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Unformatted text preview: MEEN 315—Fall 2008, Exam 2 Name Section Number I. Multiple choice problems worth 2 pts each. Circle the answer that is the most appropriate or
closest (numerically) to your answer. 1. The generalized Conservation of Mass principle states ' dm 0 o
A Mass In = Mass Out —— = m; — e
’ [air 1.. 2 2m 2. Flow work (or flow energy) is the work required to push a mass flow into or out of a control volume. B) is the work transferred out of a control volume, i.e. WW. C) ish=u+Pv. D) is eﬂaw=6=h+ 3. Steam flowing at 30 kg/s, a pressure of 10.0 MPa, and a temperature of 600°C must enter a
turbine with a velocity no less than 40 m/s. What is the maximum turbine inlet pipe diameter to accomplish this? A) 3.7 cm B) 2.9 cm C) 22.1 cm 19.1 cm Z '2,—
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ol MEEN 315—Fall 2008, Exam 2 Name Section Number 4. Select the device which does _not_ produce work: Throttle C. Heat engine B. Turbine (Ii) Compressor
Emmet» )4 or D :5 COLL/drag Cor/“cat 5. In winter a heat pump is used to pump heat from the colder outdoors to the warmer
indoors. This A. Violates the first law of thermodynamics. B. Violates the second law of thermodynamic when work input occurs.
© Does not violate the second law of thermodynamic as long as work input occurs. D. Does not violate the second law of thermodynamics when no work input occurs. f 7%»
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6. A turbine accepts steam at a pressure 01%;?) psia and a temperature of 1000 °F to produce
25.2x106 Btu/hr of power. The turbine loses 1.4x106 Btu/hr of heat energy to the surroundings. if the exit state from the turbine has a pressure of 5 psia and a temperature of 200 °F, what is the flow rate of the steam? A) 3,761 lbm/S B) 19.98 lbm/S C) 17.88 lbm/s D) 3,815 lbm/s assume 11363125“; We. 2 a” @“W =' ‘l‘& C‘ +~Aj¢§€j
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L t 3, MEEN 315—Fall 2008, Exam 2 Name Section Number 7. Carnot’s first principle deals with the efficiency of heat A. Law C. Reservoirs B. Exchangers Engines 8. An adiabatic, real process is executed between states (1) and (2). The entropy change 5251 is 9. On a Ts diagram, the area under a curve depicting a process represents: A. The work done on the substance during the process.
B. The work done by the substance during the process. C. The heat exchange, whether a reversible process or not. 7 The heat exchange, but only if internally reversible. 10. A heat engine is used to propel a steam locomotive and delivers 1200 kJ/kg of work while
rejecting 800 kJ/kg of heat. The thermal'efficiency is A. 0.67 C. 0.40 a 0.60 D. 0.33 mil; dea “@e; (2,, : Quit/V: (90494—1200)%: i3
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lE MEEN 315—Fall 2008, Exam 2 Name Section Number ll. Four workout problems valued as noted. 1) An adiabatic diffuser on the front of a jet engine decelerates air (treat as an ideal gas) with an inlet pressure of 100 kPa, inlet temperature of 288 K, and inlet velocity of 110 m/s. The exit
pressure is 104 kPa and the exit temperature is 291 K. A) What is the exit velocity of the air from the diffuser? (10 points) B) If the inlet diameter of the diffuser is 1 m, what is the mass flow rate of air into the jet
engine? (3 points) C) What is the exit diameter of the diffuser? Note: If you are unable to solve either Part A or
Part B, assume the exit velocity is 90 m/s (incorrect) and the mass flow rate is 110 kg/s
(incorrect). (5 points). I ‘43 tad, W550 L m 2
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Section Number 2. A car air conditioning system removes heat energy from the cab of the car by using the high
temperature cab air to vaporize R134a through an "evaporator". An evaporator is a heat
exchanger. Suppose air flows into the heat exchanger at a temperature of 100 °F and flows out
at a temperature of 70 °F. The R—134a flows into the heat exchanger with a quality of 30% and a
temperature of 50 °F. It flows out 'of the heat exchanger as a saturated vapor. Assume the heat
exchanger is adiabatic, is constant pressure, and had negligible changes in kinetic and potential
energies. A) How much heat energy, on a Btu / lbw,“r basis, does the air reject to the R134a? (5 pts) B) How much heat energy, on a Btu / lb,n,R_1g,4a basis, does the R—134a absorb from the air? (5 pts)
C) If the heat transfer rate between the two fluids within the heat exchanger is
260,000 Btu/hr, what is the mass flow rate of air (lbm/s)? Note: if you cannot determine an
answer for Part A, assume it is 10 Btu/lbm (incorrect). (5 pts) D) What is the mass flow rate (lbm/s) of R—134a through the heat exchanger? Note: If you
cannot determine an answer for Part B, assume it is SO Btu/lbm (incorrect). (5 pts). ‘ (a ,7 QBMMA. @a gum ‘ E m ‘ l4 ‘1 “1’ f5? \
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rec $2M $14 gig: , ____ts;l  MEEN 315—Fall 2008, Exam 2 Name Section Number 3. A building owner directly heats a building with electric resistance heat (you may view this as a
heat pump with COP = 1.0 or heater with efficiency = 1.0; both viewpoints lead to the same result).
The electric resistance heating system delivers 100,000 Btu/h in order to maintain the indoor temperature at 80 °F. Assume the cost of electricity is S 0.10 / kWh. A. How much money in dollars will the electricity cost if the heater operates for 24 hours?(7 pts) B. The owner changes to a reversible heat pump that extracts heat energy from the outside air
at 15 °F. What will be the electricity bill in dollars for the new heat pump for 24 hours of operation? (10 pts) C. Compare answers from (A) and (B) and calculate the savings in dollars (if any) realized over a
24 hour period as a result of using the heat pump. (3 pts) @ ' r l .., {722;
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i “r r?“ é’cﬂwiﬁ MEEN 315—Fall 2008, Exam 2 ‘ Name Section Number
4. An engineer proposes the following process: R134a occupies a frictionless pistoncylinder
device at T1: —— 16 °C and P1 = 0.200 MPa. The piston keeps a constant pressure on the R134a
while energy in the form of 231 kJ/kg of heat is transferred to the refrigerant from a cold space
reservoir until the refrigerant reaches T2 = 10 °C. The combined system of pistoncylinder device
and cold space are well—insulated, and the cold space is at 17 °C. A. Compute the entropy change of the refrigerant (5 pts)
B. Compute the entropy change of the cold space (5 pts)
C. Compute the combined entropy change for both the refrigerant and cold space (5 pts) 1 D. Based on the entropy changes, state if the process is possible or not possible, and state why it is
possible or not possible (5 pts) 4» 55$. (4495:) Q.2.M§/%.Q——£W,+ ﬁ<[email protected]/%s J. 5.1:»; (462;) : 0.3;493 5; = 93; C \Oct) (Lima) ll as _ ~— , ...... § *“""”7
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