The autoignition of acetylene, released from a finite-sized circular nozzle
into a turbulent coflow of hot air confined in a pipe, has been the subject of a recent
experimental study to supplement previous work for hydrogen and n-heptane. As
with hydrogen and n-heptane, autoignition appears in the form of well-defined localized
spots. Quantitative information is presented concerning the effects of turbulence
intensity, turbulent lengthscale and injector diameter on the location of autoignition.
The effects of these parameters on inhomogeneous autoignition have not been
investigated experimentally before. The present study establishes that increasing
the bulk velocity increases the autoignition length, as was reported for hydrogen
and n-heptane. For the same turbulence intensity, the autoignition length increases
as the injector diameter increases and as the turbulent lengthscale decreases. A
simultaneous decrease in turbulence intensity and increase in lengthscale causes
a reduction in autoignition length. Further, the frequency of appearance of the
autoignition spots has also been measured. It is found to increase when autoignition
occurs closer to the injector, and also at higher velocities. The observed trends are
consistent with expectations arising from the dependence of the mixture fraction and
the scalar dissipation rate on the geometrical and flow parameters. The data can be
used for the validation of turbulent combustion models.