Thermal models for fire safety : calculation of flame spread on surfaces and heating of structures

Abstract

The studies presented in this thesis under the title "Thermal models for fire safety – calculation of flame spread on surfaces and heating of structures" consist of two parts: flame spread on combustible surfaces and calculation of heating of structures.

This work consists of the development of thermal models for Fire Safety purposes. The main objective of the present thesis work is to produce new information for fire safety related to the development of models for flame spread on surfaces and to develop engineering calculation methods in heating of structures.

In the context of Fire Safety, the word "fire" means accidental unwanted fires. The primary goal in Fire Safety is to protect life and property. The research field is relatively young and multi-disciplinary.

The growth rate of a fire depends on how fast the flame will spread and involve more burning surfaces. In an enclosure, the burning rate is enhanced due to feedback effects but it is still the increasing area of the fire that affect the flame size. In fully developed fire, as in enclosures, the availablilty of air limits the rate of heat release. Fire growth rate and the rate of heat release depend highly on how rapidly the initialized fire propagates on surfaces. Thus, it is the flame spread that controls the rate of heat release in large or open spaces. This shows the importance of modeling flame spread, due to its direct impact on fire safety.

This first part of the work discusses upward surface flame spread on a combustible solid surface. The flame spread is a process of a moving flame in the vicinity of a pyrolysing region on the surface which acts as a fuel source. The flame itself results from the combustion in the atmosphere of the pyrolysed gases leaving the surface. The oxygen and fuel concentrations together with the heat transfer phenomena between the flame and the solid phase affect strongly the process. Flame spread models of various levels of complexity are developed. A novel thermal pyrolysis upward flame spread model is also developed to predict the fire growth of combustible charring wall linings.

Heat release rate in fires is of primary importance. When a structure is present, a part of the calorific energy dissipated in the fire is fed back to the structure via thermal radiation and convection with consequence of raising its temperature. As the performance of structures decrease with the increasing of temperature, knowledge of temperature distribution within the structure it is important to estimate the safe-escape time for occupants, safe-operational time for firemen and fire resistance. It is therefore essential to model heat transfer in structures. In the second part of the thesis heating of structures and temperature calculations in solids are addressed.

Efficient engineering temperature calculation algorithms for various fire heated structures are developed.