Research On The Characteristics Of The Flow Field And The Mechanism Of The Circulation Zone In A Circular Jet With Confined Space

As the foundation and core part of the national economy,Chinese industry has played an important role in promoting economic development.However,at the current stage,Chinese industry still focuses on industries that are high in energy consumption and pollution.Especially in recent years,the demand for industrial welding technology tends to be diversified,and it is not uncommon for the problem that industrial smoke is difficult to discharge when personnel are operating in confined spaces.When controlling the ventilation,the flow field distribution of the jet,the diffusion law of the pollutants,the position of the operator,the shape and size of the confined space all affect the overall ventilation effect of the confined space,and brings difficulty to the ventilation of the confined space..This paper explores the influence of different jet-jet boundary factors and the influence of obstacles and pollution sources on the basic flow field and the development of flow regimes in circular jets with cylindrical confined space.According to the specific confined space shape,worker's operating position,and the location of smoke generation,reasonable suggestions are put forward for personnel operation methods,and effective elimination measures for contaminant retention in the circulation area are proposed.The analysis of the formation mechanism of air entrainment and circulation area in the cylindrical confined space has important significance for improving the design method of the air supply terminal device in the confined space.In order to effectively control the air quality of the air in a confined space,the study of the basic flow regime and its regularity is the key.In this paper,the circular confined jet flow pattern in columnar space was partitioned by numerical simulation and experimental research.It was found that the existence of circulation area and the retention of contaminants in the circulation area lead to low ventilation efficiency.Secondly,the influence of boundary factors on the flow-limited jet is studied,including inlet shape,velocity,diameter ratio of confined structure,turbulence intensity and velocity distribution and direction.The dimensionless curve of the axis velocity attenuation under the influence of boundary factors was analyzed.It was found that the relationship between u/u₀ and x/d was exponential and the linear relationship between the two was in the attenuation of the free jet axis.In addition,the typical cross-sectional flow pattern of a confined circular jet is analyzed.It is found that the size of the circulation area decreases with the increase in the ratio d/D of the confined structure,and the size of the circulation area and the deceleration trend of the axis velocity within the common inlet velocity range basically unchanged.The characteristics of the flow field of a cylindrical confined circular section jet under the joint influence of Re and d/D are analyzed and the circulation area corresponding to the jet air supply velocity u0=0.1-10m/s and the diameter ratio d/D=0.2-0.8 is given.The calculation formula of length Xr provides a reference for the design of cylindrical confined space ventilation.It was found that when the air supply is in the form of a strong central momentum velocity distribution,the area of the circulation area is long and the kinetic energy carried by the unit volume of the vortex is the smallest.The larger the turbulence in the boundary influence factor is,the more forward the axis attenuation is,and the greater the kinetic energy transmitted to the Y-axis when the jet sucks the ambient air,the more energy for the positive gradient of the counter pressure gradient is.By analyzing the influence factors of these jet boundaries,the regularity of the flow field of the confined circular section jet is obtained,which provides a basis for improving the design of the air supply terminal device in the confined space.In addition,according to the actual welding conditions in the industrial limited environment,the operator is used as a restricted ventilation obstacle,and the pollution source is added circumferentially in the circulation area and the non-circulation area near the wall.The flow field characteristics of the restricted cylinder jet under the influence of various factors are studied and operational suggestions are proposed.For example,when the pollution source is in front of the staff,the worker should stand sideways or try to separate the legs.Finally,the mechanism of formation of the circular jet circulation area in the cylindrical confined space is analyzed as follows: 1.In the cylindrical confined space,the jet encounters the wall of the sudden expansion,because the turbulent transverse pulse action will collide with the original stationary near the restricted wall surface.The quality of air.The jet “spurs” the stationary air micro-cluster near the wall to produce sucking action.2.The separation of the boundary layer caused by the counter pressure gradient changes the rate of jet expansion,the growth rate of the boundary layer,and the shape of the velocity distribution,thereby expanding the size of the jet.Vortex caused by suction.According to the formation of the circulation area,this paper proposes a method of pushing the stagnation point of the near-wall airflow velocity to reduce the circulation area: 1.Adding the porous medium to eliminate the confined jet circulation area near the wall;2.The double jet reduces the circulation Area.Therefore,it provides a reference for the design of jet limited-end circular jet end when there are obstacles and pollution sources.