问题 问答题 简答题

如何防止发生汽蚀现象?

答案

参考答案:

为防止汽蚀现象出现,必须做到:有效汽蚀余量(NPSH)a大于必需汽蚀余量(NPSH)γ,有效吸上真空度Hsa小于泵的允许吸上真空度Hs,具体要求是:

(NPSH)a≥(Δhr)(1+α)

α=0.1~0.3

或(NPSH)a=(NPSH)r+S1

S1值对一般常压操作泵取0.6~1.0米,对真空塔底泵取2~3米。

为满足上述要求,在选泵时可选取必需汽蚀余量值较小的泵。当泵选定之后,要设法提高有效汽蚀余量,办法是合理确定吸入容器液面与泵之间的高差,使之灌注操作时有较大的灌注头,吸上操作时,吸上高度较小;合理地选用吸入管线的直径,而且尽量短而直,从而减小吸入管系的阻力。

为了防止汽蚀,在泵的结构上采用以下几种措施:

①采用双吸叶轮;

②增大叶轮入口面积;

③增大叶轮进口边宽度;

④增大叶轮前后盖板转弯处曲率半径;

⑤叶片进口边向吸入侧延伸;

⑥叶轮首级采用抗汽蚀材料;

⑦设前置诱导轮;

对于现有泵,防止汽蚀的措施有:

①通流部分断面变化率力求小,壁面力求光滑;

②吸入管阻力要小;

③正确选择吸上高度;

④汽蚀区域贴补环氧树脂涂料。

阅读理解

A kind of little cars may some day take the place of today’s cars. If everyone drives such cars in the future,there will be less pollution in the car. There will also be more space for parking cars in cities,and the streets will be less crowded. Three such cars can fit in the space now needed for one car of the usual size.

The little cars will cost much less to own and to drive. Driving will be safer,too,as these little cars can go only 65 kilometers an hour. The cars of the future will be fine for getting around a city,but they will not be useful for long trips. Little cars will go 450 kilometers before needing to stop for more gasoline.

If big cars are still used along with the small ones,two sets of roads will be needed in the future. Some roads will be used for the big,fast cars and other roads will be needed for the slower,small ones.

小题1:There is much pollution in the car today because ________.

A.people drive big cars

B.people drive little cars

C.the cars go 65 kilometers an hour

D.the cars can go 450 kilometers an hour小题2:The usual size of cars today is ________that of future cars.

A.much smaller than

B.much the same as

C.three times as large as

D.a little larger than小题3:We can learn from the passage that________.

A.big cars cost less to own and to drive

B.big cars are not useful for long trips

C.the cars of the future will be much smaller than today’s cars

D.small cars are faster than big ones小题4:The street will be less crowded because ________.

A.there will be fewer cars in the future

B.there will be fewer passengers in the street

C.driving future cars will be safe

D.future cars will be much smaller小题5:Two sets of roads may be needed in the future because ________.

A.there will be too many cars in the future

B.more and more people will get around a city

C.big cars and little cars may be used along with each other

D.it looks more beautiful to have two sets of roads

单项选择题