通气孔进气条件下水电站尾水系统水力过渡过程研究

As the important pressure-stabilizing structures in the pipeline systems, vents are widely used in tailrace tunnels of hydropower stations and long distance water supply projects. When hydraulic transients happen, the transient process of air inlet and release from vents, as well as the air-water two phrase flows in the tunnels is very complicated. During the process, the water hammer is serious and may cause damage accidents. The description of the phenomenon is inadequate in the existing theories of fluid transient and cannot meet the requirement of relevant engineering designs. The research aims to systematically investigate the thermodynamic properties of air, the dynamic characteristics of interface between water and air and mechanism of water hammer during the transient process by using theoretical analysis, numerical simulations and laboratory experiments. A coupling mathematical model between the traditional one-dimensional (1D) hydraulic transient model and three-dimensional (3D) transient model of the vent will be established to simulate the transient process of air inlet and release from vent, as well as the air-water two phrase flows in the tunnels. The dynamic impacts of the transient process will be investigated. The relationship among the shape parameters of vents, characteristics of air inlet and release and maximum water hammer will be developed. The study is of great value for design and safe operation in relevant projects.

通气孔作为输水系统中一项重要的稳压措施，广泛应用于水电站尾水隧洞及长距离供水工程中。当系统发生过渡过程时，通气孔处会出现进气、排气及局部气液两相流的瞬态过程，整个动态过程较为复杂，且常伴随较大的瞬变压力，危及系统安全。现有瞬变流理论和研究成果对于该现象的描述不够深入，难以满足相关工程设计的需要。本项目拟采取理论分析、数值模拟和模型实验相结合的方法，深入探索通气孔进气至排气整个瞬态过程中气体的热力学特性、水气交界面的动态特征以及伴随较大瞬变压力的内在机理；建立水电站输水发电系统一维过渡过程与通气孔边界三维瞬态流动耦合的数学模型及计算方法，精细模拟水电站瞬变过程中通气孔进排气过程及局部气液两相流，研究其瞬态过程对系统的动态影响；建立通气孔体型参数与进排气特性及最大水锤压力的影响关系，并寻求规律性特征。本研究对相关工程的设计及安全运行具有重要的理论意义和实用价值。

通气孔是输水系统中一项重要的稳压措施，广泛应用于水电站尾水隧洞、长距离供水及城市排水工程中。当系统发生过渡过程时，通气孔处会出现进气、排气及局部气液两相流的瞬态过程，整个动态过程较为复杂，且常伴随较大的瞬变压力，危及系统安全。本项目通过模型试验，观测了水电站甩负荷工况下通气孔处的进排气过程；基于模型试验，建立了尾水通气孔的水力过渡过程一维数学模型，模拟了通气孔的进排气过程，揭示了排气结束伴随较大瞬变压力的机理，其来源于被空气分隔的液柱重新弥合，发生直接水锤现象。通气孔面积较大时，撞击压力随着面积的增加而减小；通气孔面积较小时，撞击压力随着面积的增加而增加。根据其特性，提出通气孔结构优化方案，即进气时孔口全部打开，排气时只有部分孔口排气，当排气面积占进气面积10%左右时，压力振荡得到较好的控制，解决了通气孔水流喷涌问题；采用FLUENT及VOF 方法，建立了通气孔边界三维瞬态流动数学模型，结合水电站输水发电系统一维过渡过程计算，精细模拟水电站瞬变过程中通气孔进排气过程及局部气液两相流。本研究对水电站尾水通气孔以及城市排水管道通气竖井的设计及安全运行具有重要的理论意义和实用价值。

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