Role of heat and mass transfer on the effluent generated by fire flames

Abstract

The precise chemical composition of fire effluent has eluded scientists for decades. When solid materials burn, gaseous chemical species are initially generated. The production of gaseous volatile species occurs via pyrolysis, initiated by heat transfer. Pyrolysis products are subsequently oxidised by a flame to produce combustion products. These combustion products, alongside a proportion of partially oxidised pyrolysis products, are the species entrained in air forming the aforementioned effluent. It is therefore important to consider both preceding processes in order to fully understand the composition of fire effluent.

The resultant analytical problem is twofold: to be able to identify the combustion products being produced, a study must be confident in identifying the initial volatile species being generated. This generation stems from a combination of the initial material composition, the rate at which mass is being lost, the geometry of the flame and the imposed combustion environment. Increasing the oxidative environment using existing methodologies results in the mass loss rate (MLR) of a sample increasing as the fire intensifies. This elevated MLR causes the pyrolysis rate to increase, enabling a greater proportion of oxidation reactions to occur. This increased volume of exothermic oxidative processes causes the overall heat release rate (HRR) to rise. Such effects result in the surface of the sample reaching greater temperatures, resulting in thermal gradients being established within the material under investigation. These thermal gradients alter the pyrolysis pathways available, preventing comparisons between experiments conducted under differing oxidative environments being linked to a single changeable variable.

This thesis attempts to decouple the link between the MLR of a sample and the oxidative environment, thus enabling an insight into the role of heat and mass transfer on the composition of fire effluent. Initial room-scale experiments were conducted to attempt to obtain representative fire effluent for analysis. The combustion environment obtained during these room-scale experiments enabled the interaction between a descending smoke layer and the flames to be assessed, however the flow of oxidiser could not be independently adjusted without altering other variables such as the MLR, thus, an alternative approach was required.

Reducing the scale of the problem was found to enable a greater level of control over experimental variables. The modified means of controlling the Fire Propagation Apparatus (FPA) allowed the MLR of a sample to remain fixed under differing oxidative environments. A proportional, integral derivative (PID) controller was used to adjust the voltage controlling the FPA lamps based upon a live reading from the inbuilt load cell. This resulted in a means of enabling a steady state MLR whilst recording the mass and observing the sample throughout an experiment. The combination of such data enabled near real-time yields to be calculated and linked to observed flame geometries.

Furthermore, the heat flux being sent to control the MLR was recorded, enabling the quantification of various oxidative processes when used in conjunction with the effluent analysis. The use of a combination of analytical techniques enabled a greater proportion of the species generated in the effluent to be identified, providing insights into both pyrolysis and combustion processes. Such a setup enabled the effects of oxygen on pyrolysis and flaming combustion over a range of materials to be investigated in a novel manner.

Decoupling the MLR from the oxidiser has enabled the effect of flame geometry to be linked to the effluent composition for condensed-phase fuels. By demonstrating that these flames behave as diffusion flames, the fire science field can begin to move away from treating fire flames as point sources for chemical species to enter the effluent. It is hoped that the methodology developed in this thesis will continue to enable effluent composition to be linked to changing oxidative environments in a manner that successfully decouples the symbiotic relationship between the MLR and the oxidiser.