天然气引射器内部熵产分析

Analysis of entropy generation in natural gas ejector

应用前苏联热工研究所提出的方法设计了某工况下天然气引射器的尺寸,采用RNG k-ε涡黏模型对其进行了数值模拟,并利用实验数据进行验证。分别计算了引射器内部的黏性熵产、湍流熵产、有限温差传热熵产以及壁面熵产,其中湍流熵产约占总熵产的97%,说明湍流脉动引起的摩擦损耗是其主要能耗形式。引射器的湍流熵产与其内部钻石激波、斜激波以及流体间的剪切扩散过程密切相关。轴向上湍流熵产的分布及峰值与其混合室中的钻石激波以及扩散室中的斜激波产生的位置及强度呈正相关。径向上引射器内部的湍流熵产的分布由射流核心与引射流体间的剪切边界层逐渐向整个横截面过渡。此外,扩散室中的斜激波的出现会增大湍流熵产,使引射器工作性能变差。

The size of natural gas ejector under a certain working condition was designed by using the method proposed by the UUSR Institute of Thermal Engineering.The simulations covering 70 groups of ejector models with different structures under different working conditions were conducted adopting RNG k-εeddy viscosity model and the results were verified by experimental data,and the comparison between the exergy calculated by experiment data and the entropy generation by simulation indicated that the entropy generation analysis method was reliable.The value of entropy generation involving with viscosity,turbulent dissipation,heat transfer with finite temperature difference,and the laminar boundary layer near the wall in ejector was calculated respectively.It was found that the entropy generation due to turbulent dissipation accounts for about 97%,indicating that the friction causing by the turbulent fluctuation was the major part of energy loss.Turbulent entropy generation in ejector was closely related to its diamond shock,oblique shock and shear diffusion between primary and entrained fluid.The axial distribution and peak value of turbulent entropy generation were positively correlated with the position and intensity of the diamond shock in mixing chamber and oblique shock in diffusion chamber,while the radial distribution of turbulent entropy generation gradually transited from the shear boundary layer locating in the middle of jet core and the secondary fluid to the entire cross section.In addition,the performance of the ejector became worse with increasing turbulent entropy generation due to the oblique shock in the expansion chamber.For example,the entrainment ratio would decrease with increasing turbulent entropy generation due to the oblique shock in the expansion chamber caused by excessive expansion ratio.And the energy would be wasted due to the oblique shock in the expansion chamber in a lower compression ratio,although the entrainment ratio remained stable in this condition.