Abstract
The spontaneous-ignition and ignition delays of liquid fuel droplets falling (i) on a heated surface and (ii) through heated air are investigated. The effects of fuel properties, droplet size, initial temperature, material of the surface and additives in the fuel on the ignition delays of liquid fuel droplets falling on a heated surface are studied. It is shown that for droplets falling on a heated surface, the ignition delay/temperature curves for certain fuels show minima at or slightly above the maximum boiling rate points. Two new terms namely 'the heating up delay' and the 'evaporation delay' are introduced to represent the so called 'physical delay'. The heating up delay is isolated as the truly physical part of the ignition delay and is defined as the time taken for the droplet to reach a temperature where a stoichiometric mixture can exist at or near the fuel surface. The evaporation delay is identified as partly physical and partly chemical and is defined as a function of the evaporation rates. Expressions are derived for the heating up delay and the evaporation delay for various cases (a theoretical expression is derived for the evaporation rate of a droplet in the 'spheroidal' state on a heated surface) and the experimental results for the effects of droplet size and the initial temperature of the fuel on the ignition delay are shown to be as indicated by these expressions. It is also shown that for droplets of high boiling point fuels such as kerosine and diesel fuels, in stagnant hot atmospheres or falling through heated air, the heating up delay occupies a significant proportion of the ignition delay and at high temperatures the ignition delay is virtually the heating up delay. It is shown that additives in the fuel produce significant reductions in the ignition delays of kerosine droplets falling on a heated surface and that the dosages for certain additives are critical.