Thermal diffusivity measurements in sheared neutrally buoyant granular suspension*

Heat transport in granular suspensions is an important yet underexplored area with applications in inkjet printing, geological flows, and fluidized bed reactors. However, current understanding of their heat transfer properties is limited, with most research focusing on flow anomalies. This study examines the effective thermal diffusivity of sheared granular suspensions with neutrally buoyant particles in a thin gap Taylor-Couette cell. A steady canonical shear flow  is generated with Taylor instabilities suppressed by outer cylinder rotation. Thermal diffusivity of the medium is deduced by temporal temperature decay on the inner cylinder surface. Spherical 1 mm polystyrene and 2 mm PMMA particles were used with density-matched propylene glycol-glycerol solution. The study documents the effects of Peclet(<100) and particle Reynolds numbers(<30) on a suspension with four different volume fractions from 10-40%.

Results show that particle concentration has a non-monotonic effect on thermal transport, with the Reynolds number playing a crucial role in the transition from shear-induced diffusion to inertially driven mixing. The microscale wakes generated behind particles play a key role in this transition, with the interactions between particle spacing and wake regions creating a complex relationship for the Nusselt number. Our findings demonstrate that thermal diffusivity trends observed in the Stokes flow regime can be used to separate the effect of shear-induced diffusion from inertial flow.