[Applications of Science: Fireball]

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ABSTRACT

Recent advances in the numerical simulation of transient flame behavior demonstrating the application of Computational Fluid Dynamics (CFD) to fire safety problems are presented. Two complex problems are considered: short-duration escape of hydrocarbon fuel into the atmosphere resulting in the development of a large-scale fireball, and fire development in underventilated enclosure leading to the flame exhaust into the atmosphere. It is shown that modern CFD approaches provide a powerful tool for the qualitative and quantitative analysis of hazards associated with uncontrolled fuel combustion.

Keywords: fuel releases; fireballs; enclosure fires; flame exhaust

TABLE OF CONTENTS

ABSTRACT2

CHAPTER 1: INTRODUCTION4

Vapour Cloud Explosion Development6

CHAPTER 2: LITERATURE SURVEY13

2 Simulation of Hydrocarbon Fireballs13

2.1 Baseline Scenario13

2.2 RANS Simulation13

2.3 LES Simulation15

2.4 Combustion Characteristics of Fireballs16

2.5 Scaling of Fireball Lifetime17

3 Flame exhaust in compartment fires21

3.1 Overview of experimental data21

3.2 Large Eddy Simulation of underventilated fires23

Formulation of the model29

Experimental setup32

REFERENCES34

CHAPTER 1: INTRODUCTION

Vapour cloud explosions, along with boiling liquid expanding vapour explosions (BLEVEs), are one of the major hazards in the chemical process industries [1, 2]. They are commonly accompanied with the formation of large fireballs, which present a great danger to personnel and firefighters. Quite often such accidents involve grave damage and human casualties. The most well-known examples are the explosions at Flixborough (United Kingdom, 1974) and Mexico City (1984).

Such explosions are caused by the release and vaporization of hydrocarbon fuels into the atmosphere as a result of the rupture of vessels, tanks or pipelines. Types of release and fireball development may be different depending on the particular fuel, the actual scenario of vessel failure and the specifics of the ignition process. Quite typical is a release in the form of a vertical or inclined jet. If a source of ignition is present, the jet is ignited and a fireball is formed. If the ignition process is substantially delayed, the released fuel accumulates in the atmosphere. In the case of fuels that are heavier than air, the vaporized mixture sticks to the ground and forms a flammable cloud. Such a process represents a much more dangerous situation compared with quick jet ignition, since the amount of accumulated fuel may be extremely high. Eventually, the fuel cloud may explode upon accidental ignition. The latter scenario with delayed ignition is the case for vapour cloud explosions.

It should be noted that generally both single-phase (gas) and two-phase (dispersed flow) releases may occur. In the latter case, fuel is released in the form of a vapour-droplet mixture. For such releases, consideration of the two-phase flow, including the droplet evaporation process, is essential. Fireball development and evolution have been the subject of scientific analysis for the last few decades. Most studies have used small- to medium-scale experiments and focused primarily on measuring the integral characteristics, such as the maximum diameter, elevation, lifetime, surface temperature and emissive power of fireballs resulting from LPG releases.

Mathematical modelling of fireballs is less developed. Several analytical models have been developed using fireball approximation as a rising buoyant sphere [6-9]. With revolutionary development in computational fluid dynamics (CFD) techniques ...