The Research Of Natural Ventilation Efficiency And Numerical Model In Drainage Stack-Sewer System

As an important part of urban infrastructure,sewer network is the key to urban environment and people’s secure residence.However,in recent years,the potential safety hazard caused by harmful gases such as hydrogen sulfide（H₂S）,methane（CH₄）and carbon monoxide（CO）has brought a threat to the normal operation of sewer network system.Although the existing hazardous gas control technology can achieve a certain control effect,but still need to consume energy and chemical substances.This study focuses on the ventilation efficiency of building vertical pipeline connected with sewer network,and draws the following conclusions:（1）When the flowrate of drainage is constant,and the drainage height H remains unchanged,the position of the maximum negative pressure and zero pressure point in drainage riser are basically unchanged.But the value of maximum negative pressure is related to flowrate Q,and under the condition that the drainage flow Q remains unchanged,the value of negative pressure becomes larger with the decrease of the drainage height H.（2）When the flowrate of drainage is constant,there is an approximate linear relationship between the value of maximum positive pressure and the maximum negative pressure in system and Reynolds number.The more violent the flow in the riser,the greater the Reynolds number,and the greater maximum and minimum of pressure value in drainage rise are obtained.（3）When the flowrate of drainage is unconstant,the fresh air inhaled from stack vent can be used to reduce the concentration of hazardous gases in a certain range.When the drainage height is high enough,the process above is completed by the specific vent stack and stack vent to relieve the air pressure fluctuation;While the drainage height is low,specific vent stack no longer plays the role of filling the gas.（4）In the absence of water flow in drainage stack,the difference of gas composition inside and outside sewer is one of the driving forces of natural ventilation effect.The higher the volume fraction of the CH₄ in the pipeline,the greater the density difference between sewer and external environment.The longer the vertical pipe,the greater the flow rate of the natural ventilation airflow.When volume fraction of the CH4 in sewer headspace is 5%,2.5%,1.25%and 0.625%,natural ventilation airflow velocity is about0.6m/s,0.4m/s,0.25m/s and 0.05m/s,respectively,The error between gas flow quantification model based on Bernoulli equation and CFD simulation is within 15%.（5）Temperature difference is another driving force of natural ventilation.The greater the temperature difference and height difference,the more significant the natural ventilation effect can be achieved.When the temperature difference inside and outside the pipeline are 8℃,6℃,4℃and 2℃,the velocity of natural ventilation are 0.61m/s,0.51m/s,0.40m/s and 0.27m/s respectively.And the error between CFD simulation and measured results is less than 10%.（6）Natural ventilation effect can reduce the concentration of hazardous gas in a certain range.Taking CH₄ as an example,by establishing the relationship between methane release and natural ventilation flow rate,it can be calculated that when temperature difference are 8℃,6℃,4℃and 2℃,the length range where volume fraction of CH₄ in sewer is keeped below 5%are relatively 4640m-5300m,4120m-4700m,3540m-4040m and 2730m-3120m.