TOPIC 6 Flashcards
What is Entropy?
The ENTROPY of a system can in fact be shown to be a measure of its disorder and of the UNAVAILABILITY OF ENERGY TO DO WORK.
simple definition of energy
the ability to do work
is a measure of how much energy is not available to do work.
entropy
That unavailable energy is of interest in thermodynamics, because
the field of thermodynamics arose from efforts to convert heat to work.
Unit of Entropy
J/K
KJ/K
Btu/°R
Unit for Specific Entropy
KJ/kg K (for s)
KJ/kmol K (for s line)
Btu/lb °R (for s)
Btu/lbmol °R (for s line)
Btu/°R to J/K
1899.1 J/K
Btu to J
1055.06 J
lb to kg
0.454 kg
K to R
1.8 R
Linear Interpolation Formula
y = y1 + x - x1 / x2 - x1 (y2 - y1)
For saturation states, the values of sf and sg are tabulated as a function of
either saturation pressure or saturation temperature
entropy of a two-phase liquid–vapor mixture is calculated using the quality
s = (1 - x) sf + xsg
= sf + x (sg - sf)
the quality (x) of the mixture can be determined from the knows specific internal energy
x = u - uf / ug - uf
Temperature–Entropy Diagram is also known as
T-s Diagram
Enthalpy–Entropy Diagram is also known as
h-s diagram or Mollier diagram
intended for evaluating properties at superheated vapor states and for two-phase liquid–vapor mixtures
h-s diagram or Mollier diagram
is an expression of the second law that is particularly effective for thermodynamic analysis
entropy balance
Entropy Balance for CLOSED Systems
(change in the amount of entropy contained within the system during some time interval)
=
(net amount of entropy transferred in across the system boundary during the time interval)
+
(amount of entropy produced within the system during the time interval)
(Equation) Entropy transfer
it is associated with heat transfer to or from the system during the process
(Equation) Entropy transfer: it can be interpreted
entropy transfer accompanying heat transfer.
(Equation) Entropy transfer
The direction of entropy transfer is the same as the direction of the heat transfer, and the same sign convention applies as for heat transfer:
Positive value in entropy means
entropy is transferred into the system
Negative value in entropy means
entropy is transferred out the system
When there is NO HEAT TRANSFER
there is NO ENTROPY TRANSFER.
The term σ is positive when
internal irreversibilities are present during the process and vanishes when no internal irreversibilities are present
entropy is produced (or generated) within the system by
by the action of irreversibilities.
Entropy production is a property?
Entropy production is not a property
The second law requires that
entropy production be positive, or zero, in value:
σ > 0
irreversibilities present within the system
σ = 0
no irreversibilities present within the system
The value of the entropy production (σ)
CANNOT BE NEGATIVE
In contrast, the CHANGE IN ENTROPY (Δ S) of the system may be positive, negative, or zero:
. > 0
S2 - S1 = 0
< 0
To evaluate the entropy transfer term of the
entropy balance requires
information regarding both the heat transfer and the temperature on the boundary where the heat transfer occurs.
