[Numerical Modelling of Combustion of Fuel-Droplet-Vapour Releases in the Atmosphere]

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Acknowledgement

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Abstract

The Large-Eddy Simulation (LES) approach is used to model partially-premixed combustion (PPC) in confined and unconfined fuel vapor clouds. The model is based on the concept of a filtered reaction progress variable to describe the premixed combustion. The premixed combustion model is implemented into the Fire Dynamics Simulator (FDS), developed at the National Institute of Standards and Technology, USA, and is coupled with either an equilibrium-chemistry, mixture-fraction based model (FDS Version 4) or an eddy dissipation model (FDS Version 5) for non-premixed combustion. Modifications to the model are developed and implemented with the goal of reducing the grid resolution requirement while still producing physically sound results. The modified formulation is tested using both versions of the non-premixed combustion model, and the results are compared. It is found that the modifications are capable of reducing errors associated with poorly-resolved simulations in both versions of the model.

Table of Contents

ACKNOWLEDGEMENTII

DECLARATIONIII

ABSTRACTIV

CHAPTER 1. INTRODUCTION1

CHAPTER 2: LITERATURE REVIEW5

CHAPTER 3. MODEL DESCRIPTION30

CHAPTER 4. UNCONFINED CONFIGURATION43

CHAPTER 5. SIMULATION OF A LAMINAR DEFLAGRATION/DIFFUSION FLAME SEQUENCE65

CHAPTER 6. CONCLUSIONS67

BIBLIOGRAPHY69

Chapter 1. Introduction

There are a number of papers devoted to numerical investigations of the combustion

of vapour fuel clouds in the atmosphere (Utyuzhnikov, 2002: 137-152). The flow of a fuel-droplet-air mixture from a ruptured tank is considered. The process of a tank failure is complicated since it is accompanied by the destruction of the tank, fuel release under high pressure, the intermixing of fuel and air, and combustion. Therefore, a simple model of the initial stage of the process is desirable (Utyuzhnikov, 2002: 137-152). It was shown that it is acceptable to use a pure gas model to simulate fuel cloud behaviour in the case of nondelayed ignition (Utyuzhnikov, 2002: 137-152) .

When a flammable substance escapes into the atmosphere, in many cases two-phase outflow occurs so that the fuel cloud which builds up in the atmosphere, contains a mixture of fuel vapor and liquid fuel droplets. Two-phase releases are considered the most probable outcome of depressurization of vessels containing pressure-liquefied gases (liquefied petroleum gas [LPG]). Even if the breach in the containment is located above the liquid surface, bubble formation and swelling of liquefied gas results in discharge of a liquid-vapor mixture (Makhviladze, Roberts & Yakush, 1999: 583-605). Dispersion and combustion of two-phase releases are affected by heat and mass transfer between the phases which makes these flows more scaledependent than those in the case of singlephase fuel gas release. Over the past few years a number of numerical studies of high-temperature clouds and their effects have been ...