Flame spread behaviour of blended fuel droplet array

Experimental investigation of the effect of blended fuel on flame spread along droplet array has been conducted. Flame spread rate is measured using high‐speed chemiluminescence images of an OH radical. The flame spread is observed with the initial droplet diameter, droplet spacing, and the mixing ratio of n‐heptane and n‐hexadecane. The mode of flame spread is categorized into two types: a continuous mode and an intermittent one. It is seen that flame spread rate is sensitively dependent on the relative flame position to droplet spacing. For a large droplet, the flame spread time is governed by a volatile fuel (heptane), but for a small droplet, it is controlled by a less volatile fuel (hexadecane). Copyright © 1999 John Wiley & Sons, Ltd.     

Measurement and correlation of laminar flame speeds of CO and C2 hydrocarbons with hydrogen addition at atmospheric and elevated pressures

The laminar flame speeds of mixtures of ethane, ethylene, acetylene, and carbon monoxide with small amount of hydrogen addition at atmospheric and elevated pressures were experimentally and computationally determined. It was found that the approximate linear correlation identified previously between the laminar flame speeds and an appropriate definition of the amount of hydrogen addition for methane, propane and n-butane at atmospheric pressure also largely applies to ethane, ethylene, and acetylene at atmospheric as well as elevated pressures. The linear correlation, however, does not hold for carbon monoxide, at all pressures, due to the strong catalytic effect of hydrogen on the oxidation of carbon monoxide. A mechanistic analysis shows that both the Arrhenius and diffusive contributions to the laminar flame speed are nearly linear functions of the hydrogen addition, which explains this overall approximate linear correlation.     

Experimental and kinetic study of laminar flame characteristics of H2/O2/diluent flame under elevated pressure

Laminar burning velocity, Markstein length, and critical flame radius of an H₂/O₂ flame with different diluents, He, Ar, N₂ and CO₂, were measured under elevated pressure with different diluent concentrations. The effects of pressures, diluents, and dilution and equivalence ratios were studied by comparing calculated and experimental results. The laminar burning velocity showed non-monotonic behavior with pressure when the dilution ratio was low. The reason is the radical pool reduced with increasing pressure and leads to the decrease of overall reaction order from larger than 2 to smaller than 2, and further leads to this non-monotonic phenomenon. A modified empirical equation was presented to capture the relationship between active radicals and laminar burning velocity. Critical radii and Markstein lengths both decrease with initial pressure and increase with equivalence ratio and dilution ratio. The calculated critical radii indicate that the Peclet number and flame thickness control the change of R_cr. It can be found that Le_eff has a significant influence on Peclet number and leads to the decrease of critical flame radii of Ar, N_2, and CO₂ diluted mixture. Interestingly, the CO₂ diluted mixture has the lowest Markstein length under stoichiometric conditions and a high value under fuel-rich conditions, consistent as the flame instability observed on the flame images. The reason is that the Le_eff of CO₂ diluted mixture increased rapidly with the equivalence ratio.     

Turbulent flame topology and the wrinkled structure characteristics of high pressure syngas flames up to 1.0 MPa

The turbulent flame topology characteristics of the model syngas with two different hydrogen ratios were statistically investigated, namely CO/H₂ ratio at 65/35 and 80/20, at equivalence ratio of 0.7. The combustion pressure was kept at 0.5 MPa and 1.0 MPa, to simulate the engine-like condition. The model syngas was diluted with CO₂ with a mole fraction of 0.3 which mimics the flue gas recycle in the turbulent combustion. CH₄/air flame with equivalence ratio of 1.0 was also tested for comparison. The flame was anchored on a premixed type Bunsen burner, which can generate a controllable turbulent flow. Flame front, which is represented by the sharp increased interface of the OH radical distribution, was measured with OH-PLIF technique. Flame front parameters were obtained through image processing to interpret the flame topology characteristics. Results showed that the turbulent flames possess a wrinkled character with smaller scale concave/convex structure superimposed on a larger scale convex structure under high pressure. The wrinkled structure of syngas flame is much finer and more corrugated than hydrocarbon fuel flames. The main reason is that scale of wrinkled structure is smaller for syngas flame, resulting from the unstable physics. Hydrogen in syngas can increase the intensity of the finer structure. Moreover, the model syngas flames have larger flame surface density than CH₄/air flame, and hydrogen ratio in syngas can increase flame surface density. This would be mainly attributed to the fact that the syngas flames have smaller flame intrinsic instability scale l_i than CH₄/air flame. S_T/S_L of the model syngas tested in this study is higher than CH₄/air flames for both pressures, due to the high diffusivity and fast burning property of H₂. This is mainly due to smaller L_M and l_i. V_f of the two model syngas is much smaller than CH₄/air flames, which suggests that syngas flame would lead to a larger possibility to occur combustion oscillation.     

Explosion bomb measurements of ethanol-air laminar gaseous flame characteristics at pressures up to 1.4 MPa

The principal burning characteristics of a laminar flame comprise the fuel vapour pressure, the laminar burning velocity, ignition delay times, Markstein numbers for strain rate and curvature, the stretch rates for the onset of flame instabilities and of flame extinction for different mixtures. With the exception of ignition delay times, measurements of these are reported and discussed for ethanol-air mixtures. The measurements were in a spherical explosion bomb, with central ignition, in the regime of a developed stable, flame between that of an under or over-driven ignition and that of an unstable flame. Pressures ranged from 0.1 to 1.4 MPa, temperatures from 300 to 393 K, and equivalence ratios were between 0.7 and 1.5. It was important to ensure the relatively large volume of ethanol in rich mixtures at high pressures was fully evaporated. The maximum pressure for the measurements was the highest compatible with the maximum safe working pressure of the bomb. Many of the flames soon became unstable, due to Darrieus-Landau and thermo-diffusive instabilities. This effect increased with pressure and the flame wrinkling arising from the instabilities enhanced the flame speed. Both the critical Peclet number and the, more rational, associated critical Karlovitz stretch factor were evaluated at the onset of the instability. With increasing pressure, the onset of flame instability occurred earlier. The measured values of burning velocity are expressed in terms of their variations with temperature and pressure, and these are compared with those obtained by other researchers. Some comparisons are made with the corresponding properties for iso-octane-air mixtures.     

Effect of hydrogen enrichment and electric field on lean CH4/air flame propagation at elevated pressure

Electric assisted combustion for hydrogen enriched hydrocarbons may even extend the lean burn limit and provide the further improvement on combustion stability. This study investigates the effect of hydrogen enrichment and DC electric field on lean CH₄/air flame propagation. Electric field inside the chamber was generated by mesh and needle electrodes. Effect of hydrogen enrichment on the ion mole fraction in the flame was discussed based on reaction mechanism included neutral and ion reactions. The flame propagation images, flame displacement speed were used to evaluate the combined influences of hydrogen enrichment and electric field on propagating flame. Results showed that the hydrogen addition would increase positive ions mole fraction and the peak value is mainly determined by H₃O⁺. This would be due to that CH increases with hydrogen fraction, which is the main species in the initial reaction for the ion reactions. Electric field effect about flame propagation was suppressed with hydrogen addition due to the competition between the increment in ion mole fraction and the decrement in flame time. Electric assisted combustion is more evident at leaner conditions and elevated pressure. The ratio of ionic wind velocity to flow velocity may be the determined factor to predict the electric field effect about propagating flame. The tendency based on this ratio is in accordance with the experimental results for various hydrogen fraction and equivalence ratio at elevated pressure.     

Effects of steam dilution on laminar flame speeds of H2/air/H2O mixtures at atmospheric and elevated pressures

The laminar flame speeds of H₂/air with steam dilution (up to 33 vol%) were measured over a wide range of equivalence ratio (0.9–3.0) at atmospheric and elevated pressures (up to 5 atm) by an improved Bunsen burner method. Burke, Sun, HP (High Pressure H₂/O₂ mechanism), and Davis mechanisms were employed to calculate the laminar flame speeds and analyze different effects of steam addition. Four studied mechanisms all underestimated the laminar flame speeds of H₂/air/H₂O mixtures at medium equivalence ratios while the Burke mechanism provided the best estimates. When the steam concentration was lower than 12%, increasing pressure first increased and then decreased the laminar flame speed, the inflection point appeared at 2.5 atm. When the steam concentration was greater than 12%, increasing the pressure monotonously decrease the laminar flame speed. The chemical effect was amplified by elevated pressure and it played an important role for the inhibiting effect of the pressure on laminar flame speed. The fluctuations of the chemical effect at 1 atm were mainly caused by three-body reactions, while the turn at 5 atm was mainly caused by the direct reaction effect. Elevated pressure and steam addition amplified the influences of uncertainties in the rate constants for elementary reactions, which might leaded to the disagreement between experimental and simulation results.     

Experimental and kinetic study on laminar flame speeds of ammonia/syngas/air at a high temperature and elevated pressure

The laminar flame speeds of ammonia mixed with syngas at a high pressure, temperature, and different syngas ratios were measured. The data obtained were fitted at different pressures, temperatures, syngas ratios, and equivalence ratios. Four kinetic models (the Glarborg model, Shrestha model, Mei model, and Han model) were compared and validated with experimental data. Pathway, sensitivity and radical pool analysis are conducted to find out the deep kinetic insight on ammonia oxidation and NO formation. The pathway analysis shows that H abstraction reactions and NH_i combination reactions play important roles in ammonia oxidation. NO formation is closely related to H, OH, the O radical produced, and formation reactions. NO is mainly formed from reaction, HNO+ H= NO+ H₂. Furthermore, both ammonia oxidation and NO formation are sensitive to small radical reactions and ammonia related reactions.     

A theoretical analysis of the capture of greenhouse gases by single water droplet at atmospheric and elevated pressures

Gas absorption by droplets is an important route to reduce greenhouse gas emissions, especially for carbon dioxide. To recognize the fundamental absorption processes of greenhouse gases by single droplets, the mass transport phenomena of greenhouse gas uptake by a quiescent water droplet at atmospheric and elevated pressures are analyzed theoretically and four common greenhouse gases of CO₂, N₂O, CH₄ and O₃ are taken into consideration. On account of piecewise function encountered at the droplet surface, it is impossible to obtain a fully analytical solution for describing the mass transfer process. Instead, a semi-analytical method is developed to predict the mass diffusion between the gas phase and the liquid phase. The obtained results indicate that, by virtue of the four greenhouse gases characterized by low mass diffusion number, the entire mass transfer is controlled by the liquid phase. A unified formula has been successfully established to aid in estimating the dimensionless solute uptake process and the dimensionless aqueous diffusion time of 0.45 is sufficiently long the implement the absorption process. For the ambient temperature and pressure in the ranges of 280–350 K and 1–20 atm, respectively, it is found that increasing the two parameters will intensify the solute absorption amount significantly and the absorption process can be accelerated by increasing temperature.     

Effect of hydrogen ratio on turbulent flame structure of oxyfuel syngas at high pressure up to 1.0 MPa

The CO/H₂/CO₂/O₂, CO/H₂/CO₂/air turbulent premixed flames as the model of syngas oxyfuel and syngas/air combustion were studied experimentally and compared to that of CH₄/air mixtures at high pressures up to 1.0 MPa. Hydrogen ratio in syngas was set to be 35%, 50% and 65% in volumetric fraction. Four perforated plates are used to generate wide range of turbulence intensity and scales. The instantaneous flame structure was measured with OH-PLIF technique and then statistic flame structure parameters and turbulent burning velocity were derived to interpret the multi scale turbulence-flame interaction. Results show that the flame structure of syngas is wrinkled and convex cusps to the unburned mixtures are sharper and deeper comparing to that of CH₄ flames. Pressure has a dominating effect on flame wrinkling other than mixtures composition at high pressure of 1.0 MPa. The flame surface density, Σ of syngas is larger than that of CH₄. The Σ of syngas flames is almost independent on pressure and hydrogen ratio especially when hydrogen ratio is over 50% which is a significant feature of syngas combustion. Larger flame surface density for syngas flames mainly comes from the finer structure with smaller wrinkles which is the result of more intensive flame intrinsic instability. The S_T/S_L of syngas is larger than CH₄ and it slightly increases with the pressure rise. The S_T/S_L of syngas oxyfuel is similar to that of syngas/air flames in the present study. The S_T/S_L increases with the increase of hydrogen ratio and keeps almost constant when hydrogen ratio is over 50%.     

Ethanol electrooxidation on a carbon-supported Pt catalyst at elevated temperature and pressure: A high-temperature/high-pressure DEMS study

The electrooxidation of ethanol on a Pt/Vulcan catalyst was investigated in model studies by on-line differential electrochemical mass spectrometry (DEMS) over a wide range of reaction temperatures (23–100 °C). Potentiodynamic and potentiostatic measurements of the Faradaic current and the CO₂ formation rate, performed at 3 bar overpressure under well-defined transport and diffusion conditions reveal significant effects of temperature, potential and ethanol concentration on the total reaction activity and on the selectivity for the pathway toward complete oxidation to CO₂. The latter pathway increasingly prevails at higher temperature, lower concentration and lower potentials (∼90% current efficiency for CO₂ formation at 100 °C, 0.01 M, 0.48 V), while at higher ethanol concentrations (0.1 M), higher potentials or lower temperatures the current efficiency for CO₂ formation drops, reaching values of a few percent at room temperature. These trends result in a significantly higher apparent activation barrier for complete oxidation to CO₂ (68 ± 2 kJ mol⁻¹ at 0.48 V, 0.1 M) compared to that of the overall ethanol oxidation reaction determined from the Faradaic current (42 ± 2 kJ mol⁻¹ at 0.48 V, 0.1 M). The mechanistic implications of these results and the importance of relevant reaction and mass transport conditions in model studies for reaction predictions in fuel cell applications are discussed.