Transcript 3 - CFD - Anna University

ENGINEERING THERMODYNAMICS
Dr. M.R.SWAMINATHAN
Assistant Professor
Internal Combustion Engineering Division
Department of Mechanical Engineering
ANNA UNIVERSITY
CHENNAI-25.
PROPERTIES- contd.
CYCLE
A process (or a series of connected
processes) with identical end states is
called a cycle.
A cycle composed of two processes,
A and B.
all other thermodynamic properties
must also change
Other
thermodynamic properties
must also change so that the
pressure is a function of volume as
described by these two processes
2
P
Process
B
1
Process
A
V
EQUILIBRIUM
A system is said to be in
thermodynamic equilibrium if it
maintains
Thermal (Uniform Temperature)
Mechanical (Uniform Pressure)
Phase equilibrium
Chemical equilibrium.
20 °C
30 °C
30 °C
35 °C
32 °C
32 °C
32 °C
40 °C
No thermal equilibrium
32 °C
32 °C
Thermal equilibrium
QUASISTATIC PROCESS
Process proceeds in such a way that
the system remains infinitesimally
close to an equilibrium state at all
times, it is called a quasistatic,
or quasi-equilibrium process
QUASISTATIC PROCESS
Quasi- equilibrium process can be
viewed as a sufficiently slow process
that allows the system to adjust itself
internally so that properties in one
part of the system do not change any
faster than those at other parts
QUASISTATIC PROCESS
QUASISTATIC PROCESS
State 2
P
Process path
Intermediate
states
State 1
20
V
20 pa
20 pa
20 pa
20 pa
20 pa
(a) Slow compression (quasi-equilibrium)
ZEROTH LAW
Zeroth law was first formulated and
labeled by R. H. Fowler in 1931
• Two bodies are in thermal
equilibrium if both have the same
temperature even if they are not in
contact.
HEAT
• Heat is defined as the form of
energy that is transferred between
two systems (or a system and its
surroundings) by virtue of a
temperature difference.
• Heat is energy in transition.
HEAT
• Heat is energy in transition. It is
recognized only as it crosses the
boundary of a system
• The potato contains energy, but
this energy is heat transfer only as
it passes through the skin of the
potato (the system boundary) to
reach the air, as shown in figure.
HEAT TRANSFER
HEAT
HEAT
INTERNAL ENERGY
• Energy
possessed
by
the
molecules by the virtue of its
temperature
• Higher the temperature higher is
the internal energy possessed by
the medium ( solid / liquid/ gas )
• Internal energy is zero at absolute
zero
INTERNAL ENERGY-contd.
• Internal energy is the sum of all
microscopic forms of energy of a
system.
• Internal energy will be highest for gas
phase and minimum for the solid phase
THERMODYNAMIC PROCESSES
• There are many thermodynamic
processes in practice.
• In each of the processes we
normally allow one of the
properties to remain a constant
during a process.
THERMODYNAMIC PROCESSES
•
•
•
•
•
•
Isobaric
Isochoric
Isothermal
Polytropic
Adiabatic
Isentropic
(P=c)
(V=c)
(T=c)
(PVn=c)
(PVγ=c)
(PVγ=c)
WORK
• Work, like heat, is an energy
interaction between a system and
its surroundings.
• Energy can cross the boundary of
a closed system in the form of heat
or work.
WORK
• Work is the energy transfer
associated between a system & the
surrounding in the absence of any
temperature difference.
• An example is the work done in
expansion of a piston assuming
there is no temperature difference
between
the
cylinder
and
surroundings
WORK contd.
•
•
•
•
Displacement work (pdV work)
Force exerted, F= p. A
Work done dW= F.dL= p. A dL= p.dV
If the piston moves through a finite
distance say 1-2,Then work done has
to be evaluated by integrating
dW= ∫pdV
DISPLACEMENT WORK
FLOW WORK
• Open systems involves flow of
mass in and out of the system
unlike closed system
• Energy associated with pushing
mass or volume of a gas in or out
of the system is called FLOW
WORK or FLOW ENERGY
FLOW WORK contd.
• FLOW WORK is always associated
with open systems or a stream of
liquid or gas which is in motion
• Flow work is given by
p1v1- p2v2 or simply as pv
TYPES OF WORK
•
•
•
•
•
•
Stretching of a wire
Electrical Energy
Work of a reversible chemical cell
Work in stretching of a liquid surface
Work done on elastic solids
Work of polarization and magnetization
HEAT & WORK TRANSFER
• All our efforts are oriented towards
how to convert heat to work or vice
versa:
• Heat to work
• Work to heat
Thermal power plant
Refrigeration
HEAT AND WORK
• Heat and work are directional
quantities.
WORK INTERACTION
• Work done by a system or work given
by a system is considered + ve
e.g work supplied by turbine
• Work done on the system or work
supplied to system is considered –ve
e.g compressor or pump
HEAT INTERACTION
• Heat supplied to the system is
considered + ve
e.g heat supplied to boil water in a
boiler
• Heat rejected by a system is
considered – ve
e.g condenser of a thermal power plant
HEAT AND WORK
POINT & PATH FUNCTIONS
• Both heat and work are path functions
• They depend only on the path of travel
and not on end states or end points
• BOTH HEAT AND WORK ARE PATH
FUNCTIONS (INEXACT DIFFERENTIAL)
POINT & PATH FUNCTIONS
• Properties like pressure , temperature
depend only on end state
• They are POINT FUNCTIONS & are
known as EXACT DIFFERENTIALS.
•
All Exact differentials are property of
a system
HEAT & WORK SIMILARITY
• Heat and work are energy transfer
mechanisms between a system and
its surroundings.
• Systems possess energy, but not
heat or work.
• Both are recognised at the
boundaries of a system as they
cross the boundaries.
• Both are path functions
WORK INTERACTION
Consider
following
example
the
An electric oven
heat
by
a
heating element
HEAT INTERACTION
• Consider the
same
example
• An electric
oven heat by
a
heating
element