Multi-scale Fire Modelling Method in Longitudinally Ventilated Tunnels for FDS6.1
Edmund Ang, Guillermo Rein, Joaquim Peiro, Roger Harrison, Izabella Vermesi
Contents
Introduction
Multi-scale Modelling
Implementation of Multi-scale Modelling
Cold Flow Multi-scale Modelling Results
Multi-scale Fire Modelling Results
Conclusion
Reference
Introduction - Team
Work carried out in 2014
Led by Edmund Ang (Imperial College London), with collaborators:
Guillermo Rein (Imperial College London)
Joaquin Peiro (Imperial College London)
Roger Harrison (AECOM UK)
Izabella Vermesi (Imperial College London)
Special thanks to software support from Thunderhead Engineering
Introduction - Multi-scale Modelling
Multi-scale modelling based on Colella
et al
[1]
Previous work based on commercial CFD package and Reynolds Averaged Navier Stokes
Multi-scale modelling using FDS6:
Not attempted before
Enabled by the FDS HVAC feature
Proof of concept by Vermesi
et al
[2]
Constant flow specified at boundary
No coupling of jet fans to the fire behaviour
Multi-scale Modelling - What and Why
Increasingly denser and taller buildings
Scarcity of land resources
High rise buildings
Options for above ground rail or utility network limited
Tunnel is often the more practical solution
Tunnels can be used for rail, roads or utilities
Multi-scale Modelling - What and Why
Ventilation is needed to most tunnels
Life safety (fumes or CO from fire)
Regulating temperature (train operation)
Three main types of ventilation system
Longitudinal (jet fans)
Transverse (ducted supply and extract)
Semi-transverse (hybrid of ducted and non-ducted)
Multi-scale Modelling - What and Why
Figure
1
.
ECRL Rail Tunnel, Australia
Multi-scale Modelling - What and Why
Design of tunnel ventilation system
1D / Subway Environment Simulation (SES)
Early stages of design
Fast computation
Provides global averaged prediction
Limitations on gas species or high resolution calculations
Multi-scale Modelling - What and Why
3D / Full CFD simulation
To validate the design carried out at the earlier stages
Time consuming
Provides high resolution predictions, e.g. gas species and combustion
Tunnel section models often shorten to reduce computational time
Is there a best of both world?
Multi-scale Modelling - What and Why
Multi-scale modelling method
Using 1D for far field tunnel sections
Full CFD for near field, or tunnel section of interest
Significantly reduce the computational time
Off-set computational time for longer tunnel section
Multi-scale Modelling - What and Why
Multi-scale modelling method
Direct and indirect coupling methods
FDS6.1 is based on indirect coupling method
Implemented using HVAC feature
Acknowledge this is not the intended use of HVAC
Multi-scale Modelling - Implementation
Model is based on Dartford Tunnel West
Cold flow field measurement data available [1]
Dartford Tunnel West properties:
1.5 km long
14 jet fans pairs (JFP) with 8.3 m
3
/s per fan
Key of multi-scale model
Interface between 1D and full CFD sections
Flow need to be fully developed
Multi-scale Modelling - Implementation Diagram
Figure
2
.
Representation of multi-scale model
Multi-scale Modelling - Implementation Details
L
p
is length of portal = 50 m
L
JF,UP
and L
JF,DW
are length up and downstream of jet fans = 35 m and 130 m respectively
L
fire
is length each side of fire = 170 m
Calibrated from running multiple models
Cold Flow Modelling Results
Cold flow modelling results presented separately in Tunnelling and Underground Space Technology [3]
Good correlation from 80 m downwind of the jet fans
Poorer correlation nearer to jet fans:
Lack of detailed information of installed jet fans
Difficulty to accurately model the jet fans
No accurate measurement of tunnel walls' surface roughness
Cold Flow Modelling Results
Figure
3
.
Average velocities measured in the tunnel [3]
Cold Flow Modelling Results
Figure
4
.
Multi-scale model velocity profile downwind of jet fans [3]
Cold Flow Modelling Results
Figure
5
.
Multi-scale model velocity profile downwind of jet fans [3]
Cold Flow Modelling Results
Figure
6
.
Reduction in computational time for the multi-scale model
Multi-scale Fire Modelling Results
Adapted from the same cold flow multi-scale model
Introduction of a fire in the middle of the tunnel
Three fire sizes considered, 35 MW, 55 MW and 75 MW
Validation study conducted (Arup Tunnel case)
Mass flow rate is the measured variable
Interesting behaviour observed in the multi-scale model
Multi-scale Fire Modelling Results
Oscillatory mass flow observed
Mass flow rates in multi-scale models do not stabilise compared to the full CFD model
It is yet to be determined if the oscillation is numerical
Similar oscillation for velocity and temperature observed by Vermesi
et al
[2]
Multi-scale Fire Modelling Results
Figure
7
.
Mass flow rates (35 MW) in the tunnel. Left: Mass flow in and out. Right: Mass flow along the tunnel.
Multi-scale Fire Modelling Results
Figure
8
.
Mass flow rates (55 MW) in the tunnel. Left: Mass flow in and out. Right: Mass flow along the tunnel.
Multi-scale Fire Modelling Results
Figure
9
.
Mass flow rates (75 MW) in the tunnel. Left: Mass flow in and out. Right: Mass flow along the tunnel.
Multi-scale Fire Modelling Results