Interaction of high speed flow features with flames


What?

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A major research effort is underway to investigate the fundamentals of the interactions between shock waves and high speed vortices with flames, with the eventual applied goal of optimising a system that could be used to regain control over a remote wildfire before it can threaten people, infrastructure, and large swathes of wilderness regions.

The technique of using blast waves from high explosives to knock out a fire isn't new - the oil and gas industry have been doing it for almost a century, and Russian researchers several decades ago proposed a similar approach for wildfires. What's been missing has been a detailed, thorough understanding of the physics involved - without that, it becomes difficult to determine what would make for an effective scenario for a fire raging in the forest canopy.

Undergraduates and postdoctoral researchers have been working with our international collaborators at the Energetic Materials Research and Testing Center in New Mexico, USA,

including Dr. Michael Hargather, to undertake experiments that are leading to the first proper visualisation of the events. Numerical work accompanying this program is building up in complexity to offer additional insight into the problem.

One (two?) of the most frequently asked questions is "isn't that dangerous, and won't you kill all the koalas and birds?", to which the answer would be i) yes there are clear hazards, which is why we need to fully understand the flowfields and their effects in order to think about how this could be deployed as a controlled, effective technique (i.e. do not try this at home!), and ii) burning to death is not fun for any animal - millions of native fauna perished in the Victorian Black Saturday fires alone, with extinction-potential fire events not uncommon in various parts of the world now given some species are barely hanging on in tiny pockets of their remaining habitat. We'd like to have a role to play in preventing that happening.

How?

In the most basic sense, it's a really over-kill way of blowing out a big candle. The blast, be it with an explosion or with compressed air, creates a fast moving shock wave that's trailed by more high-speed flow - this high speed flow effectively pushes the fire off the fuel source if it's at close range... once the flame can't access fuel, combustion stops almost instantly. If the flame is further away, the supersonic shock wave has more of a role to play, introducing small-scale disturbances in the flame structures which grow and destabilise the flames. If the flame is offset to the side of the tube from which the blast is coming, then large vortices interact with the flame - their rotational energy introduces a lot of turbulence and disruption and the flame spins itself to death. These effects are quite different at lab scale vs. field scale, so it's important to get a much better understanding of the relationships between the variables involved.



What are the next steps?

There are some obvious steps to take now in continuing the research - firstly, scaling the experiments up another order of magnitude, and having an application-realistic fire scenario. Computational modeling of large turbulent flames is no easy task in itself - incorporating high speed flow features into the simulations is another big job on top of that, but it will help us predict with greater accuracy what will happen in a multitude of potential scenarios. It's expensive and tricky to do physical tests, so the better the modeling gets, the more we can understand about how this can be applied. 

The sequence below shows the interaction of a blast wave with a propane flame, from the New Mexico tests. Shock waves are normally invisible to the naked eye, so with conventional high speed video we can see the flame going out but that's about it. With a shadowgraph technique, we can effectively see changes in density (i.e. where it's hot, or where the shock wave is) because focused light will refract by a certain amount through these features - the light that refracts can be cut off from entering the camera. Videos like these, coupled with high-frequency pressure measurements and infra-red videos showing heat, give us the best possible understanding of the flow features at work - understanding that is necessary to scale up to the more complex world of a large wildfire.

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