Essay On Heat Transfer

Chapter one

Introduction

Heat transfer

Heat is defined in physics as the transfer of thermal energy across a well-defined boundary around a thermodynamic system. Three modes of heat transfer are considered. Conduction is the transfer of energy between objects that are in physical contact. Convection is the transfer of energy between an object and its environment, due to fluid motion. Radiation is the transfer of energy from the movement of charged particles within atoms is converted to electromagnetic radiation. In this work heat Transfer by conduction and convection will be considered only.

Heat transfer by conduction and convection simultaneously occurs in the majority of engineering applications, such as heat exchangers and fins. Efficiently

Conduction-only problem will be solved analytically. On the other hand, convection-only problem will be solved analytically. The two solutions will be matched together to achieve the interface conditions.

Motivation

The engineering objective of modeling any heat transfer problem is to obtain a simple relation between surface heat flux and a representative temperature difference. Governing equations are partial differential equations together with adequate boundary conditions. Since problem considered is hydrodynamically fully developed, has a given (known) profile (space dependence) over any cross section normal to fluid flow, which remains the same over the whole duct. Adequate boundary conditions will be added later. Governing energy equation for heat transfer is written as: ρCv(r)∂T/∂z=k((∂^2 T)/(∂z^2 )+(1 ∂)/(r ∂r) (r ∂T/∂r)) (1)

Solving the system is usually done numerically, which is rather a cumbersome procedure. A simple expression is desired by engineers for design purposes. The simple expression is usually cast in the form of the so-called Newton law of cooling, which for internal convection