Thermal, Optical & Electrical Properties of Grains
Subtopic: Thermal conductivity and diffusivity definitions
Question Statement:
Which of the following statements concerning thermal properties of food grains is/are TRUE?
Options:
(A) Thermal conductivity of grains increases linearly with increase in moisture content because water has much higher conductivity ($0.6\text{ W/m K}$) than dry grain matter
(B) Thermal diffusivity is defined as $\alpha = \frac{k}{\rho c_p}$, representing the rate of temperature equalization in unsteady heat transfer
(C) Bulk grain thermal conductivity is lower than individual single kernel conductivity due to insulating air voids
(D) Specific heat of dry starch is higher than the specific heat of liquid water
Answer Key:
A, B, C
Step-by-Step Solution & Explanation
1. Water is a better thermal conductor than dry carbohydrates and air, so $k$ rises with moisture (A is true).
2. By definition, $\alpha = k / (\rho c_p)$ (B is true).
3. Stagnant air has very low $k \approx 0.026\text{ W/m K}$, lowering effective bulk bed conductivity (C is true).
4. Water has exceptionally high specific heat ($4.184\text{ kJ/kg K}$), while dry starch is only $\approx 1.2-1.5\text{ kJ/kg K}$ (D is false).