**1. Introduction**

The tenth chapter reports a numerical study on unsteady mixed convection heat transfer

A computational modeling of vehicle radiators using porous medium approach is presented in the final chapter. A CFD analysis and results of a radiator are also demonstrated in this

This book is intended to be a useful source of information for researchers, postgraduate stu‐ dents, and academics, as well as designers and engineers working in the fields of heat ex‐

We would like to thank all the authors for their high-quality contributions and the publish‐ ing process manager for providing continuous support, which have made the completion of

Finally, we would like to express our appreciation to our family members for their contin‐

**S M Sohel Murshed and Manuel Matos Lopes**

University of Lisbon, Lisbon, Portugal

ued support and patience during the preparation of this book.

from two isothermal semicircular cylinders in tandem arrangement inside a channel.

chapter.

VIII Preface

changers and related industries.

this book possible.

This chapter aims to provide an overview of various aspects of heat exchangers as well as to briefly highlight research and findings from each contribution of this book.

Heat exchangers are devices that facilitate the exchange/transfer of heat between two media/ matters (fluids, solid surface/particulates and fluids) at different temperatures. Heat exchang‐ ers are commonly used in practice in a wide range of applications from heating and air‐ conditioning systems in a household to chemical processing and power production plants, bioprocess, and heavy industries. One of the well‐known heat exchangers is car radiator in which heat is transferred from the hot water flowing through the radiator tubes to the air flowing through the closely spaced thin plates outside attached to the tubes.

Heat transfer in a heat exchanger usually involves convection in each fluid and conduction through the wall separating the two fluids. Different thermal applications demand different types of hardware and different configurations of heat transfer equipment, and thus, wide range of heat exchangers are manufactured and available in the market. Heat exchangers are classified based on various features such as transfer processes, fluids flows direction (i.e., par‐ allel flow and counter flow), number of fluids (e.g., two, three fluids), surface compactness, heat transfer mechanisms, construction etc. In general, the classification of heat exchangers by transfer processes includes (i) indirect‐contact and (ii) direct‐contact exchangers [1, 2].

In indirect‐contact heat exchangers, the two flowing fluids are separated by a wall and heat exchange between these two fluids occurs through this wall. As a separating wall hinders the heat flow, these heat exchangers are less effective than the direct‐contact ones. However,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

these heat exchangers are widely used because most of the practical cases fluids cannot be allowed to contact or mix. Common examples of indirect‐contact exchangers are shell and tube, bayonet, concentric tube, plate, spiral plate, radiator, storage or regenerators, and com‐ pact exchangers. On the other hand, two fluids streams come into direct contact with each other and exchange heat before separating. The direct‐contact heat exchangers include mov‐ ing bed contactor, fluidized bed, moving belt conveyor, immiscible fluids, boiling and immis‐ cible fluids, and cooling tower exchangers.

The analysis of heat exchangers is commonly performed through two well‐known methods, which are log mean temperature difference (LMTD) and effectiveness‐number of transfer units (ε‐NTU) [3]. LMTD is easy to use in heat exchanger analysis when the inlet and the out‐ let temperatures of the hot and cold fluids are known or can be determined from the energy balance, and it is very suitable for determining the size of a heat exchanger to realize certain/ required outlet temperature. On the other hand, NTU is directly a measure of the heat transfer surface area, and therefore, the smaller the NTU the smaller the heat exchanger. Heat transfer enhancement in heat exchangers is usually accompanied by increased pressure drop and thus by higher pumping power. Therefore, any gain from the enhancement in heat transfer should be weighed against the cost of the accompanying pressure drop.
