The concept of a heat engine is very common in thermodynamics engineering, and much of the energy used by humans passes through a heat engine. A heat engine is a thermodynamic device placed between a high temperature reservoir and a low temperature reservoir. As heat flows from one to the other, the engine converts some of the heat energy to work energy. This principle is used in steam turbines and internal combustion engines, while refrigerators reverse the direction of flow of both the heat and work energy. Rather than using heat energy from the burning of fuel, OTEC power draws on temperature differences caused by the suns warming of the ocean surface.
The only heat cycle suitable for OTEC, is the Rankine cycle, using a low-pressure turbine. Systems may be either closed-cycle or open-cycle. Closed-cycle engines use working fluids that are typically thought of as refrigerants such as ammonia or R-134a. Open-cycle engines use the water heat source as the working fluid.
考生在备考过程中应针对一个核心内容尽可能多的去了解相关的外延信息,这样才能保证在考试的时候遇见这样的新题也能够游刃有余。
旧瓶新酒型
这种新话题很具迷惑性,考生往往很难辨识,会误把其归为老话题一类。而其实这种话题恰如旧瓶装新酒,在表面一样的瓶子里,正真的内容早就是五花八门了。我们就拿蚂蚁这个雅思考试生物类的明星动物举个例子。它在雅思阅读考试文章当中出现频率相当之高。考生在备考过程中补充背景知识的时候如果只粗浅一看,文章都是讲述蚂蚁的,那么肯定是一样的内容,不必细究了。其实不然。仔细研究每次的考题就会发现,虽然都是讲述蚂蚁,但是各篇文章所阐述的角度和内容是大有不同的。有的是介绍蚂蚁如何觅食及消化的,有的是介绍蚂蚁的农耕能力的,有的是介绍蚂蚁队列的,有的讲蚂蚁对于果树的好处,当然还有是从人的角度讲述如何捕捉蚂蚁的。09年8月8日考察了有关蚂蚁性息传递,主要涉及蚂蚁如何在彼此之间进行信息传递。文章还对包括如何教育后代在内的各种蚂蚁间的交流进行了分析,并比较了有关人和蚂蚁的教育方式之间的差别。
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