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.