Presence Of Swirl Research Articles

Measurement of bubble velocity components in a swirling gas–liquid pipe flow using a local four-sensor conductance probe

This paper presents a novel technique for measuring the local axial, radial and azimuthal velocity components of the gas in bubbly gas–liquid flows using a local four-sensor conductance probe. A mathematical model is presented showing how the velocity vector of a gas bubble can be calculated from seven time intervals taken from the output signals from each of the four conductance sensors located within the probe. The paper goes on to describe the construction of a local four-sensor probe and the associated electronic measurement circuitry. Results are presented showing the distributions of the mean local axial, radial and azimuthal gas velocity components in vertical, bubbly gas–liquid flows, both with and without swirl. These results were obtained using the four-sensor probe in a vertical 80 mm diameter pipe into which a swirl generator could be installed. Additional results are presented showing the local gas volume fraction distribution, also obtained from the probe, in bubbly gas–liquid flows with and without swirl. It was found, as expected, that the presence of swirl caused a significant increase in the magnitude of the measured azimuthal velocity of the gas, particularly at the pipe walls. It was also found that, at a comparatively high water flow rate, the presence of swirl caused the gas bubbles to preferentially accumulate at the centre of the pipe.

An experimental and numerical investigation of under-expanded turbulent jets

AbstractThe work described here concentrates on under-expanded, axisym-metric turbulent jets issuing into quiescent conditions. Under-expanded turbulent jets are applicable to most aircraft propulsion applications that use convergent nozzles. Experimental studies used laser doppler velocimetry (LDV) and pitot probe measurements along the jet centreline. These measurements were made for two nozzle pressure ratios (2·5 and 4·0) and at various streamwise positions up to 10 nozzle diameters downstream of the nozzle exit plane. A computational fluid dynamics (CFD) model was developed using the Fluent code and utilised the RNG K-ε two-equation turbulence model. A mesh resolution of approximately one hundredth of nozzle exit diameter was found to be sufficient to establish a mesh independent solution.Comparison of the jet centreline axial velocity (LDV data) and pressure ratio (pitot probe data) showed good agreement with the CFD model. The correct number of shock cells had been predicted and the shock strength agreed well between the experiments and numerical model. The CFD model was, however, found to over-predict the shock cell length resulting in a longer supersonic core. There was some evidence, based on analysis of the LDV measurements that indicates the presence of swirl and jet unsteadiness, which could contribute to a shortening of the shock cell length. These effects were not modelled in the CFD. Correlation between the LDV and pitot probe measurements was generally good, however, there was some evidence that probe interference may have caused the premature decay of the jet. Overall, this work has indicated the good agreement between a CFD simulation using the RNG k-ε turbulence model and experimental data when applied to the prediction of the flowfield generated by under-expanded turbulent jets. The suitability of the LDV technique to jet flows with velocities up to 500ms-1has also been demonstrated.

An experimental and numerical investigation of under-expanded turbulent jets

AbstractThe work described here concentrates on under-expanded, axisym-metric turbulent jets issuing into quiescent conditions. Under-expanded turbulent jets are applicable to most aircraft propulsion applications that use convergent nozzles. Experimental studies used laser doppler velocimetry (LDV) and pitot probe measurements along the jet centreline. These measurements were made for two nozzle pressure ratios (2·5 and 4·0) and at various streamwise positions up to 10 nozzle diameters downstream of the nozzle exit plane. A computational fluid dynamics (CFD) model was developed using the Fluent code and utilised the RNG K-ε two-equation turbulence model. A mesh resolution of approximately one hundredth of nozzle exit diameter was found to be sufficient to establish a mesh independent solution.Comparison of the jet centreline axial velocity (LDV data) and pressure ratio (pitot probe data) showed good agreement with the CFD model. The correct number of shock cells had been predicted and the shock strength agreed well between the experiments and numerical model. The CFD model was, however, found to over-predict the shock cell length resulting in a longer supersonic core. There was some evidence, based on analysis of the LDV measurements that indicates the presence of swirl and jet unsteadiness, which could contribute to a shortening of the shock cell length. These effects were not modelled in the CFD. Correlation between the LDV and pitot probe measurements was generally good, however, there was some evidence that probe interference may have caused the premature decay of the jet. Overall, this work has indicated the good agreement between a CFD simulation using the RNG k-ε turbulence model and experimental data when applied to the prediction of the flowfield generated by under-expanded turbulent jets. The suitability of the LDV technique to jet flows with velocities up to 500ms-1has also been demonstrated.

3D modeling of evaporation of water injected into a plasma jet

3D effects of the radial injection of water jet into the swirl and non-swirl plasma jets are clarified numerically. The plasma–droplet two-way interactions are modeled by coupling Lagrangian approach for droplet behavior with an Eulerian approach for plasma flow under the dense loading. The effect of radial injection of the water jet on the temperature and flow fields of the swirl and non-swirl plasma jets is studied numerically. Mass concentration of the evaporated vapor is also predicted. The local deformation of the thermo-fluid fields of the plasma jet is stronger for higher droplet loading. Evaporation rate is decreased with increasing the droplet loading as well as swirl velocity. Mixing of vapor with plasma is stronger in the presence of swirl.

Temporal instability of swirling gas jets injected in liquids

A temporal linear stability analysis of an inviscid incompressible swirling gas jet injected into a co-flowing liquid was conducted. The ratio of the tangential velocity to the axial velocity (swirl number) played a significant role in the destabilization of the gas jet. Even at small gas Weber numbers, the presence of swirl caused the higher order azimuthal modes to become unstable; the growth rates, and the dominant and limiting wave numbers of the higher order modes were greater than those of the varicose and sinuous modes. The differences in growth rates, limiting and dominant wave numbers of the various azimuthal modes became significant at large gas Weber numbers. An increase in liquid viscosity resulted in a reduction in the growth rates and the dominant wave numbers. The liquid co-flow velocity controlled the phase velocity of the unstable modes.

Arachnoid cysts: entrapped collections of cerebrospinal fluid variably communicating with the subarachnoid space.

Despite decades of investigation and discussion of the mechanisms involved in the pathophysiology of arachnoid cysts, fundamental issues concerning these entities remain poorly defined and controversial. Cine-mode magnetic resonance imaging (MRI) has shown two patterns of cerebrospinal fluid (CSF) flow within the cavity in patients harbouring arachnoid cysts. Some cysts present a harmonic flow with a patent flow entry zone. All these patients had intermittent, non-progressive and non-localizing symptoms requiring no surgery according to our criteria. The second pattern of CSF flow is more chaotic and is characterized by the presence of swirls throughout the entire cardiac cycle. This pattern is associated with a more disabling clinical picture. Some of these patients required surgical treatment. During surgery, an endoscope was used for inspection purposes revealing, above all, that arachnoid cysts always and variably communicate with the subarachnoid space. The CSF enters the cyst either through a patent flow entry zone or through minute perforations in areas more loosely packed of the arachnoid network that behave as a flexible mesh able to modify the area of flowing CSF. The slipstreams of CSF within arachnoid cysts may not be channelled properly leading to possible damage of the surrounding brain parenchyma.

An Oseen-type model for swirling internal separated flows

The viscous, laminar, separated flow downstream of a sudden expansion in a pipe is studied. The flow is modeled by an Oseen-type equation, but with the additional feature that the nonlinearity in the swirl is retained. Exact solutions are obtained for a high-Reynolds-number limit and for arbitrary Reynolds number by use of an eigenfunction-expansion procedure, in the presence of swirl. This leads to a non-standard eigenvalue problem. When the swirl is sufficiently large, a central recirculating region is observed. The effect of the pressure gradients on the velocity profiles and the central recirculating eddy is discussed. The low-Reynolds-number solutions go over smoothly to the large Reynolds number solution as the Reynolds number increases. Good agreement is obtained with the numerically computed value of the reattachment length.

Absolute inviscid instability of a ring jet with back-flow and swirl

The absolute instability of a ring jet with back-flow and swirl has been investigated on the basis of the inviscid linearized theory for incompressible flow. An axisymmetric disturbance mode is found to be only convectively unstable. The first azimuthal mode can become absolutely unstable, if the ring jet has a back-flow on the jet axis, and an additional swirl can increase the instability. However, for large swirl the absolute instability is suppressed. A ring jet without back-flow becomes absolutely unstable only in the presence of swirl.

Investigations on an axial flow fan stage subjected to circumferential inlet flow distortion and swirl

The combined effects of swirl and circumferential inlet flow distortion on the flow field of an axial flow fan stage are reported in this paper. The study involves measurements at the inlet of the rotor and exit of the rotor and stator at design and off design flow conditions. The study indicated that at the design flow condition, swirl had caused deterioration of the performance in addition to that caused by distortion. Pressure rise imparted in the distortion zone is higher than in the free zone. The attenuation of distortion is high in the presence of swirl.

Effect of the ejector configuration on the gas suction rate and gas hold-up in ejector loop reactors

An experimental study has been aimed at examining the effect of ejector configuration on the rate and energy effectiveness of gas suction and on the values of gas hold-up in ejector loop reactors. Experimental data showed that insertion of a swirl body into the ejector nozzle increased the suction rate and dispersion efficiency of the ejector distributor and significantly improved its operating flexibility. In the absence of swirl elements, the gas suction rate increased slightly with the mixing tube length up to the length-to-diameter ratio equal to six. In the presence of swirls, however, the mixing tube length exhibited negative effect on the ejector performance and the highest values of gas suction rate and dispersion efficiency were observed for the configurations without the mixing tube. The ejector suction efficiency increased sharply with increasing swirl number Sw b (related to the swirl body parameters) in the region of its values 0.06–0.12 while the increase of Sw b above 0.2 resulted in a decrease of the gas suction rate. For Sw b = 0.12–0.20, the gas suction rate was significantly higher than that achieved in the absence of swirls even at the optimum mixing bube length. Comparison of the energy effectiveness of gas suction indicated superiority of the single-orifice nozzles with swirl inserts over the other tested variants. Good agreement of gas holdup data from all experimental runs with values calculated from the relation u s = u oG ge G − u oL (1−ϵ G) , for the slip velocity value u s = 0.224 m s −1, proved adequacy of the slip velocity concept for description of gas-liquid flow in ejector loop reactors.

Effect of Swirl on Strained Premixed Flames for Mixtures with Lewis Number Distinct from Unity

Abstract The behavior of strained premixed flames in the presence of swirl has been examined for mixtures, such as lean butane/air and propane/air, for which the Lewis number is significantly larger than one. Solutions are sought using the asymptotic limits of high activation energy and large Reynolds number without restricting the Lewis number to be near one. In this formulation the flame is not necessarily embedded in the viscous layer at the stagnation plane and extinction may occur with the flame located in the upstream inviscid flow. The structure of the viscous layer however is essential in locating the additional stagnation planes that arise at high rate of rotation and hence the flame standoff distance. A detailed description of the flow field is given and the extinction characteristics are determined showing qualitative agreement with experimental observations.

X-ray measurement of minute lattice strain in perfect silicon crystals

Abstract The residual strain still present in nearly perfect silicon single crystals has been measured quantitatively by using double crystal topography at high reflexion orders. High quality float zone crystals from different suppliers were found to vary in residual strain from 2 × 10−8 to 100 × 10−8. No direct correlation with resistivity or choice of dopant was found for slightly doped crystals. The possible correlation of the local strain variation with the presence of swirls is discussed. The high sensitivity strain measurements are very useful to measure in a quantitative manner the degree of lattice perfection of so-called nearly perfect crystals. The results are already useful for neutron interferometry and, possibly, may become so for microintegration of devices.

Coaxial jets with and without swirl

Measured values of mean velocity, Reynolds stresses and probability density distributions of fluctuating velocity are reported for the turbulent coaxial jets, with and without swirl, emerging into stagnant surroundings from a long pipe and an annulus concentric with the pipe. They were obtained using hot-wire anemometry and on-line data processing with the aid of a mini-computer. The results show that non-swirling coaxial flow configurations approach a self-similar state in a much smaller distance than that of the round jet, for velocity ratios ranging between 0·65 and 1·5; this is due to the mixing layer and vortex shedding that occur in the region downstream of the separation wall between the two streams. In the presence of swirl, the coaxial jet was found to develop at a faster rate. An assessment of turbulence models, based on Reynolds stress closures, suggests that previous assumptions for turbulent diffusion of turbulent kinetic energy are in error.

レーザ流速計による倒立Ｕ字型く形管内の流れ Ｉ 流速分布と圧力分布の測定

When water flows around 180°-bend, centrifugal force acts on the flow resulting in secondary currents within the bend. This paper describes the results of an investigation of the spiral motion around the bend using a laser-Doppler velocimeter. Flow measurement in presence of swirl and manifestation of its velocity-vectors are difficult. This difficulty was solved by using a process such alike tomography and velocity-vactors could be represented stereo-graphically.Furthermore, an investigation was made of the occurrence of Dean's instability by measuring pressure distribution in the channel. This may be found to appear at lower Dean number in vertical channel than horizontal.

Laminar boundary layer on the walls of an axisymmetric channel in the presence of swirls in the external stream

The numerical solution of the problem of laminar, viscous, incompressible fluid flow in the boundary layer on the walls of an axisymmetric channel is considered for the presence of swirls in the external stream. The fundamental boundary-layer characteristics are obtained for several channels.

Laminar boundary layer in axisymmetric swirling ducted gas flow

Several studies have been published [1–3] in which the authors solve the problem of the laminar boundary layer in an incompressible fluid on the walls of an axisymmetric duct in the presence of swirl in the outer flow. In [3], Loitsyanskii's parametric method of [4, 5], generalized to the case of three-dimensional flow, is used to solve this problem.

Laminar Swirling Flow in the Entrance Region of a Circular Pipe

Treated in this paper is the laminar swirling flow in the hydrodynamic entrance region of a circular pipe, in which the entering flow has uniform axial-and linearly varying tangential-velocity components. The analysis is carried out within the framework of the boundary layer theory, the solutions being obtained by means of an extended form of the finite difference method invented by Leigh and Terril. the following results are obtained from the analysis: (1) The entrance length is significantly increased by the presence of swirl at the entrance section. (2) The additional pressure drop based on the center-line pressure at the entrance section is considerably decreased as the swirl is increased. (3) If the wall temperature is constant, the local Nusselt number is increased by a few percent at a certain distance downstream from the entrance section when Prandtl number is taken as 0.72. (4) The damping coefficients of the swirl show an exponential decay shortly after the entrance section. However, in the immediate neighbourhood of the entrance section, the damping of the swirl is a little faster than the exponential decay.