冰灵蜜蜜
殷血丹霞
排放冷却是对流冷却的另一种。与再生冷却不同,用于排放冷却的冷却剂对推力室冷却吸热后不进入燃烧室参与燃烧,而是排放出去。直接排放冷却剂会降低推力室比冲,因此需要尽可能减少用于排放冷却的冷却剂流量,同时只在受热相对不严重的喷管出口段采用排放冷却。还有一种是辐射冷却,其热流由燃烧产物传给推力室,再由推力室室壁想周围空间辐射散热。辐射冷却的特点是简单、结构质量小。主要应用于大喷管的延伸段和采用耐高温材料的小推力发动机推力室。在组织推力室内冷却时,是通过在推力室内壁表面建立温度相对较低的液体或气体保护层,以减少传给推力室室壁的热流,降低壁面温度,实现冷却。内冷却主要分为头部组织的内冷却(屏蔽冷却)、膜冷却和发汗冷却三种方法。推力室采用内冷却措施后,由于需要降低保护层的温度,所以燃烧室壁面附近的混合比不同于中心区域的最佳混合比(多数情况下采用富燃料的近壁层),造成混合比沿燃烧室横截面分布不均匀,使燃烧效率有一定程度的降低。膜冷却与屏蔽冷却类似,是通过在内壁面附近建立均匀、稳定的冷却液膜或气膜保护层,对推力室内壁进行冷却,只是用于建立保护层的冷却剂不是喷注器喷入的,而是通过专门的冷却带供入。冷却带一般布置在燃烧室或喷管收敛段的一个横截面上。沿燃烧室长度方向上可以有若干条冷却带。为提高膜的稳定性,冷却剂常常经各冷却带上的缝隙或小孔流入采用发汗冷却时,推力室内壁或部分内壁由多孔材料制成,其孔径为数十微米。多孔材料通常用金属粉末烧结而成,或用金属网压制而成。此情况下,尽可能使材料中的微孔分布均匀,是单位面积上的孔数增多。液体冷却剂渗入内壁,建立起保护膜,使传给壁的热流密度下降。当用于发汗冷却的液体冷却剂流量高于某一临界值,在推力室内壁附近形成的是液膜。当冷却剂流量低于临界值流量时,内壁温度会高于当前压力下的冷却剂沸点,部分或全部冷却剂蒸发,形成气膜。除了以上热防护外,还有其他热防护方法如:烧蚀冷却、隔热冷却、热熔式冷却以及室壁的复合防护等。3 高焓气体发生器热防护方案综合上述方法结合实际情况,便得到高焓气体发生器的热防护方法。高焓气体发生器的燃烧室与液体火箭发动机的不同,省去前面的推力室部分,使得其结构更简单而有效。那么,所涉及到的热防护即为对燃烧室室壁的热防护部分。由于燃料进入燃烧室内迅速分解并放出大量
海飄愿瓶
一、引出开头1:It is well-known to us that……(我们都知道……)==As far as my knowledge is concerned, …( 就我所知…)2:Recently the problem of…… has been brought into focus. ==Nowadays there is a growing concern over ……(最近……问题引起了关注)3:Nowadays(overpopulation)has become a problem we have to face.(现今,人口过剩已成为我们不得不面对的问题)4:Internet has been playing an increasingly important role in our day-to-day life. It has brought a lot of benefits but has created some serious problems as well.(互联网已在我们的生活扮演着越来越重要的角色,它给我们带来了许多好处但也产生了一些严重的问题)5:With the rapid development of science and technology,more and more people believe that……(随着科技的迅速发展,越来越多的人认为……)6:It is a common belief that……==It is commonly believed that……(人们一般认为……)7:A lot of people seem to think that……(很多人似乎认为……)8:It is universally acknowledged that + 句子(全世界都知道...)二、表达不同观点1:People's views on……vary from person to person. Some hold that……However, others believe that……(人们对……的观点因人而异,有些人认为……然而其他人却认为……)2:People may have different opinions on……(人们对……可能会持有不同见解)3:Attitudes towards (drugs)vary from person to person.==Different people hold different attitudes towards(failure)(人们对待吸毒的态度因人而异)4:There are different opinions among people as to……(对于……人们的观点大不相同)三、表示结尾1:In short, it can be said that……(总之,他的意思是……)2:From what has been mentioned above, we can come to the conclusion that……(从上面提到的,我们可以得出结论……)3:Taking all these factors into consideration, we naturally/reasonably come to the conclusion that……(把所有的这些因素加以考虑,我们自然可以得出结论……)4:Hence/Therefore, we'd better come to the conclusion that……(因此,我们最好的出这样的结论……)5:There is no doubt that (job-hopping)has its drawbacks as well as merits.(毫无疑问,跳槽有优点也有缺点)6:All in all, we cannot live without……,but at the same time we must try to find out new ways to cope with the problems that would arise.(总之,我们没有……无法生活,但同时我们必须寻求新的解决办法来面对可能出现的新问题)四、提出建议1:It is high time that we put an end to the (trend).(该是我们停止这一趋势的时候了)2:There is no doubt that enough concern must be paid to the problem of……(毫无疑问,对……问题应予以足够重视)3:Obviously ,if we want to do something … it is essential that……(显然,如果我们想要做么事,很重要的是……)4:Only in this way can we ……(只有这样,我们才能……)5:Spare no effort to + V (不遗余力的)五、预示后果1:Obviously,if we don't control the problem, the chances are that……will lead us in danger.(很明显,如果我们不能控制这一问题,很有可能我们会陷入危险)2:No doubt, unless we take effective measures, it is very likely that ……(毫无疑问,除非我们采取有效措施,否则我们很可能会……)3:It is urgent that immediate measures should be taken to stop the situation(很紧迫的是应立即采取措施阻止这一事态的发展)
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