In this report the effects of fire and explosion are examined based on quantitative analysis using calorimetric methods.

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Explosion and Fire

Name:                                Andrew Holmes

Student Number:                2103086

Course:                                BSc Forensic Science

Year:                                2

Unit:                                Fire and Explosion

Date:                                17/04/03

Summary

In this report the effects of fire and explosion are examined based on quantitative analysis using calorimetric methods. The report focuses on a series of four tests using calorimetric instruments to obtain measurements, which are examined in a quantitative fashion. These tests are: the oxygen index test, the bomb calorimeter, the flash point test, and the flame stability test.

The oxygen index for the various materials, calculated from the experimental results were found to be 16.67, 27.77 and 26.6 for Calico Med, Polyester and wool mix respectively.

The flash points determined from the experimentation were, 99.5 oC, 72.0 oC and 70.5 oC for Cyclohexanone 99%, 4-Hydroxy-4-Methyl-2-Pentanon 99% and an unknown hydrocarbon respectively.

The maximum flame speed for a natural gas-air flame in a horizontal tube was calculated to be 0.68ms-1, at a stiociometric ratio of 10:1, from the flame stability calculations.

After calculations using data obtained from the bomb calorimter, the enthalpy of combustion for Perspex was determined to be 16.55 Kj.kg-1.  

1.0 Introduction

A fire is the results of a number of reactions between a combustible material (fuel) and oxygen, which may be from a variety of sources, most commonly air. During which heat and light are generated. Other products from a fire include solids and gases or vapors. Flames result from gasses or vapors burning.

There are many reasons for investigating the properties of fire and it behavior. Through an understanding of the fundamental behavior of fire and fire dynamics a fire protection engineer can apply they’re understanding at the design stage rather than relying on fire incidents to draw attention to inherent fire hazards.

There are various methods available designed for the forensic analysis of fire and burning behavior, which include, oxygen consumption calorimetry, bomb calorimetry, pensky martins flash point apparatus, flame speed measurement, NIST smoke chamber and heat release rate measurements using a cone calorimeter.

In this report the experimental apparatus of the forensic analysis of fire and burning behavior, experimentation and results of their use are disused. The following techniques were employed.

  • Determination of Flash Point by means of the Pensky-Martins Closed Tester.
  • Determination of Flame Stability.
  • Oxygen Index Test.
  • Determination of Calorific Value by means of the Bomb Calorimeter.

The aim of the flash point experimentation was to determine the flash point of several hydrocarbon liquids by means of the Pensky Martins Closed Tester.

The aims of the flame stability experiments was to measure the speed of a gas, measure the limits of flammability in air, to determine the characteristics of a gas burner, to draw a Fuidge diagram for the burner. This was achieved using vertical and horizontal gas burners.

The aim of the oxygen index test was to measure the oxygen index of textile fabrics using critical oxygen index equipment.

The aim of the Bomb calorimeter experiments was to measure the water equivalent of the bomb calorimeter and to measure the calorific value of plastic.

1.1 Aims and Objectives of the Report

The aims and objectives which will be achieved by constructing this report are

  • To examine the dynamics of fire and explosion by using calorimetric methods,
  • To quantitatively analyze the measurements obtained from calorimetric instruments,
  • To interpret the results obtained using graphical analysis.

1.2 Theory

 A fire is the result of a number of reactions between a combustible material (fuel) and oxygen, which may be from a variety of sources, most commonly air. During which heat and light are generated. Other products from a fire include solids (such as ash) and gases or vapours. Flames results from gases or vapours burning, although fires can occur on solids, without flame production. To prevent a fire form starting or progressing, one of the three components must be removed; for example, depriving the fire of oxygen by smothering the fire, or denying the fire heat by cooling. This is fundamental to the dynamics of fire and explosion.

A flammable gas or vapour will only burn in air if the composition lies between certain limits. These limits are called the “Upper” and “Lower” flammability limits. If an excess of fuel vapour is present in the fuel-air mixture, then the mixture is said to be too rich to burn and not ignite. If too little fuel vapour is present as a proportion of the fuel-air mixture, then the mixture is said to be too lean and again cannot be ignited. Flammability limits vary between fuels and pressures.

When these limits are exactly at the right proportions of oxygen to fuel, the composition will react perfectly and completely, this is known as the stoichiometric ratio. When this mixture is ignited, combustion is ideal. This theory is important in the flame stability calculations as the optimum flame speed occurs when air/fuel mixture is a stiochiometric ratio.

The rate at which a fire will develop will depend on how rapidly flame can spread from the point of ignition to involve an increasingly large area or combustible material. Flame spread can be considered as an advancing ignition front in which the leading edge of the flame acts both as the source of heat and as the source of pilot ignition. Various factors which are known to be significant in determining the rate of flame spread, such as material and environmental factors. Material factors include composition of fuel, initial temperature and environmental factors include Composition of atmosphere, pressure etc.

The burning velocity or flame speed is defined as the rate at which the plane combustion wave will propagate into a stationary, quiescent flammable mixture of infinite extent. There are many experimental parameters which vary the flame speed. The variation of mixture composition is one such factor which effects the burning velocity. The burning velocity of fuel/air mixtures is maximum for mixtures on the slightly fuel rich side of the stoichiometric. The burning velocity is increased if the proportion of oxygen in the atmosphere is increased. This is of extreme significance as there is many occasions where oxygen enriched atmospheres are produced accidentally, such as leakage from an oxygen supply system in a hospital. The critical oxygen index test measures the minimum volume concentration of oxygen, which will support combustion of the test fabric.

Increasing the temperature of the fuel increases the rate of spread. Therefore, the higher the initial fuel temperature the less heat is required to raise the unaffected fuel to the fire point ahead of the flame. Higher rates of flame spread are observed at increased atmospheric pressure because of the oxygen enrichment, which enhances flame stability at the surface.  This is demonstrated in the Bomb calorimeter where high pressure enables complete combustion of the test sample.

Flame speed is related to flame spread. A flame stability test measures the flame speed of natural gas in a horizontal tube containing the relevant gas mixture and therefore, how rapidly fire can spread through a pipe.

As fire growth progresses through flame spread, analysis of a potential fire spread situation and the ability to quantify the rate of development is extremely valuable. Thus, the practical experiments presented in this report analyze such fire and explosion dynamics using calorific methods.

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The addition of suppressants, variation of temperature and variation of pressure also affect the burning velocity.

Flame spread is most rapid when it is directed vertically upward, and an increase in flame spread is directly proportional to increase in the angle of inclination. The reason for this behavior lies in the way in which the physical interaction between the flame and the un-burnt fuel changes as the orientation is varied. Thus while downward spread (-90°) achieves a slow, steady rate of propagation, upward spread (+90°) accelerates as the flame and hot combustion products will preheat the un-burnt fuel directly ...

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