Experimental Study On Aeration Concentration And Gas Movement In Fully Aerated Water Flow With And Without Cover

The rapid development of water conservancy and hydropower projects in our country has witnessed a large number of high dam hydrop ower stations proposed or under construction in Southwest Chia.Intense turbulent mixing of the air into the water flow of dissipated energy discharged from high dam can easily cause supersaturation of total dissolved gas in downstream water bodies and the refore damage to the water ecology and environment,as well as cavitation damage in the high-velocity flow channel.The reason lie s in insufficient analysis of the law of gas migration and diffusion in water flow.Currently,the aerification characteristic s of water flow in discharging and energy dissipation are much studied;however,research on the fully aerated flow after energy d issipation is rarely found.In addition,the effect of different hydraulic elements and boundary conditions on the characteris tics of gas migration and diffusion in water flow requires further analysis.In such a context,a test model of fully aerated flow is built herein to study the distribution and change law of air concentration and various hydraulic elements in fully aerated flow,and to analyze gas migration and diffusion and the effect of different boundary conditions thereon under different depth of water,velocity of flow,initial air concentration in flow and boundary conditions.The results indicate that:(1)Division of fully aerated flow area: According to the distribution of vertical air concentration and variation of longitudinal air concentrat ion along the channel,it can be longitudinally divided into the fully aerated area,the gas migration area,the gas diffusion-overflow area and the balanced area.(2)Distribution of depth of water and velocity of flow: The surface of fully aerated water flow declines along the channel,the rate of descent of water depth being: gas diffusion-overflow area> gas migration area> balanced area> fully aerated area.As for the distribution of flow velocity,in the fully aerated area,the bottom velocity is the fastest while the middle and upper parts show little difference in velocity;in the gas migration area,the veloci ty first increases and then decreases from bottom up,the bottom and the upper parts showing little difference while the middle part having the peak velocity;in the gas diffusion-overflow area,the velocity first increases,then peaks and decreases from bottom up,middle velocity> bottom velocity> surface velocity;in the balanced area,the velocity increases gradually from bottom up,the middle part showing li ttle difference from the upper part.(3)Distribution of air concentration and characteristics of gas migration and diffusion in fully aerated flow: They change as the depth of water,velocity of flow and initial air concentration in flow change.Increase i n water depth weakens the vertical diffusion of gas;faster flow velocity intensifies the longit udinal migration of gas;higher initial air concentr ation in flow intensifies gas diffusion in water and increases gas overflow,thus influencing the horizontal,longitudinal and vertical distribution of air concentration in fully aerated flow and the leng th of different structural areas.(4)The effect of different boundary conditions on the distribution of air concentration and characteristics of gas migration a nd diffusion in fully aerated flow:On the one hand,they change the kinetic and aeration charac teristics of the hydraulic jump area in the vertical flow structure;on the other hand,they change the movement of air and air pressure above water surface.La rger water depth,faster velocity of flow and lower initial air concentration in flow will lead to more significant differences from no cover/ semi-infinite boundary conditions in the distribution of air concentration and characteristics of gas migration a nd diffusion.(5)Main contributing factors to the distribution of air concentration and characteristics of gas migration and diffu sion in different areas of fully aerated flow:The fully aerated area is mainly subject to changes in hydraulic elements;both the gas migration area and the gas diffusion-overflow area are subject to hydraulic elements and boundary conditions;the balanced area is mainly subject to changes in boundary conditions.The research serves as reference to water conservancy and hydropow er projects for reduction of the concentration of total dissolved gas in downstream water bodies,protection of the water ecology and environment downstream,and avoidance of cavitation damage to flow channel.