polygon sides n ) on secondary flow strength and pattern were investigated with E Ω . Finally, a universal variable σ s was proposed to describe non-circularity of cross section, based on which the results for ducts with different cross sections can be normalized together.

A diffusion-inertia model for predicting dispersion and deposition of low-inertia particles in turbulent flows

The objective of the paper is twofold: (i) to present a model (the so-called diffusion-inertia model) for predicting dispersion and deposition of aerosol particles in two-phase turbulent flows and (ii) to examine the performance of this model as applied to the flows in straight ducts and circular bends. The model predictions compare reasonable well with both experimental data and Lagrangian tracking simulations coupled with fluid DNS or LES.

Velocity Profiles of Suspension Flows through an Abrupt Contraction Measured by Magnetic Resonance Imaging

AbstractVelocity profiles in steady flows of fluid/particle mixtures through a duct with an abrupt contraction were measured by magnetic resonance imaging. Aqueous solutions of carboxymethyl cellulose containing particles, including spheres, disk‐like particles, and short fibers, at high volume fractions were used. As a result, a plug‐like velocity profile was observed in a straight duct flow for every suspension, but the velocity profile depends on the particle shape at contraction. Disk‐like particles caused an unsteady flow, and short fibers caused a concave shape in the velocity profile near the centerline upstream of the contraction. Spheres did not affect the flow field. The concave profile became obvious with increased volume fraction of fiber. This result is caused by the larger elongational viscosity of the fiber suspension near the centerline of the channel, as compared with that of the sphere suspension.

A general implementation of the H1 boundary condition in CFD simulations of heat transfer in swept passages

A methodology has been developed to study laminar flow and heat transfer behaviour in periodic non-straight passages with a heat transfer boundary condition of constant axial heat flux and constant peripheral temperature ( H1). The technique uses Newton iteration to determine the wall temperature distribution required to satisfy the H1 boundary condition. The methodology is validated for hydrodynamically developed and thermally developing flow, as well as for hydrodynamically and thermally developed flow in straight ducts with various cross-sections. The methodology is extended to study fully developed flow in a periodic serpentine channel, consisting of a number of bends and straight sections, with a semi-circular cross-section. The results show the existence of a non-monotonic temperature distribution along the serpentine channels which exists because increased rates of heat transfer at bends lead to reductions in the local wall temperature in order to maintain a constant axial heat flux. Hot spots within the passage cross-section, typical of the H2 boundary condition, are removed in the H1 case.

A numerical study of the similarity of fully developed turbulent flows in orthogonally rotating square ducts and stationary curved square ducts

A numerical study of a quantitative analogy of fully developed turbulent flow in a straight square duct rotating about an axis perpendicular to that of the duct and a stationary curved duct of square cross‐section was carried out. In order to compare the two flows, the dimensionless parameters KTR=Re1/4/√Ro and the Rossby number, Ro=wm/Ωdh, in the rotating straight duct flow corresponded to KTC=Re1/4/√λ and the curvature ratio, λ=R/dh, in the stationary curved duct flow, so that they had the same dynamical meaning as those parameters for fully developed laminar flow. For the large values of Ro or λ, the flow field satisfied the “asymptotic invariance property”; there were strong quantitative similarities between the two flows, such as in the flow patterns and friction factors for the same values of KTR and KTC. Based on these similarities, it is possible to predict the flow characteristics in rotating ducts by considering the flow in stationary curved ducts, and vice versa.

Turbulence enhancement by ultrasonically induced gaseous cavitation in the CO2 saturated water

Recent primary concern for the design of high performance heat exchanger and highly integrated electronic equipments is to develop an active and creative technologies which enhance the heat transfer without obstructing the coolant flows. In this study, we found through the LDV measurement that the gaseous cavitation induced by ultrasonic vibration applied to the CO2 saturated water in the square cross-sectioned straight duct flow enhances the turbulence much more than the case of non-ultrasonic or normal ultrasonic conditions without gaseous cavitation does. We also found that gaseous cavitation can enhance effectively the turbulent heat transfer between the heating surfaces and coolants by destructing the viscous sublayer.

A NEW LOW-REYNOLDS VERSION OF AN EXPLICIT ALGEBRAIC STRESS MODEL FOR TURBULENT CONVECTIVE HEAT TRANSFER IN DUCTS

This investigation concerns performance of a new low-Reynolds version of an explicit algebraic stress model (EASM) for numerical calculation of turbulent forced-convective heat transfer and fluid flow in straight ducts with fully developed conditions. The turbulent heat fluxes are modeled by a SED concept, the GGDH, and the WET methods. New versions of GGDH, WET, and EASM are presented for low Reynolds numbers. However, at high Reynolds numbers, two wall functions are used, one for velocity fields and one for the temperature field. All the models are computed in a general three-dimensional channel. The low-Reynolds version of the models presented is very stable and has been used for Reynolds numbers up to 70,000 with least demanded number of grid points, and without any convergence problem or stability problem.

Performance of RNG Turbulence Modelling for Forced Convective Heat Transfer in Ducts

This investigation concerns numerical calculation of turbulent forced convective heat transfer and fluid flow in straight ducts using the RNG (Re-Normalized Group) turbulence method. A computational method has been developed to predict the turbulent Reynolds stresses and turbulent heat fluxes in ducts with different turbulence models. The turbulent Reynolds stresses and other turbulent flow quantities are predicted with the RNG κ−ϵ model and the RNG non-linear κ-ϵ model of Speziale. The turbulent heat fluxes are modeled by the simple eddy diffusivity (SED) concept, GGDH and WET methods. Two wall functions are used, one for the velocity field and one for the temperature field. All the models arc implemented for an arbitrary three dimensional duct. Fully developed condition is achieved by imposing cyclic boundary conditions in the main flow direction. The numerical approach is based on the finite volume technique with a non-staggered grid arrangement. The pressure-velocity coupling is handled by using the SIMPLEC-algorithm. The convective terms are treated by the QUICK, scheme while the diffusive terms are handled by the central-difference scheme. The hybrid scheme is used for solving the κ and ϵ equations. The overall comparison between the models is presented in terms of friction factor and Nusselt number. The secondary flow generation is also of major concern.

Development of extended field Doppler velocimetry for turbomachinery applications

The development of a portable, single component Doppler global velocimetry (DGV) head, based around a wavelength-stabilised argonion laser and a fast digital image-processing system, is described. The normalised two-dimensional DGV image, in which intensities are linearly related to velocities, can be displayed and updated at the 25 Hz camera frame rate, greatly easing the problem of system alignment. The effect of each individual system component upon the velocity resolution achieved for the system as a whole is discussed, and correction factors are calculated to account for the finite aperture and field of view of real systems and for divergence of the illuminating light sheet. Axial velocities of up to 100 m/s in a straight duct flow have been measured, demonstrating an rms velocity resolution of 2.5 m/s. The potential of the technique for gas turbine applications has been demonstrated by measuring the position of a shock in a transonic flow. At a Mach number of 2.3 and mass flow rate of 0.79 kg/s the velocity change across the shock was measured to be approximately 130 m/s.

Solution of the laminar duct flow problem by a boundary integral equation method

A boundary integral equation method is proposed for approximate numerical and exact analytical solutions to fully developed incompressible laminar flow in straight ducts of multiply or simply connected cross-section. It is based on a direct reduction of the problem to the solution of a singular integral equation for the vorticity field in the cross section of the duct. For the numerical solution of the singular integral equation, a simple discretization of it along the cross-section boundary is used. It leads to satisfactory rapid convergency and to accurate results. The concept of hydrodynamic moment of inertia is introduced in order to easily calculate the flow rate, the main velocity, and the fRe-factor. As an example, the exact analytical and, comparatively, the approximate numerical solution of the problem of a circular pipe with two circular rods are presented. In the literature, this is the first non-trivial exact analytical solution of the problem for triply connected cross section domains. The solution to the Saint-Venant torsion problem, as a special case of the laminar duct-flow problem, is herein entirely incorporated.

A method for the study of fully developed parallel flow in straight ducts of arbitrary cross section

AbstractA method is presented for the study of fully developed parallel flow of Newtonian viscous fluid in uniform straight ducts of very general cross-section. The method is based upon the concept of contour lines of constant velocity in a typical cross section of the duct, and uses the function which describes the contour lines as an independent variable to derive the integral momentum equation. The resulting ordinary integro-differential equation is, in principle, much easier to solve than the original momentum equation in partial differential equation form. Several illustrative examples of practical interest are included to explain the method of solution. Some of these solutions are compared with available solutions in the literature. All details are explained by graphs and tables. The method has several interesting features. The study has relevance to biomedical engineering research for blood and urinary tract flow.

Secondary flows in straight ducts of rectangular cross section

In this paper we consider the “elastic effects” that a non-Newtonian fluid, a solution of 2% viscarin in water, can exhibit in fully developed laminar flow in straight ducts of rectangular cross section. The elastic effects associated with the nonlinear properties of constitutive equations are modelled by the CEF-equation. Our main concern is the nature and magnitude of the secondary flows, determined both by numerical and experimental means. In contrast to earlier work, where a perturbation method has been used, the full non-linear equations are solved numerically by a finite volume method. It is shown that two vortices in each quadrant are observed for unity and moderate aspect ratios whereas three vortices are observed for the large aspect ratio of 16. The effect of secondary flow on pressure drop is calculated using both the generalized Newtonian model and the CEF-model. At low flow rates the effect is small but significant effects are observed at high flow rates. This is partly verified through measurements of pressure drop. Calculated velocity fields are compared to data obtained by laser velocimetry and show general agreement within experimental uncertainty.

Pressure drop in fully developed, duct flow of dispersed liquid-vapor mixture at zero gravity

The dynamics of steady, fully developed dispersed liquid-vapor flow in a straight duct at 0- g is simulated by flowing water containing n-butyl benzoate droplets. Water and benzoate are immiscible and have identical density at room temperature. The theoretical basis of the simulation is given. Experiments showed that, for a fixed combined flow rate of water and benzoate, the frictional pressure drop is unaffected by large changes in the volume fraction of benzoate drops and their size distribution. Measured power spectra of the static wall pressure fluctuations induced by the turbulent water-benzoate flow also revealed that their dynamics is essentially unaltered by the presence of the droplets. These experimental findings, together with the theoretical analysis, led to the conclusion that the pressure drop in fully developed, dispersed liquid-vapor flow in straight ducts of constant cross section at 0- g is identical to that due to liquid flowing alone at the same total volumetric flow rate of the liquid-vapor mixture and, therefore, can be readily determined.

Numerical Study of Turbulent Secondary Flows in Curved Ducts

The pressure driven, fully developed turbulent flow of an incompressible viscous fluid in curved ducts of square cross-section is studied numerically by making use of a finite volume method. A nonlinear K -1 model is used to represent the turbulence. The results for both straight and curved ducts are presented. For the case of fully developed turbulent flow in straight ducts, the secondary flow is characterized by an eight-vortex structure for which the computed flowfield is shown to be in good agreement with available experimental data. The introduction of moderate curvature is shown to cause a substantial increase in the strength of the secondary flow and to change the secondary flow pattern to either a double-vortex or a four-vortex configuration.

Development of a computational method for the full solution of mhd flow in fusion blankets

An advanced numerical scheme based on the finite difference method is presented for the full solution of a steady, incompressible, viscous, and electrically conducting flow with relatively large Hartmann numbers and interaction parameters. The solution method solves the coupled Navier-Stokes and Maxwell's equations at a low magnetic Reynolds number, and focuses on predicting key performance parameters for the design of fusion liquid metal blankets. The computational algorithm includes the effects of advection and diffusion, and is intended as a full solution, which has the potential capability of treating unsteady flow and applicability to heat and mass transfer in MHD flows. The present method uses primitive variables (velocity and pressure) for hydrodynamic variables and the electric potential, and employs a finite volume approach with a staggered grid system for fast convergence and physical accuracy. A pressure equation of the Poisson type is used for pressure-velocity-potential coupling. Results for two-dimensional channel flows with a nonuniform magnetic field at a Hartmann number and an interaction parameter of about 10 3 are obtained. The extension of the present numerical scheme to a three-dimensional MHD flow has been carried out and some preliminary resutls of a straight duct flow with a rectangular cross-section are presented.

The development of the quiet flow wind tunnel for aeroacoustics.

For decreasing the noise of turbo-machineries, making clear the generation mechanism of fluid-dynamic noise is important and an air flow having quiet noise is very useful for the study of aerodynamic noise. A "Quiet Flow Wind Tunnel", whose measuring section is in the annechoic chamber, has been developed and established. The maximum velocity of air flow from the nozzle with a section size of 400 mm×400 mm is 42 m/s and its turbulence is less than 0.5 %. The noise levels around the measuring section are less than 48 dB(A) at a velocity of 35 m/s. By using this facility, further studies have been made, "vortex sound from crossing two cylinders", "flow-noise from finned-tube bank", "noise from ventilating-gallery", "noise of inner flow in straight ducts" etc.. It has been recognised, through these experiments, that this facility can measure both phenomena of fluid-dynamics and acoustics at the same time although these are still impossible by usual wind tunnels.

Turbulence Generated Secondary Flows in Ducts of Non-Circular Cross-Section

The use of algebraic stress models in the prediction of secondary flows in straight ducts of non-circular cross-section is found to be unsatisfactory and to give inconsistent results in the sub-channels within a rod bundle. An algebraic expression is presented which allows the source of axial vorticity to be calculated directly and without iteration. The expression is shown to reproduce secondary velocities in square and triangular ducts, and in a duct consisting of two inter-connected subchannels.