Study On Airflow Collision Mechanism And Flow Characteristics In Horizontal Diffusion Zone Of Square Column Attachment Ventilation

Square-column attachment ventilation（S-CAV）is a new type of down-supply ventilation mode.It is developed based on the principle of Mixing ventilation and Displacement ventilation.The air supply principle of this ventilation mode is to use the Coanda effect to make the air supply flow adhere to the column and move downward.Then the airflow collides with the ground and then uses the Extended Coanda effect to attach to the ground movement to form a horizontal"air lake"area,thereby controlling the indoor environment.However,when the airflow from two or more columns moves horizontally on the ground,they may move towards each other and collide,causing the indoor airflow distribution to deviate from the design goal.In the mode of the S-CAV mode,the introduction of multiple airflow attached to the column into the horizontal diffusion zone inevitably leads to various"airflow collision"phenomena.These phenomena include:the coaxial arrangement of the air supply jet flow attached to the adjacent column,followed by diffusion in the opposite direction upon entering the working zone,along with the"central collision"that occurs during the flow process;and the"eccentric shear"of the air supply jets originating from non-coaxial adjacent columns（such as the eccentricity of two columns and the nonlinear arrangement of three columns）when they move towards each other in the working zone.In light of the aforementioned issues,this study summarizes the layout of the columns and finds that the coaxial and eccentric arrangement of two columns and the nonlinear distribution of three columns can adequately represent the existing column layout in a room.Based on this,a numerical calculation model is established to study the airflow collision mechanism and flow characteristics under different column layouts,and a new partition form is proposed to more effectively control the indoor environment.The primary conclusions of this study are as follows:In the range of 1-3 m/s,any variation in the supply air velocity,irrespective of the occurrence of airflow collision,may be considered negligible with respect to its impact on the attenuation trend of the axial and radial velocities,as well as the location of the velocity stagnation point.The collision that occurs between the two columns when two airflow move towards each other,causing the airflow to"swell",when two columns are arranged coaxially.Based on the attenuation characteristics of axial velocity,the airflow collision area in the coaxially arranged columns is classified into impact,transition,and collision regions.Furthermore,at the same absolute value of air supply temperature difference,the cooling condition experiences a 3.1 times greater kinetic energy loss due to collision as compared to the heating condition.When the two columns are arranged eccentrically,the curve formed by the stagnation point will rotation,and the direction of rotation is biased towards the side of the column that is farther away.As the moving distance of column 1 increases,the velocity of the airflow axis decreases gradually,and the position of the velocity stagnation point also moves to column 2 accordingly.Every time column 1 moves 1 m,the speed stagnation point moves about 0.05 m.When the three columns are arranged nonlinearly,the minimum value of air velocity and the maximum value of static pressure in the airflow collision area present a"Y"-shaped distribution on the height plane when Y≤0.4 m.When Y>0.4 m,the maximum velocity presents a"Y"-shaped distribution.The velocity stagnation surface between the two columns undergoes a gradual shift towards the geometric center of the column,when the velocity ratio of the two airflow approaches unity.This shift is accompanied by a transformation in the shape of the stagnation surface,which evolves from a sharp angle to a straight line,while the horizontal extent of the collision zone gradually reduces.In the case where the two airflow velocities are unequal,the collision of the airflow between the two columns has a greater impact on the axial velocity of the column with the smaller initial air supply velocity.In the CAV mode,the airflow collision area exhibits a deflection oscillation phenomenon.At u₀=2.5 m/s,the oscillation period of the airflow at X=0.1 m and Z=9.0m is 250 s,and the primary direction of the component velocity in the Z direction also varies at different heights.This study investigates the effects of the collision of two/multiple airflow with the CAV mode,thereby extending the existing research on the Fluid mechanics and HVAC of this ventilation mode.The findings presented in this paper provide a solid theoretical basis for the engineering application and optimal design of the CAV mode.