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