Thermodynamics and Propulsion

19. Radiation Heat Transfer (Heat transfer by thermal radiation)

All bodies radiate energy in the form of photons moving in a random direction, with random phase and frequency. When radiated photons reach another surface, they may either be absorbed, reflected or transmitted. The behavior of a surface with radiation incident upon it can be described by the following quantities:

• $\alpha$ = absorptance - fraction of incident radiation absorbed
• $\rho$ = reflectance - fraction of incident radiation reflected
• $\tau$ = transmittance - fraction of incident radiation transmitted.

Figure 19.1 shows these processes graphically.

Figure 19.1: Radiation surface properties

From energy considerations the three coefficients must sum to unity

$$\alpha+ \rho + \tau= 1.$$

Reflective energy may be either diffuse or specular (mirror-like). Diffuse reflections are independent of the incident radiation angle. For specular reflections, the reflection angle equals the angle of incidence.