Effect of particle stiffness and impeller speed on heat transfer in stirred granular beds

Thermal discrete element method simulations are used to investigate contact-limited heating in a vertically stirred granular bed heated through the cylindrical wall and bottom, with heat transfer restricted to conduction through particle–particle and particle–wall contacts. Young’s modulus is varied over four decades ( 5 × 1 0 6 – 5 × 1 0 10 Pa ) and impeller speed from a static bed to strong agitation (0– 240 rpm ). At sufficiently high impeller speeds, the bulk kinematics become nearly stiffness-independent, whereas the contact network remains strongly stiffness-dependent: softer particles sustain higher connectivity, larger contact areas, and longer contact durations. Macroscopic warm-up exhibits three regimes reflecting a renewal–conduction trade-off: at low agitation, the contact network is intact but boundary renewal is weak and a warm near-wall layer limits bulk heating; at intermediate agitation, renewal and conduction are balanced, defining an optimal impeller speed; at high agitation, mixing is vigorous but the degraded contact network limits conductive heat transfer despite rapid temperature homogenization. Thus, the impeller speed maximizing bulk heating decreases systematically with increasing stiffness, from ∼ 90 rpm in the softest case to ∼ 4 rpm in the stiffest case.