Non-dimensional Turbulence Intensity Research Articles

Network topology of turbulent premixed Bunsen flame at elevated pressure and turbulence intensity

For turbulent premixed flame surface, folded regions will be formed mainly depending on the turbulence-flame interaction and are significant structures which can increase flame surface area at large scale and enhance the local displacement speed. The conventional methods, such as PDF distribution of curvature, may not properly label the folded regions of turbulent flame because these regions intrude deeply into the products, resulting in a small curvature probability. This problem may be aggravated at elevated pressure and turbulence intensity conditions when more folded regions appear at the turbulent flame front. To identify the folded regions, Network topology of turbulent premixed flame front is constructed using the “visibility” method. Results show that this method can convert the spatial signal of turbulent flame to network topology, labeling the folded regions. Compared with curvature PDF, node degree distribution of network can reflect the mechanism of turbulence-flame interaction when the non-dimensional turbulence intensity is increased by different ways. The network structure of turbulent flame will transfer from sparse to condense when the dimensionless turbulence intensity is increased by pressure and perforated plate, as these two methods will extend the turbulence-flame interaction time and promote the interaction intensity, respectively. However, although the dimensionless turbulence intensity will increase with the augment of outlet velocity, the node degree distribution of network structure of turbulent flame front keeps almost constant. This is caused by the reduced turbulence-flame interaction time. It suggests that the turbulence-flame interaction time is an factor as important as the dimensionless turbulence intensity in turbulent premixed combustion. For forced-turbulent premixed flame at elevated turbulence intensity, the “bending phenomenon” will be hidden if the outlet velocity is not taken into account, as the outlet velocity is related to the turbulence-flame interaction time.

Effects of mixture composition and turbulence intensity on flame front structure and burning velocities of premixed turbulent hydrocarbon/air Bunsen flames

The influences of the equivalence ratio, turbulence intensity, and different thermo-diffusive characteristics on the flame brush characteristics, instantaneous flame front structures, and burning velocities of premixed turbulent methane/–, ethane/–, and propane/air Bunsen flames were investigated systematically. Particle image velocimetry and Mie scattering techniques were utilized to measure the turbulence statistics and to visualize flame front corrugations, respectively. All experiments were performed under a constant bulk flow velocity of 21.0 m/s. The equivalence ratio range was from 0.7 to 1.35 for methane/air flames, 0.7–1.45 for ethane/air flames, and 0.8–1.35 for propane/air flames. Two perforated plates were used to produce different turbulence levels. A series of comprehensive parameters including the characteristic flame height, mean flame brush thickness, mean volume of the turbulent flame region, mean fuel consumption rate, two-dimensional flame front curvature, local flame front angle, two-dimensional flame surface density, wrinkled flame surface area, turbulent burning velocity, mean flamelet consumption velocity, and mean turbulent flame stretch factor were obtained. The mean turbulent flame stretch factor displayed a dependence on the equivalence ratio and turbulence intensity. Results show that the mean turbulent flame stretch factors for lean/stoichiometric and rich mixtures were not equal when the unstrained premixed laminar burning velocity, non-dimensional bulk flow velocity, non-dimensional turbulence intensity, and non-dimensional longitudinal integral length scale were kept constant.

Effect of burner diameter on the burning velocity of premixed turbulent flames stabilized on Bunsen-type burners

Influence of burner diameter on the turbulent burning velocity of premixed turbulent methane/air flames was studied on two Bunsen-type burners with nozzle inner diameters of 11.1 and 22.2mm. The equivalence ratio was varied from 0.6 to 1.0. The non-dimensional turbulence intensity, non-dimensional bulk flow velocity, and non-dimensional longitudinal integral length scale were kept constant for a given equivalence ratio for both burners. Particle image velocimetry and Rayleigh scattering techniques were used to measure the instantaneous velocity and temperature fields, respectively. The characteristic flame height decreased with increasing equivalence ratio from 0.6 to 1.0 for both burners, whereas it increased considerably by increasing the burner diameter. The transverse profiles of the leading edge and half-burning surface flame surface densities showed a tall and narrow region at each side of the burner exit, whereas they were distributed over a larger area farther downstream of the burner exit. The leading edge and half-burning surface wrinkling factors were found to be higher for the larger burner farther downstream of the burner, whereas they are about the same near the burner exit. The non-dimensional leading edge turbulent burning velocity increased with increasing non-dimensional turbulence intensity for both burners. However, the leading edge turbulent burning velocity was found to be higher with the larger size burner.

Flame brush characteristics and burning velocities of premixed turbulent methane/air Bunsen flames

The flame brush characteristics and turbulent burning velocities of premixed turbulent methane/air flames stabilized on a Bunsen-type burner were studied. Particle image velocimetry and Rayleigh scattering techniques were used to measure the instantaneous velocity and temperature fields, respectively. Experiments were performed at various equivalence ratios and bulk flow velocities from 0.7 to 1.0, and 7.7 to 17.0m/s, respectively. The total turbulence intensity and turbulent integral length scale were controlled by the perforated plate mounted at different positions upstream of the burner exit. The normalized characteristic flame height and centerline flame brush thickness decreased with increasing equivalence ratio, total turbulence intensity, and longitudinal integral length scale, whereas they increased with increasing bulk flow velocity. The normalized horizontal flame brush thickness increased with increasing axial distance from the burner exit and increasing equivalence ratio. The non-dimensional leading edge and half-burning surface turbulent burning velocities increased with increasing non-dimensional turbulence intensity, and they decreased with increasing non-dimensional bulk flow velocity when other turbulence statistics were kept constant. Results show that the non-dimensional leading edge and half-burning surface turbulent burning velocities increased with increasing non-dimensional longitudinal integral length scale. Two correlations to represent the leading edge and half-burning surface turbulent burning velocities were presented as a function of the equivalence ratio, non-dimensional turbulence intensity, non-dimensional bulk flow velocity, and non-dimensional longitudinal integral length scale. Results show that the half-burning surface turbulent burning velocity normalized by the bulk flow velocity decreased as the normalized characteristic flame height increased.

Experimental investigation of the inner structure of premixed turbulent methane/air flames in the thin reaction zones regime

The thermal flame front thickness of premixed turbulent methane/air flames stabilized on a Bunsen-type burner was investigated experimentally. The instantaneous velocity and temperature fields were measured using the particle image velocimetry and Rayleigh scattering techniques, respectively. The Karlovitz number was varied from 1.2 to 20.7, indicating that the measured data are located within the thin reaction zones regime. The normalized preheat zone and reaction zone thicknesses decreased with increasing non-dimensional turbulence intensity in ultra-lean premixed turbulent flames under a constant equivalence ratio of 0.6, whereas they increased with increasing equivalence ratios from 0.6 to 1.0 under a constant bulk flow velocity. These normalized thicknesses were higher than unity for equivalence ratios ranging from 0.8 to 1.0, indicating that the internal structure of the flame fronts deviates from the thin laminar flamelet assumption. However, the mean widening for progress variable contours of premixed turbulent flames in comparison with corresponding values of unstrained premixed laminar flames was insignificant, implying that the influence of turbulent eddies on the flame front broadening is negligible. The normalized preheat zone and reaction zone thicknesses showed no overall trend with increasing non-dimensional longitudinal integral length scale. The normalized preheat zone and reaction zone thicknesses decreased by increasing the Karlovitz number, suggesting that increasing the total stretch rate is the controlling mechanism in the reduction of flame front thickness for the experimental conditions studied in this work. The probability density functions of the progress variable in front of the preheat layers were found to be insensitive to an increase in the non-dimensional turbulence intensity.

Dynamics of Lean-Premixed Turbulent Combustion at High Turbulence Intensities

Premixed turbulent flames of methane-air stabilized on a Bunsen-type burner were studied to investigate the structure of the flame front at a wide range of turbulence intensities. The nondimensional turbulence rms velocity, rms velocity divided by the laminar flame speed, covered the range from about 3 to 24. The equivalence ratio was varied from 0.6 to stoichiometric. The flame front data were obtained using planar Rayleigh imaging, and particle image velocimetry was used to measure instantaneous velocity field for the experimental conditions studied. Flame front thickness increased slightly with increasing nondimensional turbulence rms velocity. There was no significant difference in flame thickening whether the flame thickness was evaluated at progress variable 0.5, corresponding to the reaction zone, or 0.3, corresponding to the preheat zone. Flame front curvature decreased with increasing turbulence rms velocity. Flame front curvature statistics displayed a Gaussian-like distribution, which centered about zero for all the flame conditions studied during the investigation. Flame surface densities evaluated from flame front images showed almost no dependence on the nondimensional turbulence intensity. Flame surface densities integrated over the flame brush volume also did not show any sensitivity to the nondimensional turbulence rms velocity. This was discussed in the framework of a flame surface density-based turbulent premixed flame propagation closure model. The implication is that the conceptual increase in flame surface density with turbulence may not be the dominant mechanism for flame velocity enhancement in turbulent combustion in the region specified as the flamelet combustion regime by the current turbulent premixed combustion diagrams. Further, the applicability of the flamelet approach may be limited to a much smaller range of conditions than presently believed.

Investigation of Dynamics of Lean Turbulent Premixed Flames by Rayleigh Scattering

Turbulent premixed flames of propane/air and methane/air were studied on a stabilized Bunsen-type burner to investigate the interactions between turbulence and the structure of the flame front in the thin reaction zones regime. The fuel-air equivalence ratio was varied from 0.7 to stoichiometric for propane flames, and from 0.6 to stoichiometric for methane flames. The nondimensional turbulence intensity u'/S L covered the range from 2.7 to 24.1. The flame front data were obtained using planar Rayleigh scattering technique, and particle image velocimetry was used to measure the instantaneous velocity field of the flames. Thermal structure of the flame front was observed to change with increasing U'/S L . The reaction zone and preheat zone thicknesses increased modestly with nondimensional turbulence intensity in both propane and methane flames. Flame front curvature statistics displayed the same Gaussian-like distribution, which centered around zero for all the flame conditions studied. Flame surface density results exhibited almost no dependence on the nondimensional turbulence intensity. It was found that the flame curvature was able to broaden the flame front and reduce the flame surface density.

Statistical analysis of small bubble dynamics in isotropic turbulence

The dynamics and dispersion of small air bubbles in isotropic turbulence are analyzed computationally. The flow field is simulated using a pseudospectral code, while the bubble dynamics are analyzed by integration of a Lagrangian equation of motion that accounts for buoyancy, added mass, pressure, drag, and lift forces. Probability density functions (pdfs) of bubble velocities, lift and drag forces, and of field velocities and vorticities along bubble trajectories are used to analyze bubble dynamics. Lagrangian bubble trajectories are also employed to determine dispersion characteristics, following the theoretical development of Cushman and Moroni [Phys. Fluids 13, 75 (2001)]. Consistent with available experimental data, bubble rise velocities are increasingly suppressed with increasing turbulence intensity. The analysis also reveals that the vertical bubble velocities are characterized by asymmetric pdfs that are positive or negative-skewed dependent upon the nondimensional turbulence intensity and the Taylor length scale. The role of the lift force in moving the bubbles to the down-flow side of turbulent eddies, and consequently retarding their rise, is consistently observed in all analyses. The dispersion of 40μm bubbles and transition to Fickian behavior is shown to be weakly affected by the turbulence level. Larger, 400μm bubbles are shown to be more sensitive to turbulence level with transition to Fickian behavior delayed in low turbulence fields.

FLAME SURFACE DENSITIES IN PREMIXED COMBUSTION AT MEDIUM TO HIGH TURBULENCE INTENSITIES

The surface densities of flame fronts in turbulent premixed propane/air flames were determined experimentally. The instantaneous flame fronts were visualized using laser induced fluorescence (LIF) of OH on two Bunsen type burners of 11.2 and 22.4 mm diameters. Non-dimensional turbulence intensity, u′/S L, was varied from 0.84 to 15, and the Reynolds number, based on the integral length scale, varied from 34 to 467. These flames are in the flamelet combustion regime as defined by the most recent turbulent premixed combustion diagrams. From 100 to 800 images were recorded for each experimental condition. Flame surface densities were obtained from the instantaneous maps of the progress variable, which is zero in the reactants and unity in the products. These flame surface densities were corrected for the mean direction cosines of the flame fronts, which had a typical value of 0.69 for the Bunsen flames. In the non-dimensional turbulence intensity range of up to 15, it was found that the maximum flame surface density and the integrated flame surface density across the flame brush do not show any significant dependence on turbulence intensity. This was discussed in the framework of a flame surface density-based turbulent premixed flame propagation closure model. The implication is that the conceptual increase in flame surface density with turbulence may not be the dominant mechanism for flame velocity enhancement in turbulent combustion in the region specified as the flamelet combustion regime by the current turbulent premixed combustion diagrams. Small-scale transport of heat and species may be more important and chemistry may not be decoupled from turbulence. Further, the applicability of the flamelet approach may be limited to a much smaller range of conditions than presently believed.

Fractal characterisation of high-pressure and hydrogen-enriched CH 4–air turbulent premixed flames

Flame surface characteristics were measured in turbulent premixed flames of hydrogen-enriched methane–air mixtures at elevated pressures. The equivalence ratio range was from 0.6 to 0.8 and the hydrogen mole fraction in the fuel was varied from 0 to 0.2. The pressure range covered from atmospheric to 0.9 MPa. Planar Mie scattering measurements were performed to obtain the flame front images, which were further analyzed for fractal parameters, flame front curvature statistics, and flame surface density. Non-dimensional turbulence intensity, u′/ S L, changed in proportion to the pressure and was 1.53 at atmospheric pressure and 6.51 at 0.9 MPa. With increasing pressure the flame images displayed finer structures indicating that the flame surface area was increasing with pressure. This was also indicated by the probability density function of the flame front curvature as a function of the pressure. Fractal dimension showed a strong dependence on pressure and increased from about 2.1 at atmospheric pressure to about 2.25 at 0.9 MPa. Fractal parameters, including inner and outer cut-offs, agree to a certain extend with the previous experimental data obtained at atmospheric and elevated pressures. The sensitivity of the fractal dimension to u′/ S L was found to depend on the way u′/ S L was varied. A strong correlation between the inner cut-off scale and the average flame radius of curvature was demonstrated. The implications of these results for flame surface density estimations are discussed.

Numerical Study of Wake Vortex Decay and Descent in Homogeneous Atmospheric Turbulence

Numerical simulations are performed to isolate the effect of ambient turbulence on the wake vortex decay rate within a neutrally stratified atmosphere. Simulations are conducted for a range of turbulence intensities by injecting wake vortex pairs into an approximately homogeneous and isotropic turbulence field. Consistent with field observations, the decay rate of the vortex circulation increases clearly with increasing levels of ambient turbulence. Based on the results from the numerical simulations, simple decay models for the vortex pair are proposed as functions of nondimensional ambient turbulence intensity, nondimensional radial distance, and nondimensional time. For strong atmospheric turbulence, the model predictions are in reasonable agreement with the observational data. For weak turbulence with stable stratification, the model, based on turbulence dissipation alone, underestimates circulation decay with consistent overestimation of vortex descent, unless stratification effects are included

Flame front surface characteristics in turbulent premixed propane/air combustion

The characteristics of the flame front surfaces in turbulent premixed propane/air flames were investigated. Flame front images were obtained using laser-induced fluorescence (LIF) of OH and Mie scattering on two Bunsen–type burners of 11.2-mm and 22.4-mm diameters. Nondimensional turbulence intensity, u′/ S L , was varied from 0.9 to 15, and the Reynolds number, based on the integral length scale, varied from 40 to 467. Approximately 100 images were recorded for each experimental condition. Fractal parameters (fractal dimension, inner and outer cutoffs) and corresponding standard deviations were determined by analysis of the flame front images using the caliper technique. The fractal dimensions derived from OH and Mie scattering images are almost identical. However, inner and outer cutoffs from OH images are consistently higher than those obtained from Mie scattering. The self-similar region of the flame front wrinkling is about a decade for all flames studied. In the nondimensional turbulence intensity range from 1 to 15, it was found that the mean fractal dimension is about 2.2 and it does not show any dependence on turbulence intensity. This contradicts the findings of the previous studies that showed that the fractal dimension asymptotically reaches to 2.35–2.37 when the nondimensional turbulence intensity u′/ S L exceeds 3. It is shown that the reason for this discrepancy is the image analysis method used in the previous studies. Examples are given to show the inadequacy of the circle method used in previous studies for extraction of fractal parameters from flame front images. The fractal parameters obtained so far, in this and previous studies, are not capable of correctly predicting the turbulent burning velocity using the available fractal area closure model.

The Effect of Reynolds Number on the Turbulence Characteristics of Fully Developed Turbulent Channel Flow.

Hot wire measurements of a streamwise velocity component were carried out in a fully developed turbulent channel flow under an isothermal steady condition. Reynolds number of flow varied from 8000 to 40000 (Rer=190∼617). The aspect ratio of channel is 3 : 1. We focused an attention to the effect of Reynolds number on the characteristics of velocity fluctuation in the near wall region. As a result of investigation, it is clear that the universal profile of non-dimensional velocity turbulence intensity in the viscous layer is independent of Reynolds number. The universality is also gotten in distributions of the skewness and the flatness factors of turbulence in the viscous layer. From the universality, we propose the empirical relation, urms+=Σan×(Log Y+)n, in the viscous layer, Y+<50.