Predicting natural ventilation performance using ventilation effectiveness

The challenge of mitigating climate change by reducing carbon emissions means efforts are required to move away from carbon intensive practices to low/zero carbon alternatives.

Natural ventilation allows the internal environment of buildings to be regulated without the carbon footprint associated with mechanical ventilation. This research aimed to provide a better understanding of the performance of natural ventilation particularly in relation to the design guidance publishing in several building standards (CIBSE and ASHRAE). A detailed literature review identified two key gaps in the current knowledge.

The first gap was that limited research has been conducted on the use of multiple ventilation effectiveness metrics to cover the performance of natural ventilation at regulating the three core components of the internal environment (age of air, temperature and contaminant concentration). The second knowledge gap was related to a lack of studies which have used these ventilation effectiveness metrics to review in detail the full breadth of natural ventilation guidance published by CIBSE and ASHRAE and the impact that these design decisions have on the ventilation performance.

The focus was around the use of three ventilation effectiveness metrics to predict the effectiveness of a range of natural ventilation strategies. The three ventilation effectiveness metrics were the air removal effectiveness, heat removal effectiveness and contaminant removal effectiveness. These metrics used the same calculation methods but each focussed on a different component of the internal environment (age of air, air temperature or contaminant concentration). The final outcome of this research was to propose a set of ventilation effectiveness values for a range of natural ventilation strategies in line with the guidance provided by the building standards.

To model the natural ventilation strategies and evaluate the ventilation effectiveness, computational fluid dynamic techniques were utilized. The open source CFD modelling software OpenFOAM was chosen as the primary toolkit. Studies were conducted to identify the most appropriate modelling configuration and post processing procedure to predict both the fluid mechanics associated with buoyancy driven natural ventilation and the parameters required for the ventilation effectiveness metrics. The k-ω SST model was selected as the primary turbulence model. The internal contaminant (carbon dioxide) source was defined as a uniform source within the breathing zone. Due to the skewed nature of some of the calculated fields, predominantly air temperature, the median value was chosen as the average value to use for all components of the three ventilation effectiveness metrics.

The main study of this research predicted ventilation effectiveness values for a range of natural ventilation strategies. These strategies reflected the core natural ventilation design guidance published by both CIBSE and ASHRAE. The design strategies covered aspects such as building geometry (ceiling height and building depth), ventilation type (single-sided, cross and corner), number & location of openings, window type and effective opening area. The results from this study proved that all three ventilation effectiveness metrics were capable and suitable of predicting the ventilation effectiveness of natural ventilation. For the air removal effectiveness values were predicted ranging from 1.0 to 2.1. For the heat removal effectiveness values were predicted ranging from 1.4 to 13.9, while for the contaminant removal effectiveness values were predicted ranging from 1.0 to 1.4. Changes in the ventilation flow rate had the largest impact on the heat removal effectiveness as the air temperature in the breathing zone decreased. While the contaminant removal effectiveness values remained stable throughout changes to both the ventilation flow rate and reductions in the internal contaminant concentrations.

The final outcome of this research was to propose a set of ventilation effectiveness values in relation to the natural ventilation guidance which would work within the context of the building standards. The proposed ventilation effectiveness values were predicted for the core natural ventilation guidance such as single or double openings, singlesided/cross/corner ventilation and effective opening area. Values were predicted ranging from 1.3 to 3.3 for the air removal effectiveness, 1.8 to 13.2 for the heat removal effectiveness and from 0.8 to 1.4 for the contaminant removal effectiveness. These values form a strong argument for the inclusion of natural ventilation effectiveness values into the building standards with more research required to incorporate more complex natural ventilation strategies and the effect of wind driven natural ventilation.