Swirl Velocity Research Articles

Modeling and Optimization of the Vortex Tube with Computational Fluid Dynamic Analysis

Problem statement: This study illustrated the influence of the Length to Diameter (L/D) on the fluid flow characteristics inside the counter-flow Ranque-Hilsch vortex tube predicted by Computational Fluid Dynamic (CFD) and validated through experiment. Approach: The swirl velocity, axial velocity, radial velocity component and also secondary circulation flow (back-flow) along with the pressure within the vortex tube were simulated and proved. Results: The standard k-epsilon turbulence model which is one of the standard turbulence models of FLUENTTM was being used. As a first step, results of simulation will be presented also with varying amounts of cold faction. Conclusion/Recommendations: In the present study, the velocity and the pressure were investigated along the axial and radial directions to understand the flow behavior inside the tube.

Investigation of the influence of swirl on a confined coannular swirl jet

Large Eddy Simulations are used to model a turbulent confined coannular combustor and examine the effects of swirl on the flow field and mixing. Three separate simulations with relatively high mesh resolutions and different swirl numbers have been carried out using a finite volume method on a Cartesian non-uniform structured grid. A localised dynamic Smagorinsky model is used to parameterize the sub-grid scale turbulence. The snapshots of the axial and swirl velocities and velocity vector fields show the complex flow patterns developing with increased swirl number and the rapid decay of axial momentum. Precessing vortex cores (PVC) were identified for all three cases and the mean axial velocity plots indicate that the upstream extremity of the vortex breakdown bubble shifts towards the inlet as the swirl number increases. The calculated power spectra indicate the distinct precession frequency for high swirl number. Probability density functions of axial velocity showed the changes of their distributions from approximately Gaussian to non-Gaussian with increased swirl number. The swirl has a large effect on the rate of decay of the axial velocity throughout the domain, whereas only has a significant effect on the decay of swirl velocity in the near field close to the jet inlet. The relation between swirl number and the axial extent of the recirculation zone is approximately linear. Radial plots of mean passive scalar and its variance also demonstrate an increase in the rate of mixing with increasing swirl number.

On steady rotational cyclonic flows: The viscous bidirectional vortex

This study is focused on the tangential boundary layers of a bidirectional vortex, specifically those forming at the core and the sidewall of a swirl-driven cyclonic chamber. Our analysis is based on the regularized, tangential momentum equation which is rescaled in a manner to capture the forced vortex near the chamber axis and the no slip requirement at the sidewall. After identifying the coordinate transformations needed to resolve the rapid changes in the regions of nonuniformity, two inner expansions are constructed. These expansions are then matched with the outer, free vortex solution that is sandwiched between the core and the hard wall. By combining inner and outer expansions, uniformly valid approximations are subsequently obtained for the swirl velocity, vorticity, and pressure. These are shown to be strongly influenced by a dimensionless grouping that we refer to as the vortex Reynolds number, V. This keystone parameter appears as a ratio of the mean flow Reynolds number and the product of the swirl number and the chamber aspect ratio. Based on V, several fundamental features of the bidirectional vortex are quantified. Among them are the thicknesses of the viscous core and sidewall boundary layers; these decrease with V1/2 and V, respectively. The converse may be said of the peak velocity which increases with V1/2. In the same vein, the angular speed of the rigid-body rotation of the forced vortex is found to be linearly proportional to V. Our laminar swirl velocity is reminiscent of Sullivan’s two-cell vortex except for its additional dependence on the aspect ratio of the chamber. For the purpose of verification, theoretical predictions are compared to particle image velocimetry measurements and Navier–Stokes simulations at high vortex Reynolds numbers. By properly accounting for the turbulent eddy viscosity in the analytical model, local agreement is obtained with both laboratory measurements and computer simulations.

Entropy production analysis of swirling diffusion combustion processes

A critical factor in the design of combustion systems for optimum fuel economy and emission performance lies in adequately predicting thermodynamic irreversibilities associated with transport and chemical processes. The objective of this study is to map these irreversibilities in terms of entropy production for methane combustion. The numerical solution of the combustion process is conducted with the help of a Fluent 6.1.22 computer code, and the volumetric entropy production rate due to chemical reaction, viscous dissipation, and mass and heat transfer are calculated as post-processed quantities with the computed data of the reaction rates, fluid velocity, temperature and radiative intensity. This paper shows that radiative heat transfer, which is an important source of entropy production, cannot be omitted for combustion systems. The study is extended by conducting a parametric investigation to include the effects of wall emissivity, optical thickness, swirl number, and Boltzmann number on entropy production. Global entropy production rates decrease with the increase in swirl velocity, wall emissivity and optical thickness. Introducing swirling air into the combustion system and operations with the appropriate Boltzmann number reduces the irreversibility affected regions and improves energy utilization efficiency.

Effect of Turbulence and External Exhaust Gas Recirculation on HCCI Combustion Mode and Exhaust Emissions

The influence of turbulence, through changes in swirl velocity and exhaust gas recirculation (EGR), and the combustion process was investigated theoretically and experimentally in a homogeneous charge compression ignition (HCCI) engine fuelled with diesel fuel compatible with EN590. The results show that combustion chamber geometry can play a relevant role in HCCI combustion. The combustion process in a high swirl bowl in piston combustion chamber has been compared to that in an almost flat piston (nearly disk shape). Under the same operating conditions—engine speed, injected fuel per cycle, and EGR rate—the peak of the heat release rate for the high swirl piston was lower compared to the disk-shaped combustion chamber. In addition, combustion duration was increased, which is thought to be due to the reduction of gas temperature, larger heat losses, and mixing improvement. Intensity swirl control offers a potentially effective approach to modulating the combustion process in HCCI engines—although a penalt...

Multi-Zone DI Diesel Spray Combustion Model for Thermodynamic Simulation of Engine with PCCI and High EGR Level

A multi-zone, direct-injection (DI) diesel combustion model, the so-called RK-model, has been developed and implemented in a full cycle simulation of a turbocharged engine. The combustion model takes into account: • transient evolution of fuel sprays, • interaction of sprays with swirl and walls, • evolution of near-wall flow formed after spray-wall impingement depending on impingement angle and local swirl velocity, • interaction of Near-Wall Flows (NWF) formed by adjacent sprays, • influence of temperatures of gas and walls in the zones on evaporation rate. In the model the fuel spray is split into a number of specific zones with different evaporation conditions including zone on the cylinder liner and on the cylinder head. The piston bowl is assumed to be a body of revolution with arbitrary shape. The combustion model supports central and non-central injector as well as the side injection system. NOx formation model uses Detail Kinetic Mechanism (199 reactions with 33 species). Soot formation model is phenomenological. The general equation for prediction of ignition delay period was derived as for conventional engines as for engines with PCCI where pilot injection timing achieved 130 CA deg. before TDC. The model has been validated by experimental data obtained from high-speed, mediumspeed and low-speed engines over the whole operating range; a good agreement has been achieved without recalibration of the model for different operating modes. General equations for prediction of spray tip penetration, spray angle and ignition delay for low temperature combustion and high temperature combustion were derived and validated with the published data obtained for different diesels including diesels with multiple injection system and injection timing varied from very early up to after the TDC. To make a computational optimization of multiple injection strategy possible, the full cycle thermodynamic engine simulation software DIESEL-RK has been supplied with library of nonlinear optimization procedures.

The effect of plasma-gas swirl flow on a highly constricted plasma cutting arc

Swirl gas is widely used in the plasma arc cutting industry for inducing better plasma column confinement and providing a more stable constricting arc. When the inlet gas swirl number increases, the maximum pressure location moves to the plenum chamber edge due to centrifugal force, and the minimum pressure location moves much closer to the axis. The difference between the maximum and minimum pressure increases with swirl number. The large pressure difference in the radial direction constricts the arc and leads to large current density and Ohm heating near the axis. This causes much higher temperature near the cathode. It is indicated that swirl gas accelerates cathode erosion for two reasons: increasing plenum chamber pressure and changing flow patterns in the vicinity of the cathode. It is also found that the gas with large swirl number leads the swirl velocity component to be larger at the cutting location according to angular momentum conservation. It has been suggested that this causes different bevel angles for the left and right edges of the cutting kerf.

Eddy diffusivities estimated from observations in the Labrador Sea

Eddy diffusivities in the Labrador Sea (LS) are estimated from deep eddy resolving float trajectories, moored current meter records, and satellite altimetry. A mean residence time of 248 days in the central LS is observed with several floats staying for more than 2 years. By applying a simple random walk diffusion model, the observed distribution of float residence times in the central LS could be explained by a mean eddy diffusivity of about 300 m2 s−1. Estimates from float trajectories themselves and from moored current meter records yield significantly higher eddy diffusivities in the central LS of 950–1100 m2 s−1. This discrepancy can be explained by an inhomogeneity of the eddy diffusivity at middepth with high/low values in the central LS/region between central LS and deep Labrador Current, which could be conjectured from the mean altimetric eddy kinetic energy (EKE) distribution. The different diffusivities explain both (1) a fast lateral homogenization of water masses in the central LS and (2) a weak exchange between central LS and boundary current. The mean Lagrangian length scale of 11.5 ± 0.7 km as estimated from deep float trajectories is only slightly larger than the mean Rossby radius of deformation (8.8 km). Largest eddy diffusivities within the central LS are associated with strong eddy drifts, rather than with large swirl velocities and associated large EKE.

Influence of Leak Path Friction on Labyrinth Seal Inlet Swirl

Labyrinth seals in various kinds of rotating machinery often generate driving force components that may increase the unstable vibration of the rotor. The evaluation of labyrinth seal forces has been documented by CFD analysis but the leak path geometry influence on the resulting entry and reverse pumping flow prior to the labyrinth seal geometry has only recently been studied in detail. The labyrinth seal driving forces are known to increase with an increase of the inlet swirl velocity ratio. In a previous work, a commercial CFD program was used to simulate leak path and labyrinth seal flows of various centrifugal compressor eye seal leak flow geometries. For each case, 3D models with an eccentric rotor were solved to obtain the leakage flow, velocity vector, chamber pressure, and the average chamber swirl entering the seal. The current article will present the bulk flow analysis predicted entry swirl ratio for the same geometries and sealing conditions. The influence of the effective disk friction will be considered in the calculation of the bulk flow results by using modified turbulence factors. The CFD simulation showed that the inlet geometry and the resultant reverse pumping flow in the entry channel can have a significant influence on the calculated chamber average swirl velocity ratio. Generally, the bulk flow analysis gives a pessimistic estimate of the swirl velocities as compared to the CFD calculations.

Simulation of the Internal Flow of the High-Pressure Swirl Injector

The simulation of the internal flow of a kind of high-pressure swirl injector was carried out with multiphase and cavitation models basing on the movement mesh. The spray was also simulated with the result of the internal flow simulation. The result of spray simulation was compared with experimental results. In addition, the effect on the internal flow which comes from the swirl flutes, the parts for generating the swirl flow, was investigated. It is shown that the amount and configuration of swirl flutes have crucial influence on the swirl velocity and cavitation of the internal flow.

E312 NUMERICAL SIMULATION OF HEAT LOAD DISTRIBUTION IN A 1100T/H BOILER(Boiler-3)

Numerical simulation software is developed and carried out on the furnace of a 1100t/h boiler. Heat load distribution and heat deviations on water-walls were obtained. The numerical results are basically consistent with that from experiments. Two operating conditions were adopted with different swirl velocity arrangements. In condition I, tangential velocities from two burners sharing the same height and same wall, turn upward at the side close to the central line of the furnace, which turn downward in condition II. Results show that, heat load distribution varies a lot, among water-walls and among different locations in the same water-wall, especially at the corners, where bad stress concentration may form; in condition II, the flame center appears at lower position, temperature distribution is more uniform, and the difference of heat load distribution between water-walls is less evident. These results provide important reference value for water-wall safety of large capacity utility boilers.

A constant shear stress core flow model of the bidirectional vortex

In this paper, we discuss the merits of two models for the swirl velocity in the core of a confined bidirectional vortex. The first is piecewise, Rankine-like, based on a combined-vortex representation. It stems from the notion that a uniform shear stress distribution may be assumed in the inner vortex region of a cyclone, especially at high Reynolds numbers. Thereafter, direct integration of the shear stress enables us to retrieve an expression for the swirl velocity that overcomes the inviscid singularity at the centreline. The second model consists of a modified asymptotic solution to the problem obtained directly from the Navier–Stokes equations. Both solutions we present transition smoothly to the outer, free-vortex approximation at some intermediate position in the chamber. This position is deduced from available experimental data to the extent of providing an accurate swirl velocity distribution throughout the chamber. By scaling the constant shear radius to the core layer thickness, the constant of proportionality is readily calculated using the method of least squares. Interestingly, the constant of proportionality is found to be invariant at several vortex Reynolds numbers, thus helping to achieve closure. The combined-vortex representation is validated against a large body of experimental measurements and through comparisons to a laminar core model that is enhanced through the use of an eddy viscosity. Other heuristic schemes are discussed and the two most suitable models to capture realistic flow behaviour at high vortex Reynolds numbers are identified. Our two models are first derived analytically and then anchored on the available experimental measurements.

Shelf water entrainment by Gulf Stream warm-core rings between 75°W and 50°W during 1978–1999

Previous work concerning Gulf Stream warm-core rings (WCRs) and their associated shelf water entrainments have been based upon single surveys or time series from individual WCRs. To date, estimates of annual shelf water volume entrained into the Slope Sea by WCRs and its interannual variability have not been made. Using a long time series of satellite-derived sea surface temperature (SST) observations of Slope Sea WCRs, we have completed an analysis of 22 years of WCR data (1978–1999) between 75°W and 50°W to understand the interannual variability of WCRs and their role in entraining shelf water. Satellite-derived SST data digitized at Bedford Institute of Oceanography are analyzed using an ellipse-fitting feature model to determine key WCR characteristics including WCR center position, radius and orientation. Key characteristics are then used to compute WCR swirl velocity by finite-differencing WCR orientations ( θ) obtained from the feature model time series. Global mean WCR-edge swirl velocity calculated from all observations is 105.72±10.7 km day −1 (122.36±12.4 cm s −1), and global mean WCR radius is 64.8±6.2 km. Primary and derived WCR data are incorporated into a two-dimensional ring entrainment model (RM) using the quasi-geostrophic approximation of the potential vorticity equation. The RM defines ambient water as entrained by a WCR only if the gradient of relative vorticity term (horizontal shear) dominates the potential vorticity. Proximity of a WCR to the position of the shelf-slope front (SSF) is then used to determine whether the ambient water is entrained from the outer continental shelf. WCR-induced shelf entrainment derived from the RM displays considerable spatial variability, with maximum entrainment occurring offshore of Georges Bank, advecting a mean total annual shelf water volume of 7500 km 3 year −1 from the region. Estimates of shelf water fluxes display significant interannual variability, which may be in part due to the observed covariance between WCR occurrences and the state of the North Atlantic Oscillation (NAO). Increased (decreased) occurrences of WCRs are evidenced during positive (negative) phases of the NAO. The total mean annual shelf-wide WCR-induced shelf water transport is estimated to be 23,700 km 3 year −1 (0.75 Sv), accounting for nearly 25% of the total transport in the Slope Sea region neighboring the outer continental shelf.

A numerical technique for laminar swirling flow at the interface between porous and homogenous fluid domains

AbstractThere have been a few recent numerical implementations of the stress‐jump condition at the interface of conjugate flows, which couple the governing equations for flows in the porous and homogenous fluid domains. These previous demonstration cases were for two‐dimensional, planar flows with simple geometries, for example, flow over a porous layer or flow through a porous plug. The present study implements the interfacial stress‐jump condition for a non‐planar flow with three velocity components, which is more realistic in terms of practical flow applications. The steady, laminar, Newtonian flow in a stirred micro‐bioreactor with a porous scaffold inside was investigated. It is shown how to implement the interfacial jump condition on the radial, axial, and swirling velocity components. To avoid a full three‐dimensional simulation, the flow is assumed to be independent of the azimuthal direction, which makes it an axisymmetric flow with a swirling velocity. The present interface treatment is suitable for non‐flat surfaces, which is achieved by applying the finite volume method based on body‐fitted and multi‐block grids. The numerical simulations show that a vortex breakdown bubble, attached to the free surface, occurs above a certain Reynolds number. The presence of the porous scaffold delays the onset of vortex breakdown and confines it to a region above the scaffold. Copyright © 2008 John Wiley & Sons, Ltd.

Experimental investigation of counter-rotating four vortex aircraft wake

Abstract An experimental investigation on the wake vortex formation and evolution of a four vortex system of a generic model in the near field and extended near field as well as the behaviour and decay in the far field region has been conducted by means of hot-wire anemometry in a wind tunnel. The results were obtained during an experimental campaign as part of the EC project “FAR-Wake”. The model used consists of a wing–tail plane configuration with the wing producing positive lift and the tail plane negative lift. The circulation ratio of tail plane to wing is −0.3 and the span ratio is 0.3. Thus, a four vortex system with counter-rotating neighboured vortices exists. The model set-up was chosen on the condition to create a most promising four vortex system with respect to accelerate wake vortex decay by optimal perturbations enhancing inherent instability mechanisms. The flow field has been investigated for a half plane of the entire wake up to a distance of 48 span dimensions downstream of the model. The results obtained at 1, 12, 24 and 48 span distances are shown as non-dimensional axial vorticity and vertical turbulence intensities. A significant decay in peak vorticity, swirl velocity and circulation is observable during the downward motion of the vortices. Spectral analysis of the unsteady velocity data reveals a peak in the power spectral density distributions indicating the presence of a dominating instability. Using two hot-wire probes cross spectral density distributions have also been evaluated, which highlight the co-operative instability leading to a rapid wake vortex decay within 30 span dimensions downstream.

Numerical investigations on flow behaviour and energy separation in Ranque–Hilsch vortex tube

A three-dimensional numerical model of Ranque–Hilsch vortex tube has been developed using the commercial CFD code (Star-CD) to analyze the flow parameters and energy separation mechanism inside the tube. Investigations have been done on the variation of fluid properties and flow parameters as the fluid particles progress in the flow field by tracking different particles exiting through the hot and cold end. Fluid properties like stagnation temperature, static temperature, static pressure and total pressure and flow parameters like axial, radial and swirl velocities are obtained along the axial and radial directions to understand the flow behaviour inside the tube. The presence of free vortex zone inside the tube also has been investigated. Possible energy transfer mechanisms are discussed and an estimate has been made on the magnitude of energy transfer from the cold end exit flow to hot end exit flow. Effects of secondary circulation and length of the tube on energy separation also have been evaluated.

A simulation of the combustion of hydrogen in HCCI engines using a 3D model with detailed chemical kinetics

The influence of changes in the swirl velocity of the intake mixture on the combustion processes within a homogeneous charge compression ignition (HCCI) engine fueled with hydrogen were investigated analytically. A turbulent transient 3D predictive computational model which was developed and applied to the HCCI engine combustion system, incorporated detailed chemical kinetics for the oxidation of hydrogen. The effects of changes in the initial intake swirl, temperature and pressure, engine speed and compression and equivalence ratios on the combustion characteristics of a hydrogen fuelled HCCI engine were also examined. It is shown that an increase in the initial flow swirl ratio or speed lengthens the delay period for autoignition and extends the combustion period while reducing NO x emissions. There are optimum values of the initial swirl ratio and engine speed for a certain mixture intake temperature, pressure, compression and equivalence ratios operational conditions that can achieve high thermal efficiencies and low NO x emissions while reducing the tendency to knock

Resuspension of Dust Particles in a Chamber and Associated Environmental Factors

Experiments investigating particle resuspension from human activities were conducted in a full-scale experimental chamber. The experiments tested three types of flooring (vinyl tiles, new and old level-loop carpets) and two ventilation configurations (ceiling and side wall supply systems). The floorings were seeded with 0.1–10 μ m test particles. The airborne particle concentration was measured by an array of optical particle counters (OPCs) in the chamber. Resuspension rates were estimated in size ranges of 0.8–1, 1.0–2.0, 2.0–5.0, and 5.0–10 μm ranging from 10−5–10−2 hr−1, with higher resuspension rates associated with larger particles. Resuspension via walking activity varied from experiment to experiment. “Heavy and fast” walking was associated with higher resuspension rates than less active walking, most likely due to a combination of increased pace, increased air swirl velocity, and electrostatic field effects established by the walking. The type of floorings also influenced the particle resuspensio...

모형 가스터빈 연소기의 입구 형상변화에 따른 연소 불안정성에 관한 LES 연구

The effects of combustion instability on flow structure and flame dynamics with the inlet configurations in a model gas turbine combustor were investigated using large eddy simulation (LES). A G-equation flamelet model was employed to simulate the unsteady flame behaviors. As a result of mean flow field, the change of divergent half angle(α ) at combustor inlet results in variations in the size and shape of the central toroidal recirculation (CTRZ) as well as the flame length by changing corner recirculation zone (CRZ). The case of α = 45° show smaller size and upstream location of CTRZ than those of 90° and 30° by the development of higher swirl velocity. The flame length in the case of α = 45° is shorter than other cases, while the case of α = 30° yields the longest flame length due to the decrease of effective reactive area with the absence of CRZ. Through the analysis of pressure fluctuation, it was identified that the case of α = 45° shows the largest damping effect of pressure oscillation in all configurations and brings in the noise reduction of 2.97㏈, compared to that of α = 30° having the largest pressure oscillation. These reasons were discussed in detail through the analysis of unsteady phenomena related to recirculation zone and flame surface. Finally the effects of flame-acoustic interaction were evaluated using local Rayleigh parameter.

Computational Modeling of HART II Blade-Vortex Interaction Loading and Wake System

Correlations using a loosely coupled trim methodology of the computational fluid dynamics (OVERFLOW-2) and computational structural dynamics (CAMRAD-II) codes are presented to calculate the helicopter rotor blade-vortex interaction airloads and wake system for the higher-harmonic aeroacoustic rotor test (HART II) rotor at an advance ratio of 0.15. Five different grid models are studied to quantify the effects of grid refinement on rotor-wake resolution. The fine grid model has a total of 113 million grid points and it improves airload predictions compared with the standard grid model for three HART II test cases: baseline, minimum noise, and minimum vibration. The rotor-wake positions are well predicted by this fine grid model. The computed vorticity field for a young vortex using the fine grid model is compared with the measured particle image velocimetry data and the results are good. The fine grid model underpredicts the experimental value for the maximum vorticity by 61%. The predicted vortex core radius is Is % in chord for the fine grid while the measured data show about 5% chord length. The predicted swirl velocity is, however, higher than the measured data for this vortex. The results in this paper provide the first quantitative comparisons between the measured and computational fluid dynamics/computational structural dynamics computed flowfield for a helicopter rotor-wake system.