Free and Forced Convection of Heat in Gases and Liquids

The general problem of heat transfer requires a knowledge of the laws of conduction, radiation and convection. In 1822, Fourier gave us the first thoroughly scientific definition of conductivity and reduced the problem of heat conduction to an exact science, with a power and completeness which left little room for extension or improvement even to the present day. The law of radiation was first suggested by Stefan in 1879 as a result of an analysis of some experiments made by Tyndall. In 1884 Boltzman deduced the law theoretically from the principles of thermodynamics and electromagnetics. Thus the laws of conduction and radiation have been accurately known for a long time, while the problem of convection has received relatively little study. This fact is surprising when we consider the important part played by convection in almost all cases of heat transfer. A complete mathematical solution of a convection problem would require a knowledge of the hydrodynamic laws of viscous fluids for stream line and turbulent motion, combined with the Fouier equations of heat conduction in a moving medium. At present our lack of the hydrodynamic laws for turbulent motion renders a rigorous solution impossible. Therefore in most of the theoretical work so far attempted the simplifying assumption of an inviscid fluid has been found necessary. The theoretical results obtained when viscosity is neglected are in general far from the experimental facts. Langmuir's study of the problem showed that the viscosity is a factor of first importance which cannot be neglected.