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Chapter 1

INTRODUCTION

1.1. OVERVIEW OF HEAT AND MASS TRANSFER

Course Objectives

• Be acquainted with the thermodynamics, heat

transfer and their current engineering applications.

• Be comfortable with the metric SI commonly used in

engineering.

• Develop an intuitive systematic problem-solving

technique.

• Understand the basic concepts of mass transfer

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OVERVIEW OF THERMAL-FLUID SCIENCES

• Thermal-fluid sciences:The physical sciencesthat deal with energy and the transfer, transport, and conversion of energy.

• Thermal-fluid sciences are studied under thesubcategories of

� thermodynamics

� fluid mechanics

� heat transfer

The design of many engineering

systems, such as this solar hot

water system, involves thermal-fluid

sciences.

Application Areas of Thermal-Fluid Sciences

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Application Areas of Thermal-Fluid Sciences

THERMODYNAMICS

• Thermodynamics: The science of energy.

• Energy: The ability to cause changes.

• The name thermodynamics stems from the Greek words therme (heat) and dynamis (power).

• Conservation of energy principle:During an interaction, energy can change from one form to another but the total amount of energy remains constant.

• Energy cannot be created or destroyed.

• The first law of thermodynamics: An expression of the conservation of energy principle.

• The first law asserts that energy is a thermodynamic property.

Energy cannot be created

or destroyed; it can only

change forms (the first law).

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• The second law of thermodynamics:It asserts that energy has quality as well as quantity, and actual processes occur in the direction of decreasing quality of energy.

• Classical thermodynamics: A macroscopic approach to the study of thermodynamics that does not require a knowledge of the behavior of individual particles.

• It provides a direct and easy way to the solution of engineering problems.

• Statistical thermodynamics: A microscopic approach, based on the average behavior of large groups of individual particles.

Conservation of energy

principle for the human body.

Heat flows in the direction of

decreasing temperature.

FLUID MECHANICS

• Fluid mechanics: The science that deals with the behavior of fluids at rest (fluid statics) or in motion (fluid dynamics), and the interaction of fluids with solids or other fluids at the boundaries.

• Fluid: A substance in the liquid or gas phase.

• A solid can resist an applied shear stress by deforming, whereas a fluid deforms continuously under the influence of shear stress, no matter how small. Fluid mechanics deals with liquids

and gases in motion or at rest.

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Deformation of a rubber block placed

between two parallel plates under the

influence of a shear force. The shear

stress shown is that on the rubber—an

equal but opposite shear stress acts on

the upper plate.

The normal stress and shear stress at

the surface of a fluid element. For fluids

at rest, the shear stress is zero and

pressure is the only normal stress.

Unlike a liquid, a gas does not form a free surface,

and it expands to fill the entire available space.

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HEAT TRANSFER• Heat: The form of energy that can be

transferred from one system to another as a result of temperature difference.

• Heat Transfer: The science that deals with the determination of the rates of such energy transfers and variation of temperature.

• Thermodynamics is concerned with the amount of heat transfer as a system undergoes a process from one equilibrium state to another, and it gives no indication about how long the process will take. But inengineering, we are often interested in the rate of heat transfer, which is the topic of the science of heat transfer.

We are normally interested in how

long it takes for the hot coffee in a

thermos to cool to a certain

temperature, which cannot be

determined from a thermodynamic

analysis alone.

Example: Thermos

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Example: PC computer memories

Example: PCBs

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Example: Heat sinks

Example: Heat exchanger

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Introduction to mass transfer

• Mass transfer refers to the movement of a chemical species from a high concentration region toward a lower concentration region and requires the presence of two regions at different chemical compositions.

• The primary driving force is the pressure difference for fluid flow and the temperature difference for heat transfer, whereas it is the concentration difference for mass transfer. Therefore, we do not speak of mass transfer in a homogeneous medium.

• Physical Origins

• Both conduction and mass diffusion are transport processes that originate from molecular activity.

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Applications

• Evaporation of water into air in a cooling tower

• Drying of wood, paper, and textiles

• Leakage of helium from the laser of a copying machine

• Catalytic oxidation of carbon monoxide and unburnt hydrocarbons in an automobile catalytic converter

• Measurement of humidity using wet and dry thermocouples

• Aeration of sewage for biological treatment

• Evaporation and condensation in gas-controlled heat pipes

• Pollutant transport in air/water

• Combustion of pulverized coal in a power plant furnace

Tutorial questions

C1-1. Why is heat transfer a nonequilibrium phenomenon?

Ans. Heat transfer is a non-equilibrium phenomena since in a system that is in

equilibrium there can be no temperature differences and thus no heat flow.

C1-2. Can there be any heat transfer between two bodies that are at the same temperature but at different pressures?

Ans. There cannot be any heat transfer between two bodies that are at the same temperature (regardless of pressure) since the driving force for heat transfer is temperature difference

C1-3. What is mass transfer ?

Ans.Mass transfer refers to the movement of a chemical species from a high concentration region toward a lower concentration region