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Unformatted text preview: MEEN 315—Fall 2009, Exam 2 Name Kg! Section Number l. Multiple choice problems worth 2 pts each. Circle the answer that is the most appropriate or
closest (numerically) to your answer. 1. An unknown perfect gas has an enthalpy of 703.6 kJ/kg at 380 K and an enthalpy of
549.8 kJ/kg at 298 K. Estimate a value of cp for this gas: A. 153.8 kl/kg°C .188 kJ/kg~°C c. —153.8 kJ/kg«°C , D. 0.533 kJ/kg°C (P: %3 %”Q‘ 2 QOS‘Q'ngﬂéw wait?“ *2 W ._ l< K
h“ 7: (ago .... wax 3“ 2. For incompressible substances, the constant volume specific heat cV and the constant pressure
specific heat cp are related by which relationship? R is the gas constant. A. cp—cV=R B. cp+cV=R QZDCV=Cp D. cp—cv=1 3. In the definition of enthalpy, for an open system the Pv term represents 6} flow work (flow energy). B. heat energy. C. shaft work. D. total work. 4. A steady state (steady flow) condition exists in a system if A. volumetric flow rate in equals volumetric flow rate out.
B. flow work in equals flow work out. © at any given position properties do not vary With time.
D. properties within the system are uniform at any given time. 2 MEEN 315—Fall 2009, Exam 2 Name Section Number
5. For a fluid flowing through a constant—area circular duct under steady state (steady flow) conditions, the following is true (V: velocity, A = cross—sectional area, p = fluid density): A. pA = constant B. AV= constant C. p/V=constant .pV=constant
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U' 6. Select the machine whose purpose is to increase the pressure of a fluid while slowing it down. Diffuser B. Nozzle C. Throttle D. Pump 7. A heat engine converts 600 Btu of high temperature heat input (QH) into 250 Btu of useful work
(Wnet), and rejects the remaining heat energy to a thermal reservoir at 70F. What is the minimum
temperature the high temperature reservoir can be? A. 120 R 908 R C. 448R D. Undeterminable 43L CPHW Wm: @959 250M 336385;” E Rm“ (3%"? )T @607”.ng 22$; 2: QQQ’Q , MEEN 315—Fall 2009, Exam 2 Name Section Number 8. A heat pump requires 1 kW (Wnet) to transfer 1 kW of heat energy (Q) from a low temperature
space. The low temperature space is at 30°C; what is the heat pump’s maximum possible COP? A. 0.5 B. 1 © 2 D. Undeterminable 9. Water as a saturated liquid with initial temperature of 40°C exchanges heat energy with the
surroundings during a reversible process, until the water reaches saturated vapor state. What is the temperature of the surroundings? 40°C B. 20°C C. 100°C D. Undeterminable 10. An evaporator'coil adds heat energy to saturated liquid R—134a at P = 100 kPa until it exits with.
a pressure of 100 kPa and a temperature of 0°C. How much heat energy is transferred to the R 134a during this process? A. 216.74 kJ/kg B. 21.14 kJ/kg
c. 18.76 kJ/kg @ 238.3 kJ/kg
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il. Four workout problems valued as noted. 1. A rigid, insulated 2—ft3 tank contains helium at 100 °F and 1 atm. A paddle wheel stirrer is
operated within the tank and does 0.025 hp of work on the system in a 4minute period. A. How much work in Btu is done on the system? (5 pts)
B. Determine the final temperature of the helium. (10 pts)
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W... , W33W7W MEEN 315—Fall 2009, Exam 2 Name Section Number 2. Heat is being added to a rigid container of R—134a. At the bottom of the container is a valve
which leaks and allows R134a liquid to escape so that the system pressure is constant at
100 psia during the entire process. The chamber initially has half its volume occupied by
saturated liquid and half by saturated vapor, and the total mass of R134a is 2 lbm initially. Heat
is added for 4 minutes until the chamber contains by volume 75% vapor and 25 % liquid. A. What is the flow energy (flow work) in Btu/lbm of the escaping R—134a? (5 pts) B. What is the total energy in Btu/lbm of the escaping R134a? (5 ptslv C. What is the mass flow rate in lbm/min of the escaping R—134a? (5 pts) D. What is the rate in kW at which 2e ggy leaves the chamber? (5 pts) 52:71 “’2 W) m mg}: ”a; @0832—
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ﬁlm"); N nah MEEN 315—Fall 2009, Exam 2 Name Section Number 3. An ”economizer” in a steam power plant is a type of heat exchanger that directs hot exhaust
from the boiler combustion process to water that is about to enter the boiler. It ”preheats” the
water, so that less heat energy is added in the boiler. Hot exhaust, which has a constant
pressure specific heat of Cplexhaust = 1.1 kJ/kg—K, enters the heat exchanger at 1000 K and leaves
at 500 K. Liquid water at a pressure of 5 MPa and a temperature of 60°C enters the heat
exchanger. The mass flow rate of water is 10 kg/s, while the mass flow rate of exhaust is 1 kg/s. A) How much heat energy, in kW, is transferred out of the exhaust through the heat
exchanger? (10 Points) B) Assuming all the heat energy from the exhaust is transferred into the water (i.e., an adiabatic heat exchanger), what is the exit temperature of'the water? (10 Points)
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+33??? f; 9V 4 If}??? (‘3; .. 7;) 72:"?‘5 ‘ 55K: Ia/sK aha/5%: MEEN 315—Fall 2009, Exam 2 Name Section Number 4. One way to utilize solar energy is to construct a simple power plant (Le, a heat engine), that
uses the solar energy as a source for OH and rejects Q to the atmosphere. The solar heat ”flux”
(power applied per unit area) is reported to be 750 W/mz. A) If the surface of the solar collector that provides QH is 100°C, what is the maximum
efficiency the solar power plant could have, assuming it is rejecting heat energy to the
atmosphere at 20°C? (8 pts) B) if the solar collector has an area of 1 m2 (i.e., the surface area that collects the solar heat
flux), what is the maximum amount of power the solar power plant can make? (8 pts) C) Solar cells using the photovoltaic (PV) effect convert light energy directly into electricity;
this contrasts to the solar power plant described above, which is a type of a heat engine.
Such solar cells are reported to have efficiencies up to 30%. Based on your in Part A, do
PV solar cells violate the Second Law of Thermodynamics? Limit your answer to a
maximum of two sentences. (4 pts) .. 7 233 , WW]
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