文档介绍：Power Cycles with Two-Phase Media (Vapor Power Cycles)
[SB&VW – Chapter 3, Chapter 11, Sections to ]
In this section, we examine cycles that use two-phase media as the working fluid. These
can bined with gas turbine cycles to bined cycles which have higher efficiency
than either alone. They can also be used by themselves to provide power sources for both
terrestrial and space applications. The topics to be covered are:
i) Behavior of two-phase systems: equilibrium, pressure temperature relations
ii) Carnot cycles with two-phase media
iii) Rankine cycles
iv) Combined cycles
Behavior of Two-Phase Systems
The definition of a phase, as given by SB&VW, is “a quantity of matter that is
homogeneous throughout”. Common examples of systems that contain more than one phase are a
liquid and its vapor and a glass of ice water. A system which has three phases is a container with
ice, water, and water vapor.
We wish to find the relations between phases and the relations that describe the change of
phase (from solid to liquid, or from liquid to vapor) of a pure substance, including the work done
and the heat transfer. To start we consider a system consisting of a liquid and its vapor in
equilibrium, which are enclosed in a container under a moveable piston, as shown in Figure 2B-1.
The system is maintained at constant temperature through contact with a heat reservoir at
temperature T, so there can be heat transfer to or from the system.
Water vapor
Water vapor
Liquid water Liquid water
(a) (b) (c)
Figure 2B-1: Two-phase system in contact with constant temperature heat reservoir
For a pure substance, as
shown at the right, there is
a one-to-one correspondence
between the temperature at which
vaporization occurs and the pressure.
These values are called the saturation
pressure and saturation temperature
(see Ch. 3 in SB&VW).
P-T relation for liquid-vapor system
2B-1
This means there is an additional constraint for a