工程热力学试卷（A）
(120 minutes)
班级 班级序号 姓名 成绩 题 号 得 分 阅卷人 Section A Section B Section C 总分 Section A: Answer the following questions (8*5)
1. What is the condition of the following equations?
?q??u?w??dq?cvdT?pdv?dq??u?d(pv)?2. What is the difference between gage pressure and absolute pressure? 3. What is a perpetual-motion machine of the first kind and what is a perpetual-motion machine of the second kind? 4. In gas refrigeration cycles, can we replace the turbine by an expansion valve as we did in vapor-compression refrigeration cycles? Why? 5. Please use a T-s diagram to illustrate the influence of turbine inlet pressure to the efficiency of a simple ideal Rankine cycle. 6. What four processes make up the simple ideal Brayton cycle? 7. How do relative and specific humilities change during a simple heating process? Answer the same question for a simple cooling process. 8. If air flows into a nozzle at an ultrasonic speed, what
kind of nozzle should be used? Why?
Section B: Draw diagram according to the request (2*10)
1. Please draw the following cycle in a T-s diagram.
2. Please, show the schematic of a reheat Rankine cycle and indicate the name of equipment each.
Section C: Calculation
P
3 v 2 s 1 p 4 s 5 v v 1. (15) An ideal gas power cycle. The air is compressed adiabatically from state 1 (p1＝1.01bar、t1 =15℃、Vl＝0.014m3) to state 2 (V2 =0.0028m3). After this, heated at constant volume to state 3 ( p3 =18.5bar). Then expand adiabatically to state 4 (p4=1.01bar). Finally, the air back to state 1 at constant pressure. Please calculate: (1) The net work of this cycle. (2) The efficiency of this cycle.
(R=287 J/kg.K k=1.4 take cp, cv as constant) 2. (10) Please, use Clausuis inequality to judge the following cycle’s direction (engine or refrigerator)
T1 =300 K T2 =700 K /Q2/=3000J /W/=1000J
3. (15) As to a regenerative Rankine cycle, the enthalpy at each point has been shown in the following schematic. If each feedwater heater gives the same enthalpy increase to the feedwater and the power for pump can be neglected, then:
(1) Please derive the expression of the fraction of steam extracted for each feedwater heater. (2) The expression of efficiency
Boiler Extraction enthalpy of stage 1 h5 Extraction enthalpy of stage 2 h7 Feedwater enthalpy h4 Condenser outlet enthalpy h3 Drainage enthalpy of stage 1 h6
Turbine exhaust enthalpy h2 Turbine inlet enthalpy h1 Steam Turbine