Heat transfer characteristics of straw-core paper honeycomb plates II: Heat transfer mechanism with hot-above and cold-below conditions

Considering the large amount of cement consumed in construction and that straw and other renewable resources are discarded in considerable amounts every year, the heat transfer mechanism of solid-core functional paper honeycomb plates (FHPs) was investigated by calculating and analyzing the equivalent thermal conductivity λE of honeycomb plates (HPs) to develop functional sandwich plates and realize the utilization of straw as a resource. The results led to the following conclusions: (1) Under normal temperatures and hot-above/cold-below conditions, radiative heat transfer, which is usually neglected under normal temperatures, was the primary heat transfer mode in HPs. (2) Local convection occurred in HPs due to weak disturbances when the plate thickness h was greater than 15–20 mm. Accordingly, a technical approach was proposed for improving the heat insulation performance of HPs through multilayer substitution. (3) The filling materials could effectively prevent radiative and local convective heat transfer by drastically reducing the cavity size in the honeycomb structure. The heat conduction among solid particles had little influence on the λE of the FHPs. This paper lays a theoretical foundation for studying the heat transfer mechanism of FHPs at normal temperatures.