Ventilation and Thermal Performance Examination of Slot Line Diffuser for Perimeter Usage by CFD Simulation

Building ventilation demand increased during the “new normal” following the Covid-19 pandemic. Rather than completely renovating existing HVAC equipment, it is more practical and cost-effective to maximize their existing ventilation performance. This study focuses on slot line diffusers, which are widely used for handling perimeter heat loads. Its long throw distance and wide coverage effectively block cold drafts and heat flow from the exterior window glass. Meanwhile, the mounting position near the glass surface and the high supply velocity can create a ventilation scenario similar to wall confluent jet ventilation (WCJV) or ceiling-supplied displacement ventilation. In order to determine the ventilation and thermal performance of the slot line diffuser, CFD simulations have been conducted using the low-Reynolds number turbulence model with fine meshes, in which a detailed velocity characteristics model derived from a full-scale experiment is used to reproduce the diffuser’s supply airflow. This study priority examined the performance under heating supply conditions. This is because buoyance reduces the throw distance of heating airflow, which obviously influences thermal and ventilation performance compared with cooling conditions. The analysis space is modelled based on a typical open space between two columns of an office building in Japan. Four human simulators are seated around a table in this space’s interior to reproduce a small-scale meeting scenario. Gas is generated by a human simulator to represent pollutants (odor or droplet nuclei) from a speaker. The diffuser’s air supply mode (full-open and half-open) and the mounting location of the exhaust are adjusted as parameters. The CFD simulation results provide a comparison of the ventilation and perimeter heating performance of this type of air terminal by listing 1: the distribution of tracer gas concentration, air temperature, and airflow distribution in the occupancy zone; 2: heating capacity, air distribution performance index (ADPI), draft rate (DR), and ventilation efficiency under different use scenarios. In conclusion, decreasing the slot line diffuser’s supply area obviously improves its heating efficiency by blocking the cold draft from the exterior window. It also enhances ventilation performance by establishing a WCJV. The vertical temperature difference and average normalized concentration in the occupied zone can be kept below 1°C and around 0.7, respectively, for office heating usage in this study.