Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - FACADE FIRE (Numerical characterisation of fire growth in external facades and other vertical spaces)

Teaser

In modern highrise buildings, the exterior walls are often suspended from the concrete floor slabs. Examples include curtain walls and precast concrete walls. Fire growth through spreading up the façade of high-rise buildings can lead to catastrophic loss of life and...

Summary

In modern highrise buildings, the exterior walls are often suspended from the concrete floor slabs. Examples include curtain walls and precast concrete walls. Fire growth through spreading up the façade of high-rise buildings can lead to catastrophic loss of life and property. For this reason, façades at times are required to have a fire-resistance rating, for instance, if two buildings are very close together, to lower the likelihood of fire spreading from one building to another. The issue is becoming increasingly critical because new façade materials and assemblies are being continuously introduced for better insulation and durability against rain and wind. This development is also accompanied by the advance in ‘‘smart facade’’ technologies. Fire spread on the external walls of a building can be initiated from fully involved enclosure fires at a given floor of the building, from a fire in an adjacent building or other rare events as the impact of objects such as airplanes on the building like September 9.11 of the World Trade Centre Fire. The success of devising mitigation strategy for façade fire safety should hence be based on solid understanding about the nature and hazards of fully involved enclosure fires followed by their growth along the building façade.

The overall aim of the proposed research is to investigate numerically (taking advantage of past experimental and analytical study) the behaviour of ejected flames from enclosure fires in external facades and other vertical spaces such as atrium, void spaces and staircases. Full understanding of such external flame behaviour requires insight of the combustion within the enclosure. Hence the scope of the research includes enclosure fires to characterise the ejected flames as well as fire growth in facades and other vertical spaces.

The specific objectives are:
• Fine tune and validate the open source CFD code FireFOAM, a dedicated LES based solver for fire simulation within the OpenFOAM® Toolbox for enclosure fires;
• Investigate the combustion and aerothermodynamics of enclosure fires, including travelling fires;
• Characterise the spill flame extent, combustion efficiency and heat fluxes inside and outside different enclosures with different openings (office blocks, residential buildings, travelling fires, etc.);
• Develop and validate a predictive approach based on FireFOAM for fire growth in external facades and other vertical spaces; and
• Conduct parametric studies and draw guidance for fire safety design.

Work performed

1. Simulating façade fires using FireFOAM
Ejected fires from windows can lead to vertical fire spread, resulting in cascading effects. The present study aims to apply FireFOAM, for the first time, to this fire scenario. The computational domain contain two parts, the physical domain includes an enclosure of 0.4 m (L) × 0.4 m (W) × 0.4 m (H) with an attached façade wall of 1.2 m (W) ×1.8 m (H), and the extend air domain is 0.8 m (L) × 1.2 m (W) × 1.8 m (H). The extended eddy dissipation concept proposed and implemented in FireFOAM by Chen et al. [1] is used for combustion. The one k-equation eddy viscosity model [2] is used for sub-grid scale turbulence modelling. The numerical predictions are compared with the measurements of Hu et al. [3]. The predicted flame height and flame width are in reasonably good agreement with the measurements. The external vertical plume temperature profiles near the façade wall are also compared with the measurements, its predicted value are close to measurements. The present work has laid a good foundation to extend the application of FireFOAM to full-scale façade fire modelling.

2. Effect of parallel curtain walls (façade wall) on the upward flame spread characteristics and mass loss rate over PMMA
PMMA belongs to slow-burning characteristic materials comparing with other polymers. When the flame front reached the plate tail, the material at ignition position was still burning, and no burn-out area would emerge, namely the whole PMMA board was burning from bottom to top. Hence this scenario could be regarded as a PMMA wall fire after the upward flame spread has reached the steady process. The main material area exposed to the flame can be calculated by:
(1)
Where the S, W, L and T are total surface area, sample width, sample length, and sample thickness.
For fire safety consideration, the characteristic length transition is often used to quantify the dynamic fire behaviour when dealing with irregular fuel shape. In this work, as the sample length is much longer than the sample width, the definition of the hydraulic diameter has been adopted for characteristics length calculation following its use for irregular fuel shape consideration as the equivalent diameter method, which is most commonly used for the characteristics length, only works for rectangular shape with length and width being in the same level. The hydraulic diameter D can be calculated by:
(2)
3. Effect of sidewall constraint on the flame extension beneath a ceiling induced by ejected flames from a carriage in tunnels
The present knowledge on the characteristics of ceiling jets caused by ejected flames from carriages in tunnels is inadequate. And to the best of our knowledge, no investigations have addressed the effect of sidewall–carriage distance (the distance between tunnel sidewall and fire source) on the ceiling flame spread characteristics in tunnel fires.

Final results

Progress beyond the state of the art, expected results until the end of the project and potential impacts (including the socio-economic impact and the wider societal implications of the project so far)
External flame behavior ejected from high-rise buildings is a serious risk because of its high consequences for life safety and property conservation. These tall and complex buildings need to be taken seriously for fire safety. Early research on the external flame behavior ejected from high-rise buildings mainly focuses on small scale model experimental research. But the physics understanding of such external flame behavior requires insight of the combustion processes within the enclosure is lacking, such as serious questions have been raised in the ongoing debate regarding the fire growth in external facades and other vertical spaces, including upward fire spreading mechanism upon façade. Hence the scope of the research includes enclosure fires to characterize the ejected flames as well as fire growth in façades.

In China, there have been very serious big external flame behaviour of tall buildings in recent years along with the fast development of the urban city. Our results can promote the research of tall building external flame behavior in China and form a fundamental scientific base in developing national practical fire prevention guidelines.

Website & more info

More info: https://warwick.ac.uk/fac/sci/eng/research/grouplist/fluids/warwickfire/projects/facadefire/.