Numerical modelling of tunnel fires and water mist suppression

Hart, Robert (2006) Numerical modelling of tunnel fires and water mist suppression. PhD thesis, University of Nottingham.

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Abstract

Fires in mine tunnels and other underground space are a serious hazard, that can, if left unchecked, result in significant economic loss and human tragedy. In the UK, methods such as water deluge, foam application, and various types of handheld extinguishers have been used, but statistics show no improvement in the incidence of fire. Water mist has the potential to be an effective fire suppression system for tunnel spaces. Typical water mist systems utilise small droplets of around 100 micron that have a low terminal velocity and a high surface to volume ratio. This leads to behaviour distinct from that of traditional sprinklers. Various mechanisms of action have been identified: removal of heat; oxygen depletion; fuel cooling; attenuation of radiation; and disruption of air flow. The relative importance of each is case dependent.

Current research has focussed almost exclusively on enclosures with minimal or no ventilation, and no data relevant to the application of mist in tunnels exists.

In this thesis, a series of Computational Fluid Dynamics (CFD) simulations, based on published experimental data, are used to indirectly validate a CFD model of a hypothetical water mist system applied to a real tunnel fire, and to improve the understanding of how water mist performs in a strongly ventilated space. The water mist is represented by a Lagrangian-based particle-tracking model. This model is fully coupled to the continuous phase, accounting for transfer of momentum, heat, and mass.

A 16m3 unventilated enclosure is used first to validate a pool fire model based on 0.3m square pools of methanol (27 kW) and hexane (115 kW). The behaviour of a thermal plume in a tunnel with forced ventilation is then validated, initially using a fixed volumetric heat source of 7.5kW in a small-scale tunnel, and then on a full-scale 3m square cross-section tunnel with a 3m diesel pool using the pool fire model.

The water mist model is validated with the enclosure fire, and a sensitivity study assesses the effect of droplet diameter, spray velocity and angle, and water flow rate on the performance of the system. Finally water mist is applied to the tunnel fire At low ventilation, oxygen depletion and air-flow disruption are significant,

whereas at high ventilation the only effect of the mist is to remove heat and reduce temperature.

Item Type:Thesis (PhD)
Supervisors:Lowndes, I.S
Morvan, H.P.
Uncontrolled Keywords:Water mist, WMFSS, fire, tunnels, fire suppression, CFD, Computation Fluid Dynamics, Fluent
Faculties/Schools:UK Campuses > Faculty of Engineering > School of Chemical and Environmental Engineering
ID Code:185
Deposited By:Robert Hart
Deposited On:19 Jul 2006
Last Modified:06 Feb 2009 14:43

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