ME 322 – Mechanical Engineering Thermodynamics (Exam 1)
Spring 2013
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EXAM INSTRUCTIONS – PLEASE READ THIS CAREFULLY
You will have 50 minutes to complete this exam. This time limit will be strictly enforced. This is a
CLOSED TEXTBOOK exam. The only resources allowed are a hand-held calculator and the course
textbook supplement cited below,
Balmer, R.T., “Thermodynamic Tables to Accompany Modern Engineering Thermodynamics,
Elsevier Inc., Burlington, MA, 2011.
You may use the blank pages in the booklet to write anything you desire IN YOUR OWN
HANDWRITING. Absolutely no cutting and pasting in the book is allowed.
No computers, cell phones, iPhones, iPods, iPads, music players, or any other electronic equipment
may be used during the exam with the exception of a hand-held calculator.
Show all of your work in the space provided on the exam. Partial credit cannot be awarded if the
work is not shown. The points associated with each part of the exam are indicated on each part.
There are a total of 80 points on this exam.
COMMON MOLAR MASSES: C=12; H=1; O=16; N=14
PART I. The heat rate, expressed in Btu/(kw-hr), is often used to describe the performance of
a commercial powerplant (a heat engine). The heat rate is the ratio of thermal energy
input to net electrical energy output for a power cycle.
a) What are the fundamental dimensions of the heat rate in the FLT system? Begin by
assigning fundamental dimensions to each of the units (i.e. BTU, kw, and hr) in the heat rate
expression and then give the fundamental dimensions of the heat rate itself. (6 pts)
b) If the heat rate is 10,500 Btu/(kw-hr), convert this expression into all SI units. (5 pts)
c) Sketch a generic heat engine, labeling all flows of heat and work with appropriate variables.
Show the role of a high temperature and low temperature reservoir in conjunction with the
heat engine. Explain how to form the heat rate expression from variables shown in your
sketch. The heat rate is not the same as the thermal efficiency, but it is closely related.
(6 pts)
d) From the standpoint of the first law of thermodynamics, are there any limitations on values
that the heat rate can assume? Explain your reasoning. NOTE: On the web you can find
published heat rates for selected power production facilities around the country. (3 pts)
PART II. 30 g of ice (solid H2O) at 0 C is placed in a sealed, evacuated, and rigid container with a
volume of 200 cc (cubic centimeters). The walls of the container are thermally conducting and
there is an external resistance heater that can supply a nearly uniform heat flux through the
container walls.
a) How many gmoles of water are in the container? (4 pts)
b) What is the specific volume of water in the container? Express this in typical SI units. (4 pts)
c) If the container is heated to 200 C, what is the pressure? Cite your sources of data. (4 pts)
d) For this system, sketch a diagram that clearly shows the material in the system, the system
boundary, and all relevant energy and/or mass flows. Do not show any flows in your diagram
that have zero value. Classify the system as open, closed, and/or isolated. (3 pts)
e) After heating to 200 C, what is the quality of water in the container?
Cite your sources of data as well as all algebraic equations that you use. (5 pts)
PART III. A pulse jet engine can be modeled by an ideal gas undergoing the following closed cycle.
Note that P2>P1 and V3>V1.
1-2: Isochoric combustion from P1 to P2
2-3: Adiabatic, polytropic expansion from P2 back to P1
3-1: Isobaric compression back to state 1
a) Sketch these three processes (that make up the pulse jet engine cycle) on a pressure-volume
diagram, clearly labeling the endpoints 1, 2, and 3. Place pressure on the y-axis. (6 points)
b) Shade the area in the sketch that represents work done during process 2-3 and explain your
reasoning for this shading. (2 points)
c) Complete the following table, giving the correct sign, not the value, for each of the following.
Think like an engineer, not a physicist/chemist, in using 1st law sign conventions. Next to your
your answer, include brief notes that explain your rationale for each answer. As shown in the
example for heat transfer in process 2-3, reference relevant properties that remain constant
during each process, the caloric equation of state for an ideal gas, and/or the first law of
thermodynamics in supplying your rationale. (12 points)
Work
Heat
Process 1-2
Process 2-3
Process 3-1
Net for Entire Cycle
Example:
Q=0
Adiabatic process
Change in
Internal Energy
PART IV. Consider refrigerant R-22 at 0 F and 75 psia. Estimate properties in this compressed liquid
state using data from the saturation tables. Clearly show all algebraic equations you use.
a) Show the approximate location of this state point on a PT diagram as well as a Pv diagram,
including relevant saturation lines and isotherms. Correctly show the location of the critical
point. Validate your property diagrams against those in the book and/or those from EES. (6 pts)
b) What is the specific volume of R-22 under these conditions? Use only saturation data.
What assumptions about fluid behavior underlie your calculations? (4 points)
c) What is the internal energy of R-22 under these conditions? Use only saturation data.
What assumptions about fluid behavior underlie your calculations? (4 points)
d) What is the enthalpy of R-22 under these conditions? Account for the full effect of 75 psia
on this enthalpy value, not just the effect of the saturation pressure. (6 points)