Title: Understanding the ignition mechanism of high-pressure spray flames

Abstract

A conceptual model for turbulent ignition in high-pressure spray flames is presented. The model is motivated by first-principles simulations and optical diagnostics applied to the Sandia n-dodecane experiment. The Lagrangian flamelet equations are combined with full LLNL kinetics (2755 species; 11,173 reactions) to resolve all time and length scales and chemical pathways of the ignition process at engine-relevant pressures and turbulence intensities unattainable using classic DNS. The first-principles value of the flamelet equations is established by a novel chemical explosive mode-diffusion time scale analysis of the fully-coupled chemical and turbulent time scales. Contrary to conventional wisdom, this analysis reveals that the high Damköhler number limit, a key requirement for the validity of the flamelet derivation from the reactive Navier–Stokes equations, applies during the entire ignition process. Corroborating Rayleigh-scattering and formaldehyde PLIF with simultaneous schlieren imaging of mixing and combustion are presented. Our combined analysis establishes a characteristic temporal evolution of the ignition process. First, a localized first-stage ignition event consistently occurs in highest temperature mixture regions. This initiates, owed to the intense scalar dissipation, a turbulent cool flame wave propagating from this ignition spot through the entire flow field. This wave significantly decreases the ignition delay of lower temperature mixturemore » regions in comparison to their homogeneous reference. This explains the experimentally observed formaldehyde formation across the entire spray head prior to high-temperature ignition which consistently occurs first in a broad range of rich mixture regions. There, the combination of first-stage ignition delay, shortened by the cool flame wave, and the subsequent delay until second-stage ignition becomes minimal. A turbulent flame subsequently propagates rapidly through the entire mixture over time scales consistent with experimental observations. As a result, we demonstrate that the neglect of turbulence-chemistry-interactions fundamentally fails to capture the key features of this ignition process.« less

Authors:

Dahms, Rainer N. ^[1]; Paczko, Günter A. ^[2]; Skeen, Scott A. ^[1]; Pickett, Lyle M. ^[1]

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+ Show Author Affiliations

1. Sandia National Lab. (SNL-CA), Livermore, CA (United States)
2. RWTH Aachen Univ. (Germany)

Publication Date:

Tue Oct 25 00:00:00 EDT 2016

Research Org.:

Sandia National Lab. (SNL-CA), Livermore, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1335734

Alternate Identifier(s):

OSTI ID: 1397909

Report Number(s):

SAND-2015-10580C
Journal ID: ISSN 1540-7489; PII: S1540748916304126

Grant/Contract Number:  

AC04-94AL85000

Resource Type:

Accepted Manuscript

Journal Name:

Proceedings of the Combustion Institute

Additional Journal Information:

Journal Volume: 36; Journal Issue: 2; Conference: Proposed for presentation at the Thirty-Sixth International Symposium on Combustion, Seoul (Korea), 31 Jul - 5 Aug 2016; Journal ID: ISSN 1540-7489

Publisher:

Elsevier

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; flamelet; LLNL kinetics; optical diagnostics; auto-ignition; turbulence-chemistry-interactions