Modeling combusting diesel jets: The free jet regime

Venkatesh Gopalakrishnan, Purdue University

Abstract

The structure of reacting Diesel jets, including elements of the structure that are likely to strongly depend on chemical kinetics such as ignition and pollutant distribution, is investigated by employing an interactive flamelet model. The ignition delay and ignition location are computed for a range of ambient conditions and compared with measurements. Consistent with the measurements, the computed results reveal a two-stage ignition behavior. Ignition is predicted to occur in a rich mixture at an equivalence ratio of about 3.0, downstream of the maximum liquid-phase penetration. While qualitative trends are reproduced well, quantitative differences exist in the predictions of ignition delay. These differences are attributed to possible inaccuracies in computed strain rates and to differences between the surrogate fuel employed, n-heptane, and Diesel fuel. NO is modeled using a sub-mechanism from GRI-Meth 3.0 and soot is modeled using an empirical, a semi-empirical and a kinetic model. Qualitative comparisons of NO and soot distributions are made with the qualitative measurements of Dec and co-workers (1997). As expected, the maximum NO concentration is found in the maximum temperature region of the jet. The semi-empirical and kinetic soot models predict maximum soot in the head vortex region of the transient jet in the early stages of its development. The empirical model is inadequate for predicting the soot distribution. When modeling soot, its treatment as a particle with negligible diffusivity compared to the gas results in improved predictions relative to treating it as another gas species. Radiant heat transfer has a negligible effect on the transient quantitative and qualitative results during the injection period of Diesel sprays. The implication of the unity Lewis number assumption employed in the flamelet model is evaluated. When compared to predictions obtained using a detailed multicomponent transport model, the unity Lewis number assumption results in longer ignition delays at higher chamber pressures and lower chamber temperatures and shorter ignition delays at lower chamber pressures.

Degree

Ph.D.

Advisors

Abraham, Purdue University.

Subject Area

Mechanical engineering

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