Parametric investigation of the fire development in parallel plate systems

Abstract

The factors which influence the rate of fire growth for a parallel plate system of dissimilar materials are investigated. This work has been undertaken within the context of cladding fires and this defines the scope of the work. A parallel plate system in which one face is fixed as a PMMA sheet while the material comprising the other face, and the cavity separation are varied. This allows exploration of the mechanisms which control fire growth in these systems. Of particular interest are the thermal properties of materials, their combustibility and the geometry of the cladding system on the rates of fire growth in the system. The apparatus was designed such that the fire growth on the continuous sheet of PMMA could be recorded visually, while measurements of mass loss, and energy contribution for the PMMA sheet and the opposing material could be made in order to quantify the processes which drive fire growth.

A quadrant-based approach was used to assess the affects of different types of material based on their thermal inertia and their combustibility i.e. materials of high and low thermal inertia and high and low heat of combustion were systematically studied. Polymeric foam insulation, mineral wool insulation and vermiculite board and medium density fibreboard were chosen to satisfy these criteria. It was found that both the material combustibility and thermal inertia influence the fire development in this system. A lower thermal inertia leads to a faster flame spread on the PMMA sheet and a faster rate of fire growth as determined by the heat release rate. Conversely with an increase in the heat of combustion there is also an increase in the flame spread rate, however that is not reflected in the rate of fire growth. The contribution to the total heat release rate from the opposing panel was negligible, contributing less than 20% through the period of fire growth on the PMMA sheet which illustrates that the flame spread rate is governed by heat transfer within the system rather than the burning of the opposing plate. The effects of the presence of a foil face vapour barrier was also explored for both foams. It was found that the foil prevented some of the thermo-mechanical changes experienced by the foamed materials, but otherwise had little effect on the flame spread or heat release rate.

In line with exploring the consequence of different materials within the cladding system, so too must we consider how those materials are situated with respect to one another. The sizes of cavity vary substantially across systems. It was found that for cavity spacings greater than 100 mm, there is no significant, systematic effect on the fire growth. For small cavity spacings flame elongation became significant for each material.

This research presents a systematic analysis of the effects of different parameters on the fire development in cladding-like systems. These are complex systems and the fire hazard is shown to be governed by material, geometrical and mechanical properties. Nevertheless, extrapolation of the findings from this thesis is possible in general terms and can provide direction to further study and assessing the design of cladding systems.