課程名稱︰化工熱力學甲一 課程性質︰化工系必修 課程教師︰林祥泰 開課學院：工學院 開課系所︰化工系 考試日期（年月日）︰2013/10/15 考試時限（分鐘）：120min 是否需發放獎勵金：是 （如未明確表示，則不予發放） 試題 : Closed book and notes Note: Credit for each problem is given in the square brackets []. l. (a) Explain the major focus of thermodynamics. [4 points] (b) Describe the statements for the three laws of thermodynamics. [4 points] (c) Write down the mathematical expressions for the 1st and 2nd laws of thermodynamics. [4 points] (d) Describe what a reversible process is and how to attend such a process. Why is such a process important? [4 points] (e) What is equilibrium and how is it different from steady state? [4 points] 2. An isolated tank is divided into two equal chambers by an internal diaphragm. One chamber contains an ideal gas (of temperature-independent constant pressure heat capacity, C*p) of pressure P1, and temperature T1, and the other chamber is evacuated.Suddenly, the diaphragm bursts. (a) Determine the final temperature T2 and pressure P2 of the gas in the tank after sufficient time has passed for equilibrium to be attained.[6 points] (b) Determine the total entropy generation of the above process. [4 points] (c) Suppose a magic machine is applied such that shaft work can be extracted as the idea gas passes from one chamber to the other. Determine the maximum work that can be obtained. [6 points] (d) Determine the final temperature T3 and pressure P3 of the gas in problem(c) [4 points] 3. Suppose that n moles of an ideal gas originally in state1 (T1,P1) is changed to state2 (T2,P2) via different processes described below (note that P2>P1). For each process, determine the change in internal energy (△U), enthalpy (△H) ,and entropy (△S), and the work (W) done on the gas and the heat (Q),flowed into the gas.(Assuming the constant pressure heat capacity Cp* to be temperature independent) (a) Reversible isothermal (T1=T2) compression [5 points] (b) Irreversible compression against a constant external pressure P2.[5 points] 4. The Joule-Thomson expansion valve is commonly used in industry to reduce pressure in a gaseous flow stream. Since the gas flows through the valve rapidly, one can assume that there is no heat transfer to the gas T1 ——————————————————— T2 ╳╳╳╳╳╳╳╳╳╳╳ → ╳╳╳╳╳╳╳╳╳╳╳ → ╳╳╳╳╳╳╳╳╳╳╳ P1 ——————————————————— P2 (a) Show that the enthalpy of the gas remains unchanged when passing through the valve.[5 points] (b) A ideal gas of temperature T1 and pressure P1 is steadily exhausted to the atmosphere P2 at a constant molar flow rate n using the Joule-Thomson valve.What is the temperature of the gas after leaving the valve? [5 points] (c) What is the rate of entropy generation in (b)? [5 points] (d) A chemical engineer claimed that he designed a new valve capable of generating shaft work during the pressure-reducuing process. What is the temperature of the exhaust gas and the maximum work that can be produced from the new valve for the process describe in (b)?[10 points] 5. Illustrated below is a simple solar engine consisting of a collector and a heat engine. Solar energy is absorbed as heat (Qin) through the collector. The collector loses some of the heat it absorbs by convection (q1), and the remainder (q2) is passed through a heat engine to produce electricity. The rate of heat loss from the collector is q1=h(Tc－Ta) where h is the heat transfer coefficient and Ta is the ambient temperature. The heat flows to the heat engine(q2) at temperature Tc and leaves(q3) the engine at Ta. (a) Choose the collector and heat engine combined as the system, write down the energy and entropy balance equations, respectively. [5 points] (b) Express the work output Ws in terms of Qin,Tc,Ta,h and the rate of entropy generation inside the system. Under what condition will the efficiency of solar engine reach that of a Carnot engine? [10 points] (c) Describe how the work output of the solar engine changes with the changes in collector temperature Tc and ambient temperature Ta. [10 points] 第五題附圖 Qin ↓ ———————— ∣Collector, Tc∣→→ q1 ∣ ∣ ———————— ↓q2 ↓ ———————— ∣ Heat Engine ∣→→ Ws ∣ ∣ ———————— ↓q3 ↓ Air,Ta --

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