Velocity and Strain-Rate Characteristics of Opposed Isothermal Flows

Abstract

Velocity measurements in the isothermal flows created by an opposed nozzle configuration are reported with emphasis on the axis, stagnation plane and the distributions of mean and instantaneous strain rates. The instrumentation comprised particle image velocimetry (PIV) with silicon oil droplets added to the flows upstream of both nozzles with the laser sheet passing through the axis between the nozzles. The results identify the regions of high strain rates and quantify the development of the mean and turbulent components of the flow from the nozzle exits as a function of bulk velocities from 3 to 8.2 m/s and nozzle separations from 0.4 to 1.0 diameters. Results show, for example, the rise in the values of axial and radial normal stress towards the stagnation plane with values increasing by up to 300% and 160% respectively. The maximum mean strain rate occurred just over one nozzle radius from the axis at the smallest separation and with values that increased from 450 to 950 s −1 with decreasing separation at a bulk velocity of 3.0 m/s. Probability density functions were near Gaussian and hence much larger instantaneous strain rates were observed. The PIV image size had the advantage that it allowed the entire flow field to be viewed in terms of velocity vectors and derived quantities include mean strain rates. Small asymmetry of the flow and the higher strain rates at finite distances from the nominal impingement plane were observed. The experimental results permitted the domain of applicability of different modelling approaches to be defined more accurately and calculations were performed with different turbulence models. The results showed that two-equation turbulence models did not represent turbulence intensities close to impingement and that Reynolds stress closures produce superior agreement. It was further shown that ad hoc modifications to the dissipation equation, such as those based on the ratio of the turbulent to mean strain time scale, can improve results at the expense of generality. It is also shown that mean flows are well reproduced by a Reynolds stress closure for all nozzle separations. Comments are included on the implications of the results for investigations of reacting flows and extinction. NomenclatureD = Nozzle diameter (mm) H = Nozzle separation (mm) k = Turbulent kinetic energy (m 2 /s 2 ) L t = Integral length scale (m) P = Production (m 2 /s 3 ) r = Radius (mm) 170 R.P. LINDSTEDT ET AL. R = Nozzle radius (D/2) S b = Bulk strain rate (s −1 ) S rad = Radial component of strain rate (s −1 ) S ax = Axial component of strain rate (s −1 ) t = time (s) u 2 = Axial component of Reynolds stress (m 2 /s 2 ) U = Axial component of velocity (m/s) U b = Bulk nozzle exit velocity (m/s) uv = Shear component of Reynolds stress (m 2 /s 2 ) v 2 = Radial component of Reynolds stress (m 2 /s 2 ) V = Radial component of velocity (m/s) x = Axial distance from top nozzle (mm) √ U 2 + V 2 = Velocity magnitude (m/s) δ i j = Kronecker delta (equal to 1 when i = j) µ t = Turbulent viscosity (m 2 /s) ρ = Densit...

The impact of dilatation, scrambling, and pressure transport in turbulent premixed flames

“…To be consistent with the Reynolds-stress and scalar-flux transport equations, the present study includes the preferential acceleration terms. An alternative based on the renormalisation group (RNG) theory [46] has been evaluated in opposed jet flows [47,48],…”

“…In addition, a renormalisation group (RNG) modification to the dissipation equation has been proposed by Yakhot et al [46]. The additional strain-rate terms provide a slightly better agreement with experimental data in opposed jet flows [47,48].…”

Lean premixed opposed jet flames in fractal grid generated multiscale turbulence

“…1. The perforated plates are similar to those used by Mastorakos et al [32][33][34], Sardi et al [15,35,36] and Lindstedt et al [9,12]. Fractal grids [6,[23][24][25] subsequently replaced the perforated plates and were used to increase turbulence intensities at the nozzle exits.…”

“…Preparatory isothermal flow studies have also been presented by a number of investigators (e.g. Geipel et al [6], Kostiuk et al [7], Korusoy and Whitelaw [8], Lindstedt et al [9]) and flame structure studies include velocity and scalar field measurements by Mounaïm-Rouselle and Gökalp [10], Kostiuk et al [11] and Lindstedt et al [12]. Strain effects and extinction and relight characteristics were discussed by Kostiuk et al [13], Mounaïm-Rouselle and Gökalp [14], Sardi and Whitelaw [15], Korusoy and Whitelaw [16] and Luff et al [17] amongst others.…”

“…The same observation [9] also applies for isothermal flows at Reynolds (Re) numbers below 10,000 and higher values were required in order to provide good agreement with numerical simulations when perforated plates were used as turbulence generators. However, such conditions were found to lead to early extinction for flames featuring alkane fuels due to the presence of bulk strain at nozzle separations small enough (H/D ≤ 1) to prevent axial bulk instabilities in the flow.…”