Mixing time constant for jet flow in rooms.

This paper presents a study of the dynamics of the turbulent mixing of a hot or cold air stream with the air in the interior of a building zone. Observations and CFO results of transient temperature behaviour in a fully developed jet flow field are presented. A simple model for the characteristic time-constant of the mixing process in a room is derived. The mixing of a turbulent jet, as a function of position inside the room, is also discussed. This mixing time-constant plays an important role in total system dynamic behaviour and stability. For a turbulent jet, it is shown that the mixing time is short in the flow regions where strong shear forces are present and increases rapidly outside the jet. This implies that temperature sensors placed outside the shear flow regions respond slowly to supply air temperature or flow rate changes. This slow response may compromise the dynamic behaviour of systems where the temperature sensor's output is the primary feedback variable for indoor temperature control.