Intensive Study Of Aeration And Ventilation Characteristics Of Aeration Devices And Air Supply System For Hydraulic Structures

With the increase of flood-discharge capacity of hydraulic structures,cavitation induced by high-speed water flow is very prominent.Cavitation alleviation by air entrainment,proved to be an effective engineering method,has been widely adopted in hydraulic structures.The effect of air entrainment is not only related to the arrangement of aeration devices,but also to the performance of air supply system through which the air is sucked into the spillway tunnel.Although more researches on air entrainment have been carried out,due to the complexity of air entrainment phenomenon,currently,the predictions on various hydraulic indexes of aeration devices and air supply system still depend on empirical formulas or qualitative analysis,and the forecast results are discrete.There is still a need to further deepen the study of aeration characteristics of aeration devices and ventilation characteristics of air supply system in the spillway tunnel.In present study,based on the prototype observation on air entrainment of a majority of hydraulic structures,the distribution rules of the hydraulic characteristics of aeration devices are summarized and analyzed.The emphases are put on the calculation methods and the model simulation of air demand of aeration devices,as well as the theoretical analysis of ventilation characteristics of multi-well air supply system in the tunnel.Specific content includes:(1)By summarizing prototype observation data of projects related to air entrainment both at home and abroad,the general distribution principles of typical aeration hydraulic characteristics such as cavity sub-pressure,air demand and protection length of aerator were studied.(2)Based on the prototype data of air demand of aerators in many projects,the calculation method of air demand was studied.The prototype value of the empirical coefficient in the theoretical formula was discussed and a expression was deduced to describe the relationship between the empirical coefficient and its influencing factor.Through analyzing the distribution trends of the air entrainment rate with the Froude number and the unit width flow rate respectively,two empirical formulas to calculate the air entrainment rate were established.(3)Based on the prototype and model data of air demand and the model test considered surface roughness,the model simulation of air demand was studied.By comparing the air demand ratio between prototype and model,the influence of the model scale on air demand ratio was analyzed.By using the method of adding surface roughness to increase the turbulence intensity in model test,the effect of surface roughness on the aeration-related parameters was studied,and the relationship between flow turbulence intensity and air entrainment rate was explored.(4)Through theoretical analysis,the ventilation characteristics of multi-well air supply system in the tunnel were studied.Based on the aerodynamic equilibrium analysis and the mass conservation law,the theoretical analysis method of the ventilation characteristics of single-well air supply system was expanded and generalized,and a theoretical analysis method of the ventilation characteristics of multi-well air supply system was established.Taking the Jinping-I Dam spillway tunnel as an example,the influence of structural factors of spillway tunnel and air intake well on the ventilation characteristics of air supply system was explored.