Method for calculating volume of entrainment air of urban drainage shaft

Abstract

The invention discloses a method for calculating the volume of entrainment air of an urban drainage shaft, and relates to the technical field of urban drainage. The method comprises the following steps: step a, establishing a model of the volume of entrainment air of the urban drainage shaft: constructing a model comprising a water inlet pipe, an air inlet pipe, a falling shaft, a water falling pool and a water outlet pipe, sequentially connecting the water inlet pipe, the falling shaft, the water falling pool and the water outlet pipe, connecting the air inlet pipe to the top of the falling shaft, and communicating the air inlet pipe with the outside atmosphere; according to the method, on one hand, the physical model construction links are perfected, parameters of all parts are finely considered, water flow and air flow in an actual drainage shaft can be accurately simulated, a foundation is laid for follow-up calculation, on the other hand, water flow adjustment is accurate, the water flow form is accurately controlled by means of a professional device and an observation means, and calculation errors are reduced; and an entrainment air amount calculation method fitting physical characteristics is adopted for water column and water drop forms, so that the credibility is improved.

Description

Technical Field

[0001] The invention relates to the technical field of urban drainage, and in particular to a method for calculating the volume of air sucked in a vertical shaft of urban drainage. Background Art

[0002] The vertical shaft is a facility in the urban drainage system that guides the surface runoff to fall, and the maximum drop height can reach hundreds of meters. The jet in the vertical shaft has the characteristics of large flow and high drop. After falling for a long distance, it obtains a higher flow rate and greater energy. Due to the large jet flow, the flow pattern is complex during the jet drop, and the broken water flow carries a large amount of air to the downstream pipeline. If the air is not discharged in time, it will accumulate into a high-pressure air bag, and accidents such as "gas explosion" and "blowout" will occur when released. After the large-flow jet enters the vertical shaft, it will reduce the cross-sectional area of ​​the vertical shaft, and the broken water flow will carry air into the downstream, resulting in a lower negative pressure area in the vertical shaft. The shaft wall is very susceptible to erosion damage under the alternating action of air negative pressure and impact positive pressure. The flow pattern of large-flow jets in the vertical shaft is relatively complex. At present, the impact mechanism of the jet on the bottom of the vertical shaft and the air entrainment mechanism are still unclear, and it is impossible to evaluate the impact pressure of the vertical shaft and the impact of the entrained gas on the downstream pipeline, which brings difficulties to the design of deep tunnel drainage systems.

[0003] The present invention aims to provide a method for calculating the volume of air entrained in a city drainage shaft, effectively obtaining the amount of air entrained in the shaft, providing technical support for evaluating the impact of shaft entrained gas on downstream pipelines, and ensuring the safe and stable operation of the drainage system.

[0004] To this end, we provide a method for calculating the volume of air entrained in urban drainage shafts to solve the above problems. Summary of the invention

[0005] The purpose of the present invention is to provide a method for calculating the volume of air entrained in urban drainage shafts. Through the construction of precise physical models and the coordination of sophisticated water flow regulation means, the problems in the prior art such as rough models, inability to accurately simulate actual working conditions, and difficulty in accurately controlling water flow forms, which in turn lead to large errors in the calculation of the volume of air entrained, are solved.

[0006] To solve the above technical problems, the present invention is achieved through the following technical solutions:

[0007] The present invention is a method for calculating the volume of air sucked in a city drainage shaft, comprising the following steps: step a: establishing a model of the volume of air sucked in a city drainage shaft: constructing a model including a water inlet pipe, an air inlet pipe, a drop shaft, a drop pool and a water outlet pipe, connecting the water inlet pipe, the drop shaft, the drop pool and the water outlet pipe in sequence, connecting the air inlet pipe at the top of the drop shaft, and allowing the air inlet pipe to communicate with the outside atmosphere; step b: forming and adjusting a free-falling water flow: allowing the total inflow in the water inlet pipe to impact the wall of the drop shaft to form a wall-attached annular flow and a catapulted free-falling water flow, installing a flow regulating device on the water inlet pipe, and accurately adjusting the flow rate of water entering the shaft by changing the valve opening, etc., and adjusting the effective drop height of the shaft by setting adjustable water baffles at different heights of the shaft or changing the depth of the drop pool at the bottom of the shaft, and adjusting the flow rate and drop The falling height makes the free-falling water column form a falling water column and a falling water drop respectively; Step c: Calculate the entrained air volume when descending in different forms: Calculate the entrained air volume when descending in the form of a water column, and calculate the entrained air volume when descending in the form of a water drop; Step d: Consider the influence of other factors and correct the calculation results: Air resistance influence: According to the principle of aerodynamics, calculate the air resistance to water flow, and then analyze its influence on the water flow velocity and shape, and correct the calculation results of the entrained air volume, temperature and air pressure changes: Measure the temperature and air pressure at different positions in the shaft, and analyze their influence on air density and viscosity, shaft wall roughness: Consider the influence of shaft wall roughness on water flow and air flow; Step e: Build an actual urban drainage shaft model or conduct experiments in an actual drainage shaft to measure the actual entrained air volume. Compare the experimental measurement value with the calculated value and analyze the cause of the error.

[0008] The present invention is further configured such that in step a, parameters of each component in the model are set, the diameter and wall roughness of the water inlet pipe are measured and recorded; the height, diameter, inner wall material and smoothness of the drop shaft are determined; the depth, shape and size of the drop pool are clarified; and the diameter and slope parameters of the water outlet pipe are recorded.

[0009] The present invention is further configured that in step b, the water flow form is observed with the aid of equipment such as a high-speed camera to determine that the desired falling form has been achieved.

[0010] The present invention is further configured such that, in step e, the parameters and calculation methods in the model are adjusted and optimized according to the results of the error analysis to improve the accuracy and reliability of the model.

[0011] The present invention is further configured that, in step c, for a shaft of medium or low height, the free-falling water flow descends in the form of a water column, and the calculation method of the entrained air is as follows: (i) solving the average air velocity value inside the shaft based on equation (a1):

[0012]

[0013] Where Vw is the velocity of the annular flow; Vw2 is the velocity of the free-falling water column; Ds is the shaft diameter; Hs is the drop height; V0 is the initial velocity of the horizontal inflow; Q0 is the inflow rate; Da is the diameter of the middle cavity; Va is the gas velocity; dm is the diameter of the water column; dp1 / dz is the air pressure gradient; ps is the minimum gas pressure inside the shaft, located below the jet; Cd is the drag coefficient; ρa is the gas density; f is the friction coefficient; g is the acceleration due to gravity; t is time;

[0014] The velocity of the annular flow and free-falling flow in the drainage shaft can be determined by equations (1) and (2):

[0015]

[0016] After obtaining the annular flow and free-falling water column velocities, the air pressure gradients of the two water flow entrainment are calculated based on equation (3):

[0017]

[0018] Based on equations (4) and (5), the minimum gas pressure inside the shaft is determined as:

[0019]

[0020] (ii) After the average air velocity is obtained using equation (a1), combined with the drop height Hs of the shaft, the entrainment air velocity of the shaft is calculated based on equation (a2):

[0021] Q a =V a ×H s (a2).

[0022] The present invention is further configured that, in step c, for a deep vertical shaft with a large drop height, the free-falling water flow descends in the form of water droplets, and the calculation method of the entrained air is as follows: (i) solving the average air velocity value inside the vertical shaft based on equation (b1):

[0023]

[0024] Among them, Qw is the real-time inflow flow rate; Vw3 is the speed of the free-falling water drop.

[0025] After obtaining the annular flow and free-fall water flow velocities, the air pressure gradients of the two water flow entrainment are calculated based on equation (6):

[0026]

[0027] Among them, Qw is the real-time inflow flow rate; Vw3 is the speed of the free-falling water drop.

[0028] Based on equations (5) and (7), the formula for determining the minimum gas pressure inside the shaft is:

[0029]

[0030] (ii) After the average air velocity is obtained using equation (b1), combined with the drop height Hs of the shaft, the entrainment air velocity of the shaft is calculated based on equation (b2):

[0031] Q a =V a ×H s (b2).

[0032] The present invention is further configured that in step d, the formula Calculate the air resistance, where ρ 空 is the air density, C d is the resistance coefficient, A is the windward area where the water flow contacts the air, υ is the water flow velocity, and the calculation of the amount of entrained air is adjusted based on the influence of resistance on the water flow motion equation.

[0033] The present invention is further configured such that, in the step d, the entrained air volume is corrected according to the ideal gas state equation pV=nRT and the relationship between air viscosity and temperature.

[0034] The present invention is further configured that in step d, a correction coefficient related to the wall roughness is determined by experiment or empirical formula, and the calculation result of the entrained air volume is multiplied by the correction coefficient ξ, that is, the final entrained air volume V=V 气 ×ξ, where V 气 is the amount of entrained air calculated without considering the wall roughness.

[0035] The present invention has the following beneficial effects:

[0036] On the one hand, the present invention has a perfect physical model construction process, carefully considers the parameters of each component, and can accurately simulate the water flow and air flow in the actual drainage shaft, laying a solid foundation for subsequent calculations. On the other hand, the water flow regulation is precise, and with the help of professional equipment and observation methods, the water flow form is accurately controlled to reduce calculation errors. Furthermore, the entrained air volume calculation method that fits the physical characteristics is adopted for the water column and water droplet forms to improve credibility. At the same time, it comprehensively covers and corrects influencing factors such as air resistance, temperature and pressure changes, and wall roughness to enhance the applicability of the method. Finally, experimental verification and optimization links are added to form a scientific closed loop, continuously improve accuracy, and make it more in line with the needs of engineering practice. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for describing the embodiments are briefly introduced below.

[0038] Figure 1 A schematic diagram of water-gas interaction inside a drop shaft for a method of calculating the volume of air entrained in a city drainage shaft;

[0039] Figure 2 It is a schematic diagram of water-gas interaction when the falling water flow is a water column in a method for calculating the volume of air entrained in a city drainage shaft;

[0040] Figure 3 A schematic diagram of water-gas interaction when the falling water flow is water droplets in a method for calculating the volume of air entrained in a city drainage shaft;

[0041] Figure 4 is the air velocity inside the shaft at different water flow rates in Example 1 of a method for calculating the volume of air entrained in a city drainage shaft;

[0042] Figure 5 The invention is a method for calculating the volume of air entrained in a city drainage shaft, and shows the rate at which air is entrained in the shaft under different water flow rates in Example 1.

[0043] In the attached figure: 1. water inlet pipe; 2. air inlet pipe; 3. drop shaft; 4. drop pool; 5. outlet pipe; 6. total inflow; 7. annular flow; 8. free falling water flow; 9. falling water column; 10. falling water drops. DETAILED DESCRIPTION

[0044] The technical solutions in the embodiments of the present invention will be described below in conjunction with the drawings in the embodiments of the present invention. The described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0045] Example 1

[0046] Please refer to Figure 1-5, a method for calculating the volume of air entrained in a city drainage shaft, comprising the following steps: step a: establishing a model of the volume of air entrained in a city drainage shaft: constructing a model including an inlet pipe 1, an air inlet pipe 2, a drop shaft 3, a drop pool 4 and an outlet pipe 5, connecting the inlet pipe 1, the drop shaft 3, the drop pool 4 and the outlet pipe 5 in sequence, connecting the inlet pipe 2 at the top of the drop shaft 3, allowing the inlet pipe 2 to communicate with the outside atmosphere, setting parameters for each component in the model, measuring and recording the pipe diameter and wall roughness of the inlet pipe 1; determining the height, diameter, inner wall material and smoothness of the drop shaft 3; clarifying the depth, shape and size of the drop pool 4; recording the pipe diameter and slope parameters of the outlet pipe 5; step b: forming and adjusting a free falling water flow 8: allowing the total inflow 6 in the inlet pipe 1 to impact the wall of the drop shaft 3 to form a wall-attached annular flow 7 and a free ejection flow 8. The falling water flow 8 is provided with a flow regulating device on the water inlet pipe 1. The water flow rate entering the shaft is precisely adjusted by changing the valve opening, etc. The effective drop height of the shaft is adjusted by setting an adjustable water baffle at different heights of the shaft or changing the depth of the drop pool 4 at the bottom of the shaft. By adjusting the flow rate and the drop height, the water column of the free falling water flow 8 forms a falling water column 9 and a falling water drop 10 respectively. The water flow form is observed with the help of a high-speed camera and other equipment to determine whether the required drop form has been achieved. Step c: Calculate the amount of air entrained during descent in different forms: Calculate the amount of air entrained during descent in the form of a water column, and calculate the amount of air entrained during descent in the form of water droplets. For shafts of medium and low heights, the free falling water flow 8 descends in the form of a water column. The calculation method of the entrained air is as follows: (i) Solve the average air velocity value inside the shaft based on equation (a1):

[0047]

[0048] Wherein, Vw is the velocity of the annular flow 7; Vw2 is the velocity of the free-falling water column 9; Ds is the shaft diameter; Hs is the drop height; V0 is the initial velocity of the horizontal inflow; Q0 is the inflow rate; Da is the diameter of the middle cavity; Va is the gas velocity; dm is the water column diameter; dp1 / dz is the air pressure gradient; ps is the minimum gas pressure inside the shaft, located below the jet; Cd is the drag coefficient; ρa is the gas density; f is the friction coefficient; g is the gravitational acceleration; t is time;

[0049] The flow rates of the annular flow 7 and the free-falling water flow 8 in the drainage shaft can be determined by equations (1) and (2):

[0050]

[0051] After obtaining the velocities of the annular flow 7 and the free-falling water column 9, the air pressure gradients of the two water flows are calculated based on equation (3):

[0052]

[0053] Based on equations (4) and (5), the minimum gas pressure inside the shaft is determined as:

[0054]

[0055] (ii) After the average air velocity is obtained using equation (a1), combined with the drop height Hs of the shaft, the entrainment air velocity of the shaft is calculated based on equation (a2):

[0056] Q a =V a ×H s (a2),

[0057] For deep shafts with large drop heights, the free-falling water flow 8 falls in the form of water droplets. The calculation method for entrained air is as follows: (i) Solve the average air velocity value inside the shaft based on equation (b1):

[0058]

[0059] Wherein, Qw is the real-time inflow flow rate; Vw3 is the speed of the freely falling water drop 10.

[0060] After obtaining the velocities of the annular flow 7 and the free-falling water flow 8, the air pressure gradients of the two water flows are calculated based on equation (6):

[0061]

[0062] Wherein, Qw is the real-time inflow flow rate; Vw3 is the speed of the freely falling water drop 10.

[0063] Based on equations (5) and (7), the formula for determining the minimum gas pressure inside the shaft is:

[0064]

[0065] (ii) After the average air velocity is obtained using equation (b1), combined with the drop height Hs of the shaft, the entrainment air velocity of the shaft is calculated based on equation (b2):

[0066] Q a =V a ×H s (b2);

[0067] Step d: Consider the influence of other factors and correct the calculation results: Influence of air resistance: According to the principle of aerodynamics, calculate the resistance of air to water flow, and then analyze its influence on the speed and shape of water flow, and correct the calculation results of the entrained air volume. Calculate the air resistance, where ρ 空 is the air density, Cd is the resistance coefficient, A is the windward area of ​​the water flow in contact with the air, ν is the water flow velocity, and then the calculation of the amount of air entrained is adjusted according to the influence of the resistance on the water flow motion equation. Temperature and air pressure changes: measure the temperature and air pressure at different positions in the shaft, analyze their influence on the air density and viscosity, and correct the amount of air entrained according to the ideal gas state equation pV=nRT and the relationship between air viscosity and temperature. Shaft wall roughness: consider the influence of the shaft wall roughness on the flow of water and air, determine a correction coefficient related to the wall roughness through experiments or empirical formulas, and multiply the calculation result of the amount of air entrained by the correction coefficient ξ, that is, the final amount of air entrained V=V 气 ×ξ, where V 气 The entrained air volume calculated without considering the wall roughness; Step e: Build an actual urban drainage shaft model or conduct experiments in the actual drainage shaft to measure the actual entrained air volume. Compare the experimental measurement value with the calculated value, analyze the cause of the error, and adjust and optimize the parameters and calculation methods in the model according to the results of the error analysis to improve the accuracy and reliability of the model. On the one hand, the perfect physical model construction link, carefully consider the parameters of each component, can accurately simulate the water flow and air flow in the actual drainage shaft, and lay a solid foundation for subsequent calculations. On the other hand, the water flow regulation is precise, with the help of professional devices and observation methods, the water flow form is accurately controlled to reduce the calculation error. Furthermore, the entrained air volume calculation method that fits the physical characteristics is used for the water column and water droplet forms to improve the credibility. At the same time, the influencing factors such as air resistance, temperature and pressure changes, and wall roughness are fully covered and corrected to enhance the applicability of the method. Finally, experimental verification and optimization links are added to form a scientific closed loop, continuously improve the accuracy, and make it more in line with the needs of engineering practice.

[0068] Example 2

[0069] Please refer to Figure 4 and Figure 5 , Figure 4 A comparison of the measured and calculated air velocities in the shaft at different drop heights is given. Under low flow conditions, the calculated air velocity is larger than the measured value. This is because under low flow conditions, the wall-attached annular flow 7 does not completely cover the inner wall of the shaft, and the interface between air and water is very small, so the measured air velocity is slightly smaller. When the water inflow rate is greater than 35L / s, the measured air velocity is between the calculated air velocities of the two free-falling water flows 8. Figure 5A comparison of the measured and calculated air flow rates in shafts with different drop heights is given. In a shaft with a drop height of 3.38 m, the calculated air demand is in good agreement with the experimental results, taking into account the ejected free-falling water stream 8 falling in the form of a water column. When the drop height of the shaft is 7.72 m, the calculated air flow rates for both ejected free-falling water streams 8 are higher than the measured values, with a difference of about 10%-20%. Figure 4 and Figure 5 It shows that the equation has good accuracy and can be used to predict the entrained air velocity and flow rate in the shaft.

[0070] Example 3

[0071] A typical shaft structure is used to illustrate how to use equations to estimate air flow and pressure gradient. For a drop shaft 3 with a diameter of 1.2m and a drop height of 5.0m, the inflow flow and velocity are 1.5m 3 / s and 5.0m / s. When the ejected free-falling water flow 8 of the jet is α=0.1, the speeds of the annular flow 7 and the ejected free-falling water flow 8 when they reach the plunge pool 4 are 8.0m / s and 9.9m / s respectively. According to the equations of the present invention, it can be calculated that the air speed in the shaft is 11.0m / s. When the diameters of the shaft air inlet pipe 2 are 0.2m, 0.3m and 0.5m respectively, the air speeds in the drop shaft 3 are 0.45m / s, 0.95m / s and 2.18m / s respectively, and the air flow rates of the drop shaft 3 are 0.49m / s and 0.99m / s respectively. 3 / s、1.04m 3 / s and 2.38m 3 / s. Designers can evaluate the entrained air volume of the shaft based on the equations proposed in the present invention to assess whether structural safety protection measures need to be taken.

[0072] Example 4

[0073] Assume that in a newly built underground drainage system of a large commercial area in a city, there is a key drainage shaft that requires accurate calculation of the entrained air volume to optimize the performance of the drainage system.

[0074] Model building

[0075] Water inlet pipe 1: The pipe diameter is measured to be 0.5 meters, the pipe wall roughness is tested to be 0.002 meters, the material is cast iron pipe, the water inflow velocity is initially measured to be 2m / s, and the flow rate is 0.3925m3 / s.

[0076] Drop shaft 3: The height is designed to be 10 meters, the diameter is 1.5 meters, and the inner wall is coated with a smooth concrete coating. The smoothness reduces the friction of the water flow and facilitates the observation of the water flow pattern.

[0077] Plunge pool 4: 2 meters in depth, circular in shape, with a radius of 2 meters, and a certain slope (0.05) at the bottom leading to the outlet pipe 5 to facilitate rapid discharge of water.

[0078] Outlet pipe 5: pipe diameter 0.6m, slope 0.03, to ensure smooth drainage. An air intake pipe 2 with a diameter of 0.2m is connected to the top of the drop shaft 3 to make it well connected with the outside atmosphere.

[0079] Water flow regulation

[0080] Install an electric flow regulating valve on the water inlet pipe 1, and adjust the valve opening to 60% according to the drainage requirements to stabilize the water flow entering the shaft at 0.2355m3 / s.

[0081] Adjustable stainless steel water baffles were set at the heights of 4 meters and 7 meters in the shaft, and the angle of the water baffles was initially adjusted to 30 degrees. At the same time, the depth of the plunge pool 4 was appropriately lowered by 0.2 meters, so that the free-falling water flow 8 successfully formed two forms: a falling water column 9 and a falling water drop 10. The high-speed camera observed that the water flow morphology was stable and met the requirements of subsequent calculations. The average particle size of the water droplets was about 0.005 meters, and the diameter of the water column was about 0.2 meters.

[0082] Calculation of entrained air volume

[0083] For this low to medium height (10 m) shaft, the free-falling water flow 8 descends in the form of a water column:

[0084] Given that the velocity of the annular flow 7 is Vw = 1.2 m / s, the velocity of the free-falling water column 9 is Vw2 = 3 m / s, Ds = 1.5 m, Hs = 10 m, V0 = 2 m / s, Q0 = 0.2355 m3 / s, and Da = 0.8 m, the velocity of the annular flow 7 and the free-falling water flow 8 can be further accurately calculated by equations (1) and (2), and the air pressure gradient can be obtained by substituting them into equation (3), and then the minimum gas pressure ps inside the shaft can be determined by combining equations (4) and (5). Finally, equation (a1) is used to solve the average air velocity value Va = 0.5 m / s inside the shaft, and the entrainment air velocity of the shaft is calculated to be 0.025 m3 / s based on equation (a2).

[0085] For another hypothetical deep shaft with a larger drop height (20 m), the free-falling water flow 8 descends in the form of water drops:

[0086] The real-time inflow rate Qw = 0.3m3 / s, the speed of the free-falling water drop 10 Vw3 = 4m / s, and the corresponding equations (b1), (6), (5) and (7) are substituted to obtain the average air velocity Va = 0.6m / s. Combined with equation (b2), the entrainment air velocity of the shaft is calculated to be 0.036m3 / s.

[0087] Results correction

[0088] Air resistance: According to measurements, the local air density ρa = 1.2 kg / m 3 , the resistance coefficient Cd = 0.4, the windward area of ​​the water flow in contact with the air is about 0.3m2, the water flow velocity is taken as an average of 2.5m / s, and the air resistance F = 1.5N is calculated by the formula F = 0.5 × ρa × Cd × A × v2. According to the influence of resistance on the water flow motion equation, the water flow velocity and other parameters are readjusted, and the calculation results of the entrained air volume are corrected. For example, the entrained air velocity of the medium and low height shaft is corrected to 0.023m3 / s.

[0089] Temperature and air pressure changes: The temperature at different locations in the shaft fluctuates between 20-25°C, and the air pressure is close to the standard atmospheric pressure of 101.325 kPa. According to the ideal gas state equation and the relationship between air viscosity and temperature, the entrained air volume is further corrected. For example, the corrected entrained air rate in the deep shaft is 0.034 m3 / s.

[0090] Shaft wall roughness: The correction coefficient ξ=0.95 related to the current wall roughness was determined experimentally, and the entrained air volume calculated without considering the wall roughness was corrected. Finally, the actual entrained air rate for medium and low height shafts was 0.02185m3 / s, and that for deep shafts was 0.0323m3 / s.

[0091] Experimental verification and optimization

[0092] The experiment was conducted in the actual drainage shaft of the commercial area, and the actual entrained air volume was measured using a high-precision air flow meter. After multiple measurements, the average actual entrained air volume for the medium and low height shafts was 0.022m3 / s, and for the deep shafts was 0.033m3 / s. The experimental measurement values ​​were compared with the calculated values, and the errors were 0.66% and 2.1% respectively.

[0093] The cause of the error was analyzed and it was found that it was mainly due to the presence of some tiny water turbulence in the actual environment that was not accurately reflected in the model. According to the results of the error analysis, the turbulence coefficient and other parameters in the model were adjusted, the correlation coefficient in the equation was optimized, and the entrained air volume was recalculated, so that the accuracy and reliability of the model were further improved, meeting the complex drainage needs of the commercial area and ensuring the efficient and stable operation of the drainage system.

[0094] The preferred embodiments of the present invention disclosed above are only used to help explain the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific implementation methods described. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that technicians in the relevant technical field can better understand and utilize the present invention.

Claims

1. A method for calculating the volume of air entrained in a city drainage shaft, characterized in that: The following steps are involved: Step a: Establish a model of the volume of air entrained in a city drainage shaft: construct a model including an inlet pipe (1), an air inlet pipe (2), a drop shaft (3), a drop pool (4) and an outlet pipe (5), so that the water column of the inlet water forms a drop water column (9) and a drop water drop (10) respectively; the pipe, the drop shaft (3), the drop pool (4) and the outlet pipe (5) are connected in sequence, and the air inlet pipe (2) is connected to the top of the drop shaft (3), so that the air inlet pipe (2) is connected to the outside atmosphere; Step b: forming and regulating a free-falling water flow (8): allowing the total inflow (6) in the water inlet pipe (1) to impact the wall of the drop shaft (3) to form a wall-attached annular flow (7) and a free-falling water flow (8) that is ejected; installing a flow regulating device on the water inlet pipe (1); accurately regulating the flow rate of water entering the shaft by changing the valve opening, etc.; adjusting the effective drop height of the shaft by setting an adjustable water baffle at different heights of the shaft or changing the depth of a drop pool (4) at the bottom of the shaft; and adjusting the flow rate and drop height so that the water column of the free-falling water flow (8) forms a drop water column (9) and a drop water drop (10) respectively; Step c: Calculate the entrained air volume when descending in different forms: calculate the entrained air volume when descending in the form of a water column, and calculate the entrained air volume when descending in the form of water droplets; Step d: Consider the influence of other factors and correct the calculation results: Influence of air resistance: According to the principle of aerodynamics, calculate the resistance of air to water flow, and then analyze its influence on the water flow velocity and shape, and correct the calculation results of the entrained air volume. Changes in temperature and air pressure: Measure the temperature and air pressure at different positions in the shaft, and analyze their influence on air density and viscosity. Roughness of the shaft wall: Consider the influence of the roughness of the shaft wall on the flow of water and air; Step e: Build an actual urban drainage shaft model or conduct experiments in an actual drainage shaft to measure the actual amount of air entrained. Compare the experimental measurement value with the calculated value to analyze the cause of the error.

2. A method for calculating the volume of air entrained in a city drainage shaft according to claim 1, characterized in that: In the step a, parameters of each component in the model are set, the diameter and wall roughness of the water inlet pipe (1) are measured and recorded; the height, diameter, inner wall material and smoothness of the drop shaft (3) are determined; the depth, shape and size of the drop pool (4) are determined; and the diameter and slope parameters of the water outlet pipe (5) are recorded.

3. A method for calculating the volume of air entrained in a vertical drainage shaft of a city according to claim 1, characterized in that: In the step b, the water flow pattern is observed with the aid of a high-speed camera or other equipment to determine that the desired drop pattern has been achieved.

4. A method for calculating the volume of air entrained in a vertical drainage shaft of a city according to claim 1, characterized in that: In step e, the parameters and calculation methods in the model are adjusted and optimized according to the results of the error analysis to improve the accuracy and reliability of the model.

5. A method for calculating the volume of air entrained in a vertical urban drainage shaft according to claim 1, characterized in that: In step c, for a shaft of medium or low height, the free-falling water flow (8) descends in the form of a water column, and the calculation method of the entrained air is as follows: (i) Solve the average air velocity value inside the shaft based on equation (a1): where Vw is the velocity of the annular flow (7); Vw2 is the velocity of the free-falling water column (9); Ds is the shaft diameter; Hs is the drop height; V0 is the initial velocity of the horizontal inflow; Q0 is the inflow rate; Da is the diameter of the middle cavity; Va is the gas velocity; dm is the diameter of the water column; dp1 / dz is the air pressure gradient; ps is the minimum gas pressure inside the shaft, located below the jet; Cd is the drag coefficient; ρa is the gas density; f is the friction coefficient; g is the gravitational acceleration; t is time; The flow rates of the annular flow (7) and the free-falling water flow (8) in the drainage shaft can be determined by equations (1) and (2): After obtaining the velocities of the annular flow (7) and the free-falling water column (9), the air pressure gradients of the two water flows are calculated based on equation (3): Based on equations (4) and (5), the minimum gas pressure inside the shaft is determined as: (ii) After the average air velocity is obtained using equation (a1), combined with the drop height Hs of the shaft, the entrainment air velocity of the shaft is calculated based on equation (a2): Q a =V a ×H s (a2)。 6. A method for calculating the volume of air entrained in a vertical urban drainage shaft according to claim 5, characterized in that: In step c, for a deep shaft with a large drop height, the free-falling water flow (8) descends in the form of water droplets, and the calculation method of the entrained air is as follows: (i) Solve the average air velocity value inside the shaft based on equation (b1): Where Qw is the real-time inflow flow rate; Vw3 is the velocity of the free-falling water droplet (10). After obtaining the velocities of the annular flow (7) and the free-falling water flow (8), the air pressure gradients of the two water flows are calculated based on equation (6): Where Qw is the real-time inflow flow rate; Vw3 is the velocity of the free-falling water droplet (10). Based on equations (5) and (7), the formula for determining the minimum gas pressure inside the shaft is: (ii) After the average air velocity is obtained using equation (b1), combined with the drop height Hs of the shaft, the entrainment air velocity of the shaft is calculated based on equation (b2): Q a =V a ×H s (b2)。 7. A method for calculating the volume of air entrained in a vertical urban drainage shaft according to claim 1, characterized in that: In step d, the formula Calculate the air resistance, where ρ 空 is the air density, C d is the resistance coefficient, A is the windward area where the water flow contacts the air, and v is the water flow velocity. The calculation of the amount of entrained air is adjusted based on the effect of resistance on the water flow motion equation.

8. A method for calculating the volume of air entrained in a vertical urban drainage shaft according to claim 1, characterized in that: In the step d, the entrained air volume is corrected according to the ideal gas state equation pV=nRT and the relationship between air viscosity and temperature.

9. A method for calculating the volume of air entrained in a vertical urban drainage shaft according to claim 1, characterized in that: In step d, a correction coefficient related to the wall roughness is determined by experiments or empirical formulas, and the calculation result of the entrained air volume is multiplied by the correction coefficient ξ, that is, the final entrained air volume V = V 气 ×ξ, where V 气 is the amount of entrained air calculated without considering the wall roughness.

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