Chapter 7 - Momentum, heat and mass transfer in boundary layers

Abstract

This chapter focuses on the momentum, heat, and mass transfer in boundary layers. A thorough understanding of the flow in the boundary layer is necessary for a range of chemical and processing applications as the nature of flow influences the drag at a surface or on an immersed object and the rates of heat and mass transfer when temperature or concentration gradients are present. The evaluation of the resulting integral form obtained by applying Newton's second law of motion to a control volume of fluid also necessitates a knowledge of the velocity profile, and appropriate assumptions of its form are made for both laminar and turbulent flow conditions. As differential equations for flow in boundary layers require numerical solutions even when the flow is laminar and fluid behavior is Newtonian, reasonable estimates of drag on a plane surface by using the integral momentum balance approach is obtained. A momentum and a thermal boundary layer develop simultaneously whenever the fluid stream and the immersed surface are at different temperatures. In case of flow over a bluff body, the fluid elements are subjected to a rapid change in deformations near the frontal face and hence elastic effects are important in this region and the simple boundary layer approximations are not applied to viscoelastic materials in this region.