A Comparative Study on Combustion Behaviours of Polyurethane Foams with Numerical Simulations using Pyrolysis Models
Thesis DisciplineFire Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This research investigates the decomposition and burning behaviours of polyurethane foams experimentally and compares the experimental results obtained with the numerical results from the pyrolysis model of Fire Dynamics Simulator, Version 5 (FDS 5). Based on the comparison of model and experimental heat release rates, the accuracy of the pyrolysis model is quantified. In total, this research tested seven polyurethane foams consisting of three non-fire retardant (NFR) and four fire retardant (FR) foams. According to the simultaneous differential scanning calorimetry and thermogravimetric analysis (SDT) experiments, the decomposition behaviour of polyurethane foams under nitrogen environment is represented by two pyrolysis reactions. The first reaction consists of foam decomposition into melts and gases while the second reaction consists of the decomposition of the remaining melts into gases.
The kinetic properties which govern the rate of decomposition are the activation energy (E), pre-exponential factor (A), reaction order (n) and heat of reaction (Δhr). Using graphical techniques, E, A and n of the first and second reactions are determined from the thermogravimetric analysis (TGA) results. Through analysing the differential scanning calorimetry (DSC) results, Δhr is determined from the changes in heat flow and sample mass. The thermophysical properties govern the heat transfer through material and these are the thermal conductivity (λ) and specific heat (cp) which are measured experimentally at ambient temperature through the Hot Disk method.
Through the Sample Feeding Vertical Cone, the decomposition and melting behaviours of polyurethane foams in a vertical orientation are investigated and the foams tested can be categorised into those which produce melts only after ignition and those which produce melts and char after ignition. The 1-dimensional burning behaviour of foams is obtained from the cone calorimeter experiments. The NFR foams show a change from plateau burning behaviour at low heat flux to two stage burning behaviour at high heat flux while the FR foams consistently show two stage burning behaviour. The combustion property governs the amount of heat released when fuel combusts and this is the effective heat of combustion (Δhc,eff) which is determined from the heat released and mass consumed in the cone experiment.
The 1-dimensional burning behaviour is simulated using the pyrolysis model of FDS 5 and two different modelling approaches are considered. The direct method uses the material properties determined experimentally as FDS 5 inputs while the refined method uses the genetic algorithm of Gpyro to refine the kinetic properties which are later used as FDS 5 inputs. The heat release rate of the model and experiment are compared through linear regression analysis which quantifies the accuracy of both methods. The accuracy is defined as the percentage of data points within the boundary of acceptance which is bounded by 25 % of the greatest experimental heat release rate. This assessment method places greater emphasis on the accuracy of developed burning phases and lesser emphasis on the accuracy of initial growth and final decay. The accuracy of the direct method is found to be 56 % while the refined method with estimated kinetic properties achieves a higher accuracy of 75 %.
The 2-dimensional burning behaviours are investigated in the foam slab experiments for two different slab thicknesses, 120 and 100 mm. The opposed-flow spread of 120 mm slab is more intense and rapid while for the 100 mm slab, the flame spread is less intense and slow. FDS 5 is used to simulate the experimental results but when the material properties either developed experimentally or refined by Gpyro are used as inputs, the model fails to produce flame spread. This is because FDS 5 does not yet have the features which address the dynamics of foam melting and the reactive nature of the flame. In order to produce flame spread in the model, E of the reactions have been reduced to increase the decomposition rate.