Combustion leftovers

Combustion processes almost never completely exhaust all of the available fuel. In this paper, we will consider three combustion scenarios (back-to-back premixed flames in stagnation point flow, travelling combustion waves, and microgravity spherical flame balls) and show how to calculate the amount of fuel which will be left over no matter how long we allow the combustion processes to continue. Mathematical biologists will be familiar with an analogue of this in disease modelling, where there are always some susceptibles left after the passage of an epidemic. In a bushfire or forest fire context, this is seen as unburnt solid fuel. The main reason for combustion leftovers is the heat loss, which can never be completely eliminated.