Computational investigations of ignition delay and flame lift-off in diesel jets

Chetan Satish Bajaj, Purdue University

Abstract

The increasing demand for cleaner and efficient engines is forcing diesel engine designers to accelerate the pace of engine development. Multidimensional engine combustion models which numerically simulate the transient diesel engine processes such as sprays, flows and combustion are powerful tools which, when employed in conjunction with experiments and hardware testing, can reduce the development time. The accuracy of the sub-models employed determines the accuracy of the predictions of the models. Several challenges exist in achieving accuracy. These include lack of understanding of atomization, secondary breakup, drop dynamics, and turbulence/chemistry interactions. Furthermore, numerical resolution is constrained by the two-phase flow numerical model. In this work, these numerical issues are sidelined by using vapor jets instead of liquid sprays and a turbulence/chemistry interaction model is evaluated by comparing measured and computed ignition delay and flame lift-off heights in reacting diesel jets. In fact, this work shows through detailed computations of vaporizing diesel sprays that vaporization does not have a significant influence on the structure of the diesel jet. This confirms prior findings that the structure of the jet can be represented with adequate accuracy by vapor jets. An unsteady flamelet progress variable (UFPV) model is employed to represent turbulence/chemistry interactions. The model is implemented through a look-up table in which the mixture fraction, and the scalar dissipation rate and progress variable at the stoichiometric location are the independent variables and the formation rates of species are the dependent variables. Computed and measured ignition delay and flame lift-off heights are compared for a range of conditions and shown to agree qualitatively in trends and agree quantitatively within about 25% for most cases. The quantitative agreement is dependent on the measures employed for defining lift-off and ignition delay. Flame stabilization mechanisms are assessed in view of the results.

Degree

M.S.M.E.

Advisors

Abraham, Purdue University.

Subject Area

Engineering|Mechanical engineering

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