Quantification of combustion regime transitions in premixed turbulent DME flames

Abstract

The current study quantifies the probability of encountering up to five fluid states (reactants, combustion products, mixing fluid, fluids with low and high reactivity) in premixed turbulent DME flames as a function of the Damköhler number. The flames were aerodynamically stabilised in a back-to-burnt opposed jet configuration featuring fractal grid generated multi-scale turbulence (Re≃ 18,400 and Ret > 370). The chemical timescale was varied via the mixture stoichiometry resulting in a wide range of Damköhler numbers (0.08 ≤ Da ≤ 5.6). The mean turbulent strain (≥ 3200 s−1) exceeded the extinction strain rate of the corresponding laminar flames for all mixtures. Simultaneous Mie scattering, OH-PLIF and PIV were used to identify the fluid states and supporting computations show that the thermochemical state (e.g. OH and CH concentrations) at the twin flame extinction point correlates well with flames in the back-to-burnt geometry at the corresponding rate of heat release. For mixtures where the bulk strain (≃ 750 s−1) was similar to (or less than) the extinction strain rate, fluids with low and high reactivity could accordingly be segregated by a threshold based on the OH concentration at the extinction point. A sensitivity analysis of the distribution between the fluid states was performed. The flow conditions were further analysed in terms of Damköhler and Karlovitz numbers. The study provides (i) the evolution of multi-fluid probability statistics as a function of the Damköhler number, including (ii) the flow direction across fluid interfaces and OH gradients, (iii) mean flow field statistics, (iv) conditional velocity statistics and (v) a tentative combustion regime classification

Distributed Low Temperature Combustion: Fundamental Understanding of Combustion Regime Transitions

“…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].…”

“…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.…”

“…1), originally developed by Geyer et al [11], was extensively used by Geipel et al [10] and Goh et al [8,31,32,33] and is identical to the the configuration used by Hampp et al [22,23]. A cross fractal grid (CFG; [34]) was installed 50 mm upstream the UN exit, featuring a BR of 65 % with maximum and minimum bar widths of 2.0 mm and 0.50 mm.…”

“…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.…”

“…The work performed on DME 9 is here used as an example of the investigations performed. In addition, methane [22,23] and ethanol [23] have also been considered.…”