Thermodynamics I
Spring 2015
Lecture 9:
Evaluating properties
Yong Li
Shanghai Jiao Tong University
Institute of Refrigeration and Cryogenics
800 Dong Chuan Road Shanghai, 200240, P. R. China
Email : liyo@sjtu.edu.cn
Phone: 86-21-34206056; Fax: 86-21-34206056
1.1
Last Lecture
 Energy Balance – Cycles:
» Summation of all heat interactions (Qcycle) must equal the summation of all
work interactions (Wcycle)
 Power cycles deliver net work output
 Refrigeration/ heat pump cycles require net work input
Qin + Wcycle =
Qout
 Performance parameters defined for the above cycles
» Power Cycle Thermal Efficiency (η)
» Refrigeration/heat pump cycles –
Coefficient of Performance (COP)
1.2
In the following Lectures—start
Chapter 3
 The objective of the Chapter 3 is to introduce property
relations relevant to engineering thermodynamics.
» State principle
» Simple system, simple compressible systems
» Pure Substance
» Phase
» Phase Change
» Property Diagrams
 The equilibrium state of a simple compressible system
is specified of two independent, intensive properties.
1.3
Concepts
Fixing the state
 State Principle ::: A general rule determining the number of
independent properties required to fix the state of a system.
» The number of independent properties is 1+ the number of relevant work
interactions.
Based on experiments
 Simple Systems ::: There is only one way the system energy can be
significantly altered by work as the system undergoes quasiequilibrium
processes.
» Two independent properties needed to fix the state of a simple system.
» Simple compressible system
» Simple elastic system
» Simple magnetic system
1.4
Concepts
Fixing the state
 Simple Compressible Systems :::
pure substance system
» useful for a wide range of engineering applications.
» The state principle indicates that the number of independent intensive
properties is 2 for such systems.
» Intensive properties such as velocity and elevation that are assigned
values relative to datums outside the system are excluded from
present considerations.
» The only mode of energy transfer by work that can occur as a simple
compressible system undergoes quasiequilibrium processes, is
associated with volume change and is given by ∫ p dV.
Is it a model?
Yes
1.5
Concepts
Pure Substances
 A pure substance :::uniform and invariable in chemical composition.
 A pure substance can exist in more than one phase, but its chemical
composition must be the same in each phase.
» A system with two phase liquid water and water vapor is a pure substance? Yes
 A mixture of various elements or compounds is a pure substance as
long as the mixture is homogeneous.
» Air is a pure substance?
Yes
 A mixture of two or more phases is a pure substance as long as the
chemical composition of all phases is the same
» e.g. A mixture of ice and liquid water
1.6
Concepts
Phases of Pure Substances
 A phase of a substance ::: having a distinct molecular
arrangement which is homogeneous throughout and
separated from the others by easily identifiable boundary
surfaces.
 Solid ::: Molecules oscillate about “fixed positions,
tightly packed; oscillation is related to temperature
 Liquid ::: “Chunks” of molecules float about each other;
maintain molecular order
 Gas ::: No molecular order; gas molecules move at
random large distances between molecules; small
intermolecular forces
1.7
Concepts
Concepts relation
System
Simple
system
Simple Compressible
System
..
pure substance system
Phase
1.8
Concepts
Phase Change
 Phase Change::: transformation of a thermodynamic system from
one phase to another.
 Molecular motion determined by temperature
 Higher temperatures result in higher microscopic internal energy
(translational, rotational, vibrational)
 Energy content increases from solids to liquids to gases
 Phase change processes important in many practical applications
» e.g.: boiler and condenser (liquid‐vapor equilibrium) in steam power plant
cycle
 In general, liquid‐vapor phase changes will be of the most
importance for this class
1.9
Concepts
Phase Change
 A Melt
 B Freeze
 C Boil
 D Condense
 E Sublimation (升华）
 F Desublimation (凝华）
1.10
Concepts
p-v-T Surface
The substance is at equilibrium
 p-v-T Surface::: In 3-D space, indicate
relationships between p, v and T for a given pure
substance on p-v-T surface
 p-v-T surface depicts the complete behavior
of a given pure substance
 Two-phase regions::: Located between the
single-phase regions
» Two phases exist in equilibrium
» liquid–vapor(L-V), solid–liquid (S-L), and
solid–vapor(S-V)
» The state cannot be fixed by temperature and
pressure alone; however, can be fixed by v  Three phases can exist in
and either p or T
equilibrium along the line
labeled triple line.
1.11
Concepts
p-v-T Surface
 T-v, p-v and p-T diagrams are simply
projections from this surface
 Triple point ::: the triple line of the threedimensional p–v–T surface projects onto a
point on the phase diagram.
 Water, triple point 273.16K at 0.6113 kPa
P-T diagram =
Phase diagram
1.12
Concepts
p-v-T Surface
 T-v, p-v and p-T diagrams are simply
projections from this surface
 Triple point ::: the triple line of the threedimensional p–v–T surface projects onto a
point on the phase diagram.
P-T diagram =
Phase diagram
 Water, triple point 273.16K at 0.6113 kPa
Assigned
Measured
1.13
Phase diagram for water
(not to scale)
1.14
1.15
T-v, p-v diagram
1.16
Temperature and Energy
1.17
Constant Pressure Phase Change for Water
1.18
Concepts
States of Water, Liquid and Vapor
 Subcooled liquid::: liquid too cold to
vaporize at the given pressure
=::: compressed liquid
 Saturated Liquid::: liquid about to
vaporize
 Liquid‐Vapor Mixture::: liquid
and vapor co-exist in equilibrium
 Vapor dome
 Saturated Vapor::: vapor about to
condense
 Superheated Vapor::: vapor too hot
to condense at the given pressure
 Saturation state
1.19
Concepts
States of Water, Liquid and Vapor
 Subcooled liquid::: liquid too cold to
vaporize at the given pressure
=::: compressed liquid
 Saturated Liquid::: liquid about to
vaporize
 Liquid‐Vapor Mixture::: liquid
and vapor co-exist in equilibrium
 Vapor dome
 Saturated Vapor::: vapor about to
condense
 Superheated Vapor::: vapor too hot
to condense at the given pressure
 Saturation state
1.20
Concepts
T-v vs. p-v Diagrams
lines of constant P
lines of constant T
1.21
Concepts
Saturation T & p
 Saturation Temperature (Tsat)::: At a given pressure, the
temperature at which a pure substance changes phase (liquid ⇒vapor
or vapor ⇒liquid)
 Saturation Pressure (psat)::: At a given temperature, the pressure at
which the given pure substance changes phase (liquid ⇒vapor or
vapor ⇒liquid)
 Tsat increases with psat and vice‐versa
1.22
Saturation T & p
Line showing saturation
temperature and
saturation pressure
1.23
Concepts
Critical Point
 Critical Point::: A point on the phase change diagram of the given pure
substance where saturated liquid and saturated vapor states are identical
 Beyond the critical point, no distinct phase
change process
 For water, pcr ~ 221 bar; Tcr ~ 374.1C
 At critical pressure, no separation between
sub-cooled liquid and superheated vapor, i.e.,
the two-phase region is non-existent
1.24