Microconvection effects on the cooling performance of ice slurry flows within a circular tube: Immersed boundary-lattice Boltzmann simulations

This study examines the microconvection effects on the cooling performance of ice slurry flows within a circular tube based on particle-resolved simulations performed using the immersed boundary-lattice Boltzmann method. First, the Nusselt number on the tube wall, a crucial indicator of macroscopic heat-transfer characteristics, is computed at various values of the Reynolds number, ice-packing factor, and surface-area ratio. Subsequently, the dependence trends of the Nusselt number on the above parameters are explored from the viewpoint of microconvection phenomena in ice slurry flows, focusing on the temperature profile and particle distribution. The results reveal that (i) the Nusselt number increases with the Reynolds number, as more particles traverse the tube during the development of the thermal-boundary layer. (ii) The Nusselt number also increases with the ice-packing factor, as the number of particles lying close to the tube wall increases. (iii) Finally, the Nusselt number similarly increases with the surface-area ratio, as the total surface area of the particles increases. These trends persist even under gravity-driven effects, wherein buoyancy forces bias the ice particle distribution toward the outer and upper regions of the tube, subsequently altering the value of the Nusselt number. Additionally, the cooling performance of the ice slurry flow within a circular tube is compared with that in a square duct. The results reveal that the Nusselt number for the square duct is smaller than that for the circular tube. This is attributed to the undisturbed and relatively thicker state of the thermal-boundary layer around the corner of the square duct.