Experimentelle Untersuchung der Strömungsstrukturen in einer Mischlüftung

Aachen / E.ON Energy Research Center, RWTH Aachen University (2013, 2014) [Dissertation / PhD Thesis]

Page(s): XV, 94 S. : Ill., graph. Darst.

Abstract

Thermal comfort, which depends mainly on temperature and airflow velocity distributions, is an important factor to consider when designing modern building energy systems. Up until today, the transient behavior of air flow structures has only been marginally investigated. The goal of this work is to study these structures in more detail, particularly the phenomena linked to the occurrence of free and forced convection. To this purpose an experimental test facility was built to provide new data about the transient behavior of large-scale flow structures in ventilated rooms. The test facility, called the Aachen model room, is 3~m high, 4~m wide and 5~m long. Four heat sources stretching over the whole length on the room are located inside. Each heat source is 5~m long, 0.4~m wide and 0.6~m high. The total internal load in the test facility can be varied between 0~kW and 6~kW by using adjustable power supply units. The room facing side of the walls is covered in aluminium foil. The effect of radiation can thus be neglected. Directly below the ceiling, on both sides, slot diffusers are installed over the whole length of the room. Each of the air inlets is one meter long and 20~mm high. The supply air can be brought inside either on both or just one side of the test facility. Exhaust air openings are located on floor level on both sides, over the whole length of the room, with the possibility of using just the openings at the back half of the room. The air flow velocity in the room is measured with twelve omnidirectional anemometers in a rectangular grid. The sensors measure the speed of the local velocity vector. The air temperature is measured with twelve platinum resistance thermometers. Each of the twelve temperature and velocity sensors is arranged on a horizontal aluminium profile at a distance of 0.33 m from each other. The aluminium profile is fixed onto a traversing device. At isothermal boundary conditions and in the case of mixed convections with dominating forced convection the air flow structure is almost two-dimensional and stable over the whole length of the test facility. If the supply air is brought inside through both inlets two large and stable eddies occur. If the supply air is brought inside the model room on one side only one large and stable eddy is formed. The air flow structure is nearly two-dimensional over the length of the model room. If neither the free convection nor the forced convection dominates, an unstable room air flow structure occurs. The flow structure in the upper part of the model room is dominated by the forced convection from the supply air. The part directly above the heating sources is dominated by free convection. The whole room air flow structure is thus three-dimensional. The turbulence models applied in air flow simulations are based on the assumption of fully turbulent flows. However, it cannot be ruled out that there are areas within an air flow structure in a room which depend on the Reynolds number at the supply air inlet. This experimental setup was also used to provide information on such areas with low air velocities. The minimal supply air velocity required for the assumption that the room air flow structure is independent of the supply air velocity depends on the supply air inlet and the supply air momentum flow. In this study a connection between these factors was observed for the given geometries. The effect of the boundary conditions on the room air flow structure was studied by applying different supply air velocities and different internal loads. Low velocities and high internals loads lead to an unstable room air flow structure. By increasing the supply air velocity, the structure becomes more stable and can be considered two-dimensional over the whole length of the model room. The point where an unstable air flow structure becomes stable is observed at the same Archimedes number for different inlet velocities and internal loads, as long as the supply air and the heating loads are activated over the whole length of the test facility. In the case of a stable room air flow structure, the position of the exhaust opening has no further influence on the structure. Room air flow structures where no domination of forced convection is present do show an influence of the position of the exhaust opening. The room air flow structures are changing by varying the position of the exhaust opening. If only the opening at the back of the room are used, the two previously discussed stable eddies occur only in the back part of the model room, whereas the front part is characterised by a disordered flow structure.

Authors

Authors

Kandzia, Claudia

Advisors

Müller, Dirk

Identifier

  • URN: urn:nbn:de:hbz:82-opus-49811
  • REPORT NUMBER: RWTH-CONV-144713