Research On Supersaturated TDG Concentration Prediction Model Based On Bubble Dynamic

Total Dissolved Gas(TDG)is produced mainly by dam discharge,excessive photosynthesis oxygen production by plants in water and dramatic increase in water temperature,which may directly lead to the development of certain fishes and aquatic organisms suffering from Gas Bubble Disease(GBD)or even death.With the construction and operation of water conservancy projects,the TDG supersaturation in downstream water caused by dam discharge and its harm to aquatic animals and plants has become an important research focus.However,the related mitigation measures of supersaturated TDG mainly include siphon method,packing column method,activated carbon adsorption,planting water and grass,engineering measures,operation and dispatching,utilization of ecological functions,etc.,but the traditional mitigation measures can not meet the effect of reducing the generation of supersaturated TDG or accelerating the release of supersaturated TDG.Compared with previous research results,aeration significantly promotes mass transfer at the gas-liquid interface,but there are few studies on the release of supersaturated TDG by aeration.Therefore,in this paper,the generation mechanism of supersaturated TDG,the effect of aeration on the release of supersaturated TDG and its quantitative relationship and the prediction model of supersaturated TDG concentration are studied by means of laboratory mechanism test and numerical simulation.The main research contents and conclusions are as follows:(1)The gas-liquid two-phase mixture is fully mixed through a Venturi tube,and the water flow entrains gas to form a strong gas water flow.When it enters the pressurized tank,the gas pressure increases sharply,and the gas solubility significantly increases compared to normal pressure,resulting in a large amount of gas dissolution that exceeds the solubility corresponding to local atmospheric pressure,forming TDG supersaturation.The relationship between the test conditions and the generation of supersaturated TDG is established,and the relationship between the test conditions and the generation of supersaturated TDG is power function and positive correlation.The comprehensive relationship between Reynolds number and other dimensionless and the generation of supersaturated TDG is further established as follows.(2)Four aerations significantly promote the release of supersaturated TDG.Due to different aeration conditions,the average amount of time required for release of supersaturated TDG and the average variation of its mass transfer coefficient are also different.The results show that under the effect of three factors,aeration promotes the release of supersaturated TDG,while the aeration depth and aeration aperture restrain its release.However,in test of microporous aeration pipe aeration,aeration length significantly promotes the release of supersaturated TDG and solves the restraining effect of aeration aperture.With the increase of aeration,the stirring diffusion of gas bubbles is accelerated,which enhances the mass transfer at the gas-liquid interface and at the water surface.(3)In pinhole aeration test,the mass transfer coefficient ratio of supersaturated DO/TDG and its mass transfer coefficient KTDG are established as a quantitative relationship with aeration conditions.Among them,the ratio of aeration volume to mass transfer coefficient of supersaturated TDG and its mass transfer coefficient KTDG are power function and positive correlation,while the aeration depth and aeration aperture are power function and negative correlation.And a quantitative relationship the mass transfer coefficient KDO of supersaturated DO and KTDG of supersaturated TDG are established.(4)Mass transfer coefficient ratio of supersaturated TDG and its mass transfer coefficient KTDG and aeration conditions in aeration test of micro-porous aeration pipe,whirlpool aeration and micro-porous aeration distributer is the same as that established by pinhole aeration test.However,the ratio of supersaturated TDG mass transfer coefficient and its mass transfer coefficient KTDG are power function and positive correlation with the aeration length in the aeration test of microporous aeration tube.Dimensionless mathematical models such as supersaturated TDG mass transfer coefficient KTDG and Reynolds number Re are added.(5)Under different aeration conditions,the utilization rate of TDG shows common characteristics:aeration depth has the greatest effect on the utilization rate of TDG,followed by aeration,and aeration aperture/aeration length has the least effect.Among the three aeration modes,with the increase of aeration and aeration depth,the utilization ratio of TDG is as follows:microporous aeration distributor mode>whirlpool aeration mode>microporous aeration pipe mode,while with the increase of aeration depth,microporous aeration pipe mode>whirlpool aeration mode>microporous aeration distributor mode.(6)Based on the mass transfer model research of gas-liquid two-phase flow,the test conditions of micro-porous aeration disk are selected as aeration quantity Q=1.0m3/h,aeration depth H=0.8m and aeration aperture θ=260mm to study the two-dimensional supersaturated TDG concentration prediction model.When parameter is β=10.52,the results of the supersaturated TDG concentration prediction model agree well with the test conditions.The common characteristics of the model are:the concentration value on both sides of the microporous aeration disk are the highest,the concentration area on the left side is higher than those on the right side,while the concentration values of TDG are lower in the middle of the aeration disk.The hydrodynamic characteristics of the model are analyzed.The pressure of the microporous aeration disk is the smallest,but the pressure on both sides is the largest and the area is similar,contrary to the characteristics of gas pressure.The gas flow rate and turbulence intensity are symmetrically distributed.