Abstract
Practical applications typically feature high turbulent Reynolds numbers and, increasingly, low Damköhler (Da) numbers leading to distributed combustion. Such conditions are difficult to achieve under laboratory conditions that permit 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. It is shown that 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 further exemplified for fuel lean combustion with low Da numbers achieved through the stabilisation of 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 combustion regime transitions. The approach is quantified via simultaneous OH-PLIF and PIV permitting the identification of five separate states (reactant, combustion product, mixing, mildly and strongly reacting fluids). The sensitivity of the distribution between the fluid states to threshold values is also evaluated for combustion of methane. 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 is further exemplified in a flame driven shock tube and used 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.
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Acknowledgments
The authors would like to acknowledge the support of the ONR under Grant N62909-12-1-7127 and AFOSR and EOARD under Grant FA8655-13-1-3024. The authors wish to thank Dr Gabriel Roy, Dr Chiping Li, Dr Gregg Abate and Dr Russell Cummings for encouraging the work. The contributions by Dr Philip Geipel, Dr Henry Goh and Mr Tao Li are also gratefully recognised.
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Hampp, F., Lindstedt, R.P. (2016). Fractal Grid Generated Turbulence—A Bridge to Practical Combustion Applications. In: Sakai, Y., Vassilicos, C. (eds) Fractal Flow Design: How to Design Bespoke Turbulence and Why. CISM International Centre for Mechanical Sciences, vol 568. Springer, Cham. https://doi.org/10.1007/978-3-319-33310-6_3
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