Study On The Characteristics Of Gaseous Pollutants Backlayering Flow And Downstream Stratification In Channel With Forced Longitudinal Ventilation

Narrow and long restricted space refers to building structures with large width and length,such as air corridors of super high-rise buildings,underground mine walkway,traffic tunnels and so on.Due to the large ratio of length to width of narrow and long space,the flow of gas in its interior is different from that in other structures.When emergency situations occur,such as gas leak in a Mine,tunnel fire,and so on,these gaseous pollutants along the narrow corridor transmission is seriously threatened personnel safety.Longitudinal ventilation is usually used in engineering to eliminate the pollution gas.Therefore,it is of great significance to understand the flow characteristics of the gaseous pollutants in order to develop efficient control strategies and ensure the safety of personnel.In this paper,the characteristics of upstream and downstream flow of gaseous pollutants in a long and narrow space with longitudinal ventilation are studied by salt water experiment.Firstly,this paper introduces the feasibility of the saline experiment method and its similar theoretical basis,and lists the scale relation between the characteristic parameters of the saline experiment and the actual situation.The light attenuation technology and MATLAB data processing technology used in this experiment are described.The experimental equipment and measuring instruments are introduced briefly.The important experimental steps,including correction of image gray value corresponding to dye concentration,are described in detail.Then,the structure of the backlayer of gaseous pollutants is analyzed.It was found that the backlayering flow was divided into two distinct layers in the process of moving from the impact point to the stagnation point.The upper region of backlayer continues to advance,and in the middle begins to be drawn into the lower layer.However,the lower region is in the same direction as the longitudinal flow.And,backlayering flow is same as downstream,also have reduced acceleration stratification.In the area near the top,each longitudinal section of the layer has a stable distribution of reduced acceleration.The closer it is to the interface of the two fluids,the reduced gravity change from large to small from the impact point to the stagnation point,indicating that the mass exchange of two flows is stronger.In the process of backlayering flow advance,the head speed gradually slows down.After that,the empirical model is proposed to solve the backlayer length of buoyancy-driven flow under longitudinal ventilation.The empirical model is in the same form as the backlayering flow length model of smoke in tunnel proposed by predecessors,but with different coefficients.Due to this paper is carried out under the experimental condition of salt water,without boundary buoyancy loss.And buoyancy is only lost through the interaction of longitudinal flow.But the smoke will continue to transfer heat to the envelope,increasing the loss of buoyancy.Therefore,the buoyancy loss caused by boundary heat transfer can be obtained by comparing the two formulas.The results show that the boundary buoyancy loss increases with the increase of the dimensionless backlayering length.However,even if the dimensionless backlayering length reaches 20,the difference between the dimensionless wind speed（the ratio of the ventilation speed to the critical wind speed）is still less than 50%in both cases with and without boundary buoyancy loss.This means that in the process of buoyancy flow propagation with boundary buoyancy loss,the buoyancy loss is mainly caused by the shearing of longitudinal flows.Finally,the stratification characteristics of downstream buoyancy-driven flow are studied.It is found that the dimensionless thickness increases with the increase of λ=B₀^1/3/V.However,when the λ is less than 0.9,the thickness is more sensitive to λ,with a larger range of variation.The same variation law of vertical reduction acceleration in the downstream layer under different initial conditions is explored.Considering the length of the upstream and the thickness of the downstream,the product ε can represent the upstream and downstream comprehensive situation of the buoyancy flow in the long and narrow space under longitudinal ventilation,and it is found that the larger the V/B₀^1/3 is,the smaller ε it is,the better the situation will be.In this paper,the upstream and downstream flow characteristics of gaseous pollutants in a long and narrow space under longitudinal ventilation are studied by means of saltwater experiment,which has important guiding significance for the control of pollution gas and evacuation of personnel in practical project.