Strain distribution on material surfaces during combustion regime transitions

Abstract

A multi-fluid state approach is used to analyse the underlying conditions for burning mode transitions from close to the corrugated flamelet regime to distributed reactions. Turbulent (Re t 380) premixed DME/air flames were aerodynamically stabilised in a back-to-burnt opposed jet configuration with the Damköhler number varied through the mixture stoichiometry. Simultaneous Mie scattering, OH-PLIF and PIV allowed the delineation of five separate fluid states (reactants, combustion products, mixing fluid, mildly and strongly reacting fluids) with associated material surfaces. The analysis shows self-sustained flames in low strain regions with a collocated flow acceleration for higher Damköhler numbers. By contrast, in highly strained regions (e.g. beyond the twin flame extinction point) the burning mode is governed by the counter-flowing hot combustion products resulting in increased levels of vorticity and an absence of a preferential dilatation direction. The current analysis provides novel insights into combustion regime transitions by means of (i) strain rate statistics conditioned upon material surfaces and (ii) the evolution of fluid state interface probabilities as a function of the Damköhler number. The work further shows (iii) that the combustion mode influences scalar transport and that increased levels of turbulence retards the transition to non-gradient transport.

Distributed Low Temperature Combustion: Fundamental Understanding of Combustion Regime Transitions

“…The derived multi-fluid analysis is here used to delineate systematic combustion regime transitions from close to the corrugated flamelet regime well into a distributed reaction zone regime of premixed DME by means of (a) multi-fluid probability and (b) interface statistics, (c) unconditional and (d) conditional velocity statistics, (e) underlying strain statistics conditioned upon the material surfaces and (f) limitations of bimodal segregation techniques. The multi-fluid analysis is further used to delineate (g) the impact of the excess enthalpy of the stabilising hot combustion product on the burning mode transition with the impact of the fuel chemistry explored by Hampp [22] and strain (bulk and turbulent) by Hampp and Lindstedt [24]. The conventional combustion regime classification is extended by (h) a tentative delineation into a 3D regime manifold to accommodate conditions of distributed burning modes.…”

“…The determined velocity field is resolved by 115 × 88 vectors, providing a spatial resolution of 0.30 mm. The experimental and diagnostic setup were discussed in detail by Hampp et al [22,24].…”

“…Spalding [25] suggested a multi-fluid approach that permits the identification of various intermediate fluid states. The concept was explored by Hampp [22] and Hampp and Lindstedt [23,24] using simultaneous Mie scattering, PIV and OH -PLIF combined with a purpose written algorithm (summarised see below) that detects four iso-contours in each instantaneous image pair in order to distinguish between five different fluid states. The existence of relatively rare events required 3000 independent realisations to maintain statistical accuracy.…”

“…a n = f β,11 a t = f β,22 a d = e β,11 + e β,22 (6) Results presented here include the movement of the stagnation point [10] with an applied radial limit of ± 0.5 L I away from the stagnation point streamline. A more detailed discussion has been presented by Hampp and Lindstedt [24].…”

“…Hampp and Lindstedt [24] applied a PIV based segregation technique to delineate the current data series. A distinctly reduced gradient transport was shown with increasing Φ, yet the transition to countergradient transport was suppressed even for the stoichiometric flame.…”