An experimental study was conducted in a turbulent spray flame in which droplets
were produced ultrasonically at low velocity relative to the host gas. In this fashion,
injector-specific effects on the two-phase flow were minimized and a scenario generally
characteristic of the far field of practical spray systems could be simulated. Close to
the burner exit, the spray flame appeared as a dense column of drops burning with
an envelope flame. Further downstream, it opened up slowly in the radial direction
and developed a turbulent ‘brush’ appearance. Measurements of the size, velocity
and concentration of the droplets, and of gas-phase velocity and temperature were
made by combining a Phase-Doppler interferometric technique with Stokes/anti-Stokes Raman thermometry. The experimental data were used to derive scaling and
self-similarity for the Reynolds-averaged continuity and momentum equations using
suitable transformations.Results showed three distinct regions, on the basis of the behaviour of the gas
axial velocity in the spray flame. In the lower part of the flame, the gas momentum
increased because of vaporization. In the intermediate region of the spray flame,
the axial velocity decayed along the centreline as an inverse power of the distance
from the virtual origin, with exponents smaller than unity. In the upper part of
the spray flame, the flow field recovered the axial velocity decay that is typical of
incompressible jets, namely as an inverse of the axial distance. Self-similar behaviour
held for the axial velocity throughout the intermediate region. The vapour source
term in the gas continuity equation scaled approximately as an inverse power of
axial distance, and exhibited self-similarity throughout the spray flame. As a result,
a simple model of the Reynolds stress term could be formulated, in which two
competing contributions appear: one, that is due to turbulent transport, tends to
increase the value of the velocity correlation; another, that is due to the vaporization
term, tends to reduce the value of the velocity correlation and can be construed as a
vaporization-induced tendency towards relaminarization. The first term is modelled
by a classic gradient-transport approach yielding an empirical mixing length relating
the velocity correlation to the average velocity gradient. Model and experiments are
found to be in good agreement, especially sufficiently far from the injector, where
one-way coupling between the two phases holds.