Investigation Of The Horizontal Attachment Distance And Indoor Thermal Environment Of Wall-based Attachment Ventilation

As one of the main forms of air distribution in ventilation and air conditioning system,mixing ventilation has been widely used in all kinds of buildings.However,under the heating condition in winter,the thermal air is delivered to the space from the top of the room;due to the effect of thermal buoyancy,the thermal air tends to gather in the upper part of the room.To make the temperature of the occupied zone meet the design heating requirements,it will inevitably cause a large amount of inefficient energy consumption.In addition,the occupied zone is generally in the reverse-flow zone with the mixing ventilation,so the ventilation efficiency is relatively low.Although displacement ventilation is energy-saving and improves indoor air quality effectively,its air supply velocity is generally less than 0.5m/s,which limits its capacity to eliminate load,thus it is not suitable for winter heating conditions.As a novel type of air distribution strategy,wall-based attachment ventilation can not only overcome the drawbacks of high energy consumption and thermal stratification of mixing ventilation,but also make up the shortage that displacement ventilation cannot be used for winter heating.Hence,it is an ideal air distribution strategy which can be used for cooling in summer and heating in winter.Firstly,the indoor thermal environment and ventilation performance of wall-based attachment ventilation and mixing ventilation under winter heating conditions were compared and investigated in an artificial thermal comfort laboratory.The rationality of wall-based attachment ventilation as a high-efficiency air distribution in winter heating was verified.The results show that,on the one hand,the phenomenon of under-cooling and over-heating caused by vertical temperature gradient is improved when wall-basedattachment ventilation is used for heating mode.The heat can be effectively sent into the occupied zone.On the other hand,the indoor temperature distribution is more uniform.The temperature difference between head and ankles is less than 3?,and the temperature non-uniformity coefficient is about 0.05.The PD value is less than 20%,and air diffusion performance index ADPI is greater than 80%.The adverse effect of buoyancy on indoor thermal comfort was effectively improved.Secondly,the horizontal attachment distance directly effects the indoor thermal comfort and energy efficiency when wall-based attachment ventilation is used for heating in winter.The horizontal attachment distance is defined as the horizontal distance between the point where the thermal air starts to float off the floor and attached wall in horizontal air reservoir region of attachment ventilation.If the air supply parameters are unreasonable,it may lead to blank heating area.Therefore,the factors affecting the horizontal attachment distance are discussed in this paper through the CFD method verified by experiments.And the effects of air supply velocity,air supply temperature and air outlet height on indoor flow field,temperature field and horizontal attachment distance were analyzed.A linear regression model between horizontal attachment distance and Archimedes number was established,and the correlation significance test was given.The results indicate that the smaller the temperature difference of the air supply,and the greater the air supply velocity,the longer the horizontal attachment distance of thermal air is,the more uniform of indoor temperature is.Compared with the temperature difference of supply air,the influence of supply air velocity is more obvious on horizontal attachment distance.The effect of wall-based attachment ventilation in heating is dominated by inertial force,followed by the influence of thermal buoyancy within the general air supply parameters(the range of supply air temperature is 30~45?,the range of supply air velocity is 0~5m/s)used in comfort air conditioning in winter.The air supply velocity is inversely proportional to the horizontal attachment distance,and the growth rate decreases with the increase of the air supply velocity.Reducing the height of the air inlet can also significantly increase the horizontal attachment distance.The Archimedes number,dimensionless criterion for non-isothermal jets,comprehensively reflects the effect of buoyancy and inertia forces on the jet trace andrange of wall-based attachment ventilation.If the horizontal attachment distance is greater than 50% of the direction of room length,which means Ar<0.0025,it can be considered that the horizontal attachment distance meets the air supply requirements.In summary,wall-based attachment ventilation makes use of “Coanda effect” to make the airflow be delivered down to the occupied zone under the “support” of the vertical wall.As long as the design of air supply parameter is reasonable,even the air supply jet is thermal air,the jet residual momentum can still overcome the thermal buoyancy when the flow turns horizontal.Finally,the “air reservoir” is formed on the floor surface,so that thermal air can be sent into the occupied zone effectively.