Study On Transverse Mixing Characteristics Of Cooling Water In Wide And Shallow Waters

The numerical simulation of water environment impact is an important means of environmental impact prediction and assessment of thermal power plants.And it is also the main development trend in the future.The transverse mixing coefficient is an important parameter to characterize the mixing dilution characteristics of thermal discharge,which is a direct response to the self-purification ability of pollutants in the transverse diffusion section.To ensure the accuracy of calculation and selection of mixing parameters has a direct impact on the reliability of prediction results,and is of great significance to the protection of water ecological environment as well as planning and site selection of power plants.There is a lack of systematic research results about the lateral diffusion law of thermal discharge,and the directly using transverse mixing coefficient calculation method of general pollutant cannot accurately reflect the particularity of hydraulic and thermal properties of cooling water,which is prone to greater deviations.The research work is based on the combination of flume experiment and numerical simulation.Through the artificial control of meteorological conditions,the heat dissipationof water surface is similar.According to the common hydraulic characteristics in engineering waters and basic hydraulic characteristics and thermal characteristics of cooling water from thermal or nuclear power plant,the influence of the factors such as width,velocity,water depth and buoyancy characteristics of cooling water on transverse mixing is discussed.The research work lays a foundation for the study of the transport and diffusion characteristics of cooling water in the wide and shallow waters,and it offers technical support for the calculation and selection of mixing coefficients in the numerical calculation of thermal drainage.The research results also provide important reference for the study of transverse mixing of similar pollutants.The results of this study show that:(1)Under the control conditions(the ratio of width to depth is B/h = 55?150,and the ratio of ambient flow to displacement is Q/qe= 12?52.5),the change of waters width has little effect on transverse mixing of cooling water.The effects of the width is related to the mean velocity and the depth of water.In wide and shallow waters,the larger the velocity and the deeper the water depth is,the smaller the influence of the width will be.(2)The transverse mixing capacity of the thermal drainage increases with the average velocity and the depth of water,and the distribution pattern of surface temperature changes accordingly.The change of flow velocity and water depth has an important effect on the vertical temperature distribution structure of thermal drainage.The increase of flow velocity tends to mean the distribution of vertical temperature.And the increase of water depth enhances the vertical stratification effect,driving the thermal water to gather to the surface of the water body and the surface diffusion width increases.(3)The secondary flow caused by temperature difference induces the value of transverse mixing coefficient to increase obviously.The comparative analysis shows that the transverse mixing coefficient of cooling water is several times higher than that of the general neutral pollutants.This is consistent with previous research results.(4)There are some differences in flow field and temperature field of experiment results between conditions with water intaking and without water intaking.But the overall distribution and the transverse mixing coefficient of the corresponding working conditions is basically the same.It is considered that the local hydraulic factors do not have obvious influence on transverse mixing of thermal drainage.Finally,based on the experimental simulation results,the transverse mixing coefficient values of the corresponding working conditions are obtained by means of numerical simulation.The formula of transverse mixing coefficient obtained by linear regression analysis is as follows:Ey = 0.1837(U/u*)1.32hu*The verification results show that the formula is accurate,and the corresponding application conditions of the formula are as follows:the ratio of width to depth is B/h?>55,and the ratio of ambient flow to displacement is Q/qe?12.