Effects of supply air temperature and pollutant location on concentration characteristics with a night-time ventilation in a school classroom

Night-time ventilation is used to remove indoor pollutants before the occupied periods. However, a lack of knowledge exists on how well a minimum level of ventilation can dilute pollutants during the night, and how a location of a polluting source and a supply air temperature affect the concentration levels with a typical mixing ventilation strategy. Therefore, the constant polluting source of 12 mg/h was defined on the floor (60 m2), sidewall (17 m2), and floor-corner (1 m2) describing a low-polluting floor material of 0.2 mg/h,m2 (EN 15251:2007). The objective of the study was to demonstrate a minimum night-time ventilation scenario of 0.15 L/s,m2 with the isothermal supply air, underheated supply air (-4°C), and overheated supply air (4°C) compared to indoor air initial temperature. Furthermore, the temporal and spatial concentration characteristics were considered. The daytime ventilation was 3 L/s,m2 with the underheated supply air temperature. Both the night and the day periods lasted 12 hours. The room air distribution was arranged by using 2 corridor wall supply air grilles and 4 exhaust air valves. However, only 1 grille and 2 valves were used at night. ANSYS CFX tools were used for numerical modelling. The RANS and URANS simulations were carried out with the implicit pressure-based multigrid coupled solver. The second-order discretization schemes were used in space and time and the SST-model was chosen to model turbulence with the automatic wall treatment. The convection diffusion equation was used in scalar transportation. The measured data were used as boundary conditions and the computational grid adaptation was applied to improve accuracy. The results show that the underheated supply air provided the lowest concentration level in the occupied zone because the low-temperature supply air flowed down to the floor and dilute well the occupied zone. The overheated supply air, in turn, provided the largest concentration level in the occupied zone because the heated air was not mixed well in the room. The isothermal supply air provided a circulating airflow pattern increasing the concentrations beyond the supplied airflow region. The daytime ventilation decreased the cumulated night concentrations to one-tenth within an hour.