Thermodynamics 1 - Energy, Energy Transfer, and General Energy Analysis

Chapter 2 Thermodynamics 1 1

Thermodynamics I

Introduction 1. Basic Concepts of Thermodynamics 2. Energy, Energy Transfer, and General Energy Analysis 3. Properties of Pure Substances 4. Energy Analysis of Closed Systems 5. Energy and Mass Analysis of Control Volumes 6. The Second Law of Thermodynamics 7. Entropy 8. Steam Power Cycle Applications Examples


Overview Energy Transfer by Heat, Work, Mass

2-1 Forms of Energy 2-2 Energy Transfer by Heat 2-3 Energy Transfer by Work 2-4 Mechanical Forms of Work 2-5 First Law of Thermodynamics 2-6 Energy Conversion Efficiencies


Energy Conservation

A refrigerator operating with its door open in a well-sealed and well-insulated room

A fan running in a well-sealed and

well-insulated room will raise the

temperature of air in the room.


Mechanisms of Energy Transfer Heat Transfer Q

Temperature driven transient energy transfer along the boundaries of a system.

Work W Any other transient energy transfer at the boundaries of a

system. Mass Flow

For open systems the mass entering a system has energy and will increases the energy of the system. Likewise, the mass leaving the system will decrease the energy of that system.


Energy of a System Energy exists in forms such as thermal, mechanical, kinetic,

potential, electric, magnetic, chemical and nuclear. Sum of all energies of a system: Total Energy

Total Energy per unit mass of a system

Total Energy = macroscopic energy + microscopic energy

)(energiesE =

)/( kgJmEe =


Macroscopic and Total Energy Kinetic Energy KE:

Potential Energy PE:

Total Energy E:

2 2

( ) ( / )2 2

my yKE J ke J kg= =

)/()( kgJgzpeJmgzPE ==

2

2

( )2

( / )2

myE U KE PE U mgz J

ye u ke pe u gz J kg

= + + = + +

= + + = + +


Microscopic or Internal Energy Microscopic Energy is

related to the molecular structure pf a system and the degree of activity on that level.

The sum of all microscopic forms of energy is termed the internal energy U.

Thermal Energy


Volume, Mass and Energy Flow Rates

y y

3

Volume Flow Rate:

Mass Flo

[ / ]

[ / ]

[ / or ]

w Rate:

Energy Flow Rate:

avg c

avg c

V y A m s

m V y A kg s

E me kJ s kw

=

= =

=


Mechanical Energy Mechanical energy: The form of energy that can be converted to mechanical work completely and directly by an ideal mechanical device such as an ideal turbine. Associated with fluids, insignificant heat transfer, essentially isothermal. Kinetic and potential energies: The familiar forms of mechanical energy.

Mechanical energy of a flowing fluid per unit mass

= flow energy + kinetic energy + potential energy

Rate of mechanical energy of a flowing fluid

Mechanical energy change of a fluid during incompressible flow per unit mass

Rate of mechanical energy change of a fluid during incompressible flow

y

y

y y

y y


ENERGY TRANSFER BY HEAT

Temperature difference is the driving force for heat transfer. The larger the temperature difference, the higher is the rate of heat transfer.

Heat Q [kJ]: The form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference.

Energy is recognized as heat transfer only as it crosses the system boundary.


During an adiabatic process, a system exchanges no heat

with its surroundings.

Heat transfer per unit mass

Amount of heat transfer when heat transfer rate changes with time

Amount of heat transfer when heat transfer rate is constant

ENERGY TRANSFER BY HEAT

2 1( )Q U mc T T= = Amount of heat transfer to heat up a body with mass m and specific heat capacity c from T1 to T2.


Heat Transfer Mechanisms

Conduction: The transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interaction between particles.

Convection: The transfer of energy between a

solid surface and the adjacent fluid that is in motion, and it involves the combined effects of conduction and fluid motion.

Radiation: The transfer of energy due to the emission of electromagnetic waves (or photons).

Q kA T

xcond=

( )Q h A T Tconv s f=

Q A T Trad s surr= 4 4c h


ENERGY TRANSFER BY WORK Work W [kJ]: A rising piston, a rotating shaft, and an electric wire crossing the system boundaries are all associated with work interactions.

Work done per unit mass

Power is the work done per unit time (kW)

Electrical work

Boundary work:

Gravitational work:

Acceleration work:

Shaft work:

Spring work:

Work for elastic bar:

Work for liquid film:

2

1

2 1

2 22 1

2 212 2 1

2

12

1

[kJ]

[kJ]

( -z ) [kJ]

( -y ) [kJ]2 [kJ]

( -x ) [kJ]

[kJ]

[kJ]

e

b

g

a

sh

spring

bar n

film S

W VI t

W PdV

W mg z

W m yW n

W k x

W Adx

W dA

=

=

=

=

=

=

=

=


Heat vs. Work Both are recognized at the boundaries of a

system as they cross the boundaries. That is, both heat and work are boundary phenomena.

Systems possess energy, but not heat or work.

Both are associated with a process, not a state.

Unlike properties, heat or work has no meaning at a state.

Both are path functions (i.e., their magnitudes depend on the path followed during a process as well as the end states).

Properties are point functions; but heat and work are path functions (their magnitudes depend on the path followed). Properties are point functions

have exact differentials (d ). Path functions

have inexact differentials ( )


First Law of Thermodynamics Conservation of Energy

Total Energy Energy can change from one

form to the other.

Energy cannot be created or destroyed.

For all adiabatic processes between two specified states of a closed system the net work done is the same regardless of the nature of the closed system and the details of the process fundamental principle First Law of Thermodynamics.

Value of work depends on end states only must correspond to a change if a property of system Total Energy.

Energy cannot be created or destroyed; it can only change forms.

The increase in the energy of a potato in an oven is equal to the amount of heat transferred to it.


Energy Balance 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.

in out systemE E E =

The first law cannot be proven mathematically, but no process in nature is known to have violated the first law, and this should be taken as sufficient proof.


Energy Balance

The work (boundary) done on an adiabatic system is equal to the increase in the energy of the system.

The energy change of a system during a process

is equal to the net work and heat transfer

between the system and its surroundings.

For a cycle E = 0, thus Q = W.

The work (electrical) done on an adiabatic system is equal to the increase in the energy of the system.


Energy Change of a System, Esystem

Internal, kinetic, and potential energy changes

y y y y


Mechanisms of Energy Transfer, Ein and Eout

>Heat Transfer< >Work Transfer< >Mass flow<

A closed system involves only heat transfer and work.

The energy content of a control volume can be changed by mass flow as well as heat and work interactions.

Chapter 2

EFFICIENCIES Efficiency is one of the most frequently used terms in thermodynamics, and it indicates

how well an energy conversion or transfer process is accomplished.

desired outputefficiencyrequired input

=


Generator efficiency

Pump-Motor overall efficiency

Turbine-Generator overall efficiency

The overall efficiency of a turbinegenerator is the product of the efficiency of the turbine and the efficiency of the generator, and represents the fraction of the mechanical energy of the fluid converted to electric energy.

Pump efficiency

Thermodynamics IOverview Energy Transfer by Heat, Work, MassEnergy ConservationMechanisms of Energy TransferEnergy of a SystemMacroscopic and Total EnergyMicroscopic or Internal EnergyVolume, Mass and Energy Flow RatesMechanical EnergyENERGY TRANSFER BY HEATENERGY TRANSFER BY HEATHeat Transfer MechanismsENERGY TRANSFER BY WORKSlide Number 14First Law of ThermodynamicsConservation of EnergyTotal EnergyEnergy BalanceEnergy BalanceEnergy Change of a System, EsystemMechanisms of Energy Transfer, Ein and EoutEFFICIENCIESSlide Number 21

Thermodynamics 1 - Energy, Energy Transfer, and General Energy Analysis

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Transcript of Thermodynamics 1 - Energy, Energy Transfer, and General Energy Analysis