问题 问答题 简答题

离心泵的轴向推力是如何产生的?

答案

参考答案:

离心泵的轴向力产生的原因有:泵在正常工作中,出口压力作用在叶轮的前后盖板上,由于单吸叶轮缺乏对称性,其前盖板小于后盖板的面积,所以承受力就小于后盖板的承受力,液体就产生了一个方向指向入口的力,这个力叫轴向力。即由于离心泵内流体压力分布不均匀,结构上不对称,或制造上的偏差,使离心泵的转子上作用着不平衡的力,这些力可分别称为轴向力和轴向分力,因此泵在设计、安装、检修时,必须考虑轴向力对轴和轴向的影响,应采取有效措施。

单项选择题
单项选择题

Electronic mail has become an extremely important and popular means of communication.
The convenience and efficiency of electronic mail are threatened by the extremely rapid growth in the volume of unsolicited commercial electronic mail. Unsolicited commercial electronic mail is currently estimated to account for over half of all electronic mail traffic, up from an estimated 7 percent in 2001, and the volume continues to rise. Most of these messages are fraudulent or deceptive in one or more respects.
The receipt of unsolicited commercial electronic mail may result in costs to recipients who cannot refuse to accept such mail and who incur costs for the storage of such mail, or for the time spent accessing, reviewing, and discarding such mail, or for both. The receipt of a large number of unwanted messages also decreases the convenience of electronic mail and creates a risk that wanted electronic mail messages, both commercial and noncommercial, will be lost, overlooked, or discarded amidst the larger volume of unwanted messages, thus reducing the reliability and usefulness of electronic mail to the recipient. Some commercial electronic mail contains material that many recipients may consider vulgar or pornographic in nature.
The growth in unsolicited commercial electronic mail imposes significant monetary costs on providers of Internet access services, businesses, and educational and nonprofit institutions that carry and receive such mail, as there is a finite volume of mail that such providers, businesses, and institutions can handle without further investment in infrastructure. Many senders of unsolicited commercial electronic mail purposefully disguise the source of such mail.
Many senders of unsolicited commercial electronic mail purposefully include misleading information in the messages’ subject lines in order to induce the recipients to view the messages. While some senders of commercial electronic mail messages provide simple and reliable ways for recipients to reject (or "opt-out" of) receipt of commercial electronic mail from such senders in the future, other senders provide no such "opt-out" mechanism, or refuse to honor the requests of recipients not to receive electronic mail from such senders in the future, or both.
Many senders of bulk unsolicited commercial electronic mail use computer programs to gather large numbers of electronic mail addresses on an automated basis from Internet websites or online services where users must post their addresses in order to make full use of the website or service.
The problems associated with the rapid growth and abuse of unsolicited commercial electronic mail cannot be solved by the government alone. The development and adoption of technological approaches and the pursuit of cooperative efforts with other countries will be necessary as well.

What does unwanted e-mail messages do to the providers of the Internet services

A.Raising their cost.

B.Raising the Internet speed.

C.Improving their business.

D.Attracting investment.