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To study the effects of different diluents on the propagation characteristics of H 2 /CO/air mixture turbulent premixed flames, a series of experiments were carried out in a turbulent premixed flame experimental system. The effects of turbulence intensity (0.49–1.31 m/s), dilution gas content (10%, 20%, and 30%), hydrogen fraction (50%, 70%, and 90%), and equivalence ratio (0.6, 0.8, and 1.0) on the turbulent premixed flame were studied. The results show that with the increase in hydrogen fraction or turbulence intensity or equivalence ratio, the S T and u t increase at the same radius. Compared with N 2 dilution, CO 2 dilution showed a more obvious inhibition effect on S T. With the increase of Ka , S T;35mm /u' gradually decreased, and the extent of S T;35mm /u' decrease gradually became smaller. As the intensity of turbulence increases or the hydrogen fraction increases, the slope of S T,35mm /u' with Da/Le gradually decreases. In the turbulence intensity range of this experiment, the u t,35mm / μ l under nitrogen dilution condition has a larger floating range. The growth rate of u t,35mm / μ l at a low equivalence ratio is significantly higher than that at a high equivalence ratio. • Effect of diluent content on the turbulent flame propagation speed was studied. • Relationship between Ka and S T,35mm /u' of syngas/air mixture flame was evaluated. • Effect of u' , φ and X H2 on flame propagation characteristics were studied. • Effect of Da and Ze on the turbulent burning velocity was analyzed. [ABSTRACT FROM AUTHOR]

For studying the effect of carbon dioxide (CO 2) on the self-accelerating properties of syngas/air mixture's turbulent premixed flame, an experimental study was carried out on 90%H 2 /10%CO/air mixture at the turbulence intensity of 1.31 m/s. The effects of various equivalence ratios (0.6, 0.8 and 1.0) and carbon dioxide contents (10%, 20% and 30%) on the flame evolution and propagation speed were studied. The effects of the CO 2 contents and equivalence ratios on the self-acceleration properties in the flame propagation transition stage were evaluated. The effects of effective Lewis number and Zel'dovich number on the fractal excess and acceleration exponent were analyzed. The results showed that, as the CO 2 contents increased, the development process of the flame slowed down, and the rate of growth of the flame's radius and the turbulent premixed flame propagation speed decreased at the same radius. The fractal excess and acceleration exponent increased with the increasing CO 2 contents and decreased with the increasing equivalence ratio. Under different equivalence ratios and CO 2 contents, the syngas/air premixed flame acceleration exponent was less than 1.5 in the transition stage. • Effect of CO 2 content on the flame evolution and propagation speed was studied. • Effect of ϕ and CO 2 content on the self-acceleration properties were evaluated. • The self-acceleration properties in the transition stage were studied. • Effect of Le eff and Ze on the acceleration exponent was analyzed. [ABSTRACT FROM AUTHOR]

To investigate self-acceleration propagation characteristics of a laminar premixed flame, an experimental study of H 2 /CO/air mixtures with various hydrogen fractions and equivalence ratios was conducted. The acceleration exponent and fractal excess were defined to quantitatively investigated flame self-acceleration in the transition and saturation stages. Also, the influence of flame inherent instabilities on the acceleration exponent in the transition stage were investigated. The results indicate that with an increase in the hydrogen fraction, the first and second critical radius decreased, the proportion of the transition (saturation) stage in the whole flame propagation process decreased (increased), and the acceleration exponent and fractal excess of the transition and saturation stages increased. Because of the limits of flame radius and different degrees of pulsation in the saturation stage, the acceleration exponent and fractal excess at the saturation stage measured do not show obvious regularity; the values are less than 1.5 and 0.33, respectively. When the hydrogen fraction in syngas is changed, the acceleration exponent in the transition stage showed a nonlinear decreasing trend with an increase in the effective Le number. The hydrodynamic instability usually increased with a decrease in flame thickness, and the acceleration exponent in the transition stage increased. • Effect of hydrogen fraction on flame inherent instabilities was studied. • Effect of hydrogen fraction on self-acceleration propagation was studied. • Correlation between acceleration exponent and Le number was studied. • Correlation between acceleration exponent and flame thickness was studied. [ABSTRACT FROM AUTHOR]

The self-acceleration characteristics of a syngas/air mixture turbulent premixed flame were experimentally evaluated using a 10% H 2 /90% CO/air mixture turbulent premixed flame by varying the turbulence intensity and equivalence ratio at atmospheric pressure and temperature. The propagation characteristics of the turbulent premixed flame including the variation in the flame propagation speed and turbulent burning velocity of the syngas/air mixture turbulent premixed flame were evaluated. In addition, the effect of the self-acceleration characteristics of the turbulent premixed flame was also evaluated. The results show that turbulence gradually changes the radius of the premixed flame from linear growth to nonlinear growth. With the increase of turbulence intensity, the formation of a cellular structure of the flame front accelerated, increasing the flame propagation speed and burning speed. In the transition stage, the acceleration exponent and fractal excess of the turbulent premixed flame decreased with increasing equivalence ratio and increased with increasing turbulence intensity at an equivalence ratio of 0.6. The acceleration exponent was always greater than 1.5. • Effect of turbulence intensity on turbulent burning velocity was studied. • Nonlinear propagation characteristics of turbulent premixed flame were evaluated. • Fractal and acceleration characteristics in the transition stage were studied. [ABSTRACT FROM AUTHOR]

To investigate the effect of equivalence ratio and turbulence intensity on the combustion characteristics of syngas/air mixtures, experiments involving premixed combustion of 70% H 2 /30% CO/air mixtures at various equivalence ratios and turbulence intensities were conducted in a turbulent combustion bomb at atmospheric temperature and pressure. The turbulent burning velocity and flame curvature were used to study turbulent combustion characteristics. The results show that the turbulent burning velocity grew nonlinearly as the equivalence ratio increased, while the normalized turbulent burning velocity tended to decrease. When the equivalence ratio was relatively low, the turbulence intensity was a greater determinant of the burning velocity. The normalized turbulent burning velocity increased as the turbulence intensity increased. Re and Da were found to be directly and inversely proportional to u' / u L , respectively. A linear relationship was observed between u T / u L and ln Re. As the turbulence intensity increased or equivalence ratio decreased, the wrinkle degree of the flame front increased, and the maximum and minimum values of flame front curvature increased and decreased, respectively. Meanwhile, the range of the flame front curvature increased gradually. The proportion of components with smaller absolute value of flame front curvature gradually decreases. • Effect of equivalence ratios on the turbulent burning velocity were studied. • Effect of turbulence on the burning velocity were studied. • Turbulent premixed flame curvature was studied. [ABSTRACT FROM AUTHOR]

To evaluate the effect of hydrogen fraction and equivalence ratio on the competitive relationship between flame and crack development, combustion experiments of H 2 /CO/air mixtures at various equivalence ratios and hydrogen fractions were conducted in a constant volume combustion chamber. The crack length and growth rate were defined. The results indicate that the crack length and the number of the cells increase with the flame development. With the increase (decrease) in hydrogen fraction (equivalence ratio), the disturbances caused by flame inherent instabilities increases, and the crack length at the same radius increases. The appearance and generation of the new cracks dominate the cellular structure process, clear increasing the crack growth rate. The effect of hydrogen fraction on the development of cracks is more obvious at lower equivalence ratios. Flame stretch affects the competitive relationship between crack growth rate and flame propagation speed, exhibiting two completely different trends. The critical crack growth rate decreases with the increase of the equivalent ratio. [ABSTRACT FROM AUTHOR]

An experimental study was conducted using outwardly propagating flame to evaluate the laminar burning velocity and flame intrinsic instability of diluted H 2 /CO/air mixtures. The laminar burning velocity of H 2 /CO/air mixtures diluted with CO 2 and N 2 was measured at lean equivalence ratios with different dilution fractions and hydrogen fractions at 0.1 MPa; two fitting formulas are proposed to express the laminar burning velocity in our experimental scope. The flame instability was evaluated for diluted H 2 /CO/air mixtures under different hydrogen fractions at 0.3 MPa and room temperature. As the H 2 fraction in H 2 /CO mixtures was more than 50%, the flame became more unstable with the decrease in equivalence ratio; however, the flame became more stable with the decrease in equivalence ratio when the hydrogen fraction was low. The flame instability of 70%H 2 /30%CO premixed flames hardly changed with increasing dilution fraction. However, the flames became more stable with increasing dilution fraction for 30%H 2 /70%CO premixed flames. The variation in cellular instability was analyzed, and the effects of hydrogen fraction, equivalence ratio, and dilution fraction on diffusive-thermal and hydrodynamic instabilities were discussed. [ABSTRACT FROM AUTHOR]

In this study, the influence of turbulence intensity on flame local structural and local propagation characteristics was studied. In order to achieve this, 50% H2/50% CO with equivalence ratio of 0.8 were conducted under different turbulence intensities ranging from 0 to 1.31 m/s at atmospheric pressure and temperature. The distribution of the flame local radius and flame local propagation speed and the correlation between the two were investigated. The results show that the turbulence can increase the disturbance in the flame front, and enhance the fluctuation of the flame local radius, resulting in an increase in the fluctuation of the flame local propagation speed. With the development of flame, the distribution of the flame local propagation speed tended to become uniform, while the intersection zone of the flame local propagation speed and the flame local radius distribution gradually decreased. When the turbulence intensity ranged between 0.49 m/s to 0.74 m/s and between 1.08 m/s to 1.31 m/s, the sensitivity of the flame local propagation speed to its local characteristics reduced with the increase of the turbulence intensity. [ABSTRACT FROM AUTHOR]

To investigate the effect of flame inherent instabilities on the flame structural characteristics of H 2 /CO/air mixtures, experiments were conducted in a constant combustion bomb at various hydrogen fraction and equivalence ratio at room pressure and temperature. Wavelet transform was adopted to decompose the disturbances with different scales in the flame front. To study quantitatively the flame geometric structure characteristics, the maximum and minimum fluctuation radius growth rate, fluctuation range growth rate and energy of decomposition component were defined. The results indicated that with the flame development the disturbances at different scales all get greatly promotion. The higher the scale of the decomposition, the greater the amplitude and the energy of the disturbances. The disturbances at lower decomposition scales are the dominant component of the flame geometric structure characteristics; the ones at higher decomposition scales have a significance influence on the flame local geometric structure characteristics. When the flame development approached to a certain degree, the fluctuation range growth rate increased with the flame development. With the decrease of the equivalence ratio or the increase of hydrogen fraction, the thermal-diffusive instability enhances, leading to an increase of the fluctuation range growth rate and the relative energy of the approximate component at decomposition scale nine. [ABSTRACT FROM AUTHOR]

The laminar burning characteristics of syngas-premixed (H 2 /CO/CO 2 /air) laminar spherical flames have been systematically investigated through an experiment using Schlieren system. In the experiment, the volume fraction of H 2 in the H 2 /CO part of the H 2 /CO/CO 2 /air mixtures was varied from 30% to 100%, the volume fraction of CO 2 in the H 2 /CO/CO 2 mixtures was varied from 0% to 60%, and the equivalence ratio was varied from 0.4 to 1.0. The effects of hydrogen fraction, carbon dioxide fraction and equivalence ratio on flame propagation, Markstein length, and laminar burning velocities have been studied. The obtained laminar burning velocities were compared with existing data from the literature and excellent agreement was obtained. The flames become more unstable by decreasing the equivalence ratio and increasing dilution fraction. The value of laminar burning velocity decreases apparently with the increase of carbon dioxide dilution fraction due to combined effect of decrease in adiabatic temperature, thermal diffusivity and the CO 2 participation in the chemical reaction of CO + OH ↔ CO 2 + H. With the increase of hydrogen fraction, the laminar burning velocity increases significantly due to the chemical effect and diffusion effect rather than thermal effect. [ABSTRACT FROM AUTHOR]

An experimental study was performed in a constant volume vessel by using the Schlieren system to investigate the laminar burning velocities and Markstein lengths ( L b ) of H 2 /CO lean and stoichiometric ( φ = 0.4 to φ = 1.0) premixed flames under various hydrogen fractions (from 0% to 100%) at atmospheric and elevated pressures and room temperature. The unstretched laminar burning velocities ( S u 0 ) are compared with data from previous literature. Results indicate that excellent agreements are obtained. At 0.1 MPa and room temperature, the stretch behaviour is governed by both the hydrogen and carbon monoxide with hydrogen fraction of less than 50%. However, the hydrogen governs the stretch behaviour with hydrogen fractions of more than 50% because Markstein lengths are essentially the same as for H 2 /air flames. Furthermore, L b generally increases with increasing equivalence ratios for most hydrogen fractions; an opposite trend is observed when the hydrogen fraction is 0%, i.e., in pure carbon monoxide. The unstretched burning velocities of lean and stoichiometric synthesis gas (syngas) premixed flames increase with increasing hydrogen fraction and/or equivalence ratio. The value of unstretched burning velocities decreases with increasing initial pressure with a specific equivalence ratio and hydrogen fraction. [ABSTRACT FROM AUTHOR]

Syngas, a promising renewable alternative fuel, shows different combustion characteristics depending on its exact composition. In this study, the effect of different diluents on the laminar premixed flame of hydrogen-rich syngas was experimentally investigated using a 90% H 2 /10% CO/air mixture in a constant-volume combustion bomb. In particular, this investigation revealed the effects of varying the equivalence ratio (0.6, 0.8, and 1.0) and dilution content (10%, 20%, and 30%) on the flame propagation and explosion characteristics of syngas/air mixture. The results showed that for a given dilution gas content, the flame propagation speed under CO 2 dilution is less than that under N 2 dilution; however, the flame acceleration effect of CO 2 dilution is significantly higher than that of N 2. As the dilution increases, the rate of pressure rise and the maximum combustion explosion pressure gradually decrease. For a given equivalence ratio, the maximum rate of pressure rise and the deflagration index of the flame mixed with N 2 were higher than those for CO 2. • Effect of diluent gas on the flame evolution and propagation speed was studied. • Effect of φ and diluent gas on the self-acceleration properties were evaluated. • The explosion characteristics under various diluent gas were studied. • Effect of dilution fraction on the deflagration index was analyzed. [ABSTRACT FROM AUTHOR]

Effect of hydrogen fraction on the combustion characteristics of syngas/air mixtures was experimentally investigated at various hydrogen fractions and equivalence ratios in a turbulent combustion bomb under atmospheric temperature and pressure. The laminar burning velocity and turbulent burning velocity at various hydrogen fractions were measured, and the promotion of turbulence on the burning velocity at various hydrogen fractions was systematically investigated. The results show that the laminar burning velocity of the H 2 /CO/air mixture exhibits a non-linear increasing trend with increasing hydrogen fraction. With increasing equivalence ratio, the laminar burning velocity increases remarkably. Reaction CO + OH = CO 2 + H is identified as the most dominate chain branching reactions and reaction H + OH + M = H 2 O + M is the most dominant chain termination reaction. The laminar burning velocity exhibits linear and non-linear growth with the maximum concentration of radical H and OH, respectively. With increasing the hydrogen fraction, the value of u T / u L shows a non-linear decreasing trend as a whole and the growth rate gradually decreases. As the equivalence ratio increases, the u T / u L value decrease. When the hydrogen fraction is more than 50%, the promoting effect of turbulence on the burning velocity decreases gradually. • Effect of hydrogen fractions on laminar flame structures were investigated. • Effect of radical concentration on laminar burning velocity was studied. • Burning velocities of various hydrogen fractions were measured. • Effect of equivalence ratios on burning velocities were analyzed. • Enhancement of turbulence on burning velocity was studied. [ABSTRACT FROM AUTHOR]

To evaluate the effect of turbulence and inherent instabilities of flame on flame structural characteristics, experiments were conducted with a 70%-H2/30%-CO premixed flame at various equivalence ratios and turbulence intensities at atmospheric pressure and temperature. The convex and concave regions of the large-scale wrinkle in the flame front were extracted and quantitatively evaluated; wavelet transform was used to decompose disturbances in the flame front. The results show that with the development of the flame, the flame became unstable, the fluctuation range of the pulsating radius increased, the area and area ratio of the convex regions increased, and the average area and uniform degree of the convex and concave regions increased. As the turbulence intensity increased, the area and area ratio of the convex regions increased, the average area and uniform degree increased, and the disturbance energy at the same scale increased. Large-scale disturbances were the main factor affecting the flame's structural characteristics. With the increase in equivalence ratio, the effect of thermal-diffusive instability and turbulence on flame structural characteristics was reduced, decreasing the fluctuation range of the pulsation radius. The interaction between disturbances of different scales in the flame front resulted in the uneven distribution of the pulsation radius. [ABSTRACT FROM AUTHOR]