Uncoupled Laminar Boundary Layers

Abstract

In this and the following chapter, we begin our application of the concepts and equations introduced in the previous chapters with the simplest possible case: laminar flow, with temperature or concentration differences so small that the density, viscosity, and conductivity appearing in the conservation equations for mass, momentum, and enthalpy can be taken as constant, so that the equations are uncoupled. Later, in Chapters 6, 7, and 11, we discuss the more advanced problems of turbulence and of variable density, and in Chapter 9 we discuss the effect of buoyancy on momentum and heat transfer. Even in uncoupled flows, we need to define a “reference” temperature at which to evaluate the fluid properties; in weakly coupled flows it may be sufficient to evaluate the properties at some average temperature of the fluid and then use the uncoupled equations. In boundary layers an obvious choice is the arithmetic mean of the surface and external-stream temperatures, called mean film temperature and defined by (T _w + T _e)/2 = T _m ; in the duct flows discussed in Chapter 5 we use a slightly more sophisticated bulk fluid temperature, defined in Eq. (5.4).