Simulation Study On Airflow Organization Characteristics Of Sewage Pipeline Under Enhanced Ventilation Of Building Risers

Due to the poor ventilation of municipal sewage pipe,anaerobic environment was generated with methane and hydrogen sulfide and other harmful gases gathering inside with enrichment,which often causes the explosion and corrosion of the pipe.Improving the anaerobic environment of the pipe and enhancing ventilation can effectively inhibit the generation and release of harmful gases.Based on the direct connection between building stand pipe and sewage pipe without septic tank,a new method of enhancing ventilation in sewage pipe through stand pipe is proposed to control the generation and enrichment of harmful gases.A drainage stand pipe of a building in xi’an and its downstream sewer that is being directly connected were taken as the test object.Field tests were carried out to determine its performance,which proves the intensified ventilation effect of building stand pipe,and at the same time,computational fluid dynamics CFD software was used to build the pipe model with the same proportion for simulation,and the results show that the sewage pipe gas phase velocity deviation between the simulated results and measured results is within 6.5%.The CFD simulation has certain accuracy.Then,the model was modified on this basis to simulate the influence of enhanced ventilation of building stand pipe on air flow in trunk sewer with or without branches.Finally,the influence of enhanced ventilation on the damage of building water seal was discussed.The main conclusions are as follows:（1）with the actual building as prototype,establish the pipe model with the same proportion and simulate air flow in the sewer pipe after the intensified ventilation and check the condition of air flowing out from the well;conduct comparison between the simulation result and the actual measurement results;adjust the setting of simulation so that the simulated air flow setting is close to the actual measured results,and finally adjust the simulation results to ensure the deviation between the simulation results and measured results within 6.5%,which is deemed as reasonable simulation settings.（2）modify the model;build a model trunk sewer without branches;conduct simulation using the same simulation Settings.Only when the enhanced ventilation intensity reached more than 2.0 m/s,obvious effect of accelerate air flow in trunk sewer can be seen,and the increment of trunk sewer length affected by enhanced intensity showed a trend of decline.The simulation of enhanced ventilation intensity of 7.5 m/s with the influence range of 500-800 m on the trunk sewer,using enhanced ventilation to blow air into the building stand pipe to renew the air in a trunk sewer with a pipe diameter of 600 mm and a pipe length of 500 m takes about 155 s.It takes about 155 s for the enhanced ventilation to vent the air out.（3）modify the model one more time;establish the model trunk sewer with branches;simulate the impact exerted by sewer branches on the air flow condition of trunk sewer and results show that using intensified ventilation to blast air in mainly works on the air flow of trunk sewer.When the trunk sewer airflow velocity increased from 0.04 m/s to5.7 m/s,sewer branch airflow velocity increased from 0.04 m/s to 1.6-5.4 m/s.When the diameter of trunk sewer in the model was gradually increased while the ventilation intensity was kept to a certain extent,the change of airflow velocity in the trunk sewer first increased and then decreased with the increase of pipe diameter.（4）For the building stand pipe,the enhanced ventilation intensity of 7.5 m/s causes about 9.2 mm H₂O fluctuation to the water seal on the single side of the water trap of the toilet on each floor of the building,and the water loss will not be caused when the toilet is being drained.The increased intensity of enhanced ventilation will protect the water seal of the entire stand pipe to a certain extent.The enhanced ventilation of the building stand pipe only reduces the positive and negative pressure values caused by the drainage in the stand pipe and does not change the position where maximum positive and negative pressure occurs in the stand pipe and the enhanced ventilation at the top of the building drainage stand pipe is safe to operate.