Fractal-Generated Turbulence in Opposed Jet Flows

Abstract

The opposed jet configuration presents a canonical geometry suitable for the evaluation of calculation methods seeking to reproduce the impact of strain and re-distribution on turbulent transport in reacting and non-reacting flows. The geometry has the advantage of good optical access and, in principle, an absence of complex boundary conditions. Disadvantages include low frequency flow motion at high nozzle separations and comparatively low turbulence levels causing bulk strain to exceed the turbulent contribution at small nozzle separations. In the current work, fractal generated turbulence has been used to increase the turbulent strain and velocity measurements for isothermal flows are reported with an emphasis on the axis, stagnation plane and the distribution of mean and instantaneous strain rates. Energy spectra were also determined. The instrumentation comprised hotwire anemometry and particle image velocimetry with the flows to both nozzles seeded with 1 μm silicon oil droplets providing a relaxation time of 3 μs. It is shown that fractal grids increase the turbulent Reynolds number range from 48-125 to 109-220 for bulk velocities from 4 to 8 m/s as compared to conventional perforated plate turbulence generators. Low frequency motion of the order 10 Hz could not be completely eliminated and probability density functions were determined for the location of the stagnation plane. Results show that the fluctuation in the position of the stagnation plane is of the order of the integral length scale, which was determined to be 3.1±0.1 mm at the nozzle exits through the use of hot-wire anemometry. Flow statistics close to the fractal plate located upstream of the nozzle exit were also determined using a transparent glass nozzle.

Fractal Grid Generated Turbulence—A Bridge to Practical Combustion Applications

“…The opposed jet configuration (Geipel et al 2010, Goh et al 2013b) was slightly modified (see Fig. 3) to accommodate seeding of the lower nozzle (LN) by a combination of a flame stabilising mesh (FSM) and a flash back arrestor (FBA).…”

“…4) was installed 50 mm upstream the UN exit, featuring a BR of 65% with maximum and minimum bar widths of 2 mm and 0.5 mm (t r = 4). The CFG provides increased multiscale turbulence (Goh et al 2013b, Geipel et al 2010, Goh et al 2013a, Goh et al 2014) compared to conventional perforated plates. Premixed CH 4 -air mixtures with U b,U N = 11.0 ms −1 at T U N = 320 K were injected through the UN at varying equivalence ratio (Φ = 0.20, 0.40, 0.60 and 0.80).…”

“…In summary, the current contribution builds on earlier studies of isothermal and combusting flows in the opposed jet geometry (Geipel et al 2010, Goh et al 2013a, Goh et al 2014, Goh et al 2015 to cover lean premixed flames burning against hot combustion products (Goh et al 2013b). Past studies are extended through the use of fractal grid generated turbulence to quantify the transition from a conventional turbulent flame in the flamelet regime of combustion to a distributed mode using a multi-fluid description.…”

“…Despite the use of fractal based analysis techniques, the potential advantages of using fractal grids to generate turbulence in geometries suitable for systematic investigations of turbulent flames is comparatively recent. Geipel et al (2010) systematically investigated the use of fractal cross grids an opposed jet geometry. The study was conducted using Particle Image Velocimetry (PIV) and hot wire anemometry (HWA) to generate velocity statistics and energy spectra in order to assess the potential of such grids.…”

“…The opposed jet geometry has been extensively utilised to investigate -laminar as well as turbulent -non-premixed, partial premixed, and premixed combustion and, more recently, combustion regime transitions for aviation fuels (Goh et al 2013b). The configuration has notable advantages for combustion studies (Mastorakos et al 1995, Geyer et al 2005, Geipel et al 2010): (i) excellent optical access for laser-based diagnostic measurements, (ii) accurate experimental control of boundary conditions, (iii) aerodynamic flame stabilisation, rather than via pilot flames, leading to flame dynamics and extinction being related to the inherent aerothermochemistry of the combustion process; and (iv) individual control of variables affecting the chemical and turbulent time-scale. The last two points identify the facility as ideal for investigations of combustion regime transitions.…”