Dependence of autoignition delay on oxygen concentration in mild combustion of high molecular weight paraffin

New combustion technologies aim to increase the conversion efficiency and reduce the pollutant formation of combustion processes by extending the operating reaction conditions to new domains. The mild combustion mode represents one of the most promising technologies in view of its many potential advantages and numerous fields of application. The large-scale diffusion of such systems in practical applications is limited because of the lack of confidence of the system reliability, due mainly to the lack of studies carried out on the fundamentals of mild combustion processes. This paper can be considered a contribution to set up tuning and control tools for the application of the mild operating mode in practical combustion devices. Based on the definition of the conditions required to operate a combustion system in a mild mode, the specification of some requirements for the operation of high-compression ignition engines, like the homogeneous charge compression ignition engine or mild gas turbine, in mild conditions are analyzed. The role of oxygen dilution on the autoignition delay (tau) of heavy paraffins in temperature (600-1200 K) and pressure (1.3-4.0 MPa) ranges of interest in such applications is discussed on the basis of both experimental and numerical approaches. The very good agreement of results obtained by means of the two methodologies leads to the identification of a power law dependence of tau on oxygen molar fraction in the environment (tau proportional to X-O2(-n)) where the exponent is a function of temperature. Three main regimes with respect to ignition delay dependence on oxygen concentration at high pressure have been identified. The different functional dependencies found both in the modeling and experimental results have been correlated to the different reaction pathways followed in the different temperature ranges.