An Experimental Investigation on the Influence of Thermal Feedback on Pre-flashover Fires
General information
Project type: PhD Project
Funding: Rockwool International A/S for 2/3 of the project funding; Forening Østifterne, Otto Mønsted Foundation, and
Foreningen af kommunale beredskabschefer for experimental costs
Cooperation: Rockwool International A/S, Denmark; NRC-IRC, Canada
PhD student: Annemarie Poulsen
Supervisor: Grunde Jomaas
Time schedule: March 2008 - February 2012
Keywords: design fires pre-flashover fires, thermal feedback, experimental investigation
Project description
The aim of this project was to perform an experimental study on the influence of the thermal feedback on the burning behavior of well ventilated pre-flashover fires. For the purpose, an experimental method has been developed, where the same identical objects were tested under free burn conditions in two identical rooms with linings of significantly different thermal inertia. As all linings were non-combustible, the heat release rate could be found without the influence of thermal feedback and for two different levels of thermal feedback. The ISO 9705 Room Corner Test facility was chosen as the same measuring equipment could be used for all the tests. Using this method, 10 experiments were performed with 3 different sizes of heptane pools and 3 experiments were carried out with a block of flexible polyurethane foam. In addition to these 13 experiments, 16 experiments were carried out by Carleton University and NRC-IRC on 7 different types of fire loads representing commercial premises.
The results show that for some of the room test the thermal feedback occurring in the room tests did increase the heat release rate compared to free burn test for pre-flashover fires. Two observed phenomena related well to theory: i) in an incipient phase the heat release rate rose with the temperature of the smoke layer; the magnitude of the increase was found to depend on the flammability properties of the burning object, which can be described by a simple model; ii) a rapid increase of the heat release rate commenced after the incipient phase, seen as a thermal runaway caused by the energy gain in the smoke layer exceeded the energy that can be lost through the boundaries; the onset point of thermal runaway was found to depend on the thermal inertia of the linings as well as the flammability parameters of the burning object, which correlates well with theory.
At the onset point of thermal runaway the smoke layer temperature was found to be as low as 300 °C for linings with very low thermal inertia, which makes the onset point significantly below the traditional flashover criterion for the smoke layer of 5-600°C. This indicates that caution should be used when using this criterion for rooms with very low thermal inertia. The increase of the heat release rate after the onset of thermal feedback did not seem to be dominated by either temperature of the smoke layer or the type of burning materials. Given the profound difference between room burn conditions and free burn, the results show that free burn results should also be used with caution for prediction of pre-flashover design fires in rooms.