Influence of heating intensity and size of gel fuel droplets on ignition characteristics

Abstract

This experimental research studies the influence of heating intensity and typical initial size of particles (or droplets in molten state) on the main process parameter – ignition delay time – for a group of gel fuel compositions. The first group of fuels is based on oil-filled cryogels: the aqueous solution of polyvinyl alcohol (5 wt%) + 40–60 vol% of oil + 2 vol% of emulsifier. The second one was produced from identical oil-filled cryogels with fine solid combustible particles: the aqueous solution of polyvinyl alcohol (10 wt%) + 35 vol% of oil + 30% of coal (particle size 100 µm) + 2 vol% of emulsifier. Combustion was initiated in a chamber with a motionless air medium at 700–1,000°C, by introducing fuel particles into it at a rate of 0.04–0.10 m/s, their size varying from 2.5 to 3.1 mm. The consistent patterns and characteristics of processes during the induction period were recorded by a high-speed video camera and fast-response thermocouples. It has been established that the ignition delay times change in the range from 0.5 to 15 s, depending on the heating intensity (characterized by the air temperature in the chamber and the rate of introducing fuel particles into it) and component composition of the gel fuel. The calculated minimum density of the heat flux, required for the gel fuel ignition, is 40 kW/m2. At lower values, ignition does not occur even when the fuel is heated for more than 15 s, which is due to the complete evaporation of molten fuel components. The ignition delay times for all the fuel compositions and particles of different sizes differ by less than 10% (random error of measurement) at the heat flux density over 100 kW/m2. The factors under study affect the duration of the induction period only under the near-threshold (minimum) ignition conditions, when the air temperature in the chamber is 700–800°C. The higher the rates of introducing the fuel particles into the heating chamber (in the range of 0.04–0.10 m/s), the higher the oil concentration in the oil-filled cryogels (in the range of 40–60%), and the smaller the size of particles (ranging from 2.5 to 3.1 mm), the shorter the ignition delay times (by 30–40%, 25–35%, and 20–45%, respectively). At air temperatures above 900°C, the initial size of fuel particles does not have a significant effect on the ignition delay times (the difference in the recorded values does not exceed the random error of measurement).