## 9.12. Autoignition Model

A flammable mixture residing at sufficiently high temperature will ignite without further interaction after some temporal delay. This phenomenon is called ‘autoignition’ or ‘selfignition’. Depending on the application this may be either desired or adverse behavior. For example, it is an essential feature for compression-ignition (CI, Diesel, HCCI) engines. For spark-ignition (SI) engines autoignition is usually unwanted, and its occurrence is known as engine knock. A third example where autoignition may need to be considered is designing the length of a premixing section, for example, for a gas turbine combustor.

Classical combustion models like Eddy Dissipation, Flamelets, Burning Velocity Model or Extended Coherent Flame Model have been developed for high temperature combustion. The purpose of the autoignition model is to extend the primary combustion model with a special treatment for the lower temperature regime, thereby providing a combined model that is applicable over the combined range.

The low temperature regime is characterized by initial breakdown of fuel molecules and buildup of radical species. When the concentration of radicals is sufficiently high, they begin to feed the oxidation and the rate of heat release will increase. The combustion then transitions to the high temperature regime. The net effect of autoignition (or selfignition) is that significant heat release due to combustion occurs if, and only if, a certain delay time has expired. This is modeled by applying an ‘Ignition Delay Time’ to the combustion model.

Depending on the behavior of the principal combustion model, the reaction must either be suppressed for the delay time or enforced after the delay time has expired. This establishes the two kinds of autoignition models:

1. Ignition Delay: suppress combustion until delay time has expired; this is available for non-premixed models (Eddy Dissipation, Flamelet)

2. Knock: enforce combustion after delay time has expired; this is available for premixed or partially premixed models (BVM, ECFM).

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