Preliminary Study On Controlling Airborne Pollutants By Soap-bubble-assisted Local Exhaust Ventilation System

In recent years,China’s industrial construction area and total industrial output value have increased steadily.However,with the rapid development of industrial construction,a large number of harmful flue gas,dust and other airborne pollutants are emitted,resulting in indoor and outdoor environment pollution and damage,which seriously affects the health of workers.For tall industrial buildings,local ventilation is a widely used method to control airborne pollutants.It can improve the building environment and improve the cleanliness of the work area by capturing pollutants near the pollution source.In order to improve the capture efficiency of airborne pollutants and reduce the air volume of ventilation system as much as possible,so as to realize the energy saving of indoor environmental control,various local ventilation modes are constantly proposed and optimized.However,the basic principle of the existing local ventilation optimization methods is still to use airflow as a carrier to carry pollutants to the air outlet.During this process,the mixing and dilution of pollutants with ambient air is inevitable,resulting in an increase in the total volume of pollutants.Thereby,the required exhaust air volume is increased.Therefore,there is still great potential for improving ventilation system efficiency in terms of reducing the dilution and mixing rate in pollutant capture.Based on the above requirements,a new soap-bubble-assisted local exhaust ventilation(SBA-LEV)system is proposed in this paper,which is a new method to capture and control pollutants by using the physical isolation characteristics of soap bubble liquid film,so as to solve the problems of diffusion and mixing of pollutants and ambient air,and volume increase in industrial production.The process of using soap bubbles to capture and transport can be described as follows: first,the soap bubbles completely encapsulate the high-concentration pollutants produced by the pollution source at the pollution source,and then the soap bubbles carry the pollutants to the exhaust outlet,where the soap bubbles break and the pollutants are captured.In the whole process,due to the isolation effect of soap bubble film,the pollutants will not be diluted and expanded by the ambient air,so the system needs less exhaust air volume and energy consumption than the traditional local ventilation system.Based on the above discussion,this paper makes a preliminary study on soapbubble-assisted local ventilation through theoretical analysis and experimental methods.The specific contents include the following aspects:Firstly,the concept of soap-bubble-assisted local ventilation system is introduced and four modes are classified.Among them,the bubble-forming process of mode A(soap bubble generated by additional air flow and free diffusion after generation)does not need to consider the pollutant emission characteristics,the coupling mechanism between soap bubble and additional ventilation air flow does not need to be considered in the transportation process,and the boundary conditions are the easiest to control.Therefore,this model is preferentially selected for research in this paper.Secondly,according to the above determined research mode A,guar gum formula solution is prepared to form stable soap bubble and an experimental device for forming soap bubbles by single nozzle air supply is designed and built for subsequent exploration of the generation status and transportation process of soap bubbles.Thirdly,3D Laser Doppler Velocimetry(3D LDV)is used to study the air supply uniformity and diffusion range of 3D printing nozzle.The velocity measurement results show that the air supply jet does not expand at the position 50 mm away from the nozzle(bubble ring position),and its jet diameter can be regarded as the nozzle diameter.It starts to the experimental shooting of the formation situation of soap bubbles.According to the experimental phenomenon,it can be found that there is a velocity range for the formation of soap bubbles.Large diameter nozzles form soap bubbles at lower velocity,but the velocity range for the formation of soap bubbles is narrow.The minimum bubble-forming velocity is consistent with the previous theoretical values,and the relationship between the maximum bubble-forming velocity and the nozzle diameter is fitted.Fourthly,based on the above determined bubble-forming velocity range of nozzles with different diameters,the velocity conditions are reasonably set in this range to study the influencing factors of soap bubble size.The results show that for the same diameter nozzle,the soap bubble diameter is basically unchanged when the velocity is changed,and there is a certain linear relationship between the soap bubble diameter and the nozzle diameter.Fifthly,the transportation status of soap bubbles and the setting of exhaust hood of pollution sources with different heights are studied.The research on the transport process includes the moving trajectory,moving distance and diffusion range of soap bubbles.It is found that the error between the theoretical calculation value of moving distance and the experimental value is less than 20%.It is acceptable to predict the horizontal moving distance of soap bubbles as an ideal hollow sphere.The floating trajectory of soap bubbles looks like a parabola,but there are certain differences among individuals,that is,there is a diffusion range.The diffusion range of soap bubbles near the critical velocity is the smallest,and the size of the corresponding exhaust hood is the smallest,which is suitable for the bubble-forming velocity of the soap-bubble-assisted local ventilation system.The higher the falling height of soap bubbles,the larger the horizontal movement distance and diffusion range.Finally,based on the above research conclusions,this paper gives the general design process and capture efficiency evaluation method of soap-bubble-assisted local ventilation system.