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In thermodynamics, the Joule–Thomson effect (also known as the Joule–Kelvin effect, Kelvin–Joule effect, or Joule–Thomson expansion) describes the temperature change of a gas or liquid when it is forced through a valve or porous plug while kept insulated so that no heat is exchanged with the environment.〔 〕〔 〕〔 〕 This procedure is called a ''throttling process'' or ''Joule–Thomson process''.〔 〕 At room temperature, all gases except hydrogen, helium and neon cool upon expansion by the Joule–Thomson process; these three gases experience the same effect but only at lower temperatures.〔 〕〔 〕 The effect is named after James Prescott Joule and William Thomson, 1st Baron Kelvin, who discovered it in 1852. It followed upon earlier work by Joule on Joule expansion, in which a gas undergoes free expansion in a vacuum and the temperature is unchanged, if the gas is ideal. The throttling process is commonly exploited in thermal machines such as refrigerators, air conditioners, heat pumps, and liquefiers.〔M.J. Moran and H.N. Shapiro "Fundamentals of Engineering Thermodynamics" 5th Edition (2006) John Wiley & Sons, Inc.〕 Throttling is a fundamentally irreversible process. The throttling due to the flow resistance in supply lines, heat exchangers, regenerators, and other components of (thermal) machines is a source of losses that limits the performance. The physical mechanism associated with the Joule-Thomson effect is closely related to that of a shock wave.〔Hoover, Wm G., Carol G. Hoover, and Karl P. Travis. "Shock-Wave Compression and Joule-Thomson Expansion." Physical review letters 112.14 (2014): 144504. http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.144504〕 ==Description== The ''adiabatic'' (no heat exchanged) expansion of a gas may be carried out in a number of ways. The change in temperature experienced by the gas during expansion depends not only on the initial and final pressure, but also on the manner in which the expansion is carried out. *If the expansion process is reversible, meaning that the gas is in thermodynamic equilibrium at all times, it is called an ''isentropic'' expansion. In this scenario, the gas does positive work during the expansion, and its temperature decreases. *In a free expansion, on the other hand, the gas does no work and absorbs no heat, so the internal energy is conserved. Expanded in this manner, the temperature of an ideal gas would remain constant, but the temperature of a real gas may either increase or decrease, depending on the initial temperature and pressure. *The method of expansion discussed in this article, in which a gas or liquid at pressure ''P''1 flows into a region of lower pressure ''P''2 via a valve or porous plug under steady state conditions and without change in kinetic energy, is called the Joule–Thomson process. During this process, enthalpy remains unchanged (see a proof below). A throttling process proceeds along a constant-enthalpy curve in the direction of decreasing pressure, which means that the process occurs from right to left on a temperature-pressure diagram. If the pressure starts out high enough, the temperature ''increases'' as the pressure drops, until an ''inversion temperature'' is reached and a phase transition occurs, called the ''inversion point''. After this, as the fluid continues its expansion, the temperature begins immediately to drop. If we measure this point, using a specific gas, for several constant enthalpies, and join the inversion points, a curve called the ''inversion line'' is obtained. The inversion line intersects the ''T''-axis at some temperature, called the ''maximum inversion temperature'', because inversion cannot occur above that temperature, regardless of pressure. In vapor-compression refrigeration the gas must be throttled and cooled at the same time. Hydrogen must be cooled below its inversion temperature if any cooling is to be achieved by throttling. For hydrogen this temperature is ''−68 °C.'' This poses a problem for substances whose maximum inversion temperature is well below room temperature. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Joule–Thomson effect」の詳細全文を読む スポンサード リンク
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