Economics

University of Hail Faculty of Engineering DEPARTMENT OF MECHANICAL ENGINEERING

ME 315 – Heat Transfer

Lecture notes

Chapter 7

External flow: Flow over a flat plate, cylinders, spheres. Tube banks

Prepared by : Dr. N. Ait Messaoudene

Based on: “Introduction to Heat Transfer” Incropera, DeWitt, Bergman, and Lavine, 5th Edition, John Willey and Sons, 2007. 2nd semester 2011-2012

Objective:

Our primary objective is to determine convection coefficients for different flow geometries. In particular, we wish to obtain specific forms of the functions that represent these coefficients. By nondimensionalizing the boundary layer equations in Chapter 6, we found that the local and average convection coefficients may be correlated (via the Nusselt number) by equations of the form:

where

and

with

The Empirical Method

Experiment for measuring the average convection heat transfer coefficient

electrical power (E.I) = total heat transfer rate (q)

Procedure: Measure , Ts and T∞ for different conditions (I, As=W.L, u ∞, fluid) the electrical many different values of the Nusselt number corresponding to a wide range of power, E I, which is equal Reynolds and Prandtl numbers . to the total heat transfer rate q the data may then be represented by an empirical correlation of the form plot the results on a log–log scale C, m, and n are often independent of the fluid

The specific values of the coefficient C and the exponents m and n vary with the nature of the surface geometry and the type of flow.

The Flat Plate in Parallel Flow

As with all external flows, the boundary layers develop freely without constraint. Boundary layer conditions may be entirely laminar, laminar and turbulent, or entirely turbulent. To determine the conditions, compute Re   u L  u L L





and compare with the critical Reynolds number for transition to turbulence, Re x , c .

Re L  Re x,c  laminar flow throughout Re L  Re x,c  transition to turbulent...