Maarten Vanierschot

Influence of swirl on the initial merging zone of a turbulent annular jet

This paper presents an extensive study of the influence of swirl on the initial region of an annular jet. A total of five different swirl numbers S are investigated: one at zero swirl, one at low swirl (S=0.18), two at intermediate swirl (S=0.37 and 0.57), and one at high swirl (S=0.74). The flow fields are measured using the stereoscopic particle image velocimetry (PIV) technique. A detailed study on the accuracy of the PIV measurements is presented, including a validation with laser Doppler anemometry data. In this way a detailed set of accurate data is presented of the three components of velocity and the root-mean square value of their fluctuations in a plane through the central axis of the geometry. Despite its simple geometry, the immediate flow field of an annular jet is very complex. The concentric central tube of the nozzle acts as a bluff body to the flow, thus creating a central recirculation zone (CRZ) behind it. At low swirl numbers the swirl induced pressure gradients alter the structure of ...

A new method for annular jet control based on cross-flow injection

We propose a new method for controlling the outflow of a fully turbulent annular jet based on the cross-flow injection of secondary jets. Details are provided on an in-house designed nozzle with a specific geometry that causes an attachment of the annular jet to the nozzle cone when cross-flow injection is applied. When this cross injection is suddenly taken away, the nozzle attached jet evolves toward a stable wall jet. Time-resolved stereo particle image velocimetry measurements are used to characterize the three different flow patterns (annular, nozzle attached, and wall jet) and the aim of this investigation is to gain a deeper understanding into the physics of the transition from the nozzle attached jet to the wall jet.

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

Abstract 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.

Symmetry breaking and vortex precession in low-swirling annular jets

In this paper, the flow dynamics in the wake of a turbulent annular jet is studied using Time-Resolved Stereoscopic Particle Image Velocimetry and Proper Orthogonal Decomposition (POD). In this wake, a central recirculation zone is present which, under certain conditions, shows a low-frequency precessing motion. POD analysis of the measured velocity data shows that at zero swirl, an asymmetry is present in the wake, which motion is random in time. This asymmetry originates from a bifurcation of the flow once a threshold Reynolds number is exceeded. For low-swirl numbers, ranging from 0 0.12, the precession is gone and the motion of the asymmetric wake is again random in time, similar like the non-swirling jet. In this paper, a model is developed to describe the influence of swirl on the wake dynamics. The model assumes that perturbations in the inner shear layer near the bluff body wall are...

Planar Pressure Field Determination in the Initial Merging Zone of an Annular Swirling Jet Based on Stereo-PIV Measurements

In this paper the static pressure field of an annular swirling jet is measured indirectly using stereo-PIV measurements. The pressure field is obtained from numerically solving the Poisson equation, taken into account the axisymmetry of the flow At the boundaries no assumptions are made and the exact boundary conditions are applied. Since all source terms can be measured using stereo-PIV and the boundary conditions are exact, no assumptions other than axisymmetry had to be made in the calculation of the pressure field. The advantage of this method of indirect pressure measurement is its high spatial resolution compared to the traditional pitot probes. Moreover this method is non-intrusive while the insertion of a pitot tube disturbs the flow It is shown that the annular swirling flow can be divided into three regimes: a low, an intermediate and a high swirling regime. The pressure field of the low swirling regime is the superposition of the pressure field of the non-swirling jet and a swirl induced pressure field due to the centrifugal forces of the rotating jet. As the swirl increases, the swirl induced pressure field becomes dominant and for the intermediate and high swirling regimes, the simple radial equilibrium equation holds.

On the flow structures and hysteresis of laminar swirling jets

In this paper different flow patterns of an annular jet with a stepped-conical nozzle as well as the transition between these patterns are numerically investigated as a function of the swirl number S which is the ratio of tangential momentum flux to axial momentum flux. The Reynolds number of the jet based on the axial velocity and the nozzle hydraulic diameter is 180. The 3D Navier Stokes equations are solved using the direct numerical simulation. Four different flow patterns are identified and their associated flow structures are discussed. Starting from an annular jet at zero swirl, spinning vortices around the central axis originate with increasing swirl. As the swirl is further increased, the onset of vortex breakdown occurs, followed by jet attachment to the nozzle. Decreasing the swirl number back from this flow pattern, the Coanda effect near the nozzle outlet creates a wall jet. This wall jet remains till the decreasing swirl number equals to zero, showing hysteresis in flow patterns between an i...

Energy saving opportunities of energy efficient air nozzles

Compressed air is a common energy medium. The production of compressed air itself is not a very efficient process. Avoiding any unnecessary losses of air can lead to large reductions in electricity consumption. Since blowing applications are one of the main domains were compressed-air is used, any reduction in the mass flow needed for operation can lead to significant energy savings. In this paper the normal volumetric flow rate and generated impact force are compared between a stepped nozzle and a so called energy saving nozzle which allows extra air from the surroundings to be entrained. These two different nozzle geometries are used in industrial blowing applications. Until now there was no study available which compares the impact forces and volumetric flow rates for these types of nozzles. The flow field of the two nozzles was calculated by CFD simulations. The impact forces and volumetric flow rates are calculated out of this flow field. Each nozzle was simulated with three different input pressures...