INTRODUCTION TO
NAVAL
ENGINEERING
THERMODYNAMICS
II
INTRODUCTION TO
NAVAL
ENGINEERING
FIRST AND SECOND
LAWS OF
THERMODYNAMICS
FIRST LAW OF
THERMODYNAMICS
Energy can be neither created nor
destroyed but only transformed
THE GENERAL ENERGY
EQUATION
Energy In = Energy Out
or
U2 - U1 = Q - W
where
U1: internal energy of the system at the beginning
U2: internal energy of the system at the end
Q : net heat flow into the system
W : net work done by the system
ELEMENTS OF A
THERMODYNAMIC CYCLE
(1) Working Substance - medium by which
energy is carried through the cycle.
(2) Heat Source - supplies thermal energy to the
working substance.
(3) Heat Receiver - absorbs heat from the
working substance.
ELEMENTS OF A
THERMODYNAMIC CYCLE
(4) Pump - circulates the working substance; acts
as a valve between low and high pressure
(5) Engine - device which converts the thermal
energy of the working substance into useful
mechanical energy
HEAT
SOURCE
ENGINE
PUMP
HEAT
RECEIVER
THERMODYNAMIC CYCLES

CLOSED
– Working fluid never leaves the cycle,
except through accidental leakage (ex:
steam cycle)

OPEN
– Working fluid is taken in, used, then
discarded (ex: internal combustion engine)
ENGINES

HEATED:
– heat is added to the working substance in
the engine itself (ex: internal combustion
engine)

UNHEATED:
– the working substance receives heat in
some device that is separate from the
engine (ex: steam turbines)
HEAT
SOURCE
UNHEATED
ENGINE
PUMP
HEAT
RECEIVER
WORKING SUBSTANCE
(MIXTURE OF
ATMOSPHERIC AIR
AND FUEL)
LOW PRESSURE OR HIGH
PRESSURE SIDE OF THE
CYCLE, DEPENDING UPON
POSITION OF PISTON IN
THE CYLINDER
HEAT RECEIVER
(ATMOSPHERE)
HEAT SOURCE
(COMPRESSION OR SPARK)
PUMP
(PISTON)
HEATED ENGINE
(PISTON AND
CYLINDER)
FLOW PROCESSES

NON-FLOW
– One in which the working fluid does not
flow into or out of its container in the
course of the process (ex: air compressors,
internal combustion engines)

STEADY FLOW
– One in which a working substance flows
steadily and uniformly (ex: boilers,
turbines, condensers)
USING THE G.E.E. (Non-Flow)
Q12 = (U2 - U1) + wk12/J
where
Q12
U2, U1
wk12
J
total heat transferred (Btu)
total internal energy at points 1 and
2 (Btu)
work done between states 1 and 2
(ft-lbs)
constant of 778 ft-lbs/Btu
WORKING
SUBSTANCE
WORKING
SUBSTANCE
PUMP
(PISTON)
PUMP
(PISTON)
G.E.E. (Non-Flow) EXAMPLE
5 LBM of a fluid is compressed in the
cylinder using 350 Btus of work. If
internal energy initially was 100 Btu/lbm
and 150 Btu/lbm at the end of the
compression, how much heat was
added/lost?
Q12 = (U2 - U1) + wk12/J
STEADY FLOW SYSTEMS
AND THE GENERAL
ENERGY EQUATION
ENTHALPY (H or h)
 COMBINATION
OF:
– INTERNAL ENERGY (U)
– FLOW WORK
• Mechanical energy necessary to
maintain the steady flow of the fluid
• Flow work = pV/J (Btu)
H
= pV/J + U
STATE CHANGES

Isobaric
– the pressure of and on the working fluid is
constant

Isenthalpic
– the enthalpy of the working fluid does not change
(h1 = h2)

Isothermal
– temperature is constant

Adiabatic
– occurs in such a way that there is no transfer of
heat to or from the system during the process
THE SECOND LAW OF
THERMODYNAMICS
(1) All energy received as heat by a heat
engine cycle cannot be converted into
work (This means that no cycle can
have a thermal efficiency of 100%)
THE SECOND LAW OF
THERMODYNAMICS
(2) The transformation of heat to work is
dependent on a temperature difference
and on the flow of heat from a high
temperature reservoir to a low
temperature reservoir. (In other word
heat must flow from hot to cold)
THE SECOND LAW OF
THERMODYNAMICS
(3) It is impossible to construct an engine
that, operating in a cycle, will produce
no effect other than the transfer of heat
from a low temperature reservoir to a
high temperature reservoir
THE SECOND LAW OF
THERMODYNAMICS
“FRICTION HAPPENS”
ENTROPY

A THEORETICAL MEASURE OF ENERGY THAT
CANNOT BE TRANSFORMED INTO MECHANICAL
WORK IN A THERMODYNAMIC SYSTEM.

The total amount of entropy in a system always goes
up.

No thermodynamic process can occur without some
losses.

First Law: “You can’t win”
Second Law: “You can’t even break even”

ENTROPY (cont)

A MEASURE OF DISORDER
– always growing in our universe

“THE END OF THE UNIVERSE IS
UPON US”

CRUCIAL PART OF REAL
THERMODYNAMIC EQUATIONS
CARNOT CYCLE
TEMP
ENTROPY
CARNOT EFFICIENCY
work output
η
heat input
Tsource - Treceiver
η
Tsource
Source (Ts)
Heat Flowin
= CpTs = Qin
Workout = Qin - Qout
Engine

Carnot
=
Workout
Ts - Tr
=
Qin
Ts
Heat Flowout = CpTr = Qout
Receiver (Tr)
CARNOT EFFICIENCY FOR IDEAL ENGINES
Work
Output