Dispersed bubble and particle-laden turbulent flows in the two-way coupling regime

The momentum exchange of bubble and particle laden incompressible turbulent flows is investigated by means of Direct Numerical Simulations (DNS), employing the Eulerian-Lagrangian approach. The Exact Regularised Point Particle method (ERRP) is used to achieve the inter-phase momentum coupling between the two phases. The first part of the research deals with bubble-laden turbulent homogeneous shear flow. The aim of this study consists in addressing the modulation of shear turbulence and the bubble clustering geometry in presence of different inter-phase momentum coupling conditions. Suspensions with different combinations of void fraction and Kolmogorov-based Stokes number, in the dilute regime, are studied. Bubbles suppress the turbulent kinetic energy and turbulent dissipation as well. Turbulent modulation occurs via the direct change of the Reynolds shear stress. In fact, the bubble energy source is proved to be negligible in the scale-by-scale turbulent energy budget. The bubble clustering, in agreement with the literature, occurs in the form of thin elongated structures. The clusters are aligned with principal strain direction of the mean flow, as usual in shear flows. The bubble clustering and turbulent modification are strictly related: both increase with the Stokes number and are independent of the void fraction, in the range of parameters considered in our simulations. The data show that the turbulent modification is disadvantaged when the bubble distribution is homogeneous (i.e. small Stokes number). Finally, the small scale bubble clustering is slightly reduced by two-way coupling effects even though the clustering anisotropy still persists at small scales as it occurs for inertial particles. In the next stage of the research, the objective is to study multiphase wall-bounded turbulent flows. Under the same flow rate, the dispersed phase can either reduce, as in bubbly-flows, or increase, as in particles-laden flows, the viscous wall drag. However, it is well acknowledged that bubbles must be large, and deformable, in order to reduce the viscous resistance in wall turbulence. On the other hand it is known that small inertial particles lead to a wall drag increase. Since we are interested on important turbulence modifications, the second part of the research is devoted to particle-laden wall turbulence flows. In this new investigation, the turbulence modulation is addressed in an particle-laden annular pipe flow, via Direct Numerical Simulation (DNS). The alteration of the heat exchange induced by the different turbulent mixing is studied as well. The turbulence modulation induced by small particles is addressed for the first time in the annular geometry, in the context of Direct Numerical Simulations. A wall correction is included in ERPP in order to take into account wall effects in the particle disturbance. The research also focuses on the particle preferential concentration close to the wall, the so-called turbophoresis. The relation between the particle concentration and the friction wall drag and heat exchange modification is explored. The first and second moment statistics, the two-point correlation functions and the energy spectra are studied. The two-way coupled momentum exchange leads up to 30% wall drag increase. The phenomenon is controlled by the particle mass loading and the wall radius ratio Ri/Ro, where Ri is the internal wall radius and Ro the external one. The mechanism leading to the increase of resistance is attributed to the modified Reynolds shear stress. The heaviest suspensions show a drastic modification of the coherent structures by the external wall, although the flow is altered in the whole annular pipe. The TKE significantly increases close the external wall, while it is suppressed close the internal wall. The increase of the heat-exchange, induced by the different turbulent mixing, is small, below 5%. In the annular pipe the dispersed phase preferentially migrates toward the external wall. In fact, the internal peak of the particle concentration is up to 100 times lower than the external one. Moreover, the findings suggest that the particle concentration is largely overestimated in the central and internal regions, in the one-way coupling regime ( i.e. no turbulence modification ). In fact, the particle feedback promotes the turbophoresis of the external wall, while the particle accumulation close the internal side is attenuated