1 introduction

Chapter 1

Introduction

Table of Contents

1. Context of the Project ............................................................... 2 2. Numerical Models ...................................................................... 3 3. Objective of the Thesis .............................................................. 5 4. Contents of the Thesis............................................................... 6

1. Introduction

1. Context of the Project This work is the result of three and a half years of work developed in the FIRENET European research project. This European research project is focused on compartment fires developed in a ventilation-controlled situation and has a direct relevance to public safety, especially for those called to deal with emergencies such as fire fighters.

Seven research groups take part in the FIRENET project, five universities and two technological centres. The universities are: Kingston University (KU), Ulster University (FireSERT), University of Naples (UONF), National Technical University of Athens (NTUA), the University of Poitiers (LCDCNRS) and the University of Liege (ULG). And the technological centres are: ANSYS and Iceland Fire Authorities (IFA).

At the beginning of the project, each group was assigned to develop some specific tasks but during the evolution of the project new tasks were included and others were removed. In the following paragraph, a brief introduction of the main tasks of each group is provided.

Kingston University was asked to perform a study through numerical modelling of water mist by fire suppression as well as the development of a combustion model for under-ventilated fires, focusing on the simulation of backdraft and ghosting flames. The task of Ulster University was to perform an experimental investigation of glazing system response when exposed to under-ventilated enclosure fire conditions. The University of Naples was in charge of investigating solid fuel degradation and combustion, especially wood and synthetic polymers such as polyurethane, and the behaviour of different classes of flame-retardants. The University of Athens, due to administrative problems had to abandon the project. The tasks assigned to the University of Poitiers were focused on a conducting ghosting flames test that will be used for validating the results obtained by others in their numerical simulations. The task carried out by ANSYS was to model the events leading to backdraft phenomena such as gravity current and deflagration. The main tasks of the Iceland Fire Authorities were the completion of backdraft tests as well as the completion of CFD modelling of gravity currents.

The tasks for the University of Liege were to: • • • •

Perform a further analysis of the new parametric fire curves proposed by EC1 based on the existing database of full-scale fire tests from previous EC funded research.

Develop a simple numerical model for predicting and analysing backdraft phenomenon.

Develop a zone model for the prediction of glazing response Implement the previous models in OZone.

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1. Introduction To promote a European research project like this one is expensive but totally necessary. The benefit of fire research has been statistically proven. According to Cox [1999], the total cost of fire, including losses and protection, to the developed nations of the world is about 1% of gross domestic product each year.

According to Shaenmann [1991], a recent study in the USA suggested that total annual savings of $5-9 billion could be traced to the National Institute of Standards and Technology’s fire research programme, costing less that $9 million per year.

The fire safety objectives of fire engineering have been established in order to protect life and property, as well as to ensure that a disaster, which provoked the fire safety activity, “must never happen again” (Rasbash, 2004). A list of the major fire safety objectives is shown below: 1.

2.

3.

4.

Saving lives.

Life safety where is a major societal concern.

Preventing losses for individual premises and assets.

Preventing losses for premises and assets where there is a major societal concern.

5. Function maintenance.

The work presented in this thesis aims to contribute a more scientific approach towards fire engineering and fire safety of under-ventilated fires in compartments. More specifically, the main subject of this work is backdraft phenomenon. This study will allow us to fulfill the fire safety objectives shown above.

2. Numerical Models Over the last decade or so, there has been considerable activity in the development of fire computer codes. These codes have basically been aimed at assessing the hazard (toxic and thermal) associated with a potential fire. The codes vary in complexity from simple slide-rule-type, deterministic calculations to finite difference field models.

Essentially, there are two types of models available: deterministic models and probabilistic models. Whereas the former allow for a “single possible development”, the latter are able to investigate a range of possible developments.

There are three types of deterministic models used for evaluating the dynamics of fire compartments: • • •

Analytical models; Zone models; Field models (Computational fluid dynamics, CFD).

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1. Introduction Figure 1.1 shows the classification of the types of computer fire models cited above.

These tools, from the simplest to the most complex, enable the calculation of one or more aspects of compartment fires, e.g. temperature development, smoke propagation, explosions, etc.

Analytical models are usually based on simple theoretical developments or correlations obtained from experimental results. They take the form of simple equations and are suitable for hand calculations.

Zone model is the generic name given to a fire model that is based on assumption such as one in which the compartment in which the fire takes place is divided into one or more zones in which the temperature can reasonably be considered to be uniform. In zone models, the energy and mass balances are solved numerically in different zones of the fire compartment. Some fire phenomena are modelled by means of basic fundamental principles and others are modelled by means of analytical models or correlations. Zone models are then implemented in numerical software designed to solve the equations and correlations.

Fire models

Probabilistic

Analitical models

1 zone

Deterministic

Stochastic

Zone models

Field models

2 zone

Figure 1.1: Types of computer fire models The last fire model to comment on is the field model. Field models are largely based on fundamental principles but up to now still use some correlations and approximations. They are implemented in very complex software and their calculations have excellent and accurate results.

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1. Introduction It is important to know which phenomena we are trying to evaluate since some evaluations may involve only few simple calculations depending on the type of model, while others, such as CFD analyses, may require a huge number of calculations. This means an extensive computing time. Such complex analyses are costly, not only in terms of computing time but also in terms of engineer occupation due to the difficulty in defining the data and analysing and using the results produced by the tool.

3. Objective of the Thesis The main purposes of this thesis are to: •

Clarify the terminology used when describing a backdraft phenomenon.

In literature, one can find several definitions of backdraft. The existence of several definitions clearly shows that the phenomenon is not yet well understood, i.e. fire experts find it quite difficult to distinguish it from other rapid flame process phenomena such as flashover and smoke explosions.

•

Develop the first numerical model that enables us to analyse and predict the occurrence of backdraft in a fire compartment.

Backdraft is a highly dangerous fire phenomenon that can easily injure or kill people as well as cause structures to collapse. Having a tool for analysing and predicting the likelihood of backdraft will be fairly interesting not only to increase the understanding of this phenomenon but also for safety reasons, especially for those called to deal with emergencies such as fire fighters. Furthermore, the results obtained can be used for providing safety recommendations in backdraft mitigation as well as for fire fighter trainings.

This model is based on simple fundamentals and hypotheses. Developing a simple model that allows us to predict and analyse a backdraft with enough accuracy, requires a great knowledge of what the phenomenon entails.

•

Implement the backdraft model in an existing one, OZone.

OZone is a two-zone model developed at the University of Liege under the scope of the ECSC research projects “Natural Fire Safety Concept” (NFSC1, 1999) and “Natural Fire Safety Concept – Full Scale Test, Implementation in Eurocodes and Development of a User-Friendly Design Tool” (NFSC2, 2000).

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1. Introduction •

Develop a one-zone model of glazing response in a fire.

The window acts as a wall before breaking and as a vent after breaking.

In several of the fire safety engineering calculations concerned with the growth and development of room fires, it is assumed that the glazing system will completely and instantaneously fail when the upper gas layer temperature exceeds 500ºC. Others assume a percentage of the exposed area of the window to be opened from the beginning of the fire until the end. Backdraft is greatly influenced by sudden ventilation. If we try to predict backdraft, a more accurate hypothesis for glazing response than the previous one is needed. Implementation in OZone is also carried out.

•

Provide useful recommendation for tactical fire fighting.

Currently, there are only a few recommendations for tactical fire fighting to detect backdraft, i.e. oily deposits on window panes, pulsating smoke gases from small openings, and so on. New recommendations are proposed according to the results obtained in this thesis (openings, wall material, mitigation…)

4. Contents of the Thesis This research is divided into the following chapters: •

Chapter 2: “Fundamentals of Compartment Fires”

A description of compartment fire physics is given. Fire evolution in enclosures as well as the main physical phenomena is described (rate of heat release, vent flow, heat transfer to partitions, fire situation, flammability limits, deflagration…).

•

Chapter 3: “Under-ventilated Fires: Backdraft Phenomenon”

This chapter intends to explain what a backdraft phenomenon is and the different stages it comprises. Furthermore, a comparison between the definitions proposed by different fire organizations is made, providing a new one in which the weaknesses of the previous definitions are corrected.

Some backdraft incidents that took place in the past are also commented on in order to make people aware of the importance and danger of this fire situation.

The following six chapters deal with analysis and research carried out for each stage of the phenomenon.

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1. Introduction •

Chapter 4: “Pyrolysis Rate and Smouldering Combustion”.

A simplified model for calculating the pyrolysis rate of burning materials in compartments is developed and implemented in OZone. The model will be used to obtain a method for predicting the risk of backdraft as a function of the material of the partitions, ventilation factor and compartment geometry.

• • •

Chapter 5: “Combustion Products in Fires”.

Chapter 6: “Glazing Response in Fires”.

Chapter 7: “Gravity Current Prior to Backdraft”.

The effect of the opening geometry, compartment ratio and the presence of obstacles in the compartment on the gravity current structure are studied by means of CFD simulations (ANSYS-CFX).

•

Chapter 8: “Flammability Limits of Flammable Mixture”.

A model for computing the flammability limits of premixed mixtures at backdraft conditions of temperature and pressure is developed.

Furthermore, a parametric study of the possibility of having backdraft in a compartment according to the type of mixture is carried out.

• •

Chapter 9: “Ignition of the Flammable Region: Backdraft Deflagration”.

Chapter 10: “Energy Released in Backdraft”.

The last three chapters refer to the implementation of the model in OZone, its application to real life and the conclusions drawn during the FIRENET project.

• • •

Chapter 11: “Implementation in OZone”.

Chapter 12: “Application: Fire in a Building”.

Chapter 13: “Conclusions”.

Validations of the results obtained with OZone are found at the end of each chapter.

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1. Introduction

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