JSFR combustion processes of n-heptane and isooctane

Abstract

Combustion processes of n -heptane and isooctane have been studied in a Jet Stirred Flow Reactor (JSFR), which operates according to the Continuous Stirred Tank ideal model of reactor. The range of experimental conditions spanned from 202 to 1212 kPa and from 573 to 840 K. The feed consisted of stoichiometric fuel/air mixtures and flow rates were such to establish a 0.4 s residence time. The analysis of dynamic behavior of combustion processes showed that the same variety of dynamic events is exhibited by both fuels, namely: slow combustion, low temperature multistage ignition, damped and periodic cool flames, “jumps” and high temperature ignitions. Ignition diagrams have been plotted on the temperature/pressure plane. Heat release rates have been measured experimentally and their dependence on temperature clearly shows the region of existence of negative temperature coefficient of the reaction rate. The heat release rate curve provides evidence of hysteresis due to the non linearity of the process. The analyses of reaction intermediates indicate that fuel molecules are not heavily degraded at the sampling conditions which are located in the negative temperature coefficient region. The major differences between the combustion processes of the two fuels are in the pressure limit of the region of low temperature multistage ignition and in the presence of acetaldehyde among the reaction intermediates detected only for the n -heptane/air system. On this basis, the hypothesis is formulated that the different knock resistance of the two fuels could well be caused by the differences of the low temperature mechanisms and by the ability of producing acetaldehyde at an early stage of the combustion process.