Experimental study on air change effectiveness in mixing ventilation
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Providing occupant comfort and health with minimized use of energy is the ultimate purpose of heating, ventilating and air conditioning (HVAC) system. Building ventilation directly affects indoor air quality, and it influences occupant’s heath and productivity. Mixing ventilation is the most common air distribution system, and often the same diffusers provide space cooling and heating. Air distribution with all-air-heating is one of the major challenges for mixing ventilation as temperature stratification and corresponding low ventilation effectiveness may appear. The two objectives of this research were to: (1) provide design criterion for diffuser selection and location considering both thermal comfort and ventilation, emphasis on heating conditions. (2) assess the procedure of evaluating air change effectiveness in the current standard, especially focusing the variance of local air change effectiveness. The study used experimental measurements in a full-scale test room. CO2 tracer gas decay tests were conducted to simultaneously measure age of air at 18 locations in the test room with various types of ceiling diffusers, T0.25/L (air flow rates) and internal loads. Analysis of the experiments regarding first objective showed that the air change effectiveness was significantly decreased when T0.25/L went small under heating conditions. The range of air change effectiveness and the temperature effectiveness were 0.56 to 0.87 and 0.58 to 0.75 respectively within the recommended range of T0.25/L regarding ADPI. Regardless of diffuser type, air change effectiveness and the temperature effectiveness was close to or higher than 1.00 under cooling conditions. The range of T0.25/L that can achieve good mixing under heating condition was significantly shorter than the one under cooling mode. Regarding second objective, the variance analysis showed the vertical, horizontal and overall variance of local air change effectiveness was minimal in mixing ventilation. The variance of air change effectiveness in the occupied space of the room with ceiling diffusers was less than 16% in most of the cases, which is slightly larger than the experiments' uncertainty. Furthermore, the newly developed correlation of thermal effectiveness and air change effectiveness considered to be useful as an alternative method to interpret air change efficiency.