PhD Project: Characteristics of Buoyancy Driven Natural Ventilation through Horizontal Openings

Air flow through horizontal openings is an important issue of mass and energy transfer between different zones in buildings. Air flow through vertical openings has been widely investigated, but little is known about the flow in the horizontal openings, especially when they are driven by buoyancy. The lecture will present the results of two cases of full-scale measurements of buoyancy driven natural ventilation through horizontal openings: a case with one horizontal opening and a case with one horizontal opening combined with one vertical opening.

For the case of one horizontal opening the air flow patterns are highly transient, unstable and complex and the air flow rates oscillate with time. For the case of one horizontal opening combined with one vertical opening, three flow modes can be identified depending on the relation between opening areas: bidirectional flow through the bottom opening, unidirectional flow through the two openings, and bidirectional flow through the top opening. A new empirical model for calculation of the air flow rate is developed by introducing a new opening area ratio factor. Computational fluid dynamics (CFD) is also used to study these two air flow cases. The measurement data are used to compare two CFD models: standard k- ε model and large eddy simulation (LES) model. The cases calculated by the LES model agree well with the measured data.

This work is important and quite useful in relation to natural ventilation systems. The measurement results can be used in both simple calculation tools to give a rough estimate of the capacity for design of a ventilation system, but can also be implemented in more detailed models, especially multi-zone models, for simulation of the performance of natural ventilation systems.