Effects of Water Vapor Addition on the Flame Structure and Soot Formation in a Laminar Ethanol/air Coflow Flame

ABSTRACT

Ethanol is a promising alternative fuel to gasoline engines; however, the problem of particulate matter (PM) emissions still exists when ethanol is applied to gasoline direct injection (GDI) engines, especially under high load. Water injection is proved to be an effective way to reduce the PM emissions and engine knocking, simultaneously. In this work, a numerical calculation was carried out in an ethanol/air coflow diffusion flame to investigate the effects of water vapor addition on the flame structure and soot formation. A complete step-by-step decoupling method was proposed to numerically isolate the dilution, density, transport, thermal, radiative and chemical effects of water vapor by introducing several virtual species. Results show that the temperature distribution of the flame is mainly affected by the dilution effect and thermal effect of water vapor, followed by radiative effect and chemical effect. The mole fraction of OH radical increases because of the enhancement of the reaction H₂O + H <=> OH + H₂ and O + H₂O <=> OH + OH, while the conversion of CO to CO₂ is promoted through the reaction CO + OH <=> CO₂ + H, due to the chemical effect. Soot volume fraction is also reduced, where the dilution and chemical effects play the most important roles. Hydrogen-abstraction-carbon-addition (HACA) surface growth and OH oxidation are the processes controlling the soot mass addition and consumption, respectively. HACA rate is mainly affected by the dilution and chemical effects. However, chemical effect has little influence on the OH oxidation rate, since the collision probability of OH with soot particles remains almost unchanged as a result of the increase of OH concentration and the decrease of soot volume fraction under the chemical effect.

KEYWORDS:

• Water vapor addition
• laminar ethanol/air coflow flame
• complete step-by-step decoupling method
• flame structure
• soot volume fraction

Acknowledgments

The authors would like to acknowledge the financial supports to the research provided by National Natural Science Foundation of China (NSFC) through its projects of 51576083 and 51606056.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [51576083,51606056].