第六章 动力装置循环
英文习题
1. Power generation by a steam turbine
The power output of an adiabatic steam turbine is 5MW, and the
inlet and the exit conditions of the steam are as indicated in
Fig.6-1. (a) Compare the magnitudes of Δh, Δke, Δpe. (b) Determine the work done per unit mass of the steam flowing through the turbine. (c) Calculate the mass flow rate of the
steam.
2. The simple ideal Rankine cycle
Consider a steam power plant operating on the simple
ideal
Rankine cycle. The steam enters the turbine at 3 MPa and 350℃ and is condensed in the condenser at a pressure of 75 kPa. Determine the thermal efficiency of this cycle.
3. Effect of boiler pressure and temperature on efficiency
Consider a steam
power plant operating on the ideal Rankine cycle. The steam enters the turbine at 3 MPa and 350℃ and is condensed in the condenser at a pressure of 10 kPa. Determine (a) the thermal efficiency of this power plant, (b) the thermal efficiency if the steam is superheated to 600℃ instead of 350℃, and (c) the thermal efficiency if the boiler pressure is raised to 15 MPa while the turbine inlet temperature is maintained at 600℃
.
FIGURE 6-1
FIGURE 6-2
FIGURE 6-3
4. The ideal reheat Rankine cycle
Consider a steam power plant operating on the ideal reheat Rankine cycle. Steam enters the high-pressure turbine at 15 MPa and 600℃ and is condensed in the condenser at a pressure of 10 kPa. If the moisture content of the steam at the exit of the low-pressure turbine is not to exceed 10.4 percent, determine (a) the pressure at which the steam should be reheated and (b) the thermal efficiency of the cycle. Assume the steam is reheated to the inlet temperature of the high-pressure turbine.
5. The ideal regenerative Rankine cycle
Consider a steam power plant operating on the ideal regenerative Rankine cycle with one open feedwater heater. Steam enters the turbine at 15 MPa and 600℃ and is condensed in the condenser at a pressure of 10 kPa. Some steam leaves the turbine at a pressure of 1.2 MPa and enters the open feedwater heater. Determine the fraction of steam extracted from the turbine and the thermal efficiency of the cycle.
工程热力学与传热学
第六章 动力装置循环 习题
1. 试画出简单蒸汽动力装置的系统图，简单蒸汽动力装置循环的p-v 图与T-s 图。
2.
既然利用抽气回热可以提高蒸汽动力装置循环的热效率，能否将全部蒸汽抽出来用于回热？为
习 题
FIGURE 6-4
FIGURE 6-5
什么回热能提高热效率？
3.蒸汽动力装置循环热效率不高的原因是冷凝器放热损失太大，如取消冷凝器而用压缩机将乏汽直接升压送回锅炉是否可以？
4.卡诺循环优于相同温度范围的其它循环，为什么蒸汽动力循环不采用卡诺循环？
5.某朗肯循环，新蒸汽的参数为p1=4MPa，t1=400℃，乏汽的压力p2=4kPa，忽略泵功，试计算此循环的循环净功，加热量，热效率及乏汽的干度x。
若t1=550，p1和p2不变，以上各量为多少？
6.某船用理想蒸汽动力装置，汽轮机入口新蒸汽的参数为p1=6MPa，t1=560℃，冷凝器内蒸汽压力为6 kPa，忽略泵功，试求循环热效率。
若该装置的功率为10MW，试求每小时耗气量。
习题解答
1.答：简单蒸汽动力装置的系统组成锅炉，汽轮机，冷凝器，给水泵。
2.答：不能，还要保证部分蒸汽在汽轮机中继续膨胀对外作功。
回热提高了给水温度，相应地提高了吸热平均温度，从而提高了蒸汽动力装置的热效率。
3.答：不可以。
根据热力学第二定律，在热能和机械能的相互转换过程中，至少需要两个热源，才能产生动力，要想将热能转变为机械能，至少需要一个吸热过程，膨胀对外作功的过程，以及一个放热过程。
4.答：热力学第二定律证明，在相同的高温恒温热源和低温恒温热源间卡诺循环的热效率最高。
在采用气体作工质的循环中，因为定温加热和放热难于进行，而且气体的定温线和绝热线在p-v图上的斜率相差不多，以致卡诺循环所作的功并不大，故在实际上难于实现。
5.解：新蒸汽的参数为p1=4MPa，t1=400℃，乏汽的压力p2=4 kPa时，循环净功w net=1171.8 kJ/kg，加热量q=3093.2 kJ/kg，热效率ηt=37.9%，乏汽的干度x=0.79；若温度t1=550，p1和p2不变时，循环净功w net=1370.6 kJ/kg，加热量q=3437.9 kJ/kg，热效率ηt=39.9%，乏汽的干度x=0.85。
6.解：循环的热效率ηt=40.7%，每小时耗气量q m=2.59×104 kg/hr。