Flames in fractal grid generated turbulence

Abstract

Twin premixed turbulent opposed jet flames were stabilized for lean mixtures of air with methane and propane in fractal grid generated turbulence. A density segregation method was applied alongside particle image velocimetry to obtain velocity and scalar statistics. It is shown that the current fractal grids increase the turbulence levels by around a factor of 2. Proper orthogonal decomposition (POD) was applied to show that the fractal grids produce slightly larger turbulent structures that decay at a slower rate as compared to conventional perforated plates. Conditional POD (CPOD) was also implemented using the density segregation technique and the results show that CPOD is essential to segregate the relative structures and turbulent kinetic energy distributions in each stream. The Kolmogorov length scales were also estimated providing values ∼0.1 and ∼0.5 mm in the reactants and products, respectively. Resolved profiles of flame surface density indicate that a thin flame assumption leading to bimodal statistics is not perfectly valid under the current conditions and it is expected that the data obtained will be of significant value to the development of computational methods that can provide information on the conditional structure of turbulence. It is concluded that the increase in the turbulent Reynolds number is without any negative impact on other parameters and that fractal grids provide a route towards removing the classical problem of a relatively low ratio of turbulent to bulk strain associated with the opposed jet configuration.

Interscale energy transfer in the merger of wakes of a multiscale array of rectangular cylinders

“…It is worth noting that if none of the corrections described in appendix A were applied, the results of experiment 3 would be systematically lower than those of experiment 4 (by a mean value of 63%). Similar conclusions apply to the flow scales reported in table 6. Values of the Kolmogorov length scale oscillate in an interval 0.10 − 0.14 mm, which is in good agreement with Gomes-Fernandes et al [18].…”

“…The derivation of the energy budget equations, presented in section 5, requires introducing set of assumptions (6). Some of them are rather trivial (e.g.ā =ā) but some may raise a concern, in particular those governing a l ,ã l a orã lãm .…”

“…On the other hand, practical applications of such flows in different fields were studied as well, e.g. : in heat transfer (Cafiero et al [4], combustion (Verbeek et al [5], Goh et al [6]), wind energy (Weitemeyer et al [7]), mixing (Laizet and Vassilicos [8])…”

Fractal Grid Generated Turbulence—A Bridge to Practical Combustion Applications

“…It was shown that the use of fractal cross grids led to a more than 100% increase in turbulence levels, with a potential for further increases, leading to a corresponding reduction in ratio of bulk to turbulent strain and thus enabling extended studies of turbulent flames in the opposed jet configuration. Subsequent studies include the structure of turbulence in lean premixed flames (Goh et al 2013a), the determination of scalar fluxes and conditional statistics (Goh et al 2014) and turbulent transport in flames approaching extinction (Goh et al 2015). In addition, studies using rod stabilisation (Soulopoulos et al 2013) and low-swirl combustion (Verbeek et al 2015) have appeared suggesting potentially wider application.…”

“…The approach permits the identification of five fluid states (reactant, combustion product, mixing, mildly reacting and flamelet fluids) and a range of Da numbers, from the conventional propagating flamelet regime well into the distributed reaction zone regime, are investigated. Böhm et al (2009) and Goh et al (2013aGoh et al ( , 2013bGoh et al ( , 2014 have shown that the combination of PIV and OH-PLIF can be used to obtain velocity-scalar statistics describing the dynamics of opposed jet flames approaching extinction and the techniques form the basis for the investigations reported here.…”