Atrium-assisted Natural Ventilation of Multi-storey Buildings

Buoyancy-driven displacement natural ventilation in a simple multi-storey building structure is investigated. The storeys are connected to a common atrium and top-down-chimneys, and the internal heat gains of the storeys are represented by localised point heat sources on each floor. The heat sources generate thermal plumes which entrain the surrounding air and transport warm air upwards. Simple analytical models are used to describe the main flow features, such as interface height, temperature gradient and ventilation flow rate. A two-storey building structure is also investigated using computational fluid dynamics (CFD). Using the RNG k-epsilon turbulence model the predicted airflow patterns, temperature profiles and ventilation flow rates agreed favourably with the analytical models. The work demonstrates the potential of using CFD for modelling buoyancy-driven displacement ventilation in complex building structures and the accuracy that can be expected. INDEX TERMS Atrium, Natural ventilation, buoyancy, CFD, RNG turbulence model INTRODUCTION Natural ventilation has been the subject of much research over recent decades due to its potential for offering good indoor air quality for occupants and relatively low energy usage compared with mechanical ventilation. The driving force for passive natural ventilation is generated by the temperature difference between the inside and outside of a building (known as the stack effect). Architectural features such as atria can be used to enhance the stack effect. To study natural ventilation flow in buildings at the design stage it is useful to use modelling techniques to investigate the possible ventilation flow rates, temperature gradients and stratification within the ventilated spaces. For example, simple analytical models have been developed to study natural ventilation in a single and connected space by Linden et al (1990) and Holford & Hunt (2003) respectively. The work of Hunt & Holford (1998) investigated natural ventilation flow in multi-storey spaces connected to a tall atrium with top-down-chimneys (TDCs) used to bring fresh air into the spaces from high level. These analytical models were validated by small scale experiments and used to offer a better understanding of the general principles of natural ventilation. As an alternative modelling technique, CFD has been increasingly used to model natural ventilation flows. The work of Cook & Lomas (1998), Ji et al (2004) showed that CFD was successfully used to model natural ventilation flows in a single space and a single space connected to a tall atrium. The simulation results were compared with both the analytical models and the salt bath experiments, and favourable agreement was achieved. However, there was only one airflow path in the flow systems investigated. In order to further investigate the performance of CFD for modelling natural ventilation flow in multiple spaces with more than one airflow path simultaneously a general analytical model for multi-storey spaces connected to a tall atrium is presented in this paper. The model is then used to verify the CFD predictions for this type of natural ventilation flow. The intention of this research is to offer a general analytical model for multiple ventilated spaces connected to a common atrium and how best to model this type of flow using CFD. THE ANALYTICAL MODEL A natural ventilation flow system of a two-storey building connected to an atrium is shown in Figure 1. For each storey, the air flow path includes a TDC inlet, a TDC, a storey inlet, a storey outlet and a common atrium outlet. These are denoted by the labels i =1 to 5 with cross-sectional areas of 1 j a to 5 j a respectively ( j =0, 1, 2, ..., denotes the jth floor). The effective opening area of an individual opening in an airflow path and the total effective opening area for an individual storey may be written as follows: * Corresponding author email: [email protected] Proceedings: Indoor Air 2005 1312 ji ji ji a C A 2 = and ∑ = − − = 4