Non-swirling Flow Research Articles

Numerical analysis of screw swirling effects on fiber orientation in large area additive manufacturing polymer composite deposition

Large Area Additive Manufacturing (LAAM) polymer deposition employs a single screw extruder to deliver pelletized feedstock resulting in significantly higher flow rates as compared to conventional filament-based extrusion additive processes. Swirling kinematics in LAAM melt flow that result from the screw rotation generate unique particle alignment patterns within the fiber-filled polymer during deposition processing. This paper investigates the effect of the single screw swirling motion on the resulting fiber orientation in a short fiber polymer composite extrudate. An axisymmetric non-Newtonian viscoelastic flow is simulated with the finite element method, where the flow nearby the extruder screw tip, within the printing nozzle, and a short section of free extrudate compose the flow domain. Fiber orientation tensors within the flow domain are evaluated using the Wang-O’Gara-Tucker Reduced Strain Closure (RSC) fiber orientation diffusion model with the orthotropic fitted closure. The results indicate that swirling kinematics yield a longer flow path for fibers to travel and orientate within the flow domain, yielding orientation tensor results that are notably different as compared to a non-swirl event. The predicted principal elastic constant from the swirling flow and non-swirling flow models exhibit 21% difference, and those from the swirling model using the RSC closure shows a good agreement with reported experimental data for a similar material system.

The effect of swirl on transition from churn flow to annular flow in an intermediate diameter pipe

Swirling annular flow has been widely used in industries. The transition boundary of swirling annular flow which is important for its application in practice is rarely studied in published works. In this work, the effect of swirl on transition from churn flow to annular flow in an intermediate diameter pipe has been investigated by experimentation and modelling. The experiments were conducted with air–water two-phase flow in an 11-m vertical pipe (D = 62 mm) at the range of the test conditions: the liquid superficial velocity ranges from 0.0039 to 0.95 m/s and the gas superficial velocity ranges from 1.22 to 32.49 m/s. The experimental results showed that the churn flow can be transformed to swirling annular flow under the effect of swirl, and the gas velocity required for the transition from churn flow to swirling annular flow in a swirling flow is lower compared with that for the transition from churn flow to annular flow in a non-swirling flow. Then a theoretical model based on separated flow model was established to predict the transition boundary between churn flow and swirling annular flow in a vertical pipe. Calculated results were in good agreements with experimental results.

Transition from bubble flow to slug flow along the streamwise direction in a gas–liquid swirling flow

Transition from bubbly to slug flow is a well-known phenomenon. In the channel with a swirler, a transition to slug flow at a lower gas superficial velocity than in the same channel without the swirler was observed in this work. The transition phenomenon from bubble flow to slug flow under the effect of swirl in a circular pipe was observed and investigated. Compared to the flow pattern (maintaining the bubble flow along a streamwise direction) in a non-swirling flow, we observe that bubble flow is transformed to a gas column downstream of the swirler, then broken up, and finally separated into gas slugs in the streamwise direction. The dynamic condition for the occurrence of a liquid bridge is proposed, then combined with a swirl decay model of a gas–liquid two-phase swirling flow; thus, the axial position for the transition from a gas column to a slug flow along the streamwise direction can be predicted. Whether slug flow always remains along the streamwise direction is closely related to its size. Under the effect of swirl, the void fraction is decreased and the pressure drop is increased compared with that in a non-swirling flow, and gradually approximates the values of a non-swirling flow along the streamwise direction.

Flow regime identification of swirling gas-liquid flow with image processing technique and neural networks

Swirling flow is one of the commonly-recognized techniques to control working processes in various engineering fields. A fundamental understanding of the swirling flow pattern is significantly important for proper design, operation and optimization of swirling flow systems. Although a large amount of work has been done on conventional non-swirling two-phase flow, investigations on swirling flow regime classification are still scarce. In this paper, a visualization experiment was carried out to study the gas-liquid flow in a vertical pipe containing a swirler with four helical vanes. The typical flow regimes in swirling gas-liquid flow were first classified and defined by visual observation. Subsequently, the liquid holdup was measured with the proposed image processing technique and statistically analyzed through PDF and CPDF. Owing to its integral and stable feature, CPDF was then utilized along with a self-organizing neural network (SONN) to identify the swirling flow regimes objectively. Finally, a swirling flow regime map was proposed and compared with the previous non-swirling and swirling maps available in the publications. Based on this study, the influence of the centrifugal force on phase distribution of gas-liquid flow was clarified and the general relationship between swirling gas-liquid flow and non-swirling gas-liquid flow was concluded.

A mechanistic model for the prediction of swirling annular flow pattern transition

The accurate prediction of flow patterns and their transition is extremely important for proper design, operation and optimization of two-phase flow systems, since the parameters such as pressure loss and heat and mass transfer are strongly dependent on the flow pattern. So far, the non-swirling gas-liquid flow in straight pipes have been widely studied and various mechanisms that lead to flow pattern transition have been clarified and modeled. However, the dynamics of gas-liquid flow under swirling condition are not well understood, and no detailed models are available for the prediction of swirling flow pattern transition. To address this, in our previous work (Liu and Bai, 2018), a visualization experiment aimed at classifying flow regimes in swirling gas-liquid flow was presented and three typical swirling flow regimes, i.e., swirling gas column flow, swirling intermittent flow and swirling annular flow were classified and defined, respectively. As the swirling annular flow can be regarded as a special case of conventional annular flow (i.e., when tangential velocity does not equal zero), in present paper, a mechanistic model for the prediction of the swirling annular flow pattern transition was developed considering its physical interest and great practical significance. Two physical mechanisms that lead to the transition from swirling annular flow to other flow patterns were revealed and modeled, respectively. The model was evaluated against a wide range of swirling and non-swirling experimental data and based on this model, the effects of different parameters (e.g., hydraulic diameter, working pressure and swirl angle) on the boundary of flow pattern transition were presented. Results revealed that the range of swirling annular flow enlarges with the increase of the working pressure and swirl angle but narrows with the hydraulic diameter. Taking these influencing factors into account, a generalized formula for the prediction of the swirling annular flow pattern transition was proposed. Compared with existing empirical correlations for annular flow, the newly developed correlation provided more accurate and reasonable prediction of flow pattern transition for both swirling annular flow and annular flow.

A Model for Fuel Spray Formation with Atomizing Air

The formation of a liquid spray emanating from a nozzle in the presence of atomizing air was studied using a computational model approach that accounted for the deformation and break up of droplets. Particular attention was given to the formation of sprays under non-swirling flow conditions. The instantaneous fluctuating fluid velocity and velocity gradient components were evaluated with the use of a probability density function (PDF)-based Langevin equation. Motions of atomized fuel droplets were analyzed, and ensemble and time averaging were used for evaluating the statistical properties of the spray. Effects of shape change of droplets, and their breakup, as well as evaporation, were included in the model. The simulation results showed that the mean-square fluctuation velocities of the droplets vary significantly with their size and shape. Furthermore, the mean-square fluctuation velocities of the evaporating droplet differed somewhat from non-evaporating droplets. Droplet turbulence diffusivities, however, were found to be close to the diffusivity of fluid point particles. The droplet velocity, concentration, and size of the simulated spray were compared with the experimental data and reasonable agreement was found.

Adjoint-based parametric sensitivity analysis for swirling M-flames

A linear numerical study is conducted to quantify the effect of swirl on the response behaviour of premixed lean flames to general harmonic excitation in the inlet, upstream of combustion. This study considers axisymmetric M-flames and is based on the linearised compressible Navier–Stokes equations augmented by a simple one-step irreversible chemical reaction. Optimal frequency response gains for both axisymmetric and non-axisymmetric perturbations are computed via a direct–adjoint methodology and singular value decompositions. The high-dimensional parameter space, containing perturbation and base-flow parameters, is explored by taking advantage of generic sensitivity information gained from the adjoint solutions. This information is then tailored to specific parametric sensitivities by first-order perturbation expansions of the singular triplets about the respective parameters. Valuable flow information, at a negligible computational cost, is gained by simple weighted scalar products between direct and adjoint solutions. We find that for non-swirling flows, a mode with azimuthal wavenumber $m=2$ is the most efficiently driven structure. The structural mechanism underlying the optimal gains is shown to be the Orr mechanism for $m=0$ and a blend of Orr and other mechanisms, such as lift-up, for other azimuthal wavenumbers. Further to this, velocity and pressure perturbations are shown to make up the optimal input and output showing that the thermoacoustic mechanism is crucial in large energy amplifications. For $m=0$ these velocity perturbations are mainly longitudinal, but for higher wavenumbers azimuthal velocity fluctuations become prominent, especially in the non-swirling case. Sensitivity analyses are carried out with respect to the Mach number, Reynolds number and swirl number, and the accuracy of parametric gradients of the frequency response curve is assessed. The sensitivity analysis reveals that increases in Reynolds and Mach numbers yield higher gains, through a decrease in temperature diffusion. A rise in mean-flow swirl is shown to diminish the gain, with increased damping for higher azimuthal wavenumbers. This leads to a reordering of the most effectively amplified mode, with the axisymmetric ($m=0$) mode becoming the dominant structure at moderate swirl numbers.

The effect of flow swirling on flow structure, turbulence and heat transfer in turbulent droplet-laden flow in a pipe sudden expansion

This paper presents the mathematical model to simulate the swirling turbulent gas-droplet flow in a sudden pipe expansion. The set of axisymmetrical steady-state RANS equations for the two-phase flow is used. The dispersed phase is modeled by the Eulerian approach. The flow swirl causes an increase in the intensity of heat transfer (more than 1.5 times in comparison with the non-swirling mist flow at other identical inlet conditions). Evaporation of the droplets leads to a significant increase in the heat transfer intensity in the swirling two-phase flow (more than 2.5 times in comparison with the single-phase flow). It is shown that ethanol droplets evaporate much faster than water droplets due to the lower heat of phase transition. The heat transfer enhancement by using ethanol droplets is higher than the corresponding value for water droplets (up to 20%).

Dynamics and effective distance of gas–liquid two-phase swirling flow induced by vortex tools

Based on the mechanistic model of the two-phase swirling annular flow behavior, a calculation method for the effective distance of vortex tool is obtained by taking Kelvin–Helmholtz instability into account. The rationality of the proposed method is validated by comparing the predicted liquid film thickness and pressure under non-swirling flow with the commonly used correlations. Then, the influences of gas–liquid ratio, helix angle, and hub diameter of vortex tool on the liquid film thickness, pressure drop, and effective distance have been analyzed. Results show that the presence of the vortex tool causes the decrease in the liquid film thickness and increase in the pressure drop. The liquid film thickness increases gradually as the helix angle and the hub diameter increase, and the larger helix angle results in smaller pressure drop. The effective distance increases with an increase in the gas–liquid ratio and decreases with an increase in helix angle and hub diameter. A sudden decrease occurs when the helix angle exceeds 60°. The gas–liquid ratio and helix angle are more dominant factors than the hub diameter on the liquid film thickness, pressure drop, and effective distance.

Prediction of CO and NOx Pollutants in a Stratified Bluff Body Burner

The major exhaust gas pollutants from heavy duty gas turbine engines are CO and NOx. The difficulty of predicting the concentration of these combustion products originates from their wide range of chemical time scales. In this paper, a combustion model that includes the prediction of the carbon monoxide and nitric oxide emissions is tested. Large eddy simulations (LES) are performed using a compressible code (OpenFOAM). A modified flamelet generated manifolds (FGM) approach is applied with an artificially thickened flame approach (ATF) to resolve the flame on the numerical grid, with a flame sensor to ensure that the flame is only thickened in the flame region. For the prediction of the CO and NOx emissions, pollutant species transport equations and a second, CO based, progress variable are introduced for the flame burnout zone to account for slow chemistry effects. For the validation of the models, the Cambridge burner of Sweeney et al. (2012, “The Structure of Turbulent Stratified and Premixed Methane/Air Flames—I: Non-Swirling Flows,” Combust. Flame, 159, pp. 2896–2911; 2012, “The Structure of Turbulent Stratified and Premixed Methane/Air Flames—II: Swirling Flows,” Combust. Flame, 159, pp. 2912–2929.) is employed, as both carbon monoxide and nitric oxide [Apeloig et al. (2016, “PLIF Measurements of Nitric Oxide and Hydroxyl Radicals Distributions in Swirl Stratified Premixed Flames,” 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics, Lisbon, Portugal, July 4–7.)] data are available.

A LES/PDF simulator on block-structured meshes

A block-structured mesh large-eddy simulation (LES)/probability density function (PDF) simulator is developed within the OpenFOAM framework for computational modelling of complex turbulent reacting flows. The LES/PDF solver is a hybrid solution methodology consisting of (i) a finite-volume (FV) method for solving the filtered mass and momentum equations (LES solver), and (ii) a Lagrangian particle-based Monte Carlo algorithm (PDF solver) for solving the modelled transport equation of the filtered joint PDF of compositions. Both the LES and the PDF methods are developed and combined to form a hybrid LES/PDF simulator entirely within the OpenFOAM framework. The in situ adaptive tabulation method [S.B. Pope, Computationally efficient implementation of combustion chemistry using in situ adaptive tabulation, Combust. Theory Model. 1 (1997), pp. 41–63; L. Lu, S.R. Lantz, Z. Ren, and B.S. Pope, Computationally efficient implementation of combustion chemistry in parallel PDF calculations, J. Comput. Phys. 228 (2009), pp. 5490–5525] is incorporated into the new LES/PDF solver for efficient computations of combustion chemistry with detailed reaction kinetics. The method is designed to utilise a block-structured mesh and can readily be extended to unstructured grids. The three-stage velocity interpolation method of Zhang and Haworth [A general mass consistency algorithm for hybrid particle/finite-volume PDF methods, J. Comput. Phys. 194 (2004), pp. 156–193] is adapted to interpolate the LES velocity field onto particle locations accurately and to enforce the consistency between LES and PDF fields at the numerical solution level. The hybrid algorithm is fully parallelised using the conventional domain decomposition approach. A detailed examination of the effects of each stage and the overall performance of the velocity interpolation algorithm is performed. Accurate coupling of the LES and PDF solvers is demonstrated using the one-way coupling methodology. Then the fully two-way coupled LES/PDF solver is successfully applied to simulate the Sandia Flame-D, and a turbulent non-swirling premixed flame and a turbulent swirling stratified flame from the Cambridge turbulent stratified flame series [M.S. Sweeney, S. Hochgreb, M.J. Dunn, and R.S. Barlow, The structure of turbulent stratified and premixed methane/air flames I: Non-swirling flows, Combust. Flame 159 (2012), pp. 2896–2911; M.S. Sweeney, S. Hochgreb, M.J. Dunn, and R.S. Barlow, The structure of turbulent stratified and premixed methane/air flames II: Swirling flows, Combust. Flame 159 (2012), pp. 2912–2929]. It is found that the LES/PDF method is very robust and the results are in good agreement with the experimental data for both flames.

The impedance boundary condition for acoustics in swirling ducted flow

The acoustics of a straight annular lined duct containing a swirling mean flow is considered. The classical Ingard–Myers impedance boundary condition is shown not to be correct for swirling flow. By considering behaviour within the thin boundary layers at the duct walls, the correct impedance boundary condition for an infinitely thin boundary layer with swirl is derived, which reduces to the Ingard–Myers condition when the swirl is set to zero. The correct boundary condition contains a spring-like term due to centrifugal acceleration at the walls, and consequently has a different sign at the inner (hub) and outer (tip) walls. Examples are given for mean flows relevant to the interstage region of aeroengines. Surface waves in swirling flows are also considered, and are shown to obey a more complicated dispersion relation than for non-swirling flows. The stability of the surface waves is also investigated, and as in the non-swirling case, one unstable surface wave per wall is found.

Effect of Flow Swirling on Heat Transfer in Gas-Droplet Flow Downstream of Abrupt Pipe Expansion

The effect the flow swirl parameter on heat transfer in a gas-droplet flow is numerically modeled by the Euler approach. The gaseous phase is described by a system of 3D RANS equations with consideration of the back effect of particles on transfer processes in the carrier phase. The gaseous phase turbulence is calculated according to the Reynolds stress transport model with consideration of the dispersed phase effect on the turbulent characteristics. A rapid dispersion of droplets over the pipe cross section is observed in a nonswirling gas-droplet flow downstream of an abrupt pipe expansion. A swirling flow is characterized by a growing concentration of fine particles at the pipe axis due to the accumulation of particles in the zone of flow recirculation and to the turbophoresis force. In a swirling flow, the separated-flow region becomes significantly shorter (by almost a factor of two as compared to that in a nonswirling flow). It is shown that addition of droplets results in a significant growth of heat transfer intensity (by more than a factor of 2.5) in comparison with single-phase swirling flow.

Study on kinetics of flow characteristics in hot air drying of pineapple

The aim was to evaluate the kinetic parameters, total color differences (∆E*) and browning index differences (∆BI) of air flow pineapple drying. The experiments were performed on air temperatures at 60 and 70°C, and air velocities at 1.5 and 2.0m/s. The kinetic parameter (k) increased when air temperature was increased for all air velocity. The effective diffusivity coefficient (Deff) increased as high as the temperature of the heating medium. The variation of Deff of swirling flow was ranging from 6.72 × 10-9 to 10.23 × 10-9m2/s, while the variation of Deff of non-swirling flow was ranging from 6.40 × 10-9 to 9.42 × 10-9m2/s. The drying time of swirling flow was shorter than non-swirling flow in each drying condition. Moreover, the ∆E* and ∆BI of pineapple in swirling flow were lower than that obtained from non-swirling flow. Therefore, the convective drying using swirling flow can be minimized for drying time and color deterioration.

Interfacial and wall friction factors of swirling annular flow in a vertical pipe

Experiments on air–water two-phase swirling annular flows in a vertical pipe of 40 mm diameter were carried out at atmospheric pressure and room temperature to investigate interfacial and wall friction factors, fi and fw. The friction factors were evaluated using measured pressure drops and void fractions. Measurements of liquid film thickness and flow observation were also conducted. The ranges of the gas and liquid volume fluxes, JG and JL, were 12.5 ≤ JG ≤ 20.0 m/s and 0.03 ≤ JL ≤ 0.11 m/s, respectively. The main conclusions obtained are as follows: (1) fi and fw in swirling annular flows are several times larger than those in non-swirling flows, (2) fi is well correlated in terms of the liquid volume fraction and the gas Reynolds number, ReG, (3) ReG and the liquid Reynolds number, ReL, are required for correlating fw, and (4) the liquid film thicknesses in two-phase swirling flows in a one-fifth model of a BWR separator are well predicted using the two-fluid model and the correlations of fi and fw developed based on the experimental data.

Numerical investigation of turbulent swirling flows through an abrupt expansion tube

A numerical investigation of turbulent swirling flows through an abrupt expansion tube is reported. The TEFESS code, based on a staggered Finite Volume approach with the standard k-ε model and first-order numerical schemes built-in, was used to carry out all the computations. The code has been modified in the present work to incorporate the ASM and two second-order numerical schemes. The ASM, which includes the non-gradient convection terms arising from the transformation from Cartesian to cylindrical coordinates, was investigated for isothermal flows by applying it to the flow through an abrupt expansion tube with or without swirl flows. In addition, to investigate the effects of numerical diffusion on the predicted results, two second-order differencing schemes, namely, second-order upwind and the quadratic upstream interpolation, were used to compare with the first-order hybrid scheme. An abrupt expansion tube with non-swirling flow, predicted results using both the k-ε model and the ASM were in good agreement with measurements. For swirling flows, the calculated results suggested that the use of the ASM with a second-order numerical scheme leads to better agreement between the numerical results and experimental data, while the k-ε model is incapable of capturing the stabilizing effect of the swirl.

Stochastic-Based RANS-LES Simulations of Swirling Turbulent Jet Flows

Abstract Many turbulent flow simulations require the use of hybrid methods because LES methods are computationally too expensive and RANS methods are not sufficiently accurate. We consider a recently suggested hybrid RANS-LES model that has a sound theoretical basis: it is systematically derived from a realizable stochastic turbulence model. The model is applied to turbulent swirling and nonswirling jet flow simulations. The results are shown to be in a very good agreement with available experimental data of nonswirling and mildly swirling jet flows. Compared to commonly applied other hybrid RANS-LES methods, our RANS-LES model does not seem to suffer from the ’modeled-stress depletion’ problem that is observed in DES and IDDES simulations of nonswirling jet flows, and it performs better than segregated RANS-LES models. The results presented contribute to a better physical understanding of swirling jet flows through an explanation of conditions for the onset and the mechanism of vortex breakdown.

Numerical modeling of turbulent flow structure and heat transfer in a droplet-laden swirling flow in a pipe with a sudden expansion

ABSTRACTThis contribution presents the mathematical model to simulate the swirling turbulent gas-droplet flow in a sudden pipe expansion. The set of axisymmetrical steady-state Reynolds averaged Navier–Stokes equations (RANS) for the two-phase flow is utilized. The dispersed phase is modeled by the Eulerian approach. The flow swirl causes an increase in the intensity of heat transfer (more than 1.5 times compared with the nonswirling mist flow at other identical inlet conditions). Evaporation of the droplets leads to a significant increase in the heat transfer intensity in the swirling two-phase flow (more than 2.5 times compared with the single-phase flow).

Structure of the nonisothermal swirling gas-droplet flow behind an abrupt tube expansion

Flow structure and heat and mass transfer in a swirling two-phase stream is numerically modeled using the Reynolds stress transport model. The gas phase is described by the 3DRANS system of equations with account for the inverse influence of particles on the transport processes in the gas. The gas phase turbulence is calculated using the Reynolds stress transport model with account for the presence of disperse particles. The two-phase nonswirling flow behind an abrupt tube expansion contains a secondary corner vortex which is absent from the swirling flow. The disperse phase is redistributed over the tube cross-section. Large particles are concentrated in the wall region of the channel under the action of the centrifugal forces, while the smaller particles are in the central zone of the chamber.

Large eddy simulation of unconfined turbulent swirling flow

Large eddy simulations (LES) were performed to study the non-reacting flow fields of a Cambridge swirl burner. The dynamic Smagorinsky eddy viscosity model is used as the sub-grid scale turbulence model. Comparisons of experimental data show that the LES results are capable of predicting mean and root-mean-square velocity profiles. The LES results show that the annular swirling flow has a minor impact on the formation of the bluff-body recirculation zone. The vortex structures near the shear layers, visualized by the iso-surface of Q-criterion, display ring structures in non-swirling flow and helical structures in swirling flow near the burner exit. Spectral analysis was employed to predict the occurrence of flow oscillations induced by vortex shedding and precessing vortex core (PVC). In order to extract accurately the unsteady large-scale structures in swirling flow, a three-dimensional proper orthogonal decomposition (POD) method was developed to reconstruct turbulent fluctuating velocity fields. POD analysis reveals that flow fields contain co-existing helical and toroidal shaped coherent structures. The helical structure associated with the PVC is the most energetic dynamic flow structure. The latter toroidal structure associated with vortex shedding has lower energy content which indicates that it is a secondary structure.