Test: Basic Concepts of Heat Transfer - 2 - Mechanical Engineering MCQ

Because for radiation to be comparable the magnitude of temperature difference should be large enough, Convection & conduction is also predominant in boiler furnace.

According to their thermal conductivity for Air, k = 0.022 W/mK
for Aluminium, k = 205 W/mK
for Silver, k = 407 W/mK
Hence correct sequence is
Air, Lead, Aluminium, Silve

• The thermal conductivity of the liquids is more than the gasses and the metals have the highest.
• Metals have valence electrons that are free to move around the surface. When heated, these free electrons collide with each other and the majority of the Kinetic energy is transformed into heat energy, making metals a good conductor of heat.
• In the gaseous state, the molecules of a substance are spaced relatively far away, and their motion is random. This means that energy transfer by the molecular impact is much slower than in the case of a liquid, in which the motion is still random but in liquids, the molecules are more closely packed. The same is true concerning the difference between the thermal conductivity of the liquid and solid phases.

Due to increase in lattice vibratioh the thermal conductivity of metals decreases with increase in temperature.

Since thermal conductivity of air is minimum among the given material hence heat transfer will be minimum in air.