A New Type Of Ventilation-2DPIV Study Of Downward Directed Vertical Wall Jets Formed By Slot Inlet

As an important part in air-condition and ventilation,airflow organization attracted great attention of scholars and engineering designers.Since the beginning of 1ast century till present time,people have made great progress in the research of airflow organization.Airflow organization mode mainly had two types,one was traditional mixed ventilation based on dilute principle,and the other was neoteric displacement ventilation using buoyancy control as driving force.The differences of design idea between the two ventilation modes were that mixed ventilation considered architecture space while displacement ventilation considered human health and resource efficiency. Compared to mixed ventilation based on dilution principle,displacement ventilation as a high air quality,thermal comfort and less energy consumption supply mode was applied more and more in industrial,civil and public buildings.Displacement ventilation usually used bottom side air supply which was the best airflow organization, however,in view of the situation of most office equipment displacement,bottom side air supply was constrained and hard to achieve in many circumstances.After comparing and analyzing the two ventilation mode,Professor Li Angui and Zhao Hongzuo putted forward a new idea which was whether could further using the coanda effect of downward directed vertical wall air supply,thorough controlling reasonable physical parameters and extending the jet distances which could carry maximal fresh air to working area,then extended along the ground to appear "air lake" phenomenon and form air organization similar to displacement ventilation?Based on the idea above,2DPIV was used in the experiment to discuss the air-flow fluid and the main influencing factors of downward directed vertical wall jets formed by slot inlet,and the results were as follows:1.Through supplied air in the same temperature,when the circumambience temperature t_∞was 24℃,air inlet width b was 10mm,the relatively roughness k/b of side wall was 0,the relative horizontal distance of air inlet from side wall s/b was 5～15, and the air velocity was 0.3～1.5m/s,the effect of air velocity changes on the air-flow fluid of downward directed vertical wall jet ventilation mode formed by slot inlet,the experimental study showed:When the relative horizontal distance of air inlet from side wall s/b was the same (the value of s/b was separately 5,8,10,b=10mm),while the air supply velocity changed,air supply jet would evidently bend to side wall to form attached effect and flow downward along side wall until apart from side wall and flow flatly.And the eddy formed at top left corner of the model could be obviously seen.The central position of eddy would move upward as increase of air velocity,which showed that the higher of air velocity,the stronger of jet wrapping ability to surrounding air.As the increase of air supply velocity U₀,the beginning attached position y₁ would be higher to bottom of model,the shedding position y₂ would be lower to bottom of model,the formed attached distance would be longer and the air would be easier to send down to the ground.When the relative horizontal distance of air inlet from side wall s/b was certain, while the air velocity U₀ changed,the changing of centerline velocity U_m was almost the same.That was to say as the increase of distance y₃ from air inlet,centerline velocity U_m would decrease gradually till minimum and then increase to show a turning point.In the minimum velocity jet began to form attached effect(that was attached point y₁). When the relative horizontal distance of air inlet from Side wall s/b was the Same,the bigger of air velocity U₀,the higher of attached point y₁,and easier to form attached effect.Otherwise,the smaller of air velocity,the more difficult to form attached effect. It indicated that bigger air velocity should be used in permit condition when using downward directed vertical wall jets.After the attachment of jet and side wall,the change of curve tended to be steady and centerline velocity decreased obviously,which further showed that downward directed vertical wall jets could reduce air velocity and benefit to air supply.When the air velocity U₀ was 0.3m/s,the relative horizontal distance of air inlet from side wall s/b was 14(b=10mm),the jet bended to side wall obviously because of attached effect.But when the relative horizontal distance of air inlet from side wall s/b was 15(b=10mm),the bending phenomenon did not exist,the attached effect disappeared completely,and the jet fluid model changed totally to traditional mixed ventilation.When the air velocity was separately 0.5m/s,1.0m/s,and 1.5m/s it appeared the same phenomenon.It was concluded that when side wall roughness was the same, the effect of air velocity change on limit attached distance was not obvious and the relative limit attached distance was s/o=14(b=10mm).2.Through supplied air in the same temperature,when the circumambience temperature t_∞was 24℃,air inlet width b was 10mm,the relative horizontal distance of air inlet from side wall s/b was 5～15,the air velocity was 0.3～1.5m/s,and the relative roughness k/b of side wall was 0～0.2,the effect of side wall roughness change on the air-flow fluid of downward directed vertical wall jet ventilation mode formed by slot inlet,the experimental study showed:When air velocity was certain(U₀ was separately 0.5m/s,1.0m/s,1.5m/s),and the relative horizontal distance of air inlet from side wall s/b was the same,while side wall roughness changed,the changing trend was almost the same of downward directed vertical wall jets.That was to say jet all bended to side wall to form attached effect and flow downward till apart from side wall and flow flatly.And the eddy formed at top left corner of the model could be obviously seen.The central position of eddy was not affected too much by side wall roughness change but affected obviously by air velocity. As the increase of side wall roughness,the jet shedding position y₂ would be closer to the bottom of model,the attached distance formed would be longer,the air would be easier to flow to the bottom of model,and the attached effect would be stronger.The smaller of the air supply velocity,the more obvious of this effect.When the relative horizontal distance of air inlet from side wall s/b was certain, and air velocity was certain(U₀ was separately 0.5m/s,1.0m/s,1.5m/s),while side wall roughness changed,the changing of centerline velocity U_m was almost the same.It decreased gradually until to the minimum and then increase to show a turning point.In the minimum velocity jet began to form attached effect(that was attached point y₁). When the side wall roughness changed,the height of attached point did not changed obviously.After the attachment of jet and side wall,the changing of curve tended to be steady and the centerline velocity reduced distinctively,which showed that downward directed vertical wall jets could reduce air velocity and benefit to air supply.When the relative roughness k/b of side wall was 0.05(b=10mm),and the relative horizontal distance of air inlet from side wall s/b was 14,jets would bend to side wall because of attached effect.When the relative horizontal distance of air inlet from side wall s/b was 15,the bending phenomenon did not exist,the attached effect disappeared completely,and the jet flow fluid changed totally to traditional mixed ventilation.While when the relative roughness k/b of side wall was 0,0.1 and 0.2,it appeared the same phenomenon.It was concluded that when air velocity was the same,effect of side wall roughness change on limit attached distance was not obvious,and the relative limit attached distance s/b was 14(b;10mm).3.Through supplied air in the same temperature,when the circumambience temperature t_∞was 24℃,air inlet width b was 10mm,the jet supply velocity was 0.5m/s, side wall roughness k/b was 0～0.2,and the relative horizontal distance of air inlet from side wall s/b was 5～15,and the effect of the horizontal distance of air inlet from side wall change on the air-flow fluid of downward directed vertical wall jet ventilation mode formed by slot inlet,the experimental study showed:When the relative roughness k/b of side wall was the same,and the air velocity U₀ was certain,as the increase of horizontal distance of air inlet from side wall,the main body of downward directed vertical wall jet would be farther from side wall and the attached effect would be weaker.The eddy formed at top 1eft comer of model changed from could not be seen to obviously be seen,and air jet with indoor air mixed acutely.When the relative horizontal distance of air inlet from side wall s/b changed(s/b was separately 5,8 and 10,b was 10mm),the changing of centerline velocity U_m was almost the same.It decreased gradually until to the minimum and then increase to show a turning point.In the minimum velocity jet began to form attached effect(that was attached point y₁).The smaller of horizontal distance of air inlet from side wall,the higher of attached point y₁,and easier to form attached effect.Otherwise,the bigger of horizontal distance of air inlet from side wall,the more difficult to form attached effect. After the attachment of jet and side wall,the curve tended to be steady and the centerline velocity reduced obviously,which showed that downward directed vertical wall jets could reduce air velocity and benefit to air supply.In the same height,after forming attached effect between jet and side wall,the smaller of horizontal distance of air inlet from side wall,the bigger of centerline velocity,which indicated that the horizontal distance of air inlet from side wall,should be as small as possible when using downward directed vertical wall jets.