Laval喷管设计及在天然气液化中的应用研究

Design of Laval nozzle and its application in liquefaction of natural gas

提出通过超声速喷管使气体在高速流动条件下急剧膨胀而产生的低温效应液化天然气。结合双三次曲线法、BWRS真实气体状态方程、圆弧加直线方法及边界层黏性修正进行Laval喷管的设计,对喷管内甲烷气体的流动及液化过程进行研究,并分析入口温度、压力及背压对甲烷气液化过程的影响。研究结果表明:气体在喷管内流动达到超声速并导致低压低温,促使气体液化;入口温度的降低或入口压力的升高能促进气体液化,但过低温度(低于170 K)将使气体进入固相区,同样,提高压力时,由于比热比增大,当压力增大到2.5 MPa时也将使气体进入固相区,阻碍气体的液化;随着背压的升高,激波将进入喷管内,减弱或破坏气体的液化过程。利用超声速旋流分离器液化天然气时,应尽可能地回收压力能并保证激波不进入喷管和旋流分离段内。

It is proposed that natural gas is liquefied by means of the cryogenic effect produced when natural gas flows through a Laval nozzle at supersonic speed. Laval nozzle is designed based on bi-cubic curve,state equation of real gas,arc plus straight line and viscous correction of boundary layer. The flow and liquefaction process of methane gas in the Laval nozzle and the effects of inlet temperature,inlet pressure and backpressure on the liquefaction process of methane gas are studied. The results show that: the flow of methane gas in the Laval nozzle can reach to supersonic speed,and therefore methane gas can be liquefied; the reduction of inlet temperature or the rising of inlet pressure can promote the liquefaction of methane gas,but too low inlet temperature( lower than 170 K) and too high inlet pressure( higher than 2. 5 MPa) into solid-hase region so to impede the liquefaction of methane gas; with the increasing of back pressure,the shock wave occurs in the nozzle,which will weaken or destroy the liquefaction process. When supersonic cyclone is used for liquefying natural gas,the backpressure should be recycled as much as possible to ensure that the shock wave will not enter the nozzle and the cyclone separator section at the same time.