BASICS OF HEAT TRANSFER (Lecture#3)

By

Engr. Muhammad Usman Khan

THE FIRST LAW OF THERMODYNAMICS

The first law of thermodynamics, also known as the conservation of energy principle, states that energy can neither be created nor destroyed; it can only change forms

Therefore, every bit of energy must be accounted for during a process.

The conservation of energy principle (or the energy balance) for any system undergoing any process may be expressed as follows:That is,

THE FIRST LAW OF THERMODYNAMICS

The net change (increase or decrease) in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process

(Total energy entering the system) (Total energy leaving the system) =(Change in the

total energy of the system )

THE FIRST LAW OF THERMODYNAMICS

Ein- Eout =Esystem

Net energy transfer by heat, work and mass=Change in K.E and P.E etc

Open System

Open System: The open system is one in which matter crosses the boundary of the system. There may be energy transfer also. Most of the engineering devices are generally open systems e.g an air compressor in which air enter at low pressure and leaves at high pressure and there are energy transfer across the boundary

Isolated/Closed System

Isolated System: The isolated system is one in which there is no interaction between the system and surrounding it is of fixed mass and energy and there is no mass or energy transfer across the system boundary

Open System

Isolated/Closed System

Control Volume and Control Surface

In the analysis of an open system such as an air compressor attention is focused on a certain volume on a certain volume in space surrounding the compressor known as control volume bounded by a surface called control surface. Matter as well as energy crosses the control surface

Control Volume and Control Surface

Control Volume and Control Surface

A closed system is a system closed to matter flow through its volume against a flexible boundary .When there is matter flow then the system is considered to be a volume of fixed boundary, the control volume. There is thus no difference between an open system and a control volume

Homogeneous and Heterogeneous System

A quantity of matter homogeneous throughout in chemical composition and physical structure is called a phase. Every substance can exist in one of three phases i.e solid liquid and gas.

A system consist of a single phase is called a homogeneous system

A system consisting of more than one phases is called an heterogeneous system

Macroscopic and Microscopic approach

Every system has certain characteristics by which its physical condition may be described

e,g volume, temperature and pressure etc. Such characteristics are called properties of the system. These are all macroscopic in nature.

Macroscopic properties concerned with the effect of action of molecules and these effect can be perceived by human senses

Macroscopic and Microscopic approach

For example the macroscopic quantity pressure is the average change of momentum due to all the molecular collisions made on a unit area. The macroscopic point of view is not concerned with the action of individual molecules and the force on a given unit area can be measured by using a pressure gauge

Entropy, Pressure and internal energy

Macroscopic and Microscopic approach

For the microscopic point of view matter is composed of myriads of molecules if its is a gas each molecule at a given instant has a certain position velocity and energy and for each molecule these change very frequently as a result of collisions. The behavior of gas is decided by summing up the behavior of each molecule

Energy Balance for Closed Systems (Fixed Mass)

A closed system consists of a fixed mass

The total energy E for most systems encountered in practice consists of the internal energy U

This is especially the case for stationary systems since they dont involve any changes in their velocity or elevation during a process. The energy balance relation in that case reduces to

Energy Balance for Closed Systems (Fixed Mass)

Stationary closed system: Ein - Eout =U = mCv T

Where we expressed the internal energy change in terms of mass m, the specific heat at constant volume Cv, and the temperature change T of the system. When the system involves heat transfer only and no work interactions across its boundary, the energy balance relation further reduces to

Stationary closed system, no work: Q = mCv T

Energy Balance for Closed Systems (Fixed Mass)

Where Q is the net amount of heat transfer to or from the system. This is the form of the energy balance relation we will use most often when dealing with a fixed mass.

Stationary closed system, no work:

Q =mCvT

where Q is the net amount of heat transfer to or from the system. This is the form of the energy balance relation we will use most often when dealing with a fixed mass.

Energy Balance for Steady-Flow Systems

A large number of engineering devices such as water heaters and car radiators involve mass flow in and out of a system, and are modeled as control volumes

Most control volumes are analyzed under steady operating conditions

The term steady means no change with time at a specified location.

Energy Balance for Steady-Flow Systems

The opposite of steady is unsteady or transient.

Also, the term uniform implies no change with position throughout a surface or region at a specified time

The total energy content of a control volume during a steady-flow process remains constant (ECV constant).

Energy Balance for Steady-Flow Systems

That is, the change in the total energy of the control volume during such a process is zero (ECV = 0).

Thus the amount of energy entering a control volume in all forms (heat, work, mass transfer) for a steady-flow process must be equal to the

amount of energy leaving it.

The amount of mass flowing through a cross section of a flow device per unit time is called the mass flow rate, and is denoted by m

Energy Balance for Steady-Flow Systems

A fluid may flow in and out of a control volume through pipes or ducts

The mass flow rate of a fluid flowing in a pipe or duct is proportional to the cross-sectional area Ac of the pipe or duct, the density , and the velocity of the fluid.

The flow of a fluid through a pipe or duct can often be approximated to be one-dimensional. That is, the properties can be assumed to vary in one direction only (the direction of flow).

Energy Balance for Steady-Flow Systems

Under the one-dimensional flow approximation, the mass flow rate of a fluid flowing in a pipe or duct can be expressed as

m =Av (kg/s)

The volume of a fluid flowing through a pipe or duct per unit time is called the volume flow rate V and is expressed as

Energy Balance for Steady-Flow Systems

Note that the mass flow rate of a fluid through a pipe or duct remains constant during steady flow. This is not the case for the volume flow rate

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