On the Formation of Nitrogen Oxides During the Combustion of ...

3.2 Measurement Techniques and Data Acquisition
at 500 K, which is approximately 5 times higher than the respective density
of air [311]. Under normal gravity, the hot combustion products would rise
and the vaporized fuel would settle down, resulting in rapid flame extinction.
Hence, microgravity is of particular benefit in studies on vaporization, ignition,
combustion, and extinction of flames around droplets. Settling effects of
the droplet and buoyancy-induced flows around it are eliminated, leading to
a one-dimensional, spherically symmetric geometry [128, 153, 209, 235, 356].
The basic droplet combustion model, as discussed, for instance, in Law [233]
and Law and Faeth [235], was introduced with Figure 1.2. Evaporation of the
liquid fuel occurs on the droplet surface, accompanied by diffusive transport
of the gaseous fuel in the outward direction. The oxidizer, on the other hand,
diffuses in the inward direction. Both reactants meet and are consumed in a
thin, shell-like flame region. Assuming spherical symmetry, quasi-steadiness,
and flame-sheet combustion, the classical D² law is derived (cf. Chap. 2.1.3).
A large number of attempts have been made under normal gravity and microgravity
to verify the respective linear decrease of the droplet diameter
squared. Experiments under normal gravity already confirmed this trend
very well with deviations from linearity being due to second-order effects
[48, 141, 142, 151, 152, 155, 164, 202, 221, 228, 239, 308, 352, 372, 412, 462].
However, microgravity conditions alone provide for the quantification of the
flame size behavior as a result of the above mentioned dependencies, including
absence of buoyant flow, absence of flame shape elongation, and the transient
character of the whole process [233, 235, 244].
Idealized droplet vaporization and burning is dominated by Stefan flow, which
is a diffusion-controlled process [19, 443]. Since the density-weighted mass
diffusivities are pressure insensitive, experiments under reduced pressure but
normal gravity are an alternative to minimize buoyant effects. This approach
is suitable for studying particular small-scale phenomena at a pressure down
to 0.1 bar. The reduced chemical reactivity can, in turn, be partially compensated
by an oxygen-enriched environment [102, 235, 238].
Microgravity experiments, to a greater extent than normal gravity experiments,
are subjected to the formation of a soot layer between droplet and
flame. The Soret effect – also called thermodiffusion or thermophoresis –
forces an inward diffusion of soot particles formed near the flame zone. As
this inward diffusion is opposed to the Stefan flow of fuel vapor, the soot par-
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