Study On Ventilation Capacity And Air Flow Field Characteristics Of Spillway Tunnel

With the increase of flow velocity,the ventilation problem of spillway tunnel has gradually attracted people’s attention.In the past,water conservancy and hydropower projects did not pay enough attention to the ventilation problem of spillway tunnel,which led to some spillway tunnels whistling and outlet blasting vibration.If there is no ventilation tunnel in the spillway tunnel or the ventilation tunnel is set unreasonably,it will bring hidden danger to the normal operation of the spillway tunnel,and even directly affect the flood discharge safety of the project when it is serious.If the sectional area of the ventilation tunnel is too large or the number of ventilation tunnels along the way is too many,it will increase the project cost.Therefore,in-depth study of the ventilation capacity and air flow field characteristics of the spillway tunnel has important practical significance for guiding the design of the ventilation tunnel.Since Kalinske began to study ventilation in 1943,although predecessors have carried out a lot of research work,the research results of different students are very different.At present,the understanding of ventilation capacity and air flow field characteristics of spillway tunnel is still limited,and more research work is still needed.Based on this,the ventilation problem of the spillway was studied by the method of prototype observation,theoretical analysis and physical model test.The main contents include: the research on the ventilation characteristics based on the prototype observation data,the theoretical calculation method of the ventilation capacity of the single and multi-way wind tunnels,the model test of the ventilation and air flow field characteristics of the single and multi-way wind tunnels.The specific conclusions include:(1)The influence of gate opening and the ratio of residual amplitude on the prototype ventilation was studied,the quantitative relationship between the maximum allowable negative pressure behind the gate and the average wind speed of the ventilation tunnel based on the resistance loss coefficient was established,and the current situation that the ventilation formula proposed by predecessors has insufficient ability to predict the prototype ventilation was revealed.According to the prototype observation data of ventilation capacity,the ventilation capacity reaches the maximum when the relative opening of the gate is around 0.7 ~ 0.8 for the pressure flow spillway tunnel with partial opening of the gate,the ventilation capacity generally reaches the maximum when the gate is fully open for the open flow spillway tunnel,and the ventilation capacity is affected by the residual width of the tunnel top through the influence of the air carrying capacity.Based on the energy loss coefficient,the quantitative relationship(curve)between the maximum allowable negative pressure behind the gate and the average wind speed in the ventilation tunnel was established,and it is proposed that the wind speed in the ventilation tunnel is the key control factor in the ventilation design.In this study,24 kinds of ventilation formulas were summarized,and 18 of them were verified by the original data of ventilation.The validation results show that no ventilation formula can well predict all the prototype ventilation within the range of data,the formula recommended in the specification has a certain prediction ability for the ventilation of small and medium spillway tunnels,but it is insufficient for the ventilation of large open flow spillway tunnels.(2)A complete theoretical calculation method of ventilation rate for air supply system of single and multi-way wind tunnels was constructed,and a semi theoretical and semi empirical formula of ventilation rate based on theoretical analysis and prototype observation was proposed.On the basis of reasonable assumptions,the air in the spillway tunnel and the ventilation tunnel was analyzed as a whole,and the aerodynamic analysis of the air in the control body of the ventilation tunnel and the spillway tunnel was carried out.The corresponding continuity equation,energy equation,momentum equation and force balance equation were established,and then the theoretical ventilation formula of single pass wind tunnel and multi pass wind tunnel was derived.The method of substituting variables instead of polynomials and presetting initial value iteration was used to solve the problem of ventilation in multichannel wind tunnel.On the basis of the theoretical formula of ventilation in one-way wind tunnel and multi-way wind tunnel,a complete set of theoretical calculation method of ventilation was constructed,and the theoretical formula of ventilation was expanded and generalized.The semi theoretical and semi empirical formula of ventilation based on the theoretical formula and prototype observation data was also proposed in this study,and the formula is verified by the original observation data of ventilation.The verification results show that the semi theoretical and semi empirical formula of ventilation has good prediction accuracy for prototype ventilation.(3)A method to reduce the effect of scale effect of ventilation in physical model test was proposed.The quantitative relationship between drag coefficient of water flow and the root mean square of normal fluctuating velocity on the surface of flow was revealed.The effects of hydraulic and structural factors on the characteristics of ventilation and gas flow field were studied.Increasing the flow velocity can effectively increase the turbulence intensity of the model flow,and improve the dissimilarity of the flow viscosity and surface tension with the prototype.Within the scope of the research data,when the flow velocity of the model is greater than or equal to 6m/s,the effect of scale effect on the model ventilation is relatively small.Based on the physical model test of roughening and turbulence increasing,the quantitative relationship between drag coefficient of water flow and root mean square of normal fluctuating velocity on the surface of flow was established,and then the formula of drag coefficient of water flow and the more perfect theoretical formula of ventilation were proposed.Drag coefficient of water flow formula proposed in this paper is in line with the general variation law of the flow drag coefficient,and the improved ventilation formula also has higher prediction accuracy for the prototype ventilation.The discharge flow in the spillway tunnel can not directly affect the ventilation capacity.The flow velocity is the most direct hydraulic factor affecting the ventilation capacity.The greater the flow velocity is,the greater the ventilation capacity is.The increase of the depth of the spillway tunnel will reduce the shear deformation of the water surface and the turbulence intensity of the flow,thus reducing the ventilation capacity.Among the structural factors,with the gradual increase of the ventilation tunnel area,the ventilation capacity increases,but the growth rate of the ventilation capacity slows down,the proportion of the tunnel top margin affects the ventilation capacity by affecting the air flow area and the air velocity of the spillway tunnel,and with the decrease of the proportion of the tunnel top margin,the ventilation capacity first increases and then decreases.The maximum negative pressure in the spillway tunnel generally appears behind the service gate.The higher the flow velocity in the spillway tunnel,the smaller the area of the ventilation tunnel and the farther the ventilation tunnel is from the gate,the greater the negative pressure behind the gate.(4)A sound theoretical formula of ventilation in multi-way wind tunnel was established,and the influence of structural factors on ventilation and air flow field characteristics of air supply system in multi-way wind tunnel was studied.On the basis of theoretical analysis and physical model test,the ventilation theoretical formula of multi-way wind tunnel air supply system was constructed.The ventilation theoretical formula of multi-way wind tunnel was verified by using the ventilation physical model test data and the original ventilation data.The verification results show that the ventilation theoretical formula of multi-way wind tunnel constructed in this study has high prediction accuracy for the ventilation of spillway tunnel.In the air supply system of multi-way wind tunnel,using larger cross-sectional area of ventilation tunnel can reduce the wind speed of ventilation tunnel,increase the ventilation volume and reduce the negative pressure along the spillway tunnel,arranging the ventilation tunnel upstream can share the ventilation pressure,reduce the wind speed of upstream wind tunnel,effectively increase the total ventilation volume and improve the negative pressure level along the spillway tunnel,the number of ventilation tunnels has an important effect on the ventilation of spillway tunnel the results show that the air supply of multi-way wind tunnel can effectively increase the ventilation capacity in the spillway tunnel,reduce the wind speed in the tunnel and significantly improve the negative pressure level in the spillway tunnel.