The effect of heating rates on low temperature hexane air combustion
Combustion of hydrocarbon fuels is traditionally separated into slow reaction, cool flame, and ignition regimes based on pressure and temperature. Standard tests, such as the ASTM E659, are used to determine the lowest temperature required to ignite a specific fuel mixed with air at atmospheric pressure. It is expected that the initial pressure and the rate at which the mixture is heated also influences the limiting temperature and the type of combustion. This study investigates the effect of heating rate, between 4 and 15 K/min, and initial pressure, in the range of 25–100 kPa, on ignition of n-hexane air mixtures. Mixtures with equivalence ratio ranging from Φ = 0.6 to Φ = 1.2 were investigated. The problem is also modeled computationally using an extension of Semenov's classical autoignition theory with a detailed chemical mechanism. Experiments and simulations both show that in the same reactor either a slow reaction or an ignition event can take place depending on the heating rate. Analysis of the detailed chemistry demonstrates that a mixture which approaches the ignition region slowly undergoes a significant modification of its composition. This change in composition induces a progressive shift of the explosion limit until the mixture is no longer flammable. A mixture that approaches the ignition region sufficiently rapidly undergoes only a moderate amount of thermal decomposition and explodes quite violently.
© 2012 Elsevier Ltd. Received 16 August 2011, Revised 20 December 2011, Accepted 21 December 2011, Available online 31 December 2011. The work was carried out in the Explosion Dynamics Laboratory of the California Institute of Technology and was supported by The Boeing Company through a Strategic Research and Development Relationship Agreement CT-BA-GTA-1. The authors would like to thank Raza Akbar, Greg Rieker, Adrianus Indrat Aria, Bryan Hires, and David Gutschick for their help with the 2f detection, and John Ziegler and Jason Damazo for the help in refitting coefficients of the thermodynamic data to remove discontinuities.
Accepted Version - HeatingRatePaper.pdf