Test #2
Elements of Thermo
EML 3007
October 27, 2009
Open book (class text only), open notes (prepared by you, class site downloads). 2 hours.
Problem 1
(40). An ideal steam Rankine cycle with superheat operates with a condenser
temperature of 20 C. It is well known that increasing the cycle pressure ratio will increase
the cycle efficiency; here you are to estimate how sensitive the effect is. To do that:
a)
calculate the cycle efficiency when the pump discharge pressure is 10 MPa. Also
determine the maximum cycle temperature.
b)
calculate the cycle efficiency when the pump discharge pressure is 12.5 MPa.
Also determine the maximum cycle temperature.
c)
Calculate the change in efficiency per change in maximum temperature. That is,
find
"
#
"
T
max
d)
Taking the maximum temperature and condenser temperature as the external
reservoir temperatures, find the Carnot efficiency for the two cases. For which
case is the actual efficiency a greater percentage of the Carnot efficiency?
Problem 2
(30). A rigid container is divided in half by a partition. One half of the
volume contains air at 5 atm and 300K, while the other half is a vacuum. The partition is
suddenly removed, allowing the air to occupy the entire container, and the system is
allowed to reach equilibrium with negligible heat transfer.
a)
Taking the air as a perfect gas, find its change in temperature.
b)
Find the change in entropy of the air.
Problem 3
(30). Humid air in the atmosphere rises as it encounters mountains, causing
rain or snow via the decrease in pressure at higher altitude.
a)
If air is initially at 0.5 atm and 290 K, how far must the pressure drop before
freezing temperatures occurs?
b)
Now consider the water vapor in the air. The concept of
partial pressure
is such
that the total pressure is sum of the partial pressures of the water vapor and the
dry air. For condensation to begin under the conditions of part a, what would the
partial pressure of the water vapor be? What would be the ratio of water partial
pressure to total pressure (this, by the way, would be the mole fraction of water).
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Test #2
Elements of Thermo
EML 3007
March 19, 2009
Open book (class text only plus printout of chap. 23, the pdf), open notes (prepared by
you, class site downloads). 2 hours.
Problem 1
(50).
A simple Rankine cycle with superheat (the turbine inlet is superheated)
operates with water as the working fluid. The condenser pressure is 75 kPa and the boiler
pressure is 3 MPa. The cycle is ideal except that the turbine efficiency is 93%, and the
turbine exit quality is not 100%. The turbine inlet temperature is 350 °C.
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 Spring '08
 Sherif
 Thermodynamics, Heat, Cycle, turbine exit quality, maximum cycle temperature

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