Swirling Jet Research Articles

An Experimental Study of the Near-Field Mixing Characteristics of a Swirling Jet

The present experimental investigation is devoted to the mixing characteristics of a passive scalar in the near-field region of a moderately swirling jet issuing from a fully developed axially rotating pipe flow. Instantaneous streamwise and azimuthal velocity components as well as the temperature were simultaneously accessed by means of a combined X-wire and cold-wire probe. The results indicate a modification of the turbulence structures to that effect that the swirling jet spreads, mixes and evolves faster compared to its non-swirling counterpart. The high correlation between streamwise velocity and temperature fluctuations as well as the streamwise passive scalar flux are even more enhanced due to the addition of swirl, which in turn shortens the distance and hence time needed to mix the jet with the ambient air.

Scalar Mixing and Large-Scale Coherent Structures in a Turbulent Swirling Jet

The paper presents large eddy simulations of co-annular swirling jets into an open domain. In each of the annuli a passive scalar is introduced and its transport is computed. If the exit of the pilot jet is retracted strong coherent flow structures are generated which substantially impact on the transport and mixing of the scalars. Average and instantaneous fields are discussed to address this issue. A conditional averaging technique is devised and applied to velocity and scalars. This allows to quantify the impact of the coherent structures on the mixing process.

INFLUENCE OF THE NOZZLE GEOMETRY ON THE HYSTERESIS OF ANNULAR SWIRLING JETS

This study investigates the influence of the nozzle geometry on the mean flow field of an annular swirling jet. The nozzle geometry consists of a sudden expansion with step size s followed by a divergent with opening angle α and axial length L. The inner dimensions of the nozzle are changed via s, L and α and in total 87 different nozzle geometries are investigated. For each geometry the mean flow pattern as a function of the swirl is determined experimentally using pressure measurements. In these measured pressure diagrams, different regions are identified and each region corresponds to a different flow pattern. Up to four different flow patterns can exist depending on the combination of s, L, α and swirl number: Closed Jet Flow, Open Jet Flow Low Swirl, Open Jet Flow High Swirl and Coanda Jet Flow, and hysteresis exist in the flow patterns upon increasing and decreasing the swirl for certain nozzle geometries. A stability diagram is presented in the (s/L, α)- plane. This region contains the combinations of s/L and α, which give the highest number of different stable flow patterns. The information in this article can serve as a database for the design of nozzles in cold flow applications as well as in swirl-stabilized burner heads.

Study of Micro Bubble Generation by a Swirl Jet (Measurement of Bubble Distribution by Light Transmission and Characteristics of Generation Bubbles)

Characteristics of a micro bubble generator using a strong swirl jet were studied experimentally and numerically. Experiments were conducted by changing the nozzle diameter, the inflow pressure and the gas flow rate, systematically. For measurements of the bubble distribution function of PDF, a simple method using a light transmission was proposed. Principle of the method was based on changes of the light transmission due to the bubble rising to the surface of water. The bubble distribution measured by the method agreed well with that by a usual backlight method. In this study four non-dimensional parameters governing the micro bubble generation were adopted, that is, Weber number for the average diameter of bubbles, Reynolds number, the ratio of flow rate and the velocity ratio for the swirl jet. It was found that the relationship between Weber number and Reynolds number could be arranged with each velocity ratio as a parameter. An approximate experimental expression for the average diameter showed that the micro bubble diameter strongly depended on the axial flow velocity and the rotational speed of the swirl jet.

C112 複合旋回噴流の流動特性に及ぼすノズル深さの影響

A compound swirl jet which consists of an annular swirling jet and a coaxial free jet is effective as a push flow of the push-pull ventilator. In this paper, to enhance the mixing capability of the jet flow, the experiment was carried out under the condition of locating the inner tube of free jet to the upstream. The flow rate of the jet flow was examined for various nozzle depth by the measurement of velocity distribution. As a result, it was found that the optimum flow rate of the collection is obtained without the swirling flow, when the nozzle depth increases. The mechanism of the flow was clarified by the flow visualization using the smoke method.

Numerical study of hysteresis in annular swirling jets with a stepped‐conical nozzle

AbstractThis study investigates the experimentally observed hysteresis in the mean flow field of an annular swirling jet with a stepped‐conical nozzle. The flow is simulated using the Reynolds‐averaged Navier–Stokes (RANS) approach for incompressible flow with a k–ε and a Reynolds stress transport (RSTM) turbulence model. Four different flow structures are observed depending on the swirl number: ‘closed jet flow’, ‘open jet flow low swirl’, ‘open jet flow high swirl’ and ‘coanda jet flow’. These flow patterns change with varying swirl number and hysteresis at low and intermediate swirl numbers is revealed when increasing and subsequently decreasing the swirl. The influence of the inlet velocity profile on the transitional swirl numbers is investigated. When comparing computational fluid dynamics with experiments, the results show that both turbulence models predict the four different flow structures and the associated hysteresis and multiple solutions at low and intermediate swirl numbers. Therefore, a good agreement exists between experiments and numerics. Copyright © 2006 John Wiley & Sons, Ltd.

An experimental and computational study of a multi-swirl gas turbine combustor

The turbulent flow and flame dynamics within a lean direct fuel injection (LDI) multi-swirl gas turbine combustor is examined using a combination of state-of-the-art diagnostic methods, including laser doppler velocimetry (LDV), particle imaging velocimetry (PIV) and fine bead thermocouples, and modern computational methods, such as flamelet-based large eddy simulations (LES). The computations provide unsteady field data of any quantity of interest, but are to some extent model dependent, whereas the laboratory studies often can capture only end-results of the real processes with limited details. The combined perspectives can thus provide mutual validation of diagnostics and models, and a more complete understanding of the physical and chemical processes involved, including also interdependencies between processes that are very difficult to characterize in the laboratory. In turn, this provides an improved framework for modification of present gas turbine combustors and for the design of future generations. Good agreement between LES and experimental data is found both for the non-reacting and reacting regimes studied. Both cases were found to be sensitive to the inflow into the swirler, and to the confinement. The non-reacting case is dominated by an annular swirling jet, a central recirculation zone (CRZ) and a weak precessing vortex core, oscillating at ∼250 Hz. For the reacting case the CRZ remains, and dominates the flow in the upstream section of the combustor including the flame and the resulting wall jets. Longitudinal pressure fluctuations at ∼380 Hz (420 Hz in the experiments) are also observed in the reacting case.

A spectral model for the sound pressure from turbulent premixed combustion

We present a model for the spectral distribution of heat release in turbulent premixed combustion and show how it can be applied to the calculation of sound pressure. Experimental data are presented from premixed swirling jet flames at three thermal powers, three equivalence ratios and for a variation of the fuel composition. A simultaneous PIV-LIF technique was used to determine the spatial spectra of the progress variable variance and turbulence kinetic energy. It was found that the spectra of progress variable correspond well with model spectra derived for homogeneous turbulence. Mapping these Eulerian spatial spectra to Lagrangian time spectra, we show how the integral frequency spectrum of heat release fluctuation can be computed using local values of time averaged heat release rate density, turbulence kinetic energy and turbulence lengthscale. With these the spectral distribution of sound pressure from turbulent premixed combustion is calculated and compared with measured sound pressure spectra. Very good quantitative agreement is obtained in all cases.

LES of a free annular swirling jet – Dependence of coherent structures on a pilot jet and the level of swirl

The paper presents large eddy simulation of unconfined swirling jets. In the first part, an unconfined annular jet is investigated for swirl numbers ranging from 0 to 1.2. The impact of the swirl on the mean flow and the precessing vortex structures in this flow is analysed. In the second part of the paper, a co-annular pilot jet is introduced near the axis. The investigations show that the additional swirl near the axis has a stronger effect than the pilot jet itself, leading to an almost entire removal of coherent structures.

Identification and analysis of coherent structures in the near field of a turbulent unconfined annular swirling jet using large eddy simulation

Large eddy simulations of incompressible turbulent flow in an unconfined annular swirling jet at Reynolds number 81500 are reported, based on the outer radius of the jet. The results are in excellent agreement with experimental data for mean flow, turbulent statistics, and power spectral densities of velocity fluctuations. Two dominant families of large-scale coherent structures are identified in the flow. Both are orthogonal to the mean three-dimensional streamlines, which suggests that they are formed as the result of a Kelvin-Helmholtz instability. Instantaneous vortex structures as well as different types of spectra and two-point correlations are presented to further elucidate the properties of the flow.

06/01312 Studies of mean and unsteady flow in a swirled combustor using experiments, acoustic analysis, and large eddy simulations: Roux, S. et al. Combustion and Flame, 2005, 141, (1–2), 40–54

The turbulent flow and flame dynamics within a lean direct fuel injection (LDI) multi-swirl gas turbine combustor is examined using a combination of state-of-the-art diagnostic methods, including laser doppler velocimetry (LDV), particle imaging velocimetry (PIV) and fine bead thermocouples, and modern computational methods, such as flamelet-based large eddy simulations (LES). The computations provide unsteady field data of any quantity of interest, but are to some extent model dependent, whereas the laboratory studies often can capture only end-results of the real processes with limited details. The combined perspectives can thus provide mutual validation of diagnostics and models, and a more complete understanding of the physical and chemical processes involved, including also interdependencies between processes that are very difficult to characterize in the laboratory. In turn, this provides an improved framework for modification of present gas turbine combustors and for the design of future generations. Good agreement between LES and experimental data is found both for the non-reacting and reacting regimes studied. Both cases were found to be sensitive to the inflow into the swirler, and to the confinement. The non-reacting case is dominated by an annular swirling jet, a central recirculation zone (CRZ) and a weak precessing vortex core, oscillating at ∼250 Hz. For the reacting case the CRZ remains, and dominates the flow in the upstream section of the combustor including the flame and the resulting wall jets. Longitudinal pressure fluctuations at ∼380 Hz (420 Hz in the experiments) are also observed in the reacting case.

Swirling effect of jet impingement on heat transfer from a flat surface to CO 2 stream

The heat transfer of swirling and conventional CO 2–air submerged jet impingement was experimentally studied with thermo-chromic crystal liquid technique. The experiment is focused on the verification of the swirling jet effect on the distribution of the local heat transfer coefficient on the impinged target surface. In the range of Re = 7500–28 300, distributions of the local Nusselt number were obtained and compared with other researcher’s results. It is revealed that the swirling of the jet improves the radial uniformity of the heat transfer significantly. The heat transfer deteriorates in the stagnation region but gets enhanced in the wall jet region. The relative magnitude of the Nusselt number for the swirling and non-swirling jets reverse in the range of r/ D = 0.35–0.95. In addition, the swirling jet also enhances the heat transfer in terms of the mean Nusselt number in a larger r/ D area.

Linear Stochastic Estimation of a Swirling Jet

Linear stochastic estimation (LSE) was applied to study the coherent structures of a highly swirling jet The swirling jet was produced by a triple annular swirler (TAS) featuring three concentric swirling flows that merge into a turbulent swirling jet. The LSE used statistical theory to provide an estimate of the instantaneous flowfield by reproducing, in space and time, the large-scale coherent structures of the flow using minimum experimental information. LSE turned out to be very efficient for extracting coherent structures from the turbulent flowfield and for describing their dynamics. Although the LSE is highly dependent on the levels of correlation, it enabled the reconstruction of the complete coherent flowfields from the knowledge of only few near-field acoustic signals, providing an effective tool for data reduction to formulate turbulent inflow boundary conditions for large-eddy simulation.

Momentum and Heat Transfer in a Buoyant Turbulent Swirling Jet

Solutions are presented for a three-parameter model of turbulence for axisymmetric turbulent buoyant swirling jet flows in uniform and stagnant surroundings. The exit swirl number has a significant effect on the flow characteristics. The predicted results are in good agreement with experimental data. A discussion is provided for the effects of swirl and buoyancy on the growth of the jet width, the axial decay of mean flow properties, and the mass flow rates.

A Study on Flow Characteristics in a Spiral Flow Nozzle

A conventional Swirl jet is generated by applying a tangential force in the fluid injected to the nozzle. For this reason, the nozzle structure is complicated and the nozzle has a difficulty to manufacture. According to recent researches, it is known that a unique nozzle unit with an annular slit and conical cylinder can generate a spiral flow which is a kind of swirl flow without a tangential force in the jet. Pressurized fluid from the reservoir is supplied to an annular chamber and then injected into the convergent nozzle through the annular slit. In this paper, the effect Of the ratio of air pressure in the buffer to stagnant atmospheric pressure on the spiral flow field is investigated numerically. As a result, axial and tangential velocities increased with an increase of the pressure ratio. Furthermore, in case of the high-pressure ratio, the variation in axial and tangential velocities for a fully developed flow was caused by relatively high flow instability in the buffer area.

Approximate Solution for an Axisymmetric Swirling Jet Using Non-Linear k-.EPSILON. Model with Consideration of Realizability

Approximate Solution for an Axisymmetric Swirling Jet Using Non-Linear k-.EPSILON. Model with Consideration of Realizability

Coherent structures in unsteady swirling jet flow

An LDA technique and phase-averaging analysis were used to study unsteady precessing flow in a model vortex burner. Detailed measurements were made for Re=15,000 and S=1.01. On the basis of the analysis of phase-averaged data and vortex detection by the λ2-technique of Joeng and Hussain (1995), three precessing spiral vortex structures were identified: primary vortex (PV), inner secondary vortex (ISV), and outer secondary vortex (OSV). The PV is the primary and most powerful structure as it includes primary vorticity generated by the swirler; the ISV and OSV are considered here as secondary vortical structures. The jet breakdown zone is the conjunction of a pair of co-rotating co-winding spiral vortices, PV and ISV. The interesting new feature described is that the secondary vortices form a three-dimensional vortex dipole with a helical geometry. The effect of coupling of secondary vortices was suggested as a mechanism of enhanced stability reflected in their increased axial extent.

Numerical Simulation and Calculation of the Process of Interaction between a Swirling Jet and Drift Flow

A mathematical model and results of numerical calculation of the process of interaction between a swirling jet and drift flow are presented in the paper. The structure of flow and the process of chemical reactions are investigated under conditions of transverse injection of a swirling jet of combustion products into a flow of fuel-air mixture. Inferences are made of the possibility of using a transversely injected jet to provide for a gasdynamic stabilization of flame in combustors of gas-turbine engines.

Nonlinear evolution of a swirling jet instability

The nonlinear evolution of the three-dimensional instability of a viscous unsteady swirling jet, namely, the Batchelor vortex, is addressed. Two types of initial perturbations are considered: a single unstable normal mode with given azimuthal symmetry and white noise. Three different scenarios have been put into evidence according to the value of the swirl number q selected in the unstable range q<1.5. When helical symmetry is present, the dynamics can be interpreted in a two-dimensional framework. More specifically, the process is viewed as the simultaneous action of a destabilizing “instantaneous” swirling jet instability and a stabilizing accelerated viscous diffusion induced by differential rotation. For swirl numbers close to the critical value (1<q<1.5), this latter effect dominates and leads the vortex to relaminarize in the nonlinear regime. For intermediate values of swirl (q∼0.8), it breaks into an array of equal sign vortices containing most of the initial circulation and surrounding a central part with axial velocity. For lower swirl (q<0.6), the initial vortex gives birth to an array of dipoles, the ejection of which drastically increases the structure size. Similar trends are observed on simulations with an initial white noise condition.

Schmidt number effects on laminar jet diffusion flame liftoff

This work designs an axial/tangential swirl jet burner to establish stable CH4/O2/CO2 combustion under a wide range of operating conditions. As the flow rate of the axial fuel stream increases, the flame is lifted and stabilized near the downstream wall. The lifted flame is robust and shows good combustion completeness. The critical fuel flow rate for flame lift-off is affected by the oxygen mole fraction of the oxidant flow. The diagram controlling flame attachment and lift-off is investigated experimentally to derive the critical Schmidt number. In addition, high-speed photography and planar laser-induced fluorescence (PLIF) techniques are employed to resolve instantaneous flame structures near lift-off. It is visualized that as the flame height increases at larger fuel flow rates, the flame layer encounters local extinction due to the stretching of the surrounding high-speed oxidant flow. The local stretch rate measured in cold flow agrees well with the theoretic extinction strain rate. Further, a fitting line is obtained to summarize the effect of flow conditions and burner geometry on flame lift-off height. Particularly, the normalized lift-off height shows a good linear relationship with the Reynolds number of the axial fuel jet.