Transcript engineering thermodynamics - CFD

ENGINEERING THERMODYNAMICS
Dr. M.R.SWAMINATHAN
Assistant Professor
Internal Combustion Engineering Division
Department of Mechanical Engineering
ANNA UNIVERSITY
CHENNAI-25.
LIMITATIONS OF I LAW
The first law of thermodynamics is
simple,
general,
but
does
not
constitute a complete theory because
certain processes it permits do not
occur in nature!
Examples of processes which are not
prohibited by the first law, but cannot
happen in a real world.
• Perfect machine
• Transfer heat from cold to hot subject
• Gas expansion
IRREVERSIBLE PROCESSES
1. A battery discharges through a
resistor,
releasing
energy.
The
reverse process will not occur.
2. Two gases, initially in separated
adjoining
chambers,
will
mix
uniformly.
3. A free expansion of gas
Lussac-Joule experiment)
(in Gay-
4. Heat flows from a high temperature
body to a low temperature reservoir in
the absence of other effect
STATEMENTS OF THE SECOND
LAW OF THERMODYNAMICS:
Kevin-Planck
Statement:
It
is
impossible to construct a device that
operates in a cycle and produces no
other effects than the performance of
work and the exchange of heat with a
single reservoir.
Model Heat Engine
• Qhot= W+Qcold
or
• Qhot-Qcold=W
(what goes in must
come out)
II LAW OF THERMODYNAMICS
EFFICIENCY OF HEAT ENGINE
Even
the
most
efficient
heat
engines
reject almost onehalf of the energy
they
receive
waste heat.
as
It is impossible for any device that
operates on a cycle to receive heat from
a single reservoir and produce a net
amount of work.
Clausius Statement:
It is impossible to construct a device
that operates in a cycle and whose sole
effect is to transfer heat from a cooler
body to a hotter body.