Fractal Grid Generated Turbulence—A Bridge to Practical Combustion Applications

Abstract

Abstract.Practical applications typically feature high turbulent Reynolds numbers and, increasingly, low Damköhler numbers leading to distributed combustion. Such conditions are difficult to achieve on a laboratory scale that permits detailed experimental investigations. The aerodynamically stabilised turbulent opposed jet flame configuration is a case point -an exceptionally flexible canonical geometry traditionally featuring low turbulence levels. Fractal grids can be used to increase the turbulent Reynolds number, without any negative impact on other parameters, and to remove the classical problem of a relatively low ratio of turbulent to bulk strain. The use of fractal grids to ameliorate such problems is exemplified for fuel lean combustion with combustion regime transitions achieved through the stabilisation of turbulent premixed flames against hot combustion products. An analysis is presented in the context of a multi-fluid formalism that extends the customary bimodal pdf approach to include multiple fluid states. The approach is quantified via simultaneous OH-PLIF and PIV, permitting the identification of five separate states (reactant, combustion product, mixing, mildly reacting and flamelet fluids). The sensitivity of the distribution between the fluid states to threshold values is also evaluated. The work suggests that a consistent treatment of the delineating thresholds is necessary when comparing different types of simulations (e.g. DNS) and experiments for reacting fluids with multiple states. The use of fractal grids in a flame driven shock tube provides a further example and is shown to generate turbulent Re numbers of the order 10 5 for flows with Mach numbers approaching unity. The conditions are of relevance to flame stabilisation in hypersonics and are analysed through OH-PLIF and high speed PIV with optimal fractal grids selected on the basis of maximum flame acceleration.

Distributed Low Temperature Combustion: Fundamental Understanding of Combustion Regime Transitions

“…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.…”

Quantification of fuel chemistry effects on burning modes in turbulent premixed flames

“…The analysis extends bimodal descriptions [30] by including a wider range of fluid states (i.e. reactants, products, mixing, weakly and strongly reacting fluids) and has been found particularly useful when delineating low Da combustion [16,17,31]. The study further provides information on the conditions leading to burning mode transitions via (i) multi-fluid probabilities, (ii) structural flow field information and turbulence-flame interactions delineated by means of conditional (iii) velocity statistics and (iv) the rate of strain along fluid iso-contours.…”

The impact of hydrogen enrichment on the flow field evolution in turbulent explosions

“…Lindstedt and Sakthitharan [26] used laser doppler anemometry (LDA) to quantify the flow field in the recirculation region behind an obstacle at high Reynolds numbers (Re > 100,000 and 2000 < Re t < 4000) in a flame tube. Hampp and Lindstedt [27] performed 10 kHz PIV measurements in the recirculation region behind a single obstacle with a turbulence-generating cross fractal grid (CFG) installed between the obstacle and ignition spark. The influence of initial turbulence was investigated by Bauwens and Dorofeev [28] in a vented chamber with the turbu-lence generated via four symmetrically installed mixing fans.…”