It is well established that the laminar burning rate plays an important role in
turbulent combustion and previous work at Leeds has suggested that the laminar
burning velocity of an aerosol mixture is little different from that of a gaseous
mixture at similar conditions. However, it has been shown that flames within well
defined droplet suspensions (aerosols) more readily become unstable than for
gaseous ones. Flame instabilities, characterised by wrinkling and cellular surface
structure, increase the burning rate due to the associated increase in surface area. For
gaseous mixtures, the effect has been shown theoretically and experimentally to be a
function of Markstein number and critical Peclet number, which marks the flame
radius at which cellularity is first observed. In aerosol combustion, the presence of
liquid droplets has been shown to influence instabilities by causing earlier onset of
cellularity than for gaseous flames. Therefore it is imperative to conduct a
fundamental study to understand the complex interactions between droplets and
combustion.
In the present work, spherically expanding flames were employed to quantify
the burning rates in gaseous and aerosol flames and to determine their differences.
!so-octane-air aerosols were generated by expansion of the gaseous pre-mixture,
based on the Wilson cloud chamber principle of expansion cooling, to produce a
homogeneously distributed suspension of fuel droplets. Flames were centrally
ignited for quiescent aerosols at near atmospheric pressures, drop sizes of up to
30 ~m and overall equivalence ratios between 0.8 to 2.0. The flame progress was
monitored using high-speed schlieren photography, from which burning rates were
determined. In turbulent studies, measurements were made for stoichiometric
aerosols at root mean square turbulence velocities of between 1.0 and 4.0 m/s. For
companson, gaseous combustion at conditions similar to those of aerosols were
studied.
From the laminar study, it was shown that the burning rate of lean mixtures is
independent of droplet diameter. However, at higher equivalence ratios, the burning
rate became a strong function of droplet diameter and equivalence ratio. This was
associated with the onset of instabilities, which were, in tum, related to measured
values of Markstein number and critical Peclet number for aerosol flames in a
similar manner to those for gaseous flames. Heat loss from the flame due to droplet
evaporation is probably the main reason for instabilities in aerosol flames.
Interestingly, droplets which were assumed by previous workers to be fully
evaporated at the flame front, were shown, at certain conditions in the present work,to survive behind the flame front. Thus other possible mechanisms for instabilities in
aerosol flames could be important.
For very rich mixtures, gaseous flames were shown to be partially smooth,
slow, and were strongly affected by natural convection. Conversely, with the
presence of droplets at similar mixture conditions, flames were found to be fully
cellular and faster, with little sign of the effect of natural convection. This was
suggested to be due to early instabilities caused by the presence of droplets, which,
in tum, increased the burning rates.
Oscillating flames, in which the flame speed and flame structure alternated
between low and high values and smooth and cellular respectively, during flame
development, were observed for some experimental conditions. These oscillations
were most probably caused by aerodynamic interaction between droplets and gas
motion ahead of the flame. This was examined using simultaneous laser sheet
imaging and PIV analysis, with a simple model proposed by Atzler et al. (2001)
which simulated aerodynamic interaction between droplets and gas phase motion
ahead of the flame front.
A dimensionless comparison between turbulent flames of aerosol and gaseous
mixtures showed similar burning rates. The measurements were compared with
existing turbulent burning velocity expressions and correlations. In general, these
expressions are in quite good agreement with the present results, particularly at low
stretch rate
