The effect of Stokes number on particle velocity and concentration distributions in a well-characterised, turbulent, co-flowing two-phase jet

Abstract

Simultaneous measurements of particle velocity and concentration (number density) in a series of mono-disperse, two-phase turbulent jets issuing from a long, round pipe into a low velocity co-flow were performed using planar nephelometry and digital particle image velocimetry. The exit Stokes number,$Sk_{D}$, was systematically varied over two orders of magnitude between 0.3 and 22.4, while the Reynolds number was maintained in the turbulent regime ($10\,000\leqslant Re_{D}\leqslant 40\,000$). The mass loading was fixed at$\unicode[STIX]{x1D719}=0.4$, resulting in a flow that is in the two-way coupling regime. The results show that, in contrast to all previous work where a single Stokes number has been used to characterise fluid–particle interactions, the characteristic Stokes number in the axial direction is lower than that for the radial direction. This is attributed to the significantly greater length scales in the axial motions than in the radial ones. It further leads to a preferential response of particles to gas-phase axial velocity fluctuations,$u_{p}^{\prime }$, over radial velocity fluctuations,$v_{p}^{\prime }$. This, in turn, leads to high levels of anisotropy in the particle-phase velocity fluctuations,$u_{p}^{\prime }/v_{p}^{\prime }>1$, throughout the jet, with$u_{p}^{\prime }/v_{p}^{\prime }$increasing as$Sk_{D}$is increased. The results also show that the region within the first few diameters of the exit plane is characterised by a process of particle reorganisation, resulting in significant particle migration to the jet axis for$Sk_{D}\leqslant 2.8$and away from the axis for$Sk_{D}\geqslant 5.6$. This migration, together with particle deceleration along the axis, causes local humps in the centreline concentration whose value can even exceed those at the exit plane.

Propagating helical waves as a building block of round turbulent jets

“…According to far-jet asymptotics, this dynamical "skeleton" is expected to be universal also for swirling and nonaxisymmetric jets [44]. Future work should demonstrate the particular role of the observed structures and their contribution to mixing [21], aeroacoustics [45], and to the strong anisotropy and peculiarities of the concentration distribution in particle-laden jets [46]. The obtained basis can also be a good candidate for a Craya-type decomposition for inhomogeneous flows to inspect the role of helicity in the energy transfer [47][48][49].…”

Experimental Analysis of the Shot Peening Particle Stream Using Particle Tracking and Digital Image Correlation Techniques

“…Particles were located on a ring next to the centerline of the stream. Lau and Nathan [14] suggested a process of particle reorganisation (particle size < 40 μm) selected by Stokes number, resulting in significant particle migration to the jet axis or away from the centreline respectively for low values of Stokes number or for high values of Stokes number. For a given particle type and size in a fluid, the Stokes number increased with the mean velocity of the gas.…”

“…Fluid dynamics investigations have been carried out to analyse the importance of Stokes numbers on the velocity and concentration distribution of a multiphase flow [14,15]. The authors showed that the flow outside of the nozzle is characterised by a process of particle reorganisation, resulting in significant particle migration towards the jet axis or away from the centreline respectively for low values of Stokes number and for high values of Stokes number.…”

The influence of aspect ratio on the iso-thermal flow characteristics of multiple confined jets

“…For the configurations of θ j = 5 • [ Fig. 8(a)], it can be seen that the value of u /v exhibits two peaks, one in the shear layer (r/D c ≈ 0.15), consistent with an unconfined single jet, 38 and the other in the near wall region (r/D c ≈ 0.45). These locally high values imply a high degree of anisotropy in these regions, consistent with an important role of large-scale turbulence.…”