Radiation Heat Transfer

Vocabulary practice flashcards covering radiation heat transfer concepts, radiative surface properties, governing physical laws, network representations, view factors, radiation shields, and solar irradiance.

Radiation Heat Transfer

The transfer of energy across a system boundary by means of an electromagnetic mechanism caused solely by a temperature difference, occurring without requiring a medium and varying with the temperature difference to the fourth power.

Photons

Discrete packets or quanta of energy released by a radiating surface that propagate through space as electromagnetic waves along straight paths at the speed of light.

Thermal Radiation

Electromagnetic radiation emitted by a body as a result of its temperature, covering a wavelength band from $0.10$ to $100\,\mu\text{m}$.

Stefan-Boltzmann Law

The thermodynamic relation stating that the total radiant energy emitted by an ideal blackbody radiator per unit time and unit area is directly proportional to the fourth power of its absolute temperature: $E_b = \sigma T^4$, where $\sigma = 5.667 \times 10^{-8}\,\text{W/(m}^2\cdot\text{K}^4)$.

Blackbody

An ideal body that absorbs all incoming radiation incident upon it ($\alpha = 1$) with zero reflection and zero transmission, while emitting the maximum possible thermal radiation for any given temperature ($\varepsilon = 1$).

Emissive Power ($E_b$)

The radiant energy emitted per unit time per unit area by a blackbody radiator, expressed in $\text{W/m}^2$.

Absorptivity ($\alpha$)

The fraction of total incident radiant energy that is absorbed by a surface.

Reflectivity ($\rho$)

The fraction of total incident radiant energy that is reflected away from a surface.

Transmissivity ($\eta$)

The fraction of total incident radiant energy that passes through a body or surface.

Opaque Body

A body through which radiation cannot pass (transmissivity $\eta = 0$), simplifying the radiation property balance to $\alpha + \rho = 1$.

Specular Reflection

A form of reflection where the angle of incidence equals the angle of reflection ($\phi_1 = \phi_2$).

Diffuse Reflection

A form of reflection where an incident beam of radiation is scattered uniformly into all directions regardless of the angle of incidence.

Gray Body

A theoretical non-ideal radiating body whose monochromatic emissivity ($\varepsilon_\lambda$) is independent of wavelength and less than $1$, satisfying $\varepsilon = \alpha$.

Emissivity ($\varepsilon$)

The ratio of the emissive power of an actual surface ($E$) to the emissive power of a perfectly black surface ($E_b$) at the same temperature ($\varepsilon = E / E_b$).

Kirchhoff's Law

The principle stating that for a body in thermal equilibrium with its surroundings, its total emissivity is equal to its total absorptivity ($\varepsilon = \alpha$).

Monochromatic Emissivity ($\varepsilon_\lambda$)

The ratio of the monochromatic emissive power of a surface to that of an ideal blackbody at the exact same wavelength and temperature.

Planck's Distribution Law

A mathematical relationship giving the monochromatic emissive power ($E_{b\lambda}(T)$) of a blackbody as a function of wavelength ($\lambda$) and absolute temperature ($T$): $E_{b\lambda}(T) = \frac{c_1}{\lambda^5 [\exp(c_2/(\lambda T)) - 1]}$, where $c_1 = 3.743 \times 10^8\,\text{W}\cdot\mu\text{m}^4/\text{m}^2$ and $c_2 = 1.4387 \times 10^4\,\mu\text{m}\cdot\text{K}$.

Wien's Displacement Law

The law stating that the wavelength of maximum spectral emissive power is inversely proportional to absolute temperature: $\lambda_{\max} T = 2897.6\,\mu\text{m}\cdot\text{K}$, shifting the peak toward shorter wavelengths as temperature rises.

View Factor ($F_{ij}$)

The geometric fraction of diffuse radiation directly leaving surface $i$ that is intercepted by surface $j$, also termed shape factor, angle factor, or configuration factor.

Reciprocity Relation

The fundamental view factor relationship between any two surfaces $i$ and $j$: $A_i F_{ij} = A_j F_{ji}$.

Summation Rule

The view factor theorem stating that the sum of the view factors from surface $i$ to all surrounding surfaces forming an enclosure (including itself) must equal unity: $\sum_j F_{ij} = 1$.

Radiosity ($J$)

The total radiation energy streaming away from a surface per unit time and per unit area, equal to the sum of its directly emitted and reflected radiation: $J = \varepsilon E_b + \rho G$.

Space Resistance

The geometric resistance between two nodes representing surfaces in a radiation network method, formulated as $\frac{1}{A_1 F_{12}}$.

Surface Resistance

The potential resistance of a non-black surface to radiation emission and reflection in a radiation network, formulated as $\frac{1 - \varepsilon}{\varepsilon A}$.

Radiation Shield

A thin sheet of high-reflectivity (low-emissivity) material inserted between two radiating surfaces to introduce additional space and surface resistances, thereby reducing the net rate of radiative heat exchange.

Total Solar Irradiance ($G_s$)

The solar constant representing the rate at which solar energy is incident upon a surface normal to the sun's rays at the outer edge of the earth's atmosphere at mean earth–sun distance.

Radiation Heat-Transfer Coefficient ($h_r$)

A linearized heat transfer parameter enabling radiation to be combined with convection in the form $q/A_1 = h_r (T_1 - T_2)$, calculated as $h_r = \frac{\sigma(T_1^2 + T_2^2)(T_1 + T_2)}{\frac{1}{\varepsilon_1} + \frac{A_1}{A_2}\left(\frac{1}{\varepsilon_2} - 1\right)}$.