# Conditions for and Characteristics of the Dispersion of Gel Fuel Droplets during Ignition
## 1 Introduction  
The problem of the negative impact of fossil fuel combustion products on the environment can be solved by using new multicomponent fuel mixtures in practice. They can be prepared in the form of gel fuels [1], emulsions [2], or suspensions [3] containing low-grade solid fuels, waste from coal preparation or oil refining, and water. Compared to fossil fuels, multicomponent fuel mixtures are more environmentally friendly due to the content of water in the fuel mixtures and the effect of its vapor on the chemical reactions that occur during gas-phase combustion [4, 5]. Fuel droplets that are widely used in practice (gasoline, kerosene, alcohol, waste oil) are ignited and burned out with a fairly well-studied process of phase transition in the modes of evaporation and boiling of a combustible liquid. Ignition and combustion of multicomponent fuels are accompanied by droplet dispersion (secondary atomization of a droplet) under certain conditions. This is known as puffing and microexplosion [6, 7, 8]. Their essence is in the secondary atomization of liquid fuels, which contributes to an increase in the evaporation area of the fuel components and the size of the burnout area, which significantly intensifies ignition and combustion and increases the completeness of fuel burnout. Puffing is characterized by the partial disintegration of a droplet into relatively large fragments. [PERSON] et al. [6] and [PERSON] et al. [7] specifically defined puffing as the process of vapor jet discharge from a fuel droplet's surface. A microexplosion of a two-component droplet--for example, water and a combustible liquid--is characterized by explosive fragmentation of the droplet with the formation of an aerosol [8], which is also known as secondary atomization [9]. Stable microexplosive boiling or puffing occurs when a low-boiling component (usually water) is inside of a high-boiling component (for example,oil) [10]. That is, a water droplet is covered with a shell of oil. The mechanism of droplet disintegration (puffing and microexplosion) is explained by an increase in temperature at the interface between the low-boiling and high-boiling components, up to the boiling point of the low-boiling component--for example, 100-120 \({}^{\circ}\)C for water [11; 12]. This strongly depends on the difference in boiling temperatures between oil and water [13]. It is known that if the size of two-component droplets is small, then their evaporation may not be powerful enough to destroy the oil phase for the occurrence of microexplosion [13]. In this case, the puffing mode occurs [7; 14]. However, in [15; 16; 17; 18; 19; 10], the authors showed that even partial coalescence can lead to microexplosions. In [8], it was found that puffing and microexplosions occurred under conductive heating conditions and high-density heat fluxes for two-component droplets (water and oil), suspensions, emulsions, and solutions. Under convective heating conditions, these processes were achieved only with the addition of a combustible component (oil) and solid components [8]. Under radiant heating, the droplets disintegrated with the formation of a finely dispersed aerosol and fog (microexplosion), or the droplets monotonously evaporated (at relatively low densities of the supplied radiant heat flux) [8]. The puffing mode was not established in experiments with radiation heating [8]. In [20], air temperature ranges were obtained during convective heating, for which one or another mechanism was implemented--from 300 to 350 \({}^{\circ}\)C, puffing occurred, and above 400 \({}^{\circ}\)C, microexplosion of the two-component droplets occurred. At the same time, the evaporation surface area of liquid fuels increased by more than 100 times [21]. There were also studies of the puffing and microexplosion of gel fuels based on kerosene [22] and dimethylhydrazine [23].