火電廠自動(dòng)化電氣自動(dòng)化畢業(yè)設(shè)計(jì)外文翻譯-資料下載頁
2025-06-17 20:38本頁面
【正文】 he conversion of heat energy into mechanical energy. He saw that if this process were to be made reversible, heat could be converted into work and then extracted and reused to make a closed loop. In his concept (Figure ), a piston moves freely without encountering any friction inside a cylinder made of some perfectly insulating material. The piston is driven by a 39。working fluid39。. The cylinder has a head at one end that can be switched at will from being a perfect conductor to being a perfect insulator. Outside the cylinder are two bodies, one of which can deliver heat without its own temperature ( T1 ) falling, the other being a bottomless cold sink at a temperature (T2) which is also constant.The operation of the system is shown graphically in figure , which shows the pressure/volume relationship of the fluid in the cylinder over the whole cycle. As the process is a repeating cycle its operation can be studied from any convenient starting point, and we shall begin at the point A, where the cylinder head (at this time assumed to be a perfect conductor of heat), allows heat from the hot source to enter the cylinder. The result is that the medium begins to expand, and if it is allowed to expand freely, Boyle39。s law (which states that at any temperature the relationship between pressure and volume is constant) dictates that the temperature will not rise, but will stay at its initial temperature (Tl). This is called isothermal expansion.When the pressure and volume of the medium have reached the values at point B, the cylinder head is switched from being a perfect conductor to being a perfect insulator and the medium allowed to continue its expansion with no heat being gained or lost. This is known as adiabatic expansion. When the pressure and volume of the medium reach the values at point C, the cylinder head is switched back to being a perfect conductor, but the external heat source is removed and replaced by the heat sink. The piston is driven towards the head, pressing the medium. Heat flows through the head to the heat sink and when the temperature of the medium reaches that of the heat sink (at point D), the cylinder head is once again switched to bee a perfect insulator and the medium is pressed until it reaches its starting conditions of pressure and cycle is then plete, having taken in and rejected heat while doing external work. The Rankine cycleThe Carnot cycle postulates a cylinder with perfectly insulating walls and a head which can be switched at will from Being a conductor to being an insulator. Even with modifications to enable it to operate in a world where such things are not obtainable, it would have probably remained a scientific concept with no practical application, had not a Scottish professor of engineering, William Rankine, proposed a modification to it at the beginning of the twentieth century [I]. The concepts that Rankine developed form the basis of all thermal power plants in use today. Even todays binedcycle power plants use his cycle for one of the two phases of their operation.Figure illustrates the principle of the Rankine cycle. Starting at point A again, the source of heat is applied to expand the medium, this time at a constant pressure, to point B, after which adiabatic expansion is again made to occur until the medium reaches the conditions at point C. From here, the volume of the medium is reduced, at a constant pressure, until it reaches point D, when it is pressed back to its initial condition
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