  Heat transfer to a granular flow is comprised of two resistances in series:
near the wall and within the bulk particle bed, neither of which is well
understood due to the lack of experimental probes to separate their respective
contribution. Here, we use a frequency modulated photothermal technique to
separately quantify the thermal resistances in the near-wall and the bulk bed
regions of particles in flowing states. Compared to the stationary state, the
flowing leads to a higher near-wall resistance and a lower thermal conductivity
of bulk beds. Coupled with discrete element method simulation, we show that the
near-wall resistance can be explained by particle diffusion in granular flows.
