Claudia Kandzia

Flow structures and Reynolds number effects in a simplified ventilated room experiment

ABSTRACT Despite a variety of investigations, precise calculations of room airflows are still only possible to a limited extent. However, since the airflow in a room is decisive for the thermal comfort within that room, it is of particular interest to acquire comprehensive knowledge of the room airflow structures for different boundary conditions. In the model room shown here, extensive experiments were carried out in order to draw conclusions about the flow structures. By varying the supply air velocity and the thermal loads, different two- and three-dimensional flow structures can be observed. In case of isothermal boundary conditions and low supply air velocities, the flow field is partly depending on Reynolds number effects. There are regions within the flow field whose mean velocities do not scale linearly with the supply air velocity. The experiments indicate that the minimum supply flow rate for a Reynolds number independent flow field can be estimated by a Reynolds number based on the cross-sectional surface of the supply jet. Depending on the heat flux emitted by the heat sources in the room, large-scale eddies dominate the overall flow structures at higher supply air velocities. Lower flow velocities leads to undefined flow pattern.

Stability of large room airflow structures in a ventilated room

ABSTRACT This paper presents a new set of experimental data for different kinds of room airflows in a ventilated room. The data are used to examine Archimedes number effects on the behaviour of large-scale flow structures and their stability versus time. All experiments are performed in a simplified room geometry, representing a small meeting room, a train or an airplane cabin. The supply air is introduced at the ceiling and the exhaust air leaves the room at the bottom zone. The heating power is realised by four controllable heat sources in the lower part of the model room. The experimental investigations apply different supply air set-ups as well as changing supply air velocities and a variation of the heating power. The data of the experiments indicate that the room geometry-based Ar number can be used to distinguish between unstable to stable room airflow structures. Low velocities and high thermal loads cause time depended, unstable room airflows. Neither the forced nor the free convection dominates the flow structure. By increasing the inlet velocity, the airflow structure gets more stable. Large eddies occur between the heat sources and the ceiling of the model room. This experimental investigation showed that it is possible to determine an Ar number for the room geometry under investigation, in the region where a transition takes place from instable to stable airflow. The value of this critical Ar number has been extracted from a qualitative velocity profile analysis.