| The impact of sewage and thermal discharge from power plants on the aquatic ecological environment has always been an important issue of environmental protection,in which the key point is how to take effective measures to meet the environmental requirements of the mixing zone and the standard of sewage and thermal discharge.The diffuser is often used as the terminal of sewage or temperature drainage system,and its configuration determines the initial dilution of the jet,and further the range of the mixing zone of the temperature rise or concentration.With the increasing strength of water environment protection,more strictly requirements have been imposed on the sewage and thermal discharge.Therefore,it is of great practical value to carry out in-depth research on the new pr-dilution diffuser,which has broad application prospects.In this thesis,an innovative pre-dilution diffuser is proposed to achieve the pre-dilution and an annular jet differing from the conventional central straight jet,which can effectively decrease the discharge concentration(temperature)and increase the initial dilution.The hydraulic characteristics and dilution of the diffuser are systematically investigated by means of the flume experiment and three-dimensional numerical simulation.The jet flow structure,law of mixing and dilution,as well as resistance characteristics of the annular swirling jet of the diffuser with tangential inflow are thoroughly studied,based on the comparison and analysis of the initial dilution effect of the pre-dilution diffusers with radial and tangential inflow.This thesis not only provides an option of effective diffuser for wastewater and thermal drainage,but also provides a scientific basis for engineering design of wastewater treatment and environmental impact assessments,by introducing the annular jet and rotating jet theories into diffuser research.The main achievements of this paper are as follows:(1)A new type of pre-dilution diffuser is developed based on annular jet principle,and innovative design is proposed to enhance the initial dilution of the diffuser based on the"enhancement of pre-mixing" and "change of jet structure".The annular direct jet occurs for the radial inflow,while the annular swirling jet occurs for the tangential inflow.The latter is more beneficial for enhancing dilution.(2)The comparison of initial dilution effects is performed for the radial and tangential inflow pre-dilution diffusers.The amplification ratio of flow discharge,temperature and turbulent kinetic energy distribution are given for both two types of diffusers.The suction capacity and mixing dilution characteristics of the two diffusers are compared and analyzed.The results show that the tangential inflow is superior to radial inflow in regard of dilution effect.(3)The dilution and resistance characteristics are studied systematically for the tangential inflow pre-dilution diffuser in the co-current flow.The structure of an annular rotating jet is illustrated,the flow amplification ratio of diffuser is analyzed under the condition of the ambient flow,the temperature and velocity distribution at the outlet and its downstream as well as the local resistance coefficient of diffuser are obtained,some formulae are proposed to calculate the decrease of the maximum temperature rise,axial velocity and tangential velocity on the cross section with the distance,and the mechanisms of mixing dilution mechanism of annular swirling jet are discussed.(4)The hydraulic and dilution characteristics of the pre-dilution diffuser with tangential inlets are further investigated using of Reynolds Stress Model.Typical flow structure and temperature distribution of annular swirling jets are illustrated in co-current flow,and the effects of counter-current flow on its flow amplification ratio,velocity distribution,temperature rise distribution and dilution characteristics are analyzed. |
PDF Full Text Request