Which of the following factors significantly influence the effective thermal conductivity ($k_{eff}$) of a bulk grain bed?
Options:
(A) Moisture content of the individual grains.
(B) Bulk density and bed void fraction (porosity).
(C) Mean temperature of the grain bed.
(D) Color of the outer seed coat.
Answer Key:
A, B, C
Step-by-Step Solution & Explanation
Effective thermal conductivity of bulk granular grain beds depends on:
- Moisture content: Water has a thermal conductivity ($k \approx 0.6\text{ W/m}\cdot\text{K}$) much higher than dry grain matter, so higher moisture raises $k_{eff}$.
- Bulk density and porosity: Air in the voids has very low conductivity ($k \approx 0.026\text{ W/m}\cdot\text{K}$); increasing compaction/density reduces void fraction and increases $k_{eff}$.
- Temperature: Thermal conductivity of food biomaterials increases moderately with temperature.
- Seed coat color does not influence conductive thermal properties (it only affects surface radiative absorptivity).