Tuesday, 12 April 2011

Laser guns and Ignition times - research paper

We have published recently a paper in Combustion and Flame titled "Numerical Investigation of the Ignition Delay Time of a Translucent Solid at High Radiant Heat Fluxes"

This investigation revisits the theory explaining the ignition of a solid surface via a radiation external source. It led to a discovery affecting our understanding of how fires start and spread. The paper explains the failure of the classical ignition theory in polymers by using all the experimental data available to date and using a computer model to identify the missing mechanism causing the classical theory of ignition to fail when heat flux level are high. This mechanism is in-depth radiation, aka, the fact that many polymer materials are translucent to radiation.

This finding could help the US Navy fine tune their latest gadget. The BBC reports that they have fired a laser gun from one of its ships for the first time. They used a high-energy laser to carefully deliver a high flux of energy to the polymer surface covering the boat engines. This created a hot spot that reached ignition after some time, setting the engines on fire and disabling a boat. The ignition time can range from a few seconds to minutes, depending on the power of the laser. See the figure below extracted from the paper.

Time to ignition of black PMMA samples for a wide range of experimental conditions in the literature.

Our paper, and the model used in it, allows to calculate with higher precision the time required to reach ignition depending on the power of the laser and the material being heated. Higher precision calculating  the ignition time avoids failed attempts and reduces expensive laser time. It would lead to a more reliable and less costly weapon. The better tuned damage caused would be useful in non-lethal applications as well.

More at: N Bal and G Rein, Numerical Investigation of the Ignition Delay Time of a Translucent Solid at High Radiant Heat Fluxes, Combustion and Flame 158, pp. 1109–1116, 2011 http://dx.doi.org/10.1016/j.combustflame.2010.10.014

Wednesday, 6 April 2011

Oral presentation at EGU: Climate Feedbacks on Smouldering Earth

Yesterday 6 April, I gave this talk on Climate Feedbacks on Smouldering Earth at the 2011 European Geosciences Union, Vienna:

Climate Feedbacks on Smouldering Earth (talk at EGU Vienna 2011)


Climate Feedbacks on Smouldering Earth: Enhancement of Moisture deficit and self-heating of fossil and pre-fossil soils

Guillermo Rein
University of Edinburgh, School of Engineering, United Kingdom (G.Rein@ed.ac.uk)

Global smouldering phenomena, the slow, low-temperature, flameless burning of organic soils, is the most persistent type of combustion phenomena and the longest continuously fires on Earth (>6,000 years). It take place since deep times and in many ecosystems, special boreal and tropical ones. These are accidental sources of carbon emissions that during millennia have been slowly burning fuels with zero energy efficiency, consuming large amounts of fossil energy resources (coal seams), destroying natural ecosystems (peatlands) and emitting greenhouse gases and pollutants. The global problem has grown in the last decades to an estimated release varying between 10 to 40% of the man-made carbon emissions, and a coal consumption rate at least 5 times that of Germany. Because it involves the burning of fossil and pre-fossil fuels, this is a carbon-positive wildfire phenomena. This creates feedbacks in the climate system because moisture deficit and self-heating of organic soils
are enchanted under warmer climate scenarios and would lead to more frequent smouldering fires. Warmer temperatures at high latitudes are resulting already in more Artic fires and unprecedented permafrost thaw exposing large soil carbon pools to smouldering for the fist time since millennia.
While flaming fires have been a central focus in fire research, smouldering fires are as important in terms
of ecosystem damage, atmospheric emissions and socioeconomic threats but have received little attention. Moreover, these fires are difficult or impossible to detect with current remote sensing methods because the chemistry is significantly different, their thermal signature is much smaller, and the plume is much less buoyant.