課程名稱︰熱力學
課程性質︰必修
課程教師︰黃美嬌
開課學院：工學院
開課系所︰機械系
考試日期（年月日）︰2007 Jan. 17
考試時限（分鐘）：180
是否需發放獎勵金：是
（如未明確表示，則不予發放）
試題 :
Q1.(30%)Answer the following questions briefly.
(a) What is the key condition for a reservoir to be a thermal energy
reservoir?
(b) Somebody claims to have developed a new heat-engine cycle that is
operating between the temperature limits of 300K and 800K and has
a theoretical efficiency of 0.65. How do you evaluate this claim?
(c) In an effort to increase the efficiency of a power plant, somebody
suggests transferring heat from the available energy source to a higher
-temperature medium by a heat pump before energy is supplied to the
power plant. What do you think of this suggestion? Explain.
(d) A system undergoes a process between two fixed staes first in a reversible
manner an then in an irreversible manner. For which case is the entropy
changes the system greater? Why?
(e) A system undergoes a process involving a same amount of ∫δQ/T first
in a reversible manner and then in an irreversible manner. For which case
is the entropy change of the system greater? Why?
(f) What is an isentropic process? What are the conditions for a process
to be isentropic?
Q2.(20%)
An ideal Diesel cycle with air as the working fluid has a compression ratio
of 17 and cutoff ratio of 2. At the end of the compression process, the
working fluid is at 5MPa, 960K, and 110 cm^3. Determine the thermal
efficiency and the mean effective pressure by ultilizing
(a) the cold-air-standrad assumptions and (b) the air-standrad-assumptions.
Q3.(30%)
Consider a steam power plant tha operates on an ideal reheat-regenerative
Rankine cycle with one open feedwater heater and one reheater. Steam enters
the high-pressure turbine at 15MPa and 650℃. It is reheated to 650℃ at 4MPa
before entering the low-pressure turbine. Some steam is extracyed from the
trubine at 0.5MPa for the open feedwater heater. The remaining steam is
condensed in the condenser at 50kPa and pumped to 0.5MPa before entering the
feedwater heater. The steam leaves the feedwater heater as a saturated liquid.
The high-pressure turbine has an isentropic efficiency of 0.85 and the low-
pressure turbine is assumed to be isentropic.
(a) Sketch the T-s diagram of the cycle. Identify the states 1~9.
(b) Find the mass fraction of steam that is extracted for the open feedwater
heater.
(c) Compute the compressor work.
(d) Find the net work output.
(e) Find the efficiency of the cycle.
Q4.(20%)
Consider a two-stage cascade refrigeration system operating between the
pressure limits of 0.9MPa and 0.16MPa. Each stage operates on an ideal vapor-
compression refrigeration cycle with refrigerant-134a as the working fluid(the
refrigerants enter the compressors as saturated vapors and leave the condensor
as saturated liquids) except both compressors have an efficiency of 0.92. Heat
rejection form the lower cycle to the upper cycle take place in an adiabatic
counterflow heat exchanger where both steam enter at about 0.32MPa. If the
mass flow rate of the lower cycle is 0.05kg/s, sketch the T-s diagram of this
cascade refrigerator(a) and determine the mass flow rate of the refrigerant of
the upper cycle (b), the rate of heat removal from the refrigerant space,(c)
the power input to the compressor(d),the COP of this cascade refrigerator(e).
Bonus(5%)
Imagine a new application of a thermoelectric cooler or a thermoelectric power
generator.
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