Experimental study on air change effectiveness in mixing ventilation

Mixing ventilation is the most common air distribution system, and often the same diffusers provide space cooling and heating. Proper diffuser selection affects characteristics of air distribution. 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 objective of this study is to provide supporting data for air distribution system designs for several of the most common types of ceiling diffusers in the cooling and heating regime. It defines operation range for selecting diffusers with: high ADPI and ventilation and temperature effectiveness. The study is based on experimental measurements in a full-scale test room. Air change effeteness was measured using CO2 tracer gas decay tests with various types of ceiling diffusers and internal loads. Also, the air flow was changed to produce different corresponding throw for the tested diffusers (T0.25), which with characteristic space length for the diffuser jet flow (L) defined T0.25/L ratio. The results show that under heating conditions the air change effectiveness significantly decreases with decrease of T0.25/L. Even for the recommended range of T0.25/L, the value of air change effectiveness and temperature effectiveness are low: 0.56 to 0.87 and 0.58 to 0.75, respectively. Under cooling conditions, air change effectiveness and the temperature effectiveness were close to or higher than 1, regardless of diffuser type. The range of T0.25/L value that can achieve good mixing under heating condition was significantly smaller than the one under cooling mode.