11-Modes of Heat Transfer

• Radiation heat transfer involves the transfer of heat by electromagnetic radiation that arises due to the temperature of the body. Radiation does not need matter.

• Radiation is the primary source of heat generated by the sun. As the materials in your home (e.g., walls, floor, ceiling, etc.) absorb the sun’s radiant energy, this creates conductive heat. Surrounding materials further absorb the conductive heat through convection when surrounding air carries the conductive heat to other objects.

Emissive power per unit surface area: \[q = \sigma \epsilon T^4 \tag*{(Stefan's Boltzmann law)}\] where

The above equation describes a gross heat emission rather than heat transfer.

The rate of radiation heat exchange between a small surface of area \(A\) at \(T_1\) and a large surrounding at \(T_2\) is given by the following expression: \[Q = \sigma\epsilon A (T_1^4-T_2^4)\]

Surfaces with emissivities nearly unity are good absorbers and hence poor reflectors of incident radiation. Most highly polished, unoxidized metal surfaces are good reflectors of thermal radiations with emissivities less than 0.1. A roughened or an oxidized surface has correspondingly higher emissivities.

Plumeria Tree — Shining / glossy leaves have lesser emissivity (i.e., lesser absorptivity).