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Sydney is one of the most highly populated cities in Australia with a wide variety of complexity in architecture serving multiple functions. In the past, fire safety and protection systems were mainly based on prescriptive codes and they were found to be effective for traditional building developments. Owing to the rapid increase in shopping malls, as well as the increase of uniqueness in their designs, it is difficult to apply the prescriptive codes for modern buildings such as shopping malls which may comprise of a large atrium area. It presents potential risk due to smoke and flame spreading.
In fire simulations, one of the most critical challenges is the incorporation of detailed chemical description for the combustion process where intermediate chemical products are formed through a series of elementary reactions. It is essential to apply a comprehensive reaction scheme that fully describes the oxidation processes of the parent fuel and the formation processes of major intermediate chemical species. A novel in-house fire field model based on Large Eddy Simulations (LES) approach incorporating fully coupled subgrid-scale (SGS) turbulence, combustion, soot formation and radiation models for the interactive and non-linear nature of the turbulent reacting flow in compartment fire phenomena has been developed in this dissertation. It uniquely embraces the detailed reaction mechanisms for the chemical processes involved during combustion. Since the modelling of hydrocarbons by-products are enabled when considering the full chemical profile, the formation of soot particles can be related to the concentration of main incipient such as acetylene, which provides an appropriate representation of nucleation, surface growth processes. Furthermore, two alternative SGS turbulence models: Vreman model (VM) and Wall-Adapting Local Eddy Viscosity model (WALEM) are incorporated and examined for compartment fire simulations.Parametric studies have been performed in two large-scale compartment fire tests. It is found that the turbulent Prandtl and Schmidt numbers of 0.3 respectively and the Smagorinsky constant of 0.2 should be applied to correctly model the flow and thermal diffusivities. Furthermore, temperature and velocity field predications accuracies are enhanced using WALEM, owing to the wall adaptive features and the consideration of both strain and rotational rates of the turbulent field. The importance of incorporating the detailed reaction mechanisms in compartment fire simulations has been confirmed by comparing with experiments. It is discovered that species concentrations especially CO2 and CO are more accurately predicted by the detailed scheme comparing to the multi-step scheme, since the formation of hydrocarbons and nitrogen oxides are considered. This also improves the replication of the flame structure as the fire is chemically-driven within the combustion zone. In addition, the evaluation of soot particle content is also enhanced with the consideration of acetylene as the precursor.
In compartment fires, soot is an inevitable factor since it forms during incomplete combustion which consumes energy while aggregates at the ceiling. Soot particulates can act as a radiative heat transfer medium which may affect the fire development drastically. Correspondingly, when performing fire simulations using fire field models, it is essential to implement a soot model which considers the formation processes of soot particles. There are several key elementary chemical processes which may result in the generation or reduction of the number or size of soot particles including: nucleation, coagulation, surface growth, agglomeration and oxidation. In particularly, it is believed by many researchers that acetylene C2H2 plays an important role to the formation processes of soot as it is the main hydrocarbon by-product that contributes to the generation of aromatics. Furthermore, hydroxyl radiacal is the major oxidizer for the oxidation process of soot particles. In this study, a fire field model based on large eddy simulation (LES) approach has been developed which incorporates detailed chemical kinetics for combustion, as well as other essential fire modelling components. With a comprehensive description of the fuel oxidation processes as well as formation of hydrocarbons, the concentration of C2H2 and OH can be correctly simulated in the model. Therefore, the inclusion of detailed chemical products also enhances the modelling of soot particles. The Moss-Brookes two equation semi-empirical soot model is thereby utilized in the code. It considers the concentration of C2H2 as the main precursor for the reaction rates of the soot formation processes while taking OH as the oxidizer for soot oxidation. In general, the simulation showed an averaged improvement of 31.69% in the prediction of soot particle volume fraction.