Before my little incident at an airshow I knew very little about burns. As far as I was concerned hot things against your skin caused burns and they were classed as different degrees depending on how serious they were.
Burns are actually far more complex. Burns are any injury that destroys skin caused by some form of energy. That energy might be contact with a hot object, hot vapour, chemicals, friction or, in the case of flash burns, thermal radiation.
At first I thought I had received fairly minor burns to a good proportion of my body. Large parts of my skin initially turned red as my body tried to cool me down as much as possible. Beyond that I can’t remember what my initial injuries looked like, I was in shock and on painkillers so I wasn’t thinking straight.
It was only when I arrived at hospital over 6 hours after the accident that I could see the extent of my burns. I was burnt predominantly on my left leg and my right arm. Only the areas that faced the source of the heat were burnt. But I distinctly remember being inside the fireball, I would have expected to be burnt everywhere that my skin was exposed.
I set out to investigate exactly what had happened to me. I was in hospital for 10 days and had plenty of spare time so I set out to research flash burns and fireballs.
I suffered what the doctors called partial thickness burns (what most people know as second degree burns). This is where the epidermis is completely destroyed and there is partial loss of the dermis. My burns were slightly deeper on my leg and not very deep at all on my neck and upper arm.
Fireballs produce the most heat from their outer surface. This makes sense, only the portion which is exposed to oxygen can burn in a way which produces the greatest amount of heat. Inside the fireball there is incomplete combustion producing soot. These sooty products then completely combust as they meet oxygen at the edge of the fireball.
I estimate that between 500kg and 1000kg of fuel was burnt in total in the crash. This gives us a ballpark figure to estimate the amount of energy that might have been produced and investigate the thermal radiation.
There were multiple events so taking 200kg as the amount burnt in the fireball that engulfed me. Some rough models suggest that the fireball had a radius of about 16m (32m in diameter) and took 2.2 seconds to burn. This is about right looking at videos of the event. These numbers give an average power of about 4GW of which about 1.3GW was radiated. Further calculating the radiant heat flux was about 400kW/m2. My body had a surface area facing the fireball of about 0.5m2 so I experienced 200kW of heating, the equivalent of 100 2kW heaters.
Studies show that the threshold to receive a partial thickness burn is about 200kJ/m2 of energy and the threshold for a full thickness burn is 400kJ/m2. At most I could have received 880kJ/m2 but in reality being at a distance from the surface of the fireball, being within the fireball and my skin being at an angle to the fireball meant that it’s easily plausible I had between 200 and 400kJ/m2 of heat radiated into my skin.
To get an idea for the black body temperature of this radiation we can use Stefan-Boltzmann’s equation. This gives us an average black body temperature of 1300C. That’s really hot!
At this black body temperature the peak radiation is around 1.8 microns in wavelength. Well into the infrared region of the spectrum.
So why was I the only person (as far as I know) who suffered significant partial thickness burns? I’m not really sure. It probably depends on quite a few things. Many people who were within 20m of the fireball were in their cars. Glas windows reflect infrared radiation so people in cars were unlikely to get burnt. Those also at the side of the road seemed to escape unscathed, possibly due to the fact that more soot was produced at their location absorbing a large amount of the infrared energy.
Simply the angle of the skin to the heat source is enough to dramatically reduce the amount of thermal energy the skin receives. If I had taken a different position on the ground I would have been burnt completely differently.