问题 问答题 简答题

什么是离心泵的汽蚀现象?汽蚀有何危害?

答案

参考答案:

在叶轮入口处如果压力低于工作水温下的饱和压力就会引起一部分水汽化。汽化后,汽泡进入压力较高的区域时,因受压汽泡突然凝结,于是四周的液体就向此处补充造成水力冲击,这种现象就叫汽蚀现象。

汽蚀会使叶片受到连续的局部冲击负荷,使材料表面逐渐疲劳损失,引起金属表面剥蚀,进而出现大小蜂窝状蚀洞。另外,除了冲击引起部件的损坏外,还有化学腐蚀的作用也就是液体析出氧气,发生氧化作用。汽蚀过程是不稳定的会引起水泵振动和噪音,同时由于汽蚀汽泡堵塞叶轮槽道,使流量、扬程降低,效率下降。

阅读理解

阅读理解.

     Don't you think your schoolbag is too heavy to bear? The e-schoolbag will free you from

the weight.

     It is said that e-schoolbags are going to be brought into use in Chinese middle schools

soon. An experiment with several hundred e-schoolbags will begin in seven cities: Beijing,

Shanghai, Dalian, Shenzhen, Chongqing, Taiyuan and Shijiazhuang. And the e-schoolbags

are going to cover all over China if the experiment shows to be successful. In fact, the

e-schoolbag should perhaps be called an e-notebook. It is a small hand-held computers

for school students.

     Heavy schoolbags have long been a serious problem for school students. The average

schoolbag of middle school students weighs up to 5 kilos. But the e-schoolbag will change

everything. It is much lighter than a usual schoolbag, weighing under 1 kilo.

     Also, it is no bigger than a usual book, but it can still hold all the things for study, such

as a textbook, a notebook and exercise book. They could be made into chips that are as

small as a stamp. The students can read the text page by page on the screen. They can

still take notes using a special electronic pen. If they want to know the meaning or the

pronunciation of a new word, or even e-mail their teachers, it's just a press of a button.

     In some foreign countries, it is becoming common. But it is hard to tell when people

will receive this new form of study. Some say that e-textbooks can easily be broken,

some say that it is not good to students' eyesight to look at the screen for long.But only

time will tell.

1.The e-schoolbag has many functions EXCEPT _______.

A. It can pronounce the words.

B. It can send e-mails.

C. It can be used as a dictionary.

D. It can phone somebody.

2.According to the passage, the e-schoolbags _______.

A. are not heavy as before but still large

B. are used as an experiment in Tianjin

C. are made of metal

D. are going to cover China if it shows to be successful

3.Which of the following statement is TRUE?

A. All the students in China will use this new schoolbag.

B. We can do a lot of work based on the programmers of the chips.

C. The e-notebooks are very perfect.

D. We will not make notes because the e-notebook can make all by itself.

4.Which of the following can be the general title of the passage? ______

A. An Experiment In China

B. Please Change Your Schoolbag

C. A New Invention, A New Schoolbag

D. Chips

单项选择题