Fractal Grid Generated Turbulence—A Bridge to Practical Combustion Applications

Abstract

Practical applications typically feature high turbulent Reynolds numbers and, increasingly, low Damk¨ohler 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 105 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.