Non-swirling Flow Research Articles

Transient growth in vortices with axial flow

We investigate transient growth in high-Reynolds-number vortices with axial flow. Manycases of vortex instability are not fully explained by strong exponential instability modes, and transient growth could offer an alternative route to breakdown in such cases. Strong transient growth is found, in agreement with previous studies. We first discuss the problem by reference to ducted vortices which model aeroengine flow. The transient growth is inviscid in character, and in this paper we specifically interpret it as an effect of the inviscid continuous spectrum. The relevant inviscid theory explains new scalings which we find for the transient growth, which are generalizations of the quadratic scaling seen previously in two-dimensional flows and non-swirling pipe flows. We then turn to a second case, of interest for vortex breakdown, the Batchelor vortex, and present calculations of the transient growth. Large growth is possible, especially for the helical modes (with azimuthal wavenumber |m| = 1). The general trends are complicated by a number ofissues, including a long-wavelength effect and a resonance effect, both of which were recently discovered for a vortex without axial flow and are found here to be present in the Batchelor vortex also. Overall, the results suggest that strong transient effects are present in the moderate- to high-swirl regime of practical interest (swirl number q ≳ 2). Foraxisymmetric (m = 0) and higher (|m| > 1) modes, however, transient effects are not found to be significant.

Heat transfer enhancement in circular tubes using helical swirl generator insert at the entrance

This paper presents an experimental study of heat transfer and friction characteristics in decaying turbulent swirl flow generated by a short helical tape placed at the entrance of the test section. In the experiments, total mass flux was varied from 160 kg m −2 s −1 to 1628 kg m −2 s −1 and the momentum ratio ( M h / M T ) was varied from 0 to 8.6. The experiments were conducted for water flow rates in the range of 5000 ⩽ Re ⩽ 30 000 . Three different helical tapes with helical angels of 30°, 45° and 60° were used. Experimental results confirm that, the use of a helical tape inserted to the tube leads to a higher heat transfer rate than the non-swirling flow. The local heat transfer coefficients were found to be increasing to very high values along the downstream of the helical tape, and then decreasing with the distance ( x / L ). The augmentation of heat transfer was found to be the function of the momentum ratio, M h / M T and Reynolds number. No pronounced effect of the number of the helical channels ( N h ), and helical angle ( α) on heat transfer was observed. It is found that using the helical tape can help to increase the heat transfer rate up to 20% depending on M h / M T and Re at constant pumping power. Enhancement efficiency increases with increasing momentum ratio and decreases with increasing Reynolds number.

Enhancement of Mass Transfer in a Spray Tower Using Swirling Gas Flow

Spray towers are commonly used in the chemical and process industries for a number of applications including absorption, desorption and humidification. However, the main disadvantage of a spray tower compared with that of a packed tower is its lower contact efficiency. The present study is concerned with the enhancement of mass transfer between a continuous gas phase and liquid droplets in a spray tower by imparting swirl to the axial gas flow through the tower. It is well known that swirling flow has the ability to augment the rates of heat and mass transfer. Experimental investigations into the hydrodynamics and mass transfer in a laboratory-scale spray tower for air-NH3/H2O system using axial and swirling gas flows have been carried out. The hydrodynamic studies included measurements of the gas velocity distributions and overall pressure drop in the tower, and characterization of water sprays generated by a pressure-swirl nozzle where radial liquid distributions, droplet size and its distribution and mean droplet size in terms of SMD were measured. As for the mass transfer performance of the spray tower, the effect of the gas and liquid flow rates on the overall gas phase mass transfer coefficient, Kga, was investigated for both swirling and non-swirling axial gas flows in order to quantify the effect of swirl. It has been found that Kga increases with increasing gas/liquid flow rates and imparting swirl in the gas flow enhances Kga up to 20% compared with that in axial flows. Correlations of Kga as a function of the gas/liquid flow rates, and also as a function of the gas flow rate and initial droplets SMD are developed. A design methodology to determine the height of a spray tower required to achieve a specified amount of removal of a solute from a gas mixture is proposed.

A fourth order finite volume scheme for turbulent flow simulations in cylindrical domains

A finite volume scheme which is based on fourth order accurate central differences in the spatial directions and on a hybrid explicit/semi-implicit time stepping scheme was developed to solve the incompressible Navier–Stokes equations on cylindrical staggered grids. This includes a new fourth order accurate discretization of the velocity at the singularity of the cylindrical coordinate system and a new stability condition. The new method was applied in the direct numerical simulations (DNS) of the fully developed non-swirling turbulent flow through straight pipes with circular cross-section for the Reynolds number Re τ = 360 based on the friction velocity u τ and the pipe diameter. The obtained results are expressed in terms of statistical moments of the velocity components and are presented in comparison with those obtained with a second order accurate scheme and by measurements. It is shown that the fourth order spatial discretization leads to improved higher order statistical moments, while the first and the second order moments are more or less insensitive to the spatial discretization order.

4231 まっすぐな円管内の乱流旋回流の特性 : 弱い旋回流入時の流れの変化(G05-14 管内流・旋回流,G05 流体工学)

The flow changes in weak swirling flow, which are different from those of non-swirling pipe flow, are described. Tangential velocity of a combined vortex upstream changes to that of a forced vortex downstream. Axial velocity distributions which are constant in the entrance region gradually develop into non-swirling pipe flow's downstream. Also, turbulence which is controlled upstream develops far downstream.

Computational Modelling of Turbulent Mixing in Confined Swirling Environment Under Constant and Variable Density Conditions

A high-intensity swirling flow in a model combustor subjected to large density variations has been examined computationally. The focus is on the Favre-averaged Navier–Stokes computations of the momentum and scalar transport employing turbulence models based on the differential second-moment closure (SMC) strategy. An updated version of the basic, high-Reynolds number SMC model accounting for a quadratic expansion of both the pressure–strain and dissipation tensors and a near-wall SMC model were used for predicting the mean velocity and turbulence fields. The accompanied mixing between the annular swirling air flow and the central non-swirling helium jet was studied by applying three scalar flux models differing mainly in the model formulation for the pressure-scalar gradient correlation. The computed axial and circumferential velocities agree fairly well with the reference experiment [So et al., NASA Contractor Report 3832, 1984; Ahmed and So, Exp. Fluids 4 (1986) 107], reproducing important features of such a weakly supercritical flow configuration (tendency of the flow core to separate). Although the length at which the mixing was completed was reproduced in reasonable agreement with the experimental results, the mixing activity in terms of the spreading rate of the shear/mixing layer, that is its thickness, was somewhat more intensive. Prior to these investigations, the model applied was validated by computing the transport of the passive scalar in the non-swirling (Johnson and Bennet, Report NASA CR-165574, UTRC Report R81-915540-9, 1981) and swirling (Roback and Johnson, NASA Contractor Report 168252, 1983) flow in a model combustor.

Simulation of Swirling Turbulent Heat Transfer in a Vortex Heat Exchanger

ABSTRACT This article presents a numerical simulation of swirling turbulent flow and heat transfer in a novel vortex heat exchanger. A new algebraic Reynolds stress/heat flux model (ASM/AFM) is applied to the simulation. The computation is performed under different air flow rates for both swirling and nonswirling flows. The calculated mean heat transfer coefficients on both inner and outer walls of the annular duct are compared with the measured data. They are generally improved over the results predicted by the new ASM/k–ϵ model. The effects of swirl on enhancing heat transfer in the annular duct are illustrated. The heat transfer performance of the vortex heat exchanger under different air flow rates is obtained.

Analysis and Characteristics of Choked Swirling Nozzle Flows

A novel analysis is presented of swirling flows in choked nozzles. The one-dimensional compressible flow theory was extended to isentropic axisymmetric swirling flows. The most significant accomplishment of the present model is its ability to treat a general swirl type of any tangential velocity distribution. A choking criterion similar to that of one-dimensional nonswirling flows was introduced

Nonparallel local spatial stability analysis of pipe entrance swirling flows

A spatial local viscous stability analysis of a swirling flow developing in a cylindrical pipe has been carried out numerically. Even at moderately low swirl strengths, we have found the existence of centrifugal modes in addition to the shear ones found in previous stability analysis of nonswirling flows developing in pipes. It is found that these centrifugal instabilities develop at Reynolds numbers that are much lower than those required for the growing of the shear instability. Moreover, the extent of the region where centrifugal instabilities appear is much larger than that where the shear layer instability grows. We have found from the analysis that the most unstable mode was the counter-rotating one (n=−1). The critical Reynolds number for which linear analysis predicts the growth of the convective instabilities is for the centrifugal modes one hundred times smaller than for the shear layer ones.

The most general constitutive equation in motions with constant stretch history and its approximative application in numerical simulations

AbstractIn this paper we utilize the fact that in motions with constant stretch history (csh) the constitutive equation for the stress tensor can always be cast in explicit form. Great simplifications are possible in two dimensional as well as in nonswirling axisymmetric flows. For these flows one obtains the CEF equation, but the material functions depend not only on the shear rate, but on other parameters as well. For two dimensional flow this implies that the viscosity function can account for shear thinning and extension thickening. For the flow of a dilute polymer solution through a sudden contraction numerical results qualitatively agree with experimental results, quite in contrast to predictions which result if the customary generalized Newtonian fluid is used.

An investigation of heat transfer characteristics of swirling flow in a 180° circular section bend with uniform heat flux

An experiment was performed to obtain the local heat transfer coefficient and Nusselt number in a circular duct with a 180° bend for Re=6X10⁴. 8 X 10⁴ and I X 10^5 under swirling flow and non-swirling flow conditions. The test tube with a circular section was made from stainless steel having a curvature ration of 9.4. Current heat flux of 5.11 kW/m² was applied to the test tube by electrical power and the swirling motion of air was produced by a tangential inlet to the pipe axis at 180°. Measurements of local wall temperatures and the bulk mean temperatures of air were made at four circumferential positions at 16 stations. The wall temperatures showed a reduced distribution curve at the bend for the non-swirling flow. but this effect did not appear for the swirling flow. The Nusselt number distributions for the swirling flow. which was calculated from the measured wall and the bulk temperatures. were higher than that of the non-swirling flow. The average Nusselt number of the swirling flow increased by about 90­100%. compared to that of the non-swirling flow. The Nu/Nu_DB values at the 90° station for non-swirling flow and swirling flow were approximately 2.5 and 4.8 at Re=6X10⁴ respectively. The values agree well with Said's results for non-swirling flow.

Simulation of Gas Flow Instability in a Spray Dryer

In this paper, turbulent flow behind a sudden pipe expansion followed by a contraction is simulated numerically, in order to investigate the effect of the downstream contraction on the flow instability in non-swirling flows, as well as in weakly swirling flows. Calculations are carried out using a commercial CFD code (CFX4.4), in which the transient Reynolds averaged N–S equation approach and the standard k–ɛ model are implemented. The first case investigated is the effect of a sudden contraction with varying length. The diameter ratio for both the expansion and the contraction is 5. The length of the large pipe (normalized by its diameter D) is varied in the range from 1.0 to 4.0, and the results are compared with those for the case without a contraction. The current results show that a sudden contraction tends to stabilize the non-swirling flow when the large pipe length is reduced. This stabilizing effect does not apply to swirling flow, although the precessing direction and frequency are affected significantly. The second case simulated is the gas flow in a simple short-form spray dryer configuration, for which some measured data are available in the literature. A self-sustained quasi-flapping oscillation is predicted, which behaves in a similar way to the phenomenon observed in experimental measurements.

Pressure Drop in Solar Power Plant Chimneys

The paper investigates flow through a representative tall solar chimney with internal bracing wheels. It presents experimental data measured in a 0.63-m-dia model chimney with and without seven bracing wheels. The bracing wheels each had a rim protruding into the chimney and 12 spokes, each spoke consisting of a pair of rectangular section bars. The investigation determined coefficients of wall friction, bracing wheel loss, and exit kinetic energy in a model chimney, for both ideal non-swirling uniform flow and for swirling distorted flow. A fan at one end of the chimney model either sucked or blew the flow through it. The flow entering the chimney through the fan and its diffuser simulated the flow leaving the turbine at the bottom of the chimney. The swirling distorted flow increased the total pressure drop by about 28%, representing 4.7% of the turbine pressure drop. The pressure drop across the bracing wheels exceeded the frictional pressure drop by far. Designers of tall, thin-walled chimneys should take care to minimize the number of bracing wheels, reduce their rim width as much as possible, and investigate the feasibility of streamlining their spoke sections. If at all structurally possible, the top bracing wheel should be far enough from the chimney exit to allow the spoke wakes to decay and the separated flow to re-attach to the chimney wall downstream of the rims before the flow leaves the chimney, to reduce the exit kinetic energy loss.

Cathode erosion in high-current high-pressure arc

Cathode erosion rate was experimentally investigated for two types of arcs: one with tungsten cathode in nitrogen atmosphere and one with hafnium cathode in oxygen atmosphere. Conditions were typical for plasma arc cutting systems: gas pressure from 2 to 5 atm, arc current from 200 to 400 A, gas flow rate from 50 to 130 litre min−1.It was found that the actual cathode evaporation rate G is much lower than G0, the evaporation rate that follows from the Hertz–Knudsen formula: G = νG0. The difference is because some of the evaporated particles return back to the cathode. For conditions of our experiments, the factor ν could be as low as 0.01. It was shown experimentally that ν depends strongly on the gas flow pattern close to the cathode. In particular, swirling the gas increases ν many times. To explain the influence of gas swirling, model calculations of gas flows were performed. These calculations revealed difference between swirling and non-swirling flows: swirling the gas enhances the gas flow from the cathode. This enhanced flow sweeps the evaporated particles away from the cathode and thus increases the net evaporation rate.Electrode erosion associated with arc ignition (start erosion) was observed for both types of cathodes. Models of start erosion were suggested. In case of hafnium cathode the start erosion is due to cracking out the hafnium oxide layer, which was created during previous arc operation. In the case of the tungsten cathode the start erosion is produced by a cold cathode arc (vacuum arc), which exists at the cathode during first moments after arc ignition.

Self-rotation in electrocapillary flows.

The mechanism of appearance of swirl in a certain class of converging flows is investigated numerically. The analysis is motivated by the spontaneous generation of swirl, which has been observed in electrified menisci (Taylor cones). The electrical stress acting on the cone surface drives these electrified millimetric fluid flows. Numerical results show that the primarily swirl-free meridian flow is unstable within an interval of values of the Reynolds number based on the surface stress. For values of the Reynolds number outside this interval, which depends on the forcing conditions and the geometry of the flow, the nonswirling meridian flow is stable. The instability mechanism of circulation amplification, which has nothing to do with the well-known increase of swirl velocity due to the vortex stretching mechanism, is due to a convection-diffusion effect. The circulation accumulated at the axis zone by the converging meridian motion is pumped by diffusion toward the conical surface. This feedback loop mechanism shoots the circulation amplification for values of the Reynolds number larger than a critical one. The same instability mechanism of swirl amplification could also appear in other converging flows generated by body forces (natural convection, electrical forces, etc.).

Generalization of the Leveque problem for mass transfer in swirling flows in the entrance region of a cylindrical section

parameters. However, in contrast to axial non-swirling flows, the comparison [4] of experimental data [1–4] for various set-ups and flow regimes did not reveal the generalized character of Eq. (1) for swirling flows. To verify the applicability of Eq. (1) to mass transfer in swirling flows, we consider the problem of mass transfer from a swirling flow to a microelectrode flash mounted with the wall of a cylindrical tube of radius R (corresponding measurements were reported, e.g., in [4]). In order to simplify the mathematical formulation of the problem, we examine only axisymmetrically developed flow regimes (turbulent pulsations and threedimensional effects are ignored). Let the distance z0 from the entrance of the cylindrical section to the microelectrode be so small that the boundary layer developed in this length is thin (its thickness δ ! R) and Sh cReSc, Re Ud ν -------. = =

The entrainment envelope of dye-core vortices at submerged hydraulic intakes

Each winter in Canada, operational difficulties are encountered at various water works resulting from intake blockages caused by frazil ice entrainment. In a lake setting, frazil is a surface phenomenon, the strong downward current produced by a swirling flow, with an intake vortex present, provides a mechanism by which frazil is transported from the water surface to the submerged intake below. Laboratory experiments were conducted to study the entrainment envelope associated with swirling and non-swirling flows into submerged water intakes. Three-dimensional velocity measurements were made with an acoustic Doppler velocimeter. The results clearly show that the entrainment envelope for swirling flow is several times larger than that for non-swirling flow. This paper details, for a given set of conditions, the differences in the non-swirling and swirling flow entrainment envelopes and emphasizes the potential difficulties with frazil ice that vortices can cause at intakes.Key words: vortex, dye-core vortex, submerged hydraulic intake, entrainment envelope, three-dimensional velocity measurements, acoustic Doppler velocimeter.

Steady planar and non-swirling axisymmetric flows: flow classification and its use in numerical calculations of a quasi-Newtonian fluid

It is well established that the extra tensor τ in constant stretch history (csh) flow can be expressed either in terms ofD, the rate of strain tensor, and ω, the (relative) rate of rotation, or as an isotropic function ofD and its first two Jaumann derivatives. Explicit results are reported here for planar and non-swirling axisymmetric csh flows. Even the term directly proportional toD already allows for shear thinning and extension thickening. If this quasi-Newtonian fluid is used in numerical calculations a flow classification parameterX is needed which locally distinguishes weak from strong flows. It is shown thatX depends upon the type of flow, i.e.X in planar flow differs fundamentally from the classification parameter for non-swirling axisymmetric flows.

Low-Reynolds-number effects derived from direct numerical simulations of turbulent pipe flow

Fully developed, statistically steady and non-swirling turbulent flow through straight pipes of circular cross-section is investigated by means of direct numerical simulation. A finite-volume scheme of second-order accuracy and a semi-implicit time integration scheme of the same order of accuracy are used to integrate the incompressible Navier–Stokes equations on staggered grids. Significant low-Reynolds- number effects are observed in the mean axial velocity, the components of the Reynolds stress tensor, the pressure and vorticity fluctuations, the turbulent kinetic energy budget and two-point velocity correlations. It is confirmed that turbulence data obtained in the near-wall region do not scale on wall variables. Within the range of Reynolds numbers investigated, the non-dimensional streak spacing increases slightly with Re τ .

Prediction of In-Cylinder Turbulence for IC Engines

This paper presents the preliminary results of some of a few of its kind efforts in large eddy simulation (LES) of engine flows to predict turbulent fluctuations, and the statistics of turbulence quantities inside IC engine cylinders. For this purpose, the well-known engine simulation code, KIVA, is used with special precautions to keep the numerical accuracy at a sufficiently high level, as well as using relatively fine grid resolution. The capabilities of this code are tested against benchmark cases, such as lid-driven cavity flow, and swirling and non-swirling free jet flows. It is then applied to a typical engine geometry under motored conditions. In particular, turbulence generated during the intake stroke, and the instabilities induced by a typical piston-bowl assembly are investigated. The computed velocity fluctuations, correlation coefficients and energy spectra of turbulent fluctuations are compared to experimental results. The predictions seem to extend well into the inertial range of turbulence and depict a good qualitative agreement with measurements. The results also shed light into the mechanisms by which turbulence may be generated by the piston-bowl assembly.