Helium-air Experimental Study On The Characteristics Of Harmful Gas Spreading In Inclined Long And Narrow Space Under The Effect Of Buoyancy

long and narrow spaces are building structures with large aspect ratios and certain slopes,such as traffic tunnels and underground integrated corridors,and are an important part of modern society.When a toxic gas leak or fire accident occurs in a long and narrow space,the harmful gas will generally spread rapidly under the effect of buoyancy.In view of the relatively confined structure of long and narrow spaces,harmful gases are not easily discharged quickly and it is difficult to evacuate people,which may cause a large number of casualties.In addition,due to the constraints of the terrain,many long and narrow spaces have a certain degree of slope.Due to the chimney effect brought by the slope,the spreading characteristics of harmful gases in inclined long and narrow spaces are more complicated compared with horizontal long and narrow spaces.Therefore,it is of great engineering significance to study the spreading characteristics of harmful gases in inclined long and narrow spaces to provide guidance for ventilation system design and personnel evacuation.At present,there are few studies on the spread of harmful gases in inclined long and narrow spaces under the effect of buoyancy,and the research methods are mostly focused on small-size and full-size air experiments,saltwater experiments and numerical simulations.It should be noted that the former generally divided the gas leakage and fire smoke into two distinct flows for separate studies.In fact,the difference between the two mainly lies in the initial momentum of the hazardous gas release source,which is generally larger in the case of gas leakage,but very small in the case of fire smoke generation.In this paper,a relatively new research method,Helium-Air experiment,can be used to study both gas leakage and fire smoke conditions by changing the initial momentum at the source.Moreover,the Helium-Air experimental technique has good visibility and can quantify the buoyancy flux of the source term more accurately,which can better reflect the propagation characteristics of hazardous gases in inclined and narrow spaces under the condition that the similarity criterion is satisfied.Firstly,this paper introduces the Helium-Air experimental method and compares the Helium-Air experiments with small-size air experiments,full-size air experiments,saltwater experiments and numerical simulations,points out the advantages of Helium-Air experiments,and demonstrates the feasibility of using Helium-Air experiments to study the spread characteristics of harmful gases in inclined narrow and long spaces under buoyancy.The similarity principle of Helium-Air experiments is explained,and the proportional relationship between each characteristic parameter and the corresponding prototype parameter in Helium-Air experiments is obtained according to the Froude criterion.The gas distribution system and tracer system of the experimental setup are introduced and the measurement methods and MATLAB image processing techniques used in this experiment are briefly described.The effects of the source buoyancy flux and the height difference of the release source from the downstream outlet on the thickness and outlet velocity of the downstream stratified flow and the upstream back-layering length are investigated.The results show that the downstream stratified flow thickness and outlet velocity tend to increase with the increase of source buoyancy flux;the back-layering length is influenced by two mechanisms,namely,the chimney effect and the mass output of the release source,and there is a competition and counterbalance relationship between the two mechanisms,which leads to the back-layering length showing a tendency to decrease first and then increase.As the height difference between the source and the downstream outlet increases,the chimney effect is enhanced,resulting in thicker stratified flow thickness and larger outlet velocity downstream,and the upstream back-layering length decreases.After analyzing the effects of the source buoyancy flux and the release source height difference from the downstream outlet on the spreading characteristics,the dimensionless parameters representing the source buoyancy flux and the release source height difference from the downstream outlet are combined into a dimensionless mixing control factor.Finally,the relationships between the dimensionless downstream stratified flow thickness,outlet velocity and upstream back-layering length and the dimensionless mixing control factor are investigated in depth to obtain the prediction models of the above three characteristic parameters.In this paper,the characteristic parameters characterizing the spread characteristics of harmful gases in the inclined long and narrow space,i.e.,the thickness and outlet velocity of the downstream stratified flow and the upstream back-layering length,are systematically studied by Helium-Air experiments,and the trends of the characteristic parameters and their influencing factors are obtained.The results of this paper can improve the understanding of the characteristics of harmful gas spread in inclined long and narrow spaces under the action of buoyancy and provide reference for the design of ventilation systems.Also,this paper further verifies the ability of using Helium-Air experiments to study the harmful gas flow and extends the experimental means when conducting related studies.