Thermal environment in simulated office rooms generated by active ceiling diffuser with radiant panels

Abstract

Ventilation airflow rates are typically controlled based on occupancy, air quality and heat load levels in office rooms with variable air volume (VAV) system. Additional water-based cooling is often the most energy efficient to use when cooling by cold ventilation air for occupancy and indoor air quality does not cover heat gains. This can be done by air-water system, by combining ceiling diffusers with radiant panels. The target of operation of these room units is to maintain a good thermal environment for occupants and stable supply air distribution in varying occupancy/heat load levels. This can be challenging especially with non-uniform heat loads. In the earlier study, it was concluded that active ceiling diffusers were able to generate a more uniform thermal environment than static ceiling diffusers. The thrown pattern is not constant with the static diffuser, but with the active diffuser, it is more uniform due to constant supply air velocity. This study was continued by analysing differences between the design of all-air and air-water systems with active diffusers and investigating higher heat load situations enabled by air-water system. The novelty of this research is to confirm the usability of the air-water system with active ceiling diffusers in an office environment. Office room situations were measured earlier in a full-scale test room (27 m2) with partial occupancy for studying differences between air distribution. Now that was done with CFD simulations. The same 3-person office room case (46 W/m2floor) without and with radiant panels was modelled first to validate CFD-simulation in all-air system case (4.3 l/s,m2floor), and then to simulate the performance of the design with air-water system (1.8 l/s,m2floor). Then 10-person meeting room case with air-water system (4.3 l/s,m2floor) was simulated with a higher, design heat load level (81 W/m2floor). The thermal environment in the same office room cases measured in the full-scale test and modelled in the CFD simulation was near to each other. Supply air diffuser was modelled in CFD simulation with detailed geometry and other boundary conditions were similar to in full-scale test situation. RANS simulation method was used with SST turbulence model and with a fine computational grid. CFD simulations with higher heat load levels brought new findings for the air distribution with radiant panels. The increase of heat loads also increased room air velocities, but still local thermal environment remained at a good level. This confirms the usability of air-water system with active ceiling diffusers in variable conditions.