Investigation of Combustion and Flame Spread Behaviors of PS Insulation Materials Applying to Building Exterior Wall


Student thesis: Doctoral Thesis

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  • Weiguang AN


Awarding Institution
Award date22 Jun 2015


Polystyrene (PS) foam is a typical organic insulation material that is widely applied to building exterior wall for energy saving. It is mainly comprised of the extruded polystyrene foam (XPS) and the expanded polystyrene foam (EPS). PS insulation materials exhibit a number of excellent properties; however, without fire-retardant treatment, PS insulation materials become a fire hazard with a high flame spread rate. Notably, combustion and flame spread across combustible solids are typical topics in research on fire safety. Combustion and flame spread behaviors of PS insulation materials are complex, and these are affected by multiple factors; comprehensive investigations on these topics have not been conducted. Therefore, studying the combustion and flame spread behaviors of PS insulation materials under the coupling effects of multiple factors is necessary. In addition, PS flame spread models are worth establishing.
Both the experimental study and the theoretical analysis are introduced in this work. Cone calorimeter test and upward and downward flame spread experiments were conducted to investigate the effects of angle of inclination, sample width and thickness, ambient pressure, sidewalls, concave structure of exterior wall, and presence of fire blockage element on combustion and flame spread rate in PS insulation materials. Moreover, the coupling effects of these factors were investigated. Considering the influence of multiple factors, ignition and flame spread models of PS insulation materials were established based on theory of combustion science and heat transfer. Predicted results were compared with experimental values to verify and optimize these models. Ultimately, the mechanisms of combustion and flame spread in PS insulation materials influenced by multiple factors were elucidated.
Cone calorimeter tests revealed combustion characteristics of PS insulation materials, including ignition time, critical heat flux, thermal thickness, heat release rate, fire growth index, effective heat combustion, total heat release, and smoke produce rate. In addition, the effects of sample thickness and radiant heat flux on these characteristics were identified. Moreover, an optimization ignition model for PS insulation materials was established.
The effects of sample width and angle of inclination on the upward flame spread in PS insulation materials were investigated. Varying trends of flame height, pool fire characteristics, flame spread rate, and preheating length were obtained. Results indicated that with the increase in sample width, flame spread rate initially dropped and then increased when the angle of inclination was small; the opposite trend was observed in samples with a larger angle of inclination. The trend in the flame spread rate versus width in Lhasa plateau with inclines of 15° and 30° was opposite that of the Hefei plain. An upward flame spread model that incorporates the influences of the sample width, incline angle, and melting and flowing of PS insulation materials was established. The prediction of this model on the flame spread rate was consistent with the trends presented above. Moreover, the influence of fire blockage element on the upward flame spread across PS insulation materials was investigated. Results showed that heat flux and heating time were critical factors that determine whether or not the flame spreads upward. A model was established to predict whether the fire blockage element could prevent the flame from spreading upward for a certain PS length and block height. Predicted results were consistent with the experimental ones. Temperature fields of upward flame spread in PS insulation materials influenced by fire blockage element were also obtained in the experiment. In addition, an upward flame spread model that incorporates the influence of concave structure was established. This model indicated that the dimensionless flame spread rate increased with increased structure factor, i.e. sidewall width to PS width ratio, although the increase rate was dropping. Predicted results also agreed with the experimental ones.
The downward flame spread behavior of XPS foam was also investigated. The sample thickness, width, ambient pressure, and façade structure were modified to investigate their effects on flame spread in XPS insulation material. The flame height, mass loss rate (MLR), surface temperatures of the sample, gas and solid phase temperature, mass growth rate of molten XPS insulation material, and flame spread rate were measured, and their changing trends with those influencing factors are identified. The influence mechanisms involved were revealed. Increased sample thickness resulted in increased flame spread rate (νƒ) of XPS insulation material. In samples without sidewalls, νƒ initially dropped and then rose with the increased sample width. The sample width at which the minimum νƒ occurs measured at a high altitude was larger than that recorded at a lower altitude. When sidewalls were present, an increase in sample width resulted in increased νƒ. νƒ obtained without sidewalls was greater than those with sidewalls. νƒ measured at a higher altitude was smaller than that at a lower altitude. A downward flame spread model that incorporates these influencing factors was established. The predicted νƒ trends agree with the experimental results presented above.
Results obtained in this work could be employed to predict the rate of fire growth in PS insulation materials installed in buildings. These results could also provide guidance for fire hazard evaluation of exterior wall thermal insulation system and the fire safety design of buildings.