Synthetic gas (syngas) combustion has a high energy-to-mass ratio (around 50% energy density of natural gas) and exhibits high laminar flame speeds due to the presence of hydrogen. Syngas can potentially replace most of the conventional fuels, due to its lower emission rates in the case of lean combustion with acceptable energy densities. Especially, syngas can be used in small-scale combustion-related devices such as portable power generators, unmanned aerial vehicles, microsatellite thrusters, micro-chemical reactors and sensors. However, having various constituents with various burning characteristics in terms of ignition, burning speeds and diffusion properties, syngas combustion at micro scales can be more complicated than that of conventional gaseous fuels. It is therefore important to understand syngas combustion characteristics and flame dynamics to identify the potential impacts on their applications. In this work, syngas combustion characteristics are investigated through numerical simulations. Specifically, premixed syngas combustion in a horizontal, two-dimensional micro-channel of length 20 mm and width 2 mm is simulated with detailed chemistry. To reduce the computational expenses, only upper half of the channel is simulated by imposing an axisymmetric boundary condition on the lower wall of the computational domain. At the upper wall, a fixed temperature gradient is employed, to account for the conjugate heat transfer, such that the upper wall temperature grows linearly from 300 K at the inlet to 1500 K at the outlet. Stoichiometric premixed burning of a syngas comprised of 30% CO, 5% CH~4~ and 65% H~2~ is considered. The inlet velocities are in a range of 0.1 m/s to 3 m/s. The chemical kinetics is imitated by the San Diego mechanism with 46 species and 235 reactions. In all case, the preheated wall resulted in an auto ignition downstream, where a V-shape flame kernel formed and started propagating downstream exhibiting a more planar shape. It is shown that the ignition delay decreases with the inlet velocity. However, not all cases resulted in stable flames; i.e., at lower inlet velocities, the time needed to heat up the fresh inflow mixture to ignition is longer, possibly preventing flame stabilization. By the time the mixture reheats, the ignited flame can decay off and another flame ignites again further downstream and this process can keep on repeating. Such a behavior, named *flames with repetitive ignition and extinction* (FRIE), is observed in this work for the inlet velocities less than 0.25 m/s. It was shown that such instability was eliminated by increasing the inlet flow velocity to form a stable, stationary flame. In order to characterize ignition, extinction, and FRIE events in various cases, the local and total heat release rates in the simulation domain as well as the temperature distributions are analyzed. These results are further post-processed to understand the ignition process of syngas, with the ignition delay times and locations compared in all cases. In addition, the FRIE phenomenon is analyzed by investigating the FRIE periods (the time intervals between the two consecutive ignitions). Similar to the ignition delays, the FRIE periods are found to be directly related to the inlet velocity, i.e., the FRIE period decreases with the inlet velocity. The ignition location as well as the stabilized flame location (in stable cases) were found further away from the inlet as the inlet velocity increases. 


Characterizing Premixed Syngas Combustion and Flame Dynamics in Micro Scales

Division / Sponsors: 
Power Division & Nuclear Engineering Division (NED)

Power Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/POWER/POWER2020/volume/83747

International Conference on Nuclear Engineering
Paper Proceedings: https://asmedigitalcollection.asme.org/ICONE/ICONE2020/volume/83761

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