Flame propagation in the turbulent flow of a burning mixture

The propagation of a flame in the turbulent flow of a burning mixture is analyzed theoretically. An equation is derived for the gas temperature and velocity probability distributions. The solutions of this equation are analyzed and the rate of flame propagation is determined.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 3–15, September–October, 1976.The author wishes to thank V. A. Sabel'nikova for critical comments.     

The improvement of blowout limit in partially/fully premixed flames with geometrically modified bluffbody bases

The improvement of the lean blowout limit of bluffbody stabilized methane flames is investigated. The flame configuration consists of a hybrid of partially and fully premixed flames, which is produced by injecting methane jets from a streamline-shaped bluffbody into a fully premixed methane/air crossflow. Additional geometric modifications from the original bluffbody base are such that the base has two-dimensionally modified geometries and three-dimensional local cavities. We observe that the blowout limit of the hybrid configuration is extended by up to ~12% (in terms of the equivalence ratio of the crossflow) with the modified geometries. Gas chromatographic sampling and particle image velocimetry (PIV) show that high fuel mole fraction regions coexist with regions of low speed flow for the modified geometries. Further PIV analysis shows that the downstream flow fields of the modified bases generally have a larger number of incoherent vortices and lower strain rate in comparison with those of the unmodified base.     

Investigation of the flow structure in nozzles with a throat section of constant height

The results of a theoretical and experimental investigation of the flow structure in plane nozzles with a throat section of constant height are presented. The effect of longitudinal and transverse deviations in the nozzle geometry on the transonic and supersonic flow parameters is studied.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 160–166, November–December, 1990.     

LES investigation on the dependence of the flow through a centrifugal pump on the diffuser geometry

Large-Eddy Simulation is utilized to investigate the rotor–stator interaction within a centrifugal pump. Comparisons are presented across diffuser geometries for two values of the flow-rate. Decreasing the incidence angle on the diffuser blades at off-design is found the main source of higher pressure rise and lower overall values of turbulent kinetic energy within the pump, resulting in efficiency improvement. The impact on the second-order statistics of the flow is especially significant. However, the values of the pressure fluctuations acting on the diffuser blades, defining fatigue loads on them and cavitation phenomena, are found especially affected by the rotor–stator clearance. Results show that at reduced flow-rates the rotation of the diffuser blades around their mid camber is a better option than rotating them around their leading edge. They also suggest that at larger flow-rates the increased incidence on the diffuser blades causes pressure side separation and large shear layers populating the diffuser channels, not affecting substantially the region of interface between impeller and diffuser, but having detrimental effects on the performance of the latter. The rotation of the diffuser blades around their leading edge should be preferred when the pump operates at flow-rates larger than the design one, avoiding decreasing the rotor–stator gap, thus resulting in smoother rotor–stator interaction and lower pressure fluctuations.     

Field dependence of the response of a magnetorheological suspension under steady shear flow and squeezing flow

A comparison was made of the behaviour of a magnetorheological suspension under steady shear flow and constant velocity squeezing flow. The strain rates and sample dimensions were chosen to be comparable in the two deformation modes, and the dependence of the mechanical properties on the magnetic flux density B was investigated. The measurements found that the mechanical response under squeezing flow scaled as B^0.91, whereas the response under shearing scaled as B^1.4, close to theoretical predictions. This difference of the field dependence between the shearing and squeezing flows was possibly due to the different microstructural rearrangement processes which occur in the two deformation modes.     

Turbulent channel flow with 2D wedges of random height on one wall

Direct numerical simulations (DNS) of a turbulent channel flow with 2D wedges of random height on the bottom wall have been performed. In addition, two other simulations have been carried out to assess the effect of the geometry on the overlying flow. In the first simulation, the four smallest elements were removed while in the other, a uniform distribution of wedges with the same area was used. Two Reynolds numbers were studied, Re_b=2500 and Re_b=5000 which correspond in case of smooth walls to Re_τ=180 and 300, respectively. Roughness on the wall induces separated regions, the reattachment occurring on the walls of the wedges or on the bottom wall. The pressure gradients on the walls increase the ejections and inrushes towards the wall. As a consequence the flow is more isotropic. The mechanism inducing an improved isotropy has been explained in term of the spectra and budgets of Reynolds stress. The comparison of the 3 surfaces has shown that near the wall, the uniformly distributed roughness represents only a poor approximation of the surface with wedges of random height. The Reynolds stresses, pressure distribution and spectra on the modified wall agree well with those on the random surface. Energy spectra show the pitch to height ratio of the largest elements to be the more appropriate geometrical parameter to describe the geometry.     

Experimental investigation of the influence of downstream flow conditions on Mach stem height

An experimental work on the influence of downstream flow expansion process on Mach stem height has been carried out in steady hypersonic flows. The results showed clearly for the first time that the Mach stem height does not depend on the extent of expansion fan. These results contradict recent analytical findings from which a strong influence of downstream flow conditions on Mach stem height may be expected. Received 31 August 1998/ Accepted 10 September 1998     

Modelling of a Premixed Swirl-stabilized Flame Using a Turbulent Flame Speed Closure Model in LES

This paper proposes a combustion model based on a turbulent flame speed closure (TFC) technique for large eddy simulation (LES) of premixed flames. The model was originally developed for the RANS (Reynolds Averaged Navier Stokes equations) approach and was extended here to LES. The turbulent quantities needed for calculation of the turbulent flame speed are obtained at the sub grid level. This model was at first experienced via an test case and then applied to a typical industrial combustor with a swirl stabilized flame. The paper shows that the model is easy to apply and that the results are promising. Even typical frequencies of arising combustion instabilities can be captured. But, the use of compressible LES may also lead to unphysical pressure waves which have their origin in the numerical treatment of the boundary conditions.     

Effect of static bed height in the upper fluidized bed on flow behavior in the lower riser section of a coupled reactor

To study olefin reduction by using an auxiliary reactor for FCC naphtha upgrading, a large-scale cold model of a riser-bed coupled to an upper fluidized bed was established. The effect of static bed height in the upper fluidized bed on narticle flow behavior in the lower riser was investigated experimentally. A restriction index of solids holdup was used to evaluate quantitatively the restrictive effect of the upper fluidized bed. Experimental results show that, under the restrictive effect of the upper fluidized bed, the riser could be divided into three regions in the longitudinal direction： accelerating, fully developed and restriction. The axial distribution of solids holdup in the riser is characterized by large solids holdup in the top and bottom sections and small solids holdup in the middle section. Overall solids holdup increased with increasing static bed height in the upper fluidized bed, while particle velocity decreased. Such restrictive effect of the upper fluidized bed could extend from the middle and top sections to the whole riser volume when riser outlet resistance is increased, which increases with increasing static bed height in the upper fluidized bed. The upper bed exerts the strongest restriction on the area close to the riser outlet.     

Effect of static bed height in the upper fluidized bed on flow behavior in the lower riser section of a coupled reactor

To study olefin reduction by using an auxiliary reactor for FCC naphtha upgrading, a large-scale cold model of a riser-bed coupled to an upper fluidized bed was established. The effect of static bed height in the upper fluidized bed on particle flow behavior in the lower riser was investigated experimentally. A restriction index of solids holdup was used to evaluate quantitatively the restrictive effect of the upper fluidized bed. Experimental results show that, under the restrictive effect of the upper fluidized bed, the riser could be divided into three regions in the longitudinal direction: accelerating, fully developed and restriction. The axial distribution of solids holdup in the riser is characterized by large solids holdup in the top and bottom sections and small solids holdup in the middle section. Overall solids holdup increased with increasing static bed height in the upper fluidized bed, while particle velocity decreased. Such restrictive effect of the upper fluidized bed could extend from the middle and top sections to the whole riser volume when riser outlet resistance is increased, which increases with increasing static bed height in the upper fluidized bed. The upper bed exerts the strongest restriction on the area close to the riser outlet.     

The dependence of viscoelastic flow properties on the structure for styrene-butadiene copolymers

The flow properties for 300 kg/m³ solutions of four-arm, star-branched random and block styrene-butadiene-styrene copolymers in n-butylbenzene are presented. The viscosity, η, first and second normal stress differences, N₁ and N₂, and the steady sher compliance, J, were determined as a function of the shear rate from cone-plate shearing data obtained with a stiffened Model R17 Weissenberg Rheogoniometer. The normal stress differences were determined from total normal force and plate pressure distribution data. Four sensitive, miniature, variable-capacitance pressure transducers mounted in the 7.4-cm plate with their approximately 2.4 mm in diameter pressure-sensing diaphragms flush with the plate surface provided data for the pressure distribution on the plate. In general, the data extend from the zero shear-rate viscosity region somewhat into the shear thinning region. Based on limited data, the zero shear viscosities for the random copolymers increased with (M₃)^3.2, whereas those for the 28% block and 38% block polymers increased with (M_w)^?;6, respectively. The latter high exponents are believed to be a consequence of a network with junctions formed by dispersed phase polystyrene block domains. The sign for N₂ was opposite that of N₁ and the ratio N₂/M₁ for all of the star copolymers averaged –0.214 with a standard deviation of 1.5%. This value is within ±1% of the ratios for tetrachain polybutadienes and polystyrenes and is significantly lower than the –0.29 for linear polybutadiene and polystyrene solutions in normal butylbenzene. The N₂/N₁ ratio did not vary significantly over the shear-rate range investigated.     

Migration of particles in a turbulent nonisothermic flow as a result of dependence of the viscosity of the gas on the temperature

The motion is considered of a Stokes-like spherical particle in a turbulent nonisothermic gas flow whose viscosity depends on the temperature. The field of the turbulent velocity is assumed to be homogeneous, isotropic, and steady. It is shown that if there is a mean temperature gradient in the gas, and, consequently, a heat flow due to turbulent pulsations, then there may be turbulent migration of particles in a direction collinear with the gradient of the mean temperature. The migration is due to statistical correlation of turbulent pulsations of velocity and temperature, and is not connected with the phenomenon of ordinary thermophoresis. Upon the introduction of a number of simplifying assumptions, the rate of migration is calculated in dependence on the characteristics of the particle and the flow. Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 53–58, November–December, 1986. The author is grateful to V. S. Galkin, V. A. Zharov, M. N. Kogan, and V. A. Sabel'nikov for discussions of the study.     

Chain scission in transient extensional flow kinetics and molecular weight dependence

Recent experimental investigations have shown that large macromolecules can be fully stretched and fractured in an extensional flow. In this situation, the critical strain-rate for bond scission was found to depend on molecular weight as (M_W)⁻², in agreement with the theoretical predictions of the bead---rod model. One of the conditions prerequisite to full chain extension is that the residence time at the appropriate strain-rate must be much larger than the terminal relaxation time of the macromolecule. If this requirement is not fulfilled, chain fracture could still occur at sufficiently high strain-rate, but in a partially uncoiled state. In the present studies we have measured the critical strain-rate for chain scission in transient extensional flow of extremely sharp PS fractions dissolved in dekalin at the θ-temperature. The molecular weight range investigated varied from 2.86 × 10⁶ to 426,000. The critical strain-rate for chain scission was found experimentally to scale as (M_W)^−0.95 instead of (M_W)⁻² as predicted for stagnant extensional flow. Our results are in good accord with a recent theory for rupture of partly extended coils. Even in the partially uncoiled state, the degraded macromolecules showed a remarkable propensity for chain halving, indicating that midchain scission in flow is a general property that is not uniquely reserved to the fully extended chain.     

Reynolds number dependence of vortex patterns in accelerated flow around airfoils

Reynolds number dependence of vortical patterns visualized by smoke technique in accelerating flow behind airfoils is documented in photographic sequences at angles of attack 20° and 60°. At low Reynolds numbers the vortical pattern development is quite simple. With increasing Reynolds numbers these patterns become increasingly complex, and onset of turbulence occurs early and generates large scale turbulent vortex patterns.     

Penetration height correlations for non-aerated and aerated transverse liquid jets in supersonic cross flow

Experimental results on the mixing of non-aerated and aerated transverse liquid jet in supersonic cross flow (M = 1.5) are presented in this paper. The goal of this study is to investigate the effect of the gas/liquid mass ratio on the penetration and atomization of an aerated liquid jet in high speed cross flow and to develop correlations for the penetration heights. High speed imaging system was used in this study for the visualization of the injection of aerated liquid jet. The results show the effect of jet/cross flow momentum flux ratio, the gas/liquid mass ratio and the Ohnesorge number on the penetration of aerated liquid jet in supersonic cross-flow. New correlations of the spray penetration height for the non-aerated liquid jet (GLR = 0) and the net gain in spray penetration height for the aerated liquid jet (GLR > 0) are presented.