The laminar flame
speed plays a fundamental role in understanding
hydrocarbon chemistry and is the basis of turbulent flame modeling.
The main objective of this research is to visualize a flame and measure
its speed using a new optical diagnostic technique, which is based
on the natural radiative emissions of various species, mainly hydrocarbons,
water, and carbon dioxide. This new technique has two main advantages
over the Schlieren method. First, only one optical access is needed,
and second, the preheat zone and the flame front can be observed,
and their radiative emission can be quantified. In addition, the condition
of the unburned gas in front of the flame can be studied using the
new technique to evaluate some of the hypotheses behind unstretched
flame speed measurement, such as the trivial change of the unburned
gas state. To evaluate and validate the new optical diagnostic technique,
the flame propagation of methane, oxygen, and nitrogen mixtures was
studied using species infrared radiative emissions at broadband wavelengths
in the range of 3.239–3.629 μm. The flame propagation
was pictured using a high-speed infrared detector under constant pressure
condition. The study was conducted at a mean initial gas pressure
of 1.005 ± 0.0005 bar, a mean initial gas temperature of 296
± 1.1 K, and four different equivalence ratios of approximately
0.8, 0.9, 1.0, and 1.1. The flame speed measured using the new technique
aligns well with the measured data from the literature using the Schlieren
technique and simulated data using a detailed mechanism at the studied
conditions. The measured data also show the presence of the radiative
emission from the unburned gas. This observation contradicts the assumption
of trivial temperature change of the unburned gas mixture during the
flame propagation, which is widely used for unstretched flame speed
measurement relative to the unburned gas state.