Effect of thermo– and diffusiophoretic forces on the motion of flame-generated particles in the neighbourhood of burning droplets in microgravity conditions

Abstract

Numerical analysis of the e¬ect of thermo-and di¬usiophoretic forces on the motion of moderately large (0:01 . Kn . 0:3) combustion-generated (soot) particles and on the formation of soot-shell structure in the buoyancy-free spherical droplet ®ames is performed. Transient evaporation, ignition and combustion of a single sootingfuel droplet immersed into a quiescent hot environment are considered, taking into account the e¬ects of radiative heat losses, variable transport properties and the dependence of the droplet surface temperature on time. Results of numerical calculations are compared with available experimental data and recent theoretical models. We calculated thermo-and di¬usiophoretic and total velocities of combustiongenerated particles and showed that soot particles form a size-segregated soot-shell structure. Within a soot shell the particles with smaller radii are located closer to the droplet surface, while larger particles are located closer to the ®ame front. Numerical calculations performed for moderately large soot particles showed that there are two equilibrium locations, where the total velocity is equal to zero, between the droplet and the ®ame front. It is found that one of the equilibrium locations is the point of unstable equilibrium and the other position is the point of stable equilibrium. We numerically determined the dependence of soot-shell-to-droplet-diameter ratios on the particle radius.

Disruptive burning of precursor/solvent droplets in flame‐spray synthesis of nanoparticles

“…(3) Finally, as the viscous shell inhibits vaporization, the flame moves closer to the surface (contraction), heating up the shell quickly beyond the heterogeneous nucleation temperature of xylene and 2-EH acid, initiating disruptions. Moreover, disruptions are only observed if Sn(II)2-EH is present, coinciding with the observations of Wong et al 73,74 and Byun et al 75 In addition, the disruption may also be initiated by soot or nanoparticles, which are formed in the flame and transported back to the droplet by thermophoresis as proposed by Shaw et al 31 Very small particles (d p 200 nm ) from the gas phase can get close to the droplet surface, as was shown by Ben-Dor et al 76 The probability of such an effect may increase as the flame contracts due to a decreasing vaporization rate ( Figure S1 of Supporting Information), thus reducing the force (i.e., drag force due to the Stefan flow) counteracting thermophoresis. Entrapped ambient gas ( Figure 9) may also draw particles into the liquid, where they can act as nuclei.…”

Comprehensive study of initial diameter effects and other observations on convection-free droplet combustion in the standard atmosphere for n-heptane, n-octane, and n-decane

“…The data show that the SSR increases with increasing D o for the three alkanes. Though no theory currently exists for droplet burning that incorporates soot formation, prior qualitative modeling and scaling has shown that forces due to thermophoresis and drag (associated with fuel evaporation) are the dominant influences on trapping soot aggregates between the droplet and flame [3,12,15] . As such, viewing the thermophoretic forces on aggregates entirely from the perspec- tive of the flame temperature, thermophoresis will decrease with D o because the flame temperature also decreases.…”

“…6 a). This asymmetry comes from either early soot shell distortion (due to ISS igniters retraction) or incident soot aggregation at one angle such that it has less physical meaning regarding the position where the soot particles are situated as a result of the "force balance" between Stefan drag, diffusio-phoresis and thermophoresis [12,54,55] . The non-spherical part of soot shell is therefore excluded and the circle extended around the circumference.…”

“…Measurements made across such a large range of D o as reported in this study will also be valuable to test the ability of detailed ab initio numerical models of liquid fuel combustion that assume the base case of Fig. 1 [8][9][10][11][12]15,41,42] . It is important to note that the motivation for studying the burning process of the large droplets at the upper end of this range in no way is meant to suggest the relevance of droplets for D o ∼ 5 mm to practical spray systems.…”

“…The inclusion of detailed combustion chemistry is particularly significant because it provides the capability to address soot formation through prediction of its precursors (e.g., acetylene, polycyclic aromatic hydrocarbons, etc.) [3,[12][13][14][15] . The results of these detailed numerical treatments show that the droplet burning process is inherently unsteady, with the burning rate exhibiting a time dependence that originates in liquid phase unsteadiness persisting throughout burning 2 or flame extinction mechanisms that force the burning rate to decrease near the end of burning, with the flame standoff ratio continually increasing with time.…”