Monday, 12 October 2015

Fire Science is in Season

Article reprinted with permission from the Combustion Institute
 
by Guillermo Rein, Imperial College London, UK
and Naian Liu, University of Science and Technology, China


Wildfires in the United States this season are raging in California and other regions. Thousands of people have been evacuated from their communities, their homes lost. Millions of hectares of forest have burned. Countries in the southern hemisphere such as Australia and South Africa are preparing for what government agencies expect to be a severe brushfire season. Billions of U.S. dollars are spent annually around the world to fight wildfires. Particularly large firefighting budgets are approved in the United States, Australia, Canada, China and the European Union.

But let’s start with the broad context to the wildfire problem borrowing ideas from (Rein 2015). Fire is a natural phenomenon. It contributed to shaping most ecosystems on Earth and plays essential roles supporting life through the regulation of atmospheric oxygen, the carbon cycle, and the climate. However, wildfire is also a hazard to life, and when it threatens human populations or valuable ecosystems, it must be suppressed.


Despite its central importance to the planet and to humanity, our understanding of fire remains limited. For example, we currently cannot predict the location of a fire in 30 minutes time. To quote Prof HC Hottel at MIT (1984): “A case can be made for fire being, next to the life processes, the most complex of phenomena to understand”. It comes as no surprise, then, that the discipline of fire science is less mature than other combustion topics.

Fire has been a topic of interest to the Combustion Institute since its foundation in 1954. For the combustion expert, wildfires are large-scale turbulent non-premixed flames fed by pyrolysis of a condensed-phase natural fuel. Historical contributions from combustion research have been especially important in understanding ignition and flame spread of natural fuels, flame radiation and emissions. Recent contributions include work published in Combustion and Flame or Proceedings of the Combustion Institute on flame spread over porous fuel beds (Liu et al. 2014), wildfire radiation (Cruz et al. 2011), forecasting wildfire dynamics (Rochoux et al. 2013), thermofluids of fire whirls (Lei et al. 2015) and heterogeneous chemistry of smoldering wildfires (Huang and Rein 2014).

Left: Flame spread experiment over an artificial inclined canyon. Photo by JR Raposo (Laboratory for Forest Fire Studies - LEIF, Coimbra, Portugal) 2014. Right: Combination of high-speed imaging shots shows the formation of a 1kW fire whirl under different angular speeds. Image by J Lei (SKLFS, China) 2014.


We must highlight the most recent contribution of combustion science to wildfires. The work of Finney et al. (2015) just published in PNAS is a scientific breakthrough. Finney et al. have discovered the long-missing piece of the puzzle to understand wildfire dynamics. For the first time, their work puts forward a fundamental, comprehensive and verifiable theory of flaming wildfire spread. Finney’s theory relates the rate of spread to basic fluid mechanics and heat transfer, and it is strongly supported by laboratory measurements and field observations. We expect Finney’s theory to have a profound impact in the field. Once implemented into a new fire spread model, the theory would improve predictions of fire behavior and help them gain in both accuracy and consistency. This in turn would allow the simulations used by the Fire Service worldwide to provide more reliable information for deployment and disaster management of fire incidents.

Combustion science is an essential enabler of understanding of wildfire dynamics. It is expected that by strengthening the importance of fundamental knowledge and by growing the fire community in the Combustion Institute, combustion science will serve as the basis for tackling wildfires.

References
  • MG Cruz, BW Butler, DX Viegas, P Palheiro, Characterization of flame radiosity in shrubland fires, Combustion and Flame 158 (2011) 1970–1976.
  • MA Finney, JD Cohen, JM Forthofer, SS McAllister, MJ Gollner, DJ Gorham, K Saito, NK Akafuah, BA Adam, JD English (2015) The role of buoyant flame dynamics in wildfire spread. Proc. Natl. Acad. Sci. USA, 10.1073/pnas.1504498112.
  • HC Hottel, Stimulation of fire research in the United States after 1940, Combustion Science and Technology, 1984, 39:1–10.
  • X Huang, G Rein, Smouldering combustion of peat in wildfires: Inverse modelling of the drying and the thermal and oxidative decomposition kinetics, Combustion and Flame 161 (2014) 1633–1644. 
  • J Lei, N Liu, L Zhang, K Satoh, Temperature, velocity and air entrainment of fire whirl plume: A comprehensive experimental investigation, Combustion and Flame 162 (2015) 745–758.
  • N Liu, J Wu, H Chen, L Zhang, Z Deng, K Satoh, DX. Viegas, JR. Raposo, Upslope spread of a linear flame front over a pine needle fuel bed: The role of convection cooling, Proceedings of the Combustion Institute 35 (2015) 2691–2698.
  • G Rein, Breakthrough in the understanding of flaming wildfires, Proceedings of the National Academy of Science 112 (32), pp. 9795-9796, 2015. doi: 10.1073/pnas.1512432112.
  • MC Rochoux, B Delmotte, B Cuenot, S Ricci, A Trouvé, Regional-scale simulations of wildland fire spread informed by real-time flame front observations, Proceedings of the Combustion Institute (2013), 34:2641-2647.