The Prediction of Smoke Detector Activation Times in a Two-Storey House Fire through CFD Modelling

Abstract

This report describes an investigation into the prediction of the activation times of domestic ionisation and photoelectric smoke detectors within a two storey dwelling, the work undertaken being an extension to that previously presented by Brammer (2002). Three fire scenarios are considered, each having been a real test fire undertaken at the Building Research Establishment in Cardington. These three fire scenarios all involved the flaming combustion of an upholstered armchair within the lounge on the Ground floor. During the experiments various results were recorded, including temperatures, optical densities and smoke detector activation times.

The fire scenarios where modelled using FDS, Version 5. Base parameters regarding the fuel load where defined to be 0.05kgsoot/kgfuel and 20MJ/kg. Consideration was also given to the effect varying the effective heat of combustion and defined soot yield would have on derived smoke detector activation times. Additional simulations where thus run considering soot yields of 0.04kgsoot/kgfuel and 0.10ksoot/kgfuel, and an effective heat of combustion of 25MJ/kg.

Three prediction methods where applied to the results of the FDS simulations for derivation of the activation times of smoke detectors located throughout the house. These methods where the temperature correlation method, Heskestad’s method, and Cleary’s method. The temperature correlation method considered activation criterions of 4°C, 13°C and 20°C above ambient.

The Heskestad and Cleary methods were found to derive comparable activation times for each detector location. None of the prediction algorithms where however found to predict activation times consistently comparable to the test data. Rather, it was determined that for an appropriate prediction method to be adopted for accurate assessment of a given fire scenario, consideration must be given to the:

• type of detector being assessed; • location of the detector relative to the fire; • mode of combustion (i.e. flaming or smouldering); and the • growth rate of the fire.