Burning material behaviour in hypoxic environments: An experimental study examining fire dynamics of composite materials in vitiated conditions

The progression of fire safety research has contributed to improving the guiding principles of standardised building design. Research making use of pure materials, once common in homes and workplaces, are less relevant as such materials are no longer as prevalent. It is often newer, composite materials that are featured more prominently in building contents. The trend towards the evolution of commonly found materials introduces greater uncertainty into assumptions frequently made in simplified calculation methods. Hypoxic conditions, where oxygen concentrations within an enclosed environment are lowered in order to reduce the ignitability and flammability of the room’s contents, are less frequently tested. Systems that create hypoxic conditions, referred to as Oxygen Reduction Systems (ORS), are typically used to protect high value or high-risk contents. High-loss fire scenarios and less rigorously validated research data supporting system design introduce the need for further insight. It is the primary intent of this thesis to contribute to the discussion of small-scale material testing in hypoxic conditions. Acrylonitrile butadiene styrene (ABS) samples of various thicknesses (20mm, 10mm and 5mm) have been compared to a composite mix of ABS with a surface layer of cardboard and secondary layer of bubble wrap. Tested materials were considered reasonable because they represent a plastic commonly used in the formation of high-end electronic devices whilst cardboard and bubble wrap layers represent common storage components. The samples have been tested with exposure to radiant heat fluxes, namely 25kW/m2, and 50kW/m2. The oxygen concentrations compared are 20.95%, 17% and 15%. Analysis found that due to unique material behaviours in composite samples, caused by an ash forming top layer, heat release rates for hypoxic conditions could be greater than those in ambient conditions. It is argued that unique variations in composite material behaviour limit the validity of tests of pure materials in isolation from their wider application within an ORS design.It is widely accepted that by lowering the oxygen concentration below that of the typical ambient conditions (21% Oxygen) fuels are less likely to ignite and will burn slower. ORS (Oxygen Reduction Systems) make use of this to protect their contents. The oxygen concentration in these systems is done by introducing a flow rate containing a higher percentage of nitrogen into the protected area. The following study examines how other effects also impact the minimum concentration of oxygen necessary to prevent ignition of materials, often referred to as the LOC (Limiting Oxygen Concentration). These can include the shape and size of the object, its relationship with neighbouring objects, the local temperature and many other factors. There is review of the testing process of ORS and how well these tests effectively capture the true limiting oxygen concentration or if they instead are influenced by other effects which are more dependant on the tested sample and its features. The study aims to compare effects on both pure plastics and on a composite arrangement (where multiple layers of different materials are included together). The tests were small scale and used multiple oxygen concentrations to see how variables were influenced by the oxygen concentration. Other variables changed were the thickness of the materials, the material layers, and the heat applied onto the sample. The study found that the testing methods of the ORS would be often influenced by factors other than oxygen concentration. This risks the design of systems to be such that they are designed to pass the test but are not necessarily suitable to protect the full contents where other variations are considered. The study also contributed to interesting findings on the complexity of composite behaviour compared to pure materials. Results indicated the difficulty when the value of the LOC is assumed to be the lowest of all of the materials in the composite rather than an independent value influenced by the interaction of the material behaviours as a collective. Following the completion of the study there is plenty of further analysis and exploration required in order to further contribute to the study of material burning behaviour in lower oxygen environments. Avenues for the completion of computer modelling, larger scale testing and other variables are noted as being of particular interested when pursuing further knowledge gaps