Thermal comfort is the prime purpose of Heating, Ventilation, and Air Conditioning (HVAC) systems in the indoor environment. Ideally, providing the best thermal comfort with the minimum energy consumption of HVAC systems is most desired. Cold air systems are proven to put forward energy savings with relatively low supply air temperatures ranging between 4 ℃ and 10 ℃, compared to conventional systems, which supply air at around 16℃. However, cold air systems are rarely applied due to cold draft formation and thermal comfort concerns. Thermal comfort is established by the interactions of convective and radiative heat transfers within the zone. These conditions of the thermal environment are incorporated with the occupants’ characteristics of metabolic rate associated with their activities as well as clothing (thermal insulation) into the well-established Predicted Mean Vote (PMV) index. Cold air systems were narrowly studied in terms of averaged Air Diffusion Performance Index (APDI), leading to a false representation of the system compatibility to provide thermal comfort. This paper develops a User Defined Function (UDF) combined with a Computational Fluid Dynamics (CFD) model to accurately represent thermal comfort conditions of cold air systems. The model is tested for the two-dimensional indoor zone. A scenario of variable indoor conditions is considered to identify the PMV index on the cell-sized scale in the order of 9 mm x 9 mm of the airflow in the office space. The PMV index is considered for a k–ε turbulence model for indoor airflow. In order to test the validity of the two-dimensional model estimations, PMV indices for cells are compared with the results from Center for the Built Environment (CBE) thermal comfort tool against ASHRAE-55 standard. The CFD model developed has shown the effectiveness of cold air systems for the occupied zone layer with applications of PMV based reduced-order control systems.
Keywords Thermal comfort, CFD, Indoor environment, PMV index, User Defined Function (UDF), Steady state model