The Study On The Geysers Induced By The Release Of Trapped Air In A Storage Tunnel System

Underground storage tunnel system is an important municipal project to enlarge the urban storage capacity and prevent urban flooding.The trapped air pockets can easily be produced in the storage tunnel system with rapid filling water,and then the geysers phenomenon may occur in the process of the trapped air pocket releasing.The geysers can lead to the strong instantaneous pressure,and produce a geyser jet which can reach tens of meters above the ground.This will destroy the urban underground pipe network system,and have the severe impact for personal safety,social property,and public order etc.Geyser phenomenon is a complex two-phase flow of water-gas,and the mechanism is still unclear at present.It is of great scientific significance for the basic theory of water-gas two-phase flow,and have a high application value for the construction of the storage tunnel system.In this thesis,the variation of water-gas interface and instantaneous pressure were measured through experiment in the process of release of trapped air pocket.Meanwhile,the flow field,and the characteristics of the water-gas interface of geysers were analyzed by using a high-precision numerical model of two-phase flow and turbulence.Finally,based on the idea of flow control,several schemes to restrain geysers were proposed and their mechanism was analyzed.The main work and results of this thesis is as follows:Ⅰ.Experimental study on blowout hydraulic characteristicsExperimental study on hydraulic characteristics of geysers has been carried out.The geysers induced by the release of trapped air pocket were studied in detail by visual experiment,and the flow characteristics,instantaneous pressure characteristics,free surface and interface migration characteristics of geysers were analyzed.According to the hydraulic characteristics of geysers,the geysers can be divided into two types: gas-flow geyser and surge geyser.The gas-flow geyser is caused by the high-speed air flow due to the release of air pocket,the pressure of the system decreases gradually in the progress of geyser.The latter is caused by the sudden increase of the instantaneous pressure,and the geyser was propelled by the mixture of water and gas.By means of theoretical analysis and data fitting,the quantitative relationship between the geyser intensity and the main influencing factors is obtained.Genetic algorithm-support vector regression(GA-SVR)method was used to analyze the sensitivity of the influencing factors.The results show that the influence of shaft diameter,external pressure,air pocket volume and initial height of water column on the gas-flow geyser intensity can be successively reduced.Ⅱ.Numerical simulation of geysersIn order to understand the intrinsic nature of geyser,the flow field structure,pressure characteristics and interface characteristics of geysers was analyzed by using CLSVOF combined with the hybrid RANS/LES method.For gas-flow geyser,the relation between the flow field structure,gas-column velocity,liquid film thickness and pressure was analyzed in detail,the relative relation between the relative thickness of liquid film,relative axial velocity of gas-column and inverse viscosity was put forward,the relation between gas-column relative axial velocity and average water-column axial velocity,and pressure gradient of water-column was put forward.There exists a dynamic equilibrium between gas-column pressure and water-column gravity in the storage tunnel system,and the gas-column pressure is shown as periodic oscillation decreases.With the increase of shaft diameter,the difference value between gas-column pressure head and water-column height rapidly decreases,and the geyser intensity was corresponding weakening,the diameter ratio of shaft and tunnel with 0.8 can be as an indicator of the gas-flow geyser.The propagation velocity of air pocket in tunnel will cause the gas-column of geyser to be deflected or unstable,the thickness of the liquid film increases,and the relative axial velocity of the gas-column increases correspondingly.The surge geyser is the result of the combined action of inertia oscillation and the compression and expansion effect of the trapped air pocket.The extreme pressure generated by the compression and expansion effect of the air pocket is proportional to the volume of the air pocket.In the storage tunnel system,the impact on trapped air pocket decreases with the increase of shaft diameter.When the diameter ratio of shaft and tunnel is 0.67,the system pressure extreme value is basically dominated by the inertial oscillation.Ⅲ.Study on flow control of geysersBased on the flow control idea,study on flow control of geysers has been carried out.The flow control schemes of geysers such as orifice plate,storage tank and split plate are put forward to suppress the geyser intensity.The orifice plan,the storage tank,and the split plate can effectively destroy the shape of gas-column and reduce the strength of the gas-flow geyser.The storage tank can eliminate the local low pressure induced by the release of air pocket and avoid the strong instantaneous extreme pressure in the system.