Indoor environment analysis based on the relationship between supply airflow and discharge air temperature in VAV system

This study investigates the relationship between supply airflow and discharge air temperature in a variable air volume (VAV) system and evaluates its impact on indoor environmental performance and energy consumption during winter heating operation. Although minimum airflow settings are typically determined at the design stage, inappropriate airflow control may cause thermal stratification or excessive energy use. Field measurements were conducted in a university laboratory equipped with a single-duct VAV system, and a computational fluid dynamics (CFD) model was developed and validated using measured indoor temperatures. The SST k-ω model was applied as the turbulence model for the CFD simulation, and the model showed good agreement with the field measurements, with a CVRMSE of 7.96%. This result confirms the applicability of the CFD model to the laboratory-scale analysis. Indoor environmental performance was evaluated using the vertical air temperature difference criterion from ASHRAE Standard 55 and the air diffusion performance index (ADPI). In addition, a dimensionless stratification index was applied to quantitatively interpret the relative relationship between the mixing effect induced by supply airflow and the buoyancy effect caused by the discharge air temperature difference. Simulation cases were established for heating loads ranging from 500 to 2000 W and supply airflow ranging from 500 to 2500 CMH. The results showed that increasing the supply airflow reduced the vertical temperature difference and improved ADPI under higher heating loads. By simultaneously considering stratification and ADPI, an acceptable operating region was derived for the relationship between supply airflow and discharge air temperature. A supply airflow of 2000 CMH satisfied both the vertical temperature difference criterion of 3°C or less and the ADPI criterion of 80% or higher under all heating load conditions. Winter energy consumption was estimated for the acceptable operating condition using the bin method. Compared with conventional fixed-airflow operation at 1500 CMH, the proposed supply airflow operation reduced supply fan energy by 11.01%, reheat energy by 34.63%, and the combined supply fan and reheat energy consumption by 30.54%. These findings provide a CFD-based evaluation procedure for deriving an acceptable operating region between supply airflow and discharge air temperature that can maintain indoor environmental performance while reducing the analyzed supply fan and reheat energy consumption in a laboratory-scale VAV system.