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MECHENG 235 Study Guide - Final Guide: Thermodynamics, Isentropic Process, Rankine CycleExam


Department
Mechanical Engineering
Course Code
MECHENG 235
Professor
Donald Siegel
Study Guide
Final

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ME235: Thermodynamics I
Final Exam
December 19, 2016
4:00pm 6:00pm
SECTIONS 001 AND 003
Exam Rules
Open Textbook
One Page of Handwritten Notes and Calculator Allowed
Printouts of Appendices A and B Allowed
No Electronic Devices (Including E-Readers) Allowed
When you are done, staple your work pages to these sheets,
with this cover page on top.
Name: __________________________________
Section # (check one): ___ 001 (Pipe) ___ 003 (Borgnakke)
Problem 1 30 Points
Problem 2 30 Points
Problem 3 40 Points
I have observed the honor code and neither given nor received aid on this exam.
____________________________________________
Signature
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Problem 1 (30 pts). A rigid tank contains 1.5 kg of R-410A at 0°C with
quality x = 0.25. A valve on top of the tank is opened, and saturated
vapor R-410A leaving the tank is throttled by the valve and exits to a line
with pressure 200 kPa. The process stops when 20% of the mass has
flowed out. During the process, the temperature inside the tank remains
constant by heat transfer from the 20°C ambient. HINT: Consider two
steps in the process: 1) the flow from the tank to the inlet of the valve,
and 2) the flow through the valve.
a) Find the amount of heat transfer during the process.
b) Find the total entropy generation during the process.
Problem 2 (30 pts). Consider a steam-based power plant operating in an environment that has
an ambient temperature of 25°C. The cycle is identical to a Rankine cycle except that the turbine
is neither reversible nor adiabatic and has an isentropic efficiency of 60%. The pump inlet is at
10 kPa and the pump exit is at 4 MPa. The boiler exit is at 600°C. The turbine exit is a saturated
vapor.
a) Find the turbine isentropic specific work and actual specific work
b) Find the thermal efficiency of the cycle.
c) Find the specific entropy generation due to the turbine.
Problem 3 (40 pts). In the Otto cycle, heat addition is assumed to occur at constant volume. In
practice, however, some of the heat transfer occurs after the piston has started moving downward
in the expansion stroke. Consider a cycle identical to an Otto cycle except that 1300 kJ/kg of heat
is first added at constant volume and 600 kJ/kg of heat is then added at constant pressure. Note
that this means that the cycle now has 5 steps, with a constant-pressure expansion step added
before isentropic expansion. Assume that the state at the end of compression is 2000 kPa, 700 K,
and the compression ratio is 10.
a) Draw the cycle on a P-v diagram, clearly numbering all points in the process (1, 2, 3, 4, 5).
Numerical values of P and v at these points don’t need to be written on the diagram, but label
the steps of the cycle that are isentropic.
b) Find the maximum temperature during the cycle.
c) Find the maximum pressure during the cycle.
d) Find the thermal efficiency of the cycle.
e
Q
sat vap
cv
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