slayt 1 - marmara Üniversitesimimoza.marmara.edu.tr/~cem/ht/sunu6.pdf · 2011-06-01 · when the...

In Chapter 6 we considered the general and theoretical

aspects of forced convection, with emphasis on differential

formulation and analytical solutions.

In this chapter we consider the practical aspects of forced

convection to or from flat or curved surfaces subjected to

external flow, characterized by the freely growing boundary

layers surrounded by a free flow region that involves no

velocity and temperature gradients

In order to account for the variation of theproperties with temperature, the fluid propertiesare usually evaluated at the so-called filmtemperature, defined as

which is the arithmetic average of the surfaceand the free-stream temperatures. The fluidproperties are then assumed to remain constantat those values during the entire flow

When relations for local friction and convectioncoefficients are available, the average friction andconvection coefficients for the entire surface can bedetermined by

The first relation gives the average friction coefficient for the entireplate when the flow is laminar over the entire plate. The second relationgives the average friction coefficient for the entire plate only when theflow is turbulent over the entire plate, or when the laminar flow regionof the plate is too small relative to the turbulent flow region

In some cases, a flat plate is sufficiently long or theflow to become turbulent, but not long enough todisregard the laminar flow region. In such cases, theaverage friction coefficient over the entire plate is