Achieving suitable airflow rate in New Zealand classrooms: a CFD approach to inform on potential retrofitting solutions

During the COVID-19 pandemic, besides sanitising, masking, and increasing social distancing, opening classroom windows was the NZ Ministry of Education's main requirement for reopening schools. However, a pre-COVID-19 survey showed that only a third of the NZ teachers opened windows during teaching time. Achieving a suitable ventilation level could not rely on humans to open windows. Heating, Ventilation, and Air Conditioning (HVAC) systems are not affordable for most NZ schools. Consequently, an alternative and affordable ventilation method that could be retrofitted is needed to increase the airflow rate. In this project, we investigated the benefit of using trickle ventilators with connection (or not) to extraction fans in three NZ locations (Auckland, Wellington and Dunedin). The computational fluid dynamic (CFD) approach allowed us to test different scenarios for ventilation performances, such as single-sided trickle ventilators versus cross-ventilation scenarios, modelling with or without extractor fans. We carried out an aeraulic simulation of the airflows to visualise the trajectory of the flows and check the air velocity and temperature in the classroom volume where the students are located. The results for the Wellington case study (no extractor fan used) showed a suitable airflow rate in summer when the trickle ventilators were fully open. However, the trickle ventilators' effective area was reduced in winter, and insufficient air entered the classroom. In addition, despite good air mixing assisted by the inverter heat pump use in winter, there were still a few areas where the air was not well-mixed. These areas experiencing a lower mixing rate could create some “CO2 pockets”, reducing the extraction of pollutants or viruses. Acknowledging COVID-19 and its lasting impacts on NZ schools and families, we must better prepare for future learning disruptions. By investigating different cases simulated by CFD software, we compared different scenarios to improve ventilation performance in the classroom. Following the CFD project, we plan to deploy some sensors to monitor the temperature, relative humidity and CO2 in Auckland, Wellington and Dunedin classrooms and validate our modelling findings. This study will assist the NZ Ministry of Education retrofit classrooms for a healthy environment.