Effects of Microwaves on Burning Velocity, UV–VIS-Spectra, and Exhaust Gas Composition of Premixed Propane Flames

Abstract

Microwaves can interact with the flame’s combustion zone and increase the energy density. This induces changes to the combustion properties by influencing the radical formation and significantly expanding the flame thickness. It is possible to combust low calorific gas mixtures in a stable process without preheating or co-firing with microwave assistance. Furthermore, the hybrid heating of gaseous fuels and electrical energy offers a solution for the improved use of volatile energy sources and the further improvement of the efficiencies of thermal systems. Moreover, microwaves can be used to heat materials internally, and when coupled with external heating, contribute to a more homogeneous process. This can be particularly beneficial in the glass industry and metal processing. However, the literature on basic behaviour of this coupling such as dielectric properties for higher temperature is limited, which prevents a comprehensive evaluation of the application potential. Thus, investigations with a newly designed, externally cooled, and symmetric resonator were conducted. Furthermore, the optimized resonator was improved by installing ports for burner installation and filter elements for exhaust gas probing and optical access. An axisymmetric burner was designed to generate a steady conical laminar premixed flame, which is stabilized on the outlet of a contoured nozzle. Combustion regimes with propane/air were tested within a range of equivalence ratios from 0.9 to 1.5. Experiments were carried out with an initial temperature of 298 K under atmospheric pressure and with microwave inlet power with a range of 200 to 800 W. The optical techniques used in the current survey are based on flame contour detection using OH* chemiluminescence imaging. Additionally, spectrally resolved flame emission measurement techniques were used to monitor excited state species. Significantly increased production rates of OH*, C2*, and CH* radicals were measured when there was an increased microwave power. The amount of NOx and CO were increased by 20% and 55% respectively in the exhaust gas, which was determined experimentally using a probe method. Numerical simulations of the electromagnetic field and its influence on combustion were carried out to confirm these findings. This enhanced reactivity by 22%, resulted in higher burning velocity, and provided knowledge on the emission performance of the combustion process under microwave influence. The numerical electromagnetic simulations using FEKO produced results that are consistent with the experimental results.