Hot ashes in the column of rising gases above a fire.
I came up with the idea for this image somewhat by accident while I was talking to some friends by my newly made fire pit one night. The shots I took of the fire that night contained the hot ash tracers that inspired me but I didn’t have my tripod with me so I couldn’t properly frame the whole fire pit. For the final image I planned to move the camera about five feet away in order to capture the entire flame and the column of rising air above it.
The apparatus producing the flow is a wood fueled fire inside a backyard fire pit. The logs were burning for about 30 minutes before the shot and were completely charred on the outside. By disturbing the logs with an iron poker, some of the hot ashes on the surface of the logs detached. The rising hot gases propelled the ashes vertically. The particles of ash are visible because of the energy they are dissipating. The streaks of hot ash form a compact column where the flames stop; as they’re carried away from the flames and their velocity decreases it looks like they diverge from the center of the column, falling back towards the ground. The ashes that depart closer to the edge of the main flames seem to travel shorter, more turbulent paths while the ashes that start at the center of the fire leave longer and smoother vertical paths suggesting the velocity of the gases is highest in this region due to higher temperatures. Overall the flow appears to be turbulent suggesting the Reynolds number is greater than 4000. I estimate that the speed of the particles at the center of column is about 3 m/s based off a light trail 5 feet in length captured in half a second.
The camera I shoot with is a Nikon D3100. As for the technique, the shutter speed was set to a half second which was slow enough to capture the light trails of the hot ashes. The aperture was f/7.1 and I set the ISO to the minimum level of 100 to compensate for the long exposure. Even though I took the image outside at 10 pm, no external light sources were necessary to achieve this image leaving the background completely unexposed and adding most of the contrast to the image. I placed the camera on a tripod six feet away from the fire and two feet off the ground. I set the focus manually. In photoshop I increased the contrast just a little using a curves layer. I also increased the clarity in the raw editor and applied a high contrast filter with the radius set to 5 pixels and the blending mode set to overlay.