21 - Convection - Introduction

Near any wall, there is a stagnant sub layer. Since there is no fluid motion in this layer, heat transfer is by conduction in this layer. Above the sub layer is a region where viscous forces retard fluid motion; in this region some convection may occur, but conduction may well predominate. \[q = -k\left.\frac{\partial T}{\partial y}\right|_{y=0} = h(T_s-T_\infty)\]

Hence, \[h = \frac{-k\left.\dfrac{\partial T}{\partial y}\right|_{y=0}}{T_s-T_\infty}\]

\(\left.\dfrac{\partial T}{\partial y}\right|_{y=0}\) depends on the whole fluid motion, and both fluid flow and heat transfer equations are needed to find this.