Swirling Jet Research Articles

Particle Image Velocimetry Measurements of Bed-Shear Stress Induced By Wall-Bounded Swirling Jets

AbstractThis study presents measurements of the bed shear stress associated with the three-dimensional boundary layer induced by a wall-bounded swirling jet. The velocity profile within the viscous...

Characterization of Convective Transfer of Heat by Air Impingement using Single Swirling Jet with Insertion of Twisted Tapes by Computational Fluid Dynamics

This study showed the analysis of heat transfer by computational fluid dynamics (CFD) from a heated target surface by the use of jet of single swirl and impingement by air at various Reynolds numbers. The half-length downstream with insertion of twisted tape is applied in a nozzle body to get the swirl. The distance between twist ratio and nozzle to plate of twisted tape is found to be same as y = 2.93 and 21mm (H/D = 1). The characteristics of transfer of heat on heated surface are determined with varied Reynolds numbers like 12000, 17000, 22000 and 27000.

Detailed heat transfer measurements of impinging swirling and non-swirling jet arrays emitted from grooved orifice plate

An innovative type of heat exchanger with heat transfer benefits from jet impingements is proposed. The increase of surface area to volume ratio for the heat exchanger requires the minimization of the separation distance for the impinging jets, which motivates the present study to comparatively examine the heat transfer performances between the non-swirling and swirling impinging jet arrays emitted from the orifice plate with vertical and horizontal grooves at small separation distances less than one orifice diameter. As demonstrated by the selective experimental results, either the grooves on the orifice plate or the swirling jets can effectively alleviate the unfavorable crossflow effect to boost the heat trasnfer rates at small separation distances. After analyzing the isolated and coupled Rej and separation-distance effects on the local and area-averaged heat transfer properties for the non-swirling and swirling impinging jet arrays, two sets of physically consistent heat transfer and friction factor correlations are devised for practical applications.

Numerical investigation of heat transfer from a plane surface due to turbulent annular swirling jet impingement

The heat transfer characteristic of an annular turbulent swirling jet impingement on a heated plane surface is investigated numerically. The annular jet configuration causes instabilities and fluctuations in the flow. Depending on the combinations of different parameters, the annular jet impingement may have positive or negative effects on the heat transfer from the impingement surface. Swirl, on the other hand, introduces vorticity and fluid mixing in an impinging jet, which is desirable in some applications. The axisymmetric two-dimensional flow domain on a radial-axial plane is considered in the numerical model. Both non-swirling and swirling jet impingement is studied. The numerical computations are performed using the ANSYS Fluent CFD code. The Realizable k-ε turbulence model with enhanced wall treatment is used in the computation. The computational process is validated against other published data on similar flow configuration for non-swirling annular impinging jets. The flow and geometric parameters are the jet exit Reynolds number, Re (5000 to 25,000), the swirl strength Sw (0–0.77), the jet exit to the impingement surface distance, H (0.5–4.0), and the moderate blockage ratio of the annular jet, BR (0.4–0.6). The thermal-hydraulic field in the domain is computed for various combinations of these parameters. The effects of these parameters on the Nusselt number distribution on the impingement surface are analyzed. For short separation distance (H = 0.5), the swirling motion positively affects the overall heat transfer, and the average Nu is increased as high as 8% for certain combinations of Re, Sw, and BR; compared to the non-swirling annular jet impingement. For higher separation distances, the average and peak Nusselt number is initially reduced and then increased with increasing swirl strength.

Morphology and internal structure of soot particles under the influence of jet–swirl and jet–jet interactions in a diesel combustion environment

A new multi-location soot sampling method is used to enhance the knowledge about the structural evolution of in-flame particles in a light-duty optical diesel engine. Through thermophoresis-based particle sampling performed at multiple in-bowl locations, the soot structures are shown for both early formation stage and later stage from the same combustion event. Three different jet-spacing angles of 45°, 90° and 180° were studied to analyse how different levels of jet–jet interaction impact the soot particle morphology and internal structure. One selected jet–jet interaction condition was further analysed to show differences in soot structures between the up-swirl side and down-swirl side of the wall jets. From transmission electron microscopes (TEM) images of the sampled soot particles and their statistical size analysis, it was found soot particles initially formed within 45∘ separated jet–jet interaction region have un-solidified premature aggregates due to limited carbonisation in the locally fuel-rich mixtures. When these soot particles travelled on the down-swirl side of the jets, they became solidified and carbonised while the oxidation was evident from the smaller soot primary particle and longer carbon-layer fringe and lower tortuosity. The higher mixing on the up-swirl side of the jets further enhanced the soot oxidation, resulting in even smaller soot primary particle, fragmentation of large soot aggregates, and even longer and less curved carbon-layer fringes. Regarding jet–jet interaction, the 180° jet spacing angle created no jet–jet interaction condition on the soot sampler locations. For smaller jet-spacing angles, the increase in jet–jet interaction promoted the soot formation as evidenced by larger and more complex soot aggregates formed due to more active soot aggregation and agglomeration. The soot oxidation became limited at higher jet–jet interaction conditions, which led to more amorphous soot internal structures.

Explosive boiling-up in a swirl jet of superheated ethanol

Based on the knowledge of the kinetics of liquid vaporization under conditions of high and attainable superheats, experimental data on the atomization of a jet of boiling ethanol at various speeds of a flow swirl are discussed. It has been stated that under conditions of low superheats and boiling-up on solitary non-interacting heterogeneous sites, the rotation of the flow is considered to be the decisive factor in the atomization process. At high superheats and the realization of the mechanism of intense heterogeneous vaporization, the effect of boiling-up on the jet disintegration becomes dominant. When establishing conditions for explosive boiling-up (homogeneous nucleation) under attainable superheats, a complete jet breakup was observed both without a swirl and at low swirl speeds. With an increase in the swirl speed, the temperature range of the breakup narrowed. With a strong swirl the breakup disappeared completely. In the initial states of ethanol close to the thermodynamic critical point, the jet shape was similar to a gas one with or without a swirl. The 1/f fluctuations revealed were caused by oscillations (nutation) of the precessional motion of the jet. They indicate a possibility of large-scale outbursts in the jet.

Single- and double-helix vortex breakdown as two dominant global modes in turbulent swirling jet flow

In this paper, we study the shape and dynamics of helical coherent structures found in the flow field of an annular swirling jet undergoing vortex breakdown. The flow field is studied by means of time-resolved tomographic particle image velocimetry measurements. The obtained flow fields are analysed using both classic and spectral proper orthogonal decomposition. Despite the simple geometrical set-up of the annular jet, the flow field is very complex. Two distinct large-scale helical flow structures are identified: a single and a double helix, both co-rotating with the swirl direction, and it is revealed that these structures are not higher harmonics of each other. The structures have a relatively low energy content which makes it hard to separate them from other dynamics of the flow field, notably turbulent motions. Because of this, classic proper orthogonal decomposition fails to identify both structures properly. Spectral proper orthogonal decomposition, on the other hand, allows them to be identified accurately when the filter size is set at around eight times the precession period. The precession frequencies of the single and double helices correspond to Strouhal numbers of 0.273 and , respectively. A global stability analysis of the mean flow field shows that these structures correspond to two separate global modes. The precessing frequencies obtained by the stability analysis and the related spatial structures match very well with the experimental observations. The current work extends our knowledge on turbulent vortex breakdown and on mean field global stability theory in general. It leads to the following conclusions. Firstly, single- and double-helix vortex breakdown are both manifestations of global modes. Previous studies have shown that both and modes can coexist in swirling jets. However, the mode has been identified as a second harmonic of the first mode, while this study identifies both as two independent global modes. Secondly, this work shows that the simultaneous occurrence of multiple helical global modes is possible within a turbulent flow and their shapes and frequencies are very well predicted by mean field stability analysis. The latter finding is of general interest as it applies to a wide class of fluid problems dominated by multiple oscillatory structures.

Detection and analysis of coherent structures in the near-field of a turbulent annular swirling jet: an unsteady Reynolds-averaged Navier–Stokes (URANS) simulation

This paper analyses the capabilities of unsteady Reynolds-averaged Navier-Stokes (URANS) simulations to predict the coherent structures found in a swirling jet undergoing vortex breakdown. Recently, tomographic particle image velocimetry experiments of an annular swirling jet at a Reynolds number of 8500 at moderate swirl showed the presence of a double helical structure in the flow field (Vanierschot et al., Physical Review Fluids, 2018). This structure corresponds to the double helix vortex breakdown mode and is rarely observed in turbulent flows. In this study, the same flow topology is simulated using Computational Fluid Dynamics (CFD). Turbulence is modeled using the unsteady RANS methodology with a RNG k-e turbulence model. The coherent structures in the flow field were analysed using the spectral proper orthogonal decomposition technique. Despite the fact that it is known that steady-state RANS simulations using two-equation isotropic turbulence models have problems in accurately describing swirling flows which are highly anisotropic, this study shows that the unsteady variant was able to predict the large scale flow structures and their associated dynamics reasonable well. In particular, similar to the experiments, a double helical structure was found in the flow field. The structure has windings in the counter-swirl direction and it is wrapped around the central breakdown bubble. To the authors knowledge, this study is the first one to show the ability of unsteady RANS to predict not only the presence of the double helix vortex breakdown in the flow field, but also the spatial and temporal structure of it.

Transient heat transfer characteristics of swirling and non-swirling turbulent impinging jets

A multitude of research has already been undertaken to examine the effects of imparting swirl (S), Reynolds number (Re), and nozzle-to-plate distance (H/D) on the steady-state heat transfer characteristics of turbulent impinging jets. However, no studies to date have compared the transient development of such jets in both swirling and non-swirling conditions. The current paper addresses this gap by using highly resolved (time series) imaged (infrared) data in conditions spanning Re = 11,600, 24,600, and 35,000. The experiments are based on an electrically heated foil (0.025 mm) with jets over S = 0–1.05 and nozzle-to-plate-distances of H/D = 2, 4, and 6.For non-swirling impinging jets, at greater Reynolds numbers, the location of the highest Nusselt number remains fixed for all the time steps in contrast to jets with low Reynolds number. The transient characteristics for H/D = 4 are distinct compared to H/D = 2 and 6, with impingement heat transfer taking more time to reach steady-state for H/D = 4. It is also seen that heat transfer distribution over the impingement plate changes significantly over the small interval of time. For swirling jets, the impingement plate heat transfer characteristics develop simultaneously for the stagnation and wall jet region. The peak Nusselt number shifts to the wall jet region as the intensity of swirl increases. Two correlations (first for non-swirl and low-swirl, later for moderate to high swirl) are proposed to predict the time needed to reach steady-state for Re = 35,000.

Numerical investigation of passive scalar transport and mixing in a turbulent unconfined coaxial swirling jet

This paper presents the numerical results of a study on passive scalar mixing in coaxial jets under the influence of swirl. Two cases with different swirling strengths are considered and compared to a non-swirling case. Each jet is injected with an individual scalar to better understand the mixing between jets. The case with intermediate swirling strength exhibits faster centerline decay and radial spread in the mean streamwise velocity in the downstream region when compared to the non-swirling case. The case with strong swirling exhibits the formation of an internal recirculation zone. The mean scalar distributions indicate a better spreading rate of scalars in the intermediate swirling case in the far downstream region, whereas the strong swirling case exhibits maximum spread far upstream. These results are confirmed based on entropy, which is a measure of the diffusion of scalars. Furthermore, the ambient fluid reaches the centerline earlier with the introduction of swirl as a result of an increase in the entrainment rate caused by swirl. The enhanced presence of ambient fluid is observed to influence scalar fluctuations. Otherwise negligible turbulent azimuthal fluxes of both scalars exhibit higher values in the swirling cases. The positive correlation between scalar fluctuations is more evident farther upstream in the intermediate swirling case compared to the non-swirling case. This phenomenon is a result of the enhanced presence of ambient fluid. However, in the strong swirling case, this process takes place far upstream based on additional effects of the internal recirculation zone. The joint probability density function of scalar fluctuations at the leading centerline stagnation point of the internal recirculation zone contains two distinct peaks, indicating the presence of flapping between inner and outer jet scalars caused by the oscillating stagnation point.

Nozzle exit conditions and the heat transfer in non-swirling and weakly swirling turbulent impinging jets

Investigations have been conducted into turbulent impinging jets but the exact flow dynamics and mechanisms leading to the observed heat transfer distributions at the impingement plane remain outstanding. In particular, use of different swirl generators (vanes, twisted inserts) means the role of varying inflow conditions (at the nozzle exit plane x/D = 0) should be studied to resolve its role on the observed convective heat transfer trends. The present paper studies axisymmetric turbulent weakly swirling (S = 0.31) jets (D = 40 mm) impinging onto a heated plate. Parameters varied include inflow conditions and the effects of impingement distance (H/D = 2, 4, and 6). The Reynolds Averaged Navier Stokes (RANS) equations are used to model the jets using the k-kl-ω turbulence model, which is benchmarked against other models. Three azimuthal ( ) velocity profiles at a Reynolds (Re) number of 24,600 are used at the nozzle exit plane: Uniform (UP), Solid Body Rotation (SBR), and Parabolic Profiles (PP). The start of the wall jet region, designated through elevated levels of turbulent kinetic energy correlates well with the widely observed first peak in Nu distribution. This is however extremely sensitive to the imposition of any swirl, with the application of even weak swirl (S = 0.31) minimally modifying flow dynamics (in the upstream jet region) and leading to recirculation zones stabilized at the impingement plane. This occurs in near-field impingement (H/D) for some inflow conditions (S031-UP), but not others thereby highlighting the significance varied nozzle and swirl generation methods on trends observed in the literature. The imposition of elevated levels of turbulence at the nozzle inflow (x/D = 0) appreciably modifies the heat transfer distribution, particularly in far-field impingement (H/D = 6).

Theoretical model for swirling thin film flows inside nozzles with converging-diverging shapes

• A quasi-one-dimensional model was developed to describe a swirling, thin, liquid film inside nozzles. • The model quantifies the effects of swirl strength, Reynolds and Weber numbers on the film thickness. • The effect of the nozzle geometry on film thickness is explored. A quasi-one-dimensional model was developed to describe a swirling, thin, liquid film inside nozzles with different wall profiles. The model quantifies the effects of swirl strength, initial film thickness, and Reynolds and Weber numbers on the film thickness along the nozzle surface. Moreover, the model allows for a rapid (at least, qualitative) evaluation of different effects, e.g. of the swirl strength and nozzle geometry, and can serve as a benchmark case for the subsequent more involved numerical simulations. Steady-state solutions are presented as a function of various parameters. The effect of the nozzle geometry on film thickness is explored. As swirling flow entered the expanding (diverging) section of the nozzle, film thickness decreased to satisfy continuity (to conserve mass). Conversely, film thickness increased upon entering the contracting (converging) region of the nozzle. Geometric effects controlled film thicknesses much more than other flow parameters. This quasi-one-dimensional model for a swirling thin film can be useful for designing a swirl jet used in various industrial applications.

Experimental investigation of the precessing vortex core in annular swirling jet flows in the transitional regime

In this paper, we study the flow structures and associated dynamics of an annular swirling jet in the transitional flow regime at Re = 520. In literature, studies on fully laminar or turbulent jets are plentiful, but detailed quantitative data for transitional jets are very scarce. Time-resolved stereoscopic particle image velocimetry measurements are employed to study the flow dynamics in combination with a spectral proper orthogonal decomposition analysis to extract the large-scale motions. Three different swirl numbers Sw are measured: Sw = 0.38, 0.47 and 0.57, with the lowest one very close to the critical swirl number for vortex breakdown to occur. Swirl has a significant impact on the flow field. For the lowest swirl number (Sw = 0.38), a vortex breakdown bubble appears/disappears periodically corresponding to a Strouhal number St = 0.023. If the swirl is increased to Sw = 0.47, the breakdown bubble remains permanently present in the flow field, accompanied with a large-scale vortical structure called the Precessing Vortex Core (PVC). The PVC precesses around the central axis of the jet at a Strouhal number of 0.23. It was observed that the precessing frequency of this spiral vortex core decreases to St = 0.146 as the swirl is increased to Sw = 0.57. This decrease of precession frequency finds its origin in the increased radial spreading of the jet, reducing the rotation rate of the flow near the centre. The 3D structure of the PVC is reconstructed from the planar stereo-PIV measurements by rotating the measurement plane around the central axis of the jet geometry in synchronisation with the precession. It is shown that the PVC is wrapped around the central vortex breakdown bubble and the windings are directed opposite to the swirl direction. This geometrical structure is also found in fully laminar or turbulent swirling jets and this paper shows it is the same for transitional jets.

Experimental Analysis of Heat Transfer and Multi Objective Optimization of Swirling Jet Impingement on a Flat Surface

Experimental Analysis of Heat Transfer and Multi Objective Optimization of Swirling Jet Impingement on a Flat Surface

Investigation of large-scale structures of annular swirling jet in a non-premixed burner using delayed detached eddy simulation

Delayed detached eddy simulation (DDES) is accompanied with Stereo-PIV measurements to study the non-reacting flow field of a non-premixed swirl burner in this paper. Comparisons of experimental and numerical data show that the DDES results are capable of predicting the mean swirling flow features adequately. The instantaneous flow field is found to be strongly affected by the Kelvin–Helmholtz instability. The flow near the injector involves a complex behavior including a recirculation zone. The 3D flow structure at the burner exit, visualized by the iso-surface of Q-criterion, displays four instability types. The dominant instabilities are vortex ring structures induced by the Kelvin–Helmholtz instability, and finger structures induced by the swirling instability. Pressure fluctuation signal recorded in the swirling jet region show that the computational flow passes through transition instants from RANS to DDES equations. After that, the swirling jet becomes fully developed with an oscillation frequency of 222 Hz.

Swirl Impinging Cooling on an Airfoil Leading Edge Model at Large Reynolds Number

Jet impingement cooling has been extensively investigated due to its significant applications on the airfoil leading edge region; however, most of which are about normal jet impingement. The systematic research on swirl jet impinging cooling on leading edge is relatively rare. This study comprehensively investigated the heat transfer distribution of swirl jet impingement with one row of tangential jets. The location of the cross-over jets is offset from the centerline toward either suction or pressure side. Five jet Reynolds numbers varying from 10,000 to 80,000 are tested to reach real engine cooling condition. Jet plates with jet-to-jet spacing (s/d = 2, 4, and 8) and the ratio of surface diameter-to-jet diameter (D/d = 4, 6.6, and 13.3) are tested. We conducted the experiments with a test matrix of 45 cases. The optimum geometric parameters of the jet plate are revealed. Results indicate that for a given Reynolds number, the jet plate configuration with D/d = 4 and s/d = 2 provides the highest Nusselt number profile than the other jet plate configurations, while the jet plate configuration with D/d = 13.3 and s/d = 8 provides the lowest Nusselt number profiles. The best heat transfer region shifts by varying the jet plate configuration depending on the strength of swirl flow. Additionally, correlation of tangential jet impingement has been developed to predict the area-averaged Nusselt number, which is useful for airfoil leading edge cooling design and heat transfer analysis.

Flames of Swirling Double-Concentric Jets Subject to Acoustic Excitation at Resonance

The effects of acoustic excitation at resonance on the flame appearances, flame lengths, flame temperatures, and combustion product concentrations of combusting swirling dual-disk double-concentric jets were studied. The Reynolds number of the annular swirling air jet was varied, while it was fixed at 2500 for the central propane jet. The central fuel jet was acoustically forced by a loudspeaker, which was installed using downstream longitudinal irradiation. The central jet pulsation intensities were measured by a calibrated, one-component hot-wire anemometer. The instantaneous full-length and close-up flame images were captured to identify the characteristic flame modes. Long-exposure flame images were taken to measure the flame lengths. The axial and radial temperature distributions of flames were measured using a homemade, fine-wire R-type thermocouple. The concentrations of combustion products were measured by a gas analyzer. Four characteristic flame modes, blue-base wrinkled flame, yellow-base anchored flame, blue-base anchored flame, and lifted flame, were observed in the domain of central jet pulsation intensity and annular swirling jet Reynolds number. The lifted flame, which was formed at large central jet pulsation intensities, presented characteristics of a premixed flame due to significant mixing induced by violent, turbulent flow motions. It was short and stable, with high combustion efficiency and low toxic emissions, when compared with the unexcited flame and other excited characteristic flame modes, which presented characteristics of diffusion flame.

Effect of Taper Angle of Swirl Jet Pipe on Collected Flow Rate of Compound Swirl Jet

Effect of Taper Angle of Swirl Jet Pipe on Collected Flow Rate of Compound Swirl Jet

Numerical Analysis of the Dust-Air Current Near a Spherical Suction Unit Screened by a Circular Swirling Jet. Part 2. Dynamics of Dust Particles1

We study the method of catching of pollutants with the use of a local suction unit placed over an impermeable plane and in the closed domain of an aspiration hood. The suction unit is screened by a circular swirling jet. The results of investigations of the behavior of dust particles in the air flow near the suction unit are presented.

Identification of geometrical vortex parameters in tangential swirler

This experimental work identifies spatial vortex parameters at the exit of the tangential swirler based directly on velocity distributions obtained by Stereo-PIV method with using pressure pulsations in the acoustic field of swirling jets. The velocity measurements are carried out for high swirl jet close to exit of swirler nozzle (Re = 23000 and Sh = 1.54). The spatial distribution of the PVC is captured with statistical analysis of instantaneous velocity fields and phase-averaged vorticity distribution.