Modulation of turbulence and heat transfer by finite-size particles in unstably stratified turbulent channel flows

Direct numerical simulations of turbulent channel flows with thermally unstable stratification laden with finite-size particles are performed using the fictitious domain method. The effects of particle concentration, size and specific heat capacity on turbulence and heat transfer are investigated at the friction Reynolds number 180, the Prandtl number 0.7, the Richardson number 20 and both density ratio and thermal conductivity ratio being unity. The natural convection circulations in the unstable stratification case cause the occurrence of streamwise streaks of vortex, velocity and temperature. Compared with the neutral case, the particle-induced flow drag enhancement is more significant for the unstable stratification case, mainly because the particle-induced reduction in the fluid Reynolds shear stress is much less significant for the unstable stratification case, which may be caused by the weaker suppression effects of the particles on the Rayleigh–Bénard circulations and the streamwise vortex packages. In contrast to the neutral case where the particles attenuate the fluid turbulent heat flux and thereby the Nusselt number, the particles enhance the fluid turbulent heat flux and thereby the Nusselt number for the unstable stratification case. The above particle effects are stronger for higher particle volume fractions or smaller particle sizes, when the other parameters are fixed. As the specific heat capacity ratio increases, the Nusselt number increases as a result of the increase in the solid turbulent heat flux contribution.