Selected Heat Transfer Topics
This chapter is intended to review those aspects of heat transfer that are important in the design and analysis of solar collectors and systems. It begins with a review of radiation heat transfer, which is often given cursory treatment in standard heat transfer courses. The next sections review some convection correlations for internal flow and wind-induced flow.
The role of convection and conduction heat transfer in the performance of solar systems is obvious. Radiation heat transfer plays a role in bringing energy to the earth, but not so obvious is the significant role radiation heat transfer plays in the operation of solar collectors. In usual engineering practice radiation heat transfer is often negligible. In a solar collector the energy flux is often two orders of magnitude smaller than in conventional heat transfer equipment, and thermal radiation is a significant mode of heat transfer.
3.1 The Electromagnetic Spectrum
Thermal radiation is electromagnetic energy that is propagated through space at the speed of light. For most solar energy applications, only thermal radiation is important. Thermal radiation is emitted by bodies by virtue of their temperature; the atoms, molecules, or electrons are raised to excited states, return spontaneously to lower energy states, and in doing so emit energy in the form of electromagnetic radiation. Because the emission results from changes in electronic, rotational, and vibrational states of atoms ...