The results of three-dimensional Direct Numerical Simulation (DNS) of Moderate,
Intense Low-oxygen Dilution (MILD) and conventional premixed turbulent
combustion conducted using a skeletal mechanism including the effects of nonunity
Lewis numbers and temperature dependent transport properties are analysed
to investigate combustion characteristics using scalar gradient information. The
DNS data is also used to synthesise laser induced fluorescence (LIF) signals of
OH, CH2O, and CHO. These signals are analysed to verify if they can be used
to study turbulent MILD combustion and it has been observed that at least two
(OH and CH2O) LIF signals are required since the OH increase across the reaction
zone is smaller inMILD combustion compared to premixed combustion. The
scalar gradient PDFs conditioned on the reaction rate obtained from the DNS data
and synthesised LIF signals suggests a strong gradient in the direction normal to
the MILD reaction zone with moderate reaction rate implying flamelet combustion.
However, the PDF of the normal gradient is as broad as for the tangential
gradient when the reaction rate is high. This suggests a non-flamelet behaviour,
which is due to interaction of reaction zones. The analysis of the conditional
PDFs for the premixed case confirms the expected behaviour of scalar gradient in
flamelet combustion. It has been shown that the LIF signals synthesised using 2D
slices of DNS data also provide very similar insights. These results demonstrate that the so-called flameless combustion is not an idealised homogeneous reactive
mixture but has common features of conventional combustion while containing
distinctive characteristics.The financial supports of Nippon Keidanren, Cambridge Overseas Trust and
EPSRC are acknowledged. The direct simulations were made using the facilities
of HECToR, the UK’s national high-performance computing service, which is
provided by UoE HPCx Ltd at the University of Edinburgh, Cray Inc and NAG
Ltd, and funded by the Office of Science and Technology through EPSRCs High
End Computing Programme.This is the accepted manuscript. The final published version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S0010218013003799