CO₂-diluted CH₄-air premixed spherical flames with microwave-assisted spark ignition

Abstract

The performance of microwave-assisted spark ignition (MAI) under exhaust gas recirculation conditions was explored with CO₂-diluted CH₄-air premixed spherical flames in a constant volume combustion chamber. The flame kernel radius at 5 ms after spark started was selected to evaluate the property of MAI for CO₂ dilution ratio of 0–20% and equivalence ratio of 0.6–1.4 with 1 kHz microwave pulse repetition frequency under 0.2 MPa ambient pressure. The results showed that the addition of microwave induced some wrinkles on the flame surface and strongly deformed the flame. MAI expanded the limit of CO₂ dilution ratio to 16% with an equivalence ratio of 0.75, in which case the spark only (SI) failed to ignite the mixture. With the CO₂ dilution ratio increasing, the wrinkles induced by microwave pulses decreased apparently, and the enhancement value of MAI peaked at 4% CO₂ dilution ratio. The effect of microwave was considered in two aspects, namely, reaction kinetics and thermal effect, which shows a “trade-off” as CO₂ dilution ratio rose. With 8% volume of CO₂ added, the flammable interval (equivalence ratio 0.6–1.2) of mixture in SI mode shrunk, and MAI can maintain a flammable interval consistency with the case that no CO₂ was added.

Keywords

microwave / spark ignition / CO₂ dilution / spherical flame / flame surface wrinkles

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