It's widely known that high-temperature waste heat contained in solid powder from industry is quite large. Generally, the moving bed heat exchanger (MBHE) is used to recover the energy from gravity-driven particle flow, which is efficient and could keep the energy clean. In the present work, the particle flow nearby an immersed tube with heat transfer is numerically studied as a basis sub-model for flexible design of MBHE. To overcome the limitation of continuum method used before, the discrete element method (DEM) is adopted to simulate particle flow, which could get more detail information. The simulation is validated by comparing heat transfer coefficients with others' experimental data between particle flow and tube. DEM accounts for the heat conduction through interstitial fluid and thermal radiation between different surfaces. Furthermore, particle size is also a key parameter for flow and heat transfer performances, which affects contact between tube and particles. The DEM simulation shows that, contact significantly affects heat transfer, which makes the relationship between flow characteristic and thermal performance in MBHE different from the fluid heat exchanger. It's found that heat transfer coefficient increases with particle descending rate increasing only when the descending rate is low. However, heat transfer coefficient would decrease with descending rate increasing in faster descending rate.