Stochastic modeling of unsteady extinction in turbulent non-premixed combustion

• Turbulent fluctuations of the scalar dissipation rate have a major impact on extinction in non-premixedcombustion. Recently, an unsteady extinction criterion has been developed (Hewson, 2013) that predictsextinction dependent on the duration and the magnitude of dissipation rate fluctuations exceeding a criticalquenching value; this quantity is referred to as the dissipation impulse. The magnitude of the dissipationimpulse corresponding to unsteady extinction is related to the difficulty with which a flamelet is exintguished,based on the steady-state S-curve.In this paper we evaluate this new extinction criterion for more realistic dissipation rates by evolving a stochas-tic Ornstein–Uhlenbeck process for the dissipation rate. A comparison between unsteady flamelet evolutionusing this dissipation rate and the extinction criterion exhibit good agreement. The rate of predicted ex-tinction is examined over a range of Damköhler and Reynolds numbers and over a range of the extinctiondifficulty. The results suggest that the rate of extinction is proportional to the average dissipation rate andthe area under the dissipation rate probability density function exceeding the steady-state quenching value. Itis also inversely related to the actual probability that this steady-state quenching dissipation rate is observedand the difficulty of extinction associated with the distance between the upper and middle branches of theS-curve.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Annan teknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Other Engineering and Technologies (hsv//eng)

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Strömningsmekanik och akustik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Fluid Mechanics and Acoustics (hsv//eng)

Nyckelord

Extinction

Unsteady flames

Non-premixed flame

Scalar dissipation rate

Turbulence

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