Effects of ignition location on premixed hydrogen/air flame propagation in a closed combustion tube

The dynamics of premixed hydrogen/air flame ignited at different locations in a finite-size closed tube is experimentally studied. The flame behaves differently in the experiments with different ignition positions. The ignition location exhibits an important impact on the flame behavior. When the flame is ignited at one of the tube ends, the heat losses to the end wall reduce the effective thermal expansion and moderate the flame propagation and acceleration. When the ignition source is at a short distance off one of the ends, the tulip flame dynamics closely agrees with that in the theory. And both the tulip and distorted tulip flames are more pronounced than those in the case with the ignition source placed at one of the ends. Besides, the flame–pressure wave coupling is quite strong and a second distorted tulip flame is generated. When the ignition source is in the tube center, the flame propagates in a much gentler way and the tulip flame can not be formed. The flame oscillations are weaker since the flame–pressure wave interaction is weaker.     

Effects of mesh aluminium alloy and aluminium velvet on the explosion of H2/air,CH4/air and C2H2/air mixtures

MAA (mesh aluminium alloy) is one of the most widely used explosion suppression materials in military and civilian applications. To systematically research the effect of MAA on the explosion reactions of flammable gases, we investigated the effect of MAA and AV (aluminium velvet) on the explosion of hydrogen-air, methane-air and acetylene-air mixtures. The results indicated that MAA and AV suppress the methane-air mixture explosion but significantly promote the explosions of the hydrogen-air mixture and the acetylene-air mixture. MAA and AV have the dual effect of promoting and suppressing an explosion. In addition, with an increase in filling density, the promotion of MAA and AV first strengthens and then weakens. The results of this study show that the properties of flammable gas, not the shapes of the explosion suppression materials, determine whether the dominant effect of explosion suppression material is promotion or suppression. Use of explosion suppression materials is not suitable for all flammable gases, especially highly reactive chemical fuels. Applying explosion suppression materials blindly may greatly increase safety risk.     

Effects of hydrogen peroxide addition on two-stage ignition characteristics of n-heptane/air mixtures

Rising fuel cost and environmental concerns of greenhouse emissions have driven the development of advanced engine technology with optimal fuel strategy that can simultaneously yield high thermal efficiency and low emissions. Due to its strong reactivity and extra oxygen atom serving as an oxidizer, hydrogen peroxide (H₂O₂) has been used along with other hydrocarbons to promote overall combustion process. To explore the potential benefits of H₂O₂ in clean combustion technology, a numerical study with detailed chemistry is conducted to investigate the effects of H₂O₂ addition on the two-stage ignition characteristics of n-heptane/air mixtures at low-to-intermediate temperatures (below 1000 K), with due emphasis on how the negative temperature coefficient (NTC) behavior is affected. The results show that H₂O₂ addition shortens both the first-stage and total ignition delay times of n-heptane/air mixtures and suppresses the NTC behavior by reducing the upper turnover temperature. With increasing H₂O₂ addition, the lower turnover temperature, corresponding to the first-stage ignition delay minimum, is found to increase first and then decrease. Chemical kinetic analyses show that the addition of H₂O₂ promotes both first- and second-stage ignition reactivity by enhancing OH production through H₂O₂ decomposition. Furthermore, low-temperature chemistry controls the first-stage ignition, while H₂O₂ chemistry dominates the second-stage ignition.     

Extinction of premixed methane/air flames in microgravity by diluents: Effects of radiation and Lewis number

Laminar burning velocities and flammability limits of premixed methane/air flames in the presence of various diluents were investigated by combined use of experiments and numerical simulations. The experiments used a 1-m free-fall spherical combustion chamber to eliminate the effect of buoyancy, enabling accurate measurements of near-limit burning velocities and flammability limits. Burning velocities were measured for CH₄/air flames with varying concentrations of He, Ar, N₂ and CO₂ at NTP. The limiting concentration of each diluent was measured by systematically varying the composition and ignition energy and finding the limiting condition through successive experiment trials. The corresponding freely-propagating, planar 1-D flames were simulated using PREMIX. The transient spherically-expanding flames were simulated using the 1-D Spherical Flame & Reactor Module of COSILAB considering detailed radiation models. The results show that helium exhibits more complex limit behavior than the other diluents due to the large Lewis number of helium mixtures. The near-limit helium-diluted flames require much higher ignition energy than the other flames. In addition, for the spherically expanding helium-diluted flames studied here (Le > 1), stretch suppresses flame propagation and may cause flame extinction. For the CO₂-diluted flames, the flame speed predicted by the optically-thick model based on the Discrete Transfer Method (DTW) and a modified wide band model has better agreement with measurements in the near-limit region. A significant amount of heat is absorbed by the dilution gas CO₂, resulting in elevation of temperature of the ambient gases. The optically-thick model, however, still overpredicts flame speed, indicating a more sophisticated radiation property model may be needed. Finally, the chemical effect of CO₂ on flame suppression was quantified by a numerical analysis. The results show that the chemical effect of CO₂ is more important than the other diluents due to its active participation in the reaction CO₂ + H = CO + OH, which competes for H radicals with the chain-branching reactions and thus reduces flame speed.     

Photochemical ignition of premixed hydrogenoxidizer mixtures with excimer lasers

Photochemically augmented combustion has been identified as a potential technique for extending combustion associated limits. The objective of this research has been to investigate initiation of combustion by irradiation of unsensitized fueloxidizer mixtures with ultraviolet light. This light, absorbed initially by oxygen molecules, results in their photodissociation to oxygen atoms, which produce other reactive radicals via chain-branching. The presence of these radicals and subsequent thermalization results in ignition. In this study, a Lumonics excimer laser (TE 861) is used to investigate the effect of wavelength on the photochemical ignition of hydrogenair and hydrogenoxygen mixtures at several equivalence ratios and pressures. Two wavelengths are used; 157 nm emitted by fluorine and 193 nm emitted by argon fluoride. Photoignitions were achieved with the fluorine laser and not with the argon fluoride laser, even though the latter has 10-fold greater fluence. This result demonstrates the important role of the selectivity of the absorption coefficient of molecular oxygen, which sharply decreases (four orders of magnitude) as the wavelength increases from 157 nm to 193 nm.Experiments in which minimum pressures for ignition of $\text{H}{}_2\text{O}{}_2$ and $\text{H}{}_2\text{air}$ mixtures were determined at various equivalence ratios suggest fundamental differences between the performance of photochemical and conventional spark ignited systems. Overall minima for photochemical initiation occurred at equivalence ratios to the fuel-lean side of stoichiometric. This fuel-lean shift appears typical of photochemical initiation, with the explanation that increased molecular oxygen yields higher atomic oxygen concentration upon irradiation.A complementary analytical effort was undertaken to eludicate the fundamental interaction of photon absorption, subsequent dissociation, chemical kinetics, and heat loss to surroundings. These processes are described by energy and species conservation equations, including some 90 chemical reactions and 6 photodissociation reactions, the latter allowing for the temporal and spectral dependence of the incident light and absorber cross sections. Good quantitative agreement with the experimental data of the laser ignition experiments attests to the adequacy of the model for investigation of photochemical initiation and supports the postulated mechanism.     

Experimental and numerical study on premixed partially dissociated ammonia mixtures. Part I: Laminar burning velocity of NH3/H2/N2/air mixtures

Ammonia is considered as a promising hydrogen carrier, which is seen as a reliable carbon-free fuel. Improving the combustion properties of ammonia is the focus of current research. The hydrogen could be dissociated from the ammonia in real applications. For purpose of combustion, partially dissociated ammonia could be combusted directly without using extra hydrogen. Laminar burning velocity is an important combustion parameter, but there are only a few data of partially dissociated ammonia are reported. To fill the data gap, the laminar burning velocity was measured at various equivalence ratios and dissociation degrees of ammonia by the constant pressure spherical flame method in this study. Besides, fifteen kinetic models were compared with experimental data, and the model with the best consistency was obtained. The experimental results show that the laminar burning velocity increases monotonically with the increase of the dissociating degree. When ammonia is completely dissociated, the maximum laminar burning velocity increases from 7.9 cm/s to 228 cm/s, and the equivalence ratio corresponding to the peak value also shifts from 1.1 to 1.6. The laminar burning velocity predicted by the model constructed by Stagni is in best agreement with the experimental data. Moreover, data calculated by the five correlations for predicting laminar burning velocity were compared with the numerical data to verify that whether they are suitable for the mixtures with additional nitrogen. The results show that the correlation based on the activation temperature is the most accurate. However, it still has a maximum relative error of ±20% within the calculated range.     

Experimental study on the turbulent premixed combustion characteristics of 70%H2/30%CO/air mixtures

To investigate the effect of equivalence ratio and turbulence intensity on the combustion characteristics of syngas/air mixtures, experiments involving premixed combustion of 70% H₂/30% 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.     

Characteristics of hydrogen-assisted catalytic ignition of n-butane/air mixtures

External heating and hydrogen-assisted catalytic ignition characteristics of n-butane (n-C₄H₁₀) were studied experimentally in a Pt-coated monolith catalytic reactor. Special attention was paid to the chemical effect of hydrogen on hydrogen-assisted ignition. A comparison of the ignition temperature for these two ignition methods shows hydrogen can lower ignition temperature. Furthermore, the ignition experiment at low hydrogen mole fraction (1.5%) shows that hydrogen has a positive chemical effect on hydrogen-assisted ignition. At constant n-butane/air flow and within certain limits of hydrogen mole fraction of mixtures, the ignition temperature changes little, whereas the time required for ignition and the cumulative amount of hydrogen decrease substantially. Consequently, high hydrogen mole fraction is favorable to hydrogen-assisted ignition. Two startup methods and thermal insulation are discussed. The co-feed method (n-butane/air/hydrogen mixtures are fed into reactor) and thermal insulation were found to be beneficial to hydrogen-assisted ignition.     

Experimental and numerical study on laminar premixed NH3/H2/O2/air flames

Adding the product of water electrolysis (i.e. 2:1 volume of H₂ and O₂) is an effective strategy to enhance the combustion intensity of NH₃/air mixtures. In this work, the laminar burning velocity (LBV) of the obtained NH₃/H₂/O₂/air mixtures was measured at 303 K, 0.1 MPa and compared with the values predicted by seven mechanisms. To improve the prediction performance, a new mechanism is developed based on the existing mechanism and adopted for numerical simulation. The results of this study show that the LBV of NH₃ is significantly increased by additional H₂ and O₂. By comparison, it is found that H₂ shows a more significant promoting effect on LBV when the volume ratio of additional H₂ and O₂ is 2. The concentration of key radicals and the flame temperature increase remarkably due to the addition of H₂ and O₂, which promote the flame propagation. Furthermore, the experimental results also indicated that the additional H₂ and O₂ make the burned gas Markstein length decrease on the lean side and increase on the rich side.     

Effect of hydrogen and syngas addition on the ignition of iso-octane/air mixtures

There is worldwide interest in using renewable fuels within the existing infrastructure. Hydrogen and syngas have shown significant potential as renewable fuels, which can be produced from a variety of biomass sources, and used in various transportation and power generation systems, especially as blends with hydrocarbon fuels. In the present study, a reduced mechanism containing 38 species and 74 reactions is developed to examine the ignition behavior of iso-octane/H₂ and iso-octane/syngas blends at engine relevant conditions. The mechanism is extensively validated using the shock tube and RCM ignition data, as well as three detailed mechanisms, for iso-octane/air, H₂/air and syngas/air mixtures. Simulations are performed to characterize the effects of H₂ and syngas on the ignition of iso-octane/air mixtures using the closed homogenous reactor model in CHEMKIN software. The effect of H₂ (or syngas) is found to be small for blends containing less than 50% H₂ (or syngas) by volume. However, for H₂ mole fractions above 50%, it increases and decreases the ignition delay at low (T < 900 K) and high temperatures (T > 1000 K), respectively. For H₂ fractions above 80%, the ignition is influenced more strongly by H₂ chemistry rather than by i-C₈H₁₈ chemistry, and does not exhibit the NTC behavior. Nevertheless, the addition of a relatively small amount of i-C₈H₁₈ (a low cetane number fuel) can significantly enhance the ignitability of H₂-air mixtures at NTC temperatures, which are relevant for HCCI and PCCI dual fuel engines. The CO addition seems to have a negligible effect on the ignition of i-C₈H₁₈/H₂/air mixtures, indicating that the ignition of i-C₈H₁₈/syngas blends is essentially determined by i-C₈H₁₈ and H₂ oxidation chemistries. The sensitivity and reaction path analysis indicates that i-C₈H₁₈ oxidation is initiated with the production of alkyl radical by H abstraction through reaction: i-C₈H₁₈ + O₂ = C₈H₁₇ + HO₂. Subsequently, the ignition chemistry in the NTC region is characterized by a competition between two paths represented by reactions R2 (C₈H₁₇ + O₂ = C₈H₁₇O₂) and R8 (C₈H₁₇ + O₂ = C₈H₁₆ + HO₂), with the R8 path dominating, and increasing the ignition delay. As the amount of H₂ in the blend becomes significant, it opens up another path for the consumption of OH through reaction R36 (H₂ + OH = H₂O + H), which slows down the ignition process. However, for T > 1100 K, the presence of H₂ decreases ignition delay primarily due to reactions R31 (O₂ + H = OH + O) and R35 (H₂O₂ + M = OH + OH + M).     

Effects of CO2 and N2 dilution on the characteristics and NOX emission of H2/CH4/CO/air partially premixed flame

For the combustion of the mixture of blast furnace gas, natural gas, and coke oven gas in industrial burners, how to improve combustion efficiency and reduce pollutant emission are of significance. To accomplish this, an industrial partially premixed burner with a combustion diagnostic system is used to experimentally reveal the characteristics and NO_X emission of H₂/CH₄/CO/air flame under CO₂, N₂, and CO₂/N₂ (replacing half of N₂ with CO₂) dilution. NO_X emission and flame length, temperature profile, along with CO, CH₄, and CO₂ concentration profiles are analyzed with the three diluents in the fuel stream under different dilution rates (0–32% by volume). Experimental results show that for lean H₂/CH₄/CO combustion, greater proportions of CO₂ in the diluent affect flame characteristics in various ways. These effects include longer flame length, lower highest flame temperature, the highest flame temperature being located farther away from the nozzle, and the highest CO₂ concentration being located nearer the nozzle. Furthermore, results of CO, CH₄, and CO₂ concentrations indicate that chemical reactions in the flame are significantly affected by CO₂ owing to the series reaction CH₄?CH₃→CO?CO₂. Finally, increasing diluents or the ratio of CO₂ in diluents has the benefit of reducing NO_X emission.     

Combined effect of ignition position and equivalence ratio on the characteristics of premixed hydrogen/air deflagrations

Premixed hydrogen/air deflagrations were performed in a 100 mm × 100 mm × 1000 mm square duct closed at one end and opened at the opposite end under ambient conditions, concerning with the combined effect of ignition position IP and equivalence ratio ?. A wide range of ? ranging from 0.4 to 5.0, as well as multiple IPs varying from 0 mm to 900 mm off the closed end of the duct were employed. It is indicated that IP and ? exerted a great impact on the flame structure, and the corresponding pressure built-up. Except for IP₀, the flame can propagate in two directions, i.e., leftward and rightward. A regime diagram for tulip flames formation on the left flame front (LFF) was given in a plane of ? vs. IP. In certain cases (e.g. the combinations of ? = 0.6 and IP₅₀₀ or IP₇₀₀), distorted tulip flames were also observed on the right flame front (RFF). Furthermore, the combinations of IP and ? gave rise to various patterns of pressure profiles. The pressure profiles for ignition initiated at the right half part of the duct showed a weak dependence on equivalence ratio, and showed no dependence on ignition position. However, the pressure profiles for ignition initiated at the left half part of the duct were heavily dependent on the combination of IP and ?. More specifically, in the leanest (? = 0.4) and the richest (? = 4.0–5.0) cases, intensive periodical oscillations were the prime feature of the pressure profiles. With the moderate equivalence ratios (? = 0.8–3.0), periodical pressure oscillations were only observed for IP₉₀₀. The maximum pressure peaks P_max were reached at ? = 1.25 rather than at the highest reactivity ? = 1.75 irrespective of ignition position. The ignition positions that produced the worst conditions were different, implying a complex influence of the combination of IP and ?.     

Scaling-effect of explosion in H2/CH4/air mixtures

The scaling-effect of mixture explosion is an unresolved issue in explosion science. In this work, we carry out experimental measurements of explosion characteristics using hydrogen/methane/air (H₂/CH₄/air) mixtures in two tubes with lengths of 1.5 m and 60 m. The explosion overpressure of the mixtures increases exponentially with hydrogen mole fractions in the small tube, as expected. In contrast, explosion overpressure increases rapidly, causing detonation when hydrogen is added to the mixtures. Comparing measurements in both tubes, the explosion overpressure exhibits a clear scaling-effect dependence on the tube size. The scaling-effect cannot be explained by the aspect ratio (AR) of the tube. The analysis of the hotspot size, which is correlated with the ignition delay time of mixtures, is the critical factor governing the scaling-effect of explosion seen in a large tube.     

Experimental study on the effect of diluent gas on H2/CO/air mixture turbulent premixed flame

To study the effects of different diluents on the propagation characteristics of H₂/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 N₂ dilution, CO₂ 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.     

Laminar burning characteristics of ammonia/hydrogen/air mixtures with laser ignition

Ammonia, as a zero-carbon fuel, is drawing more and more attention. The major challenge of using ammonia as a fuel for the combustion engines lies in its low chemical reactivity, and therefore more fundamental researches on the combustion characteristics of ammonia are required to explore effective ways to burn ammonia in engines. In this study, the laminar burning characteristics of the premixed ammonia/hydrogen/air mixtures are investigated. In the experiment, the laser ignition was used to achieve stable ignition of the ammonia/air mixtures with an equivalence ratio range from 0.7 to 1.4. The propagating flame was recorded with the high-speed shadowgraphy. Three different processing methods were introduced to calculate the laminar burning velocity with a consideration of the flame structure characteristics induced by the laser ignition. The effects of initial pressure (0.1 MPa–0.5 MPa), equivalence ratio (0.7–1.4), hydrogen fraction (0–20%) on the laminar burning velocity were investigated under the initial ambient temperature of 360 K. The state-of-the-art kinetic models were used to calculate the laminar burning velocities in the CHEMKIN-pro software. Both the simulation and experimental results show that the laminar burning velocity of the ammonia mixtures increases at first, reaches the peak around ϕ of 1.1, and then decreases with the equivalence ratio increasing from 0.7 to 1.4. The peak laminar burning velocities of the ammonia mixture are lower than 9 cm/s and are remarkably lower than those of hydrocarbon fuels. The laminar burning velocity of the ammonia mixture decreases with the increase of the initial ambient pressure, and it can be drastically speeded up with the addition of hydrogen. While the models except for those by Miller and Bian can give reasonable predictions compared to the experimental results for the equivalence ratio from 0.7 to 1.1 in the ammonia (80%)/hydrogen (20%)/air mixtures, all the kinetic models overpredict the experiments for the richer mixtures, indicating further work necessary in this respect.     

Experimental study on the Influence of Hydrogen Fraction on Self-acceleration of H2/CO/air laminar premixed flame

To investigate self-acceleration propagation characteristics of a laminar premixed flame, an experimental study of H₂/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.     

Under-expansion jet flame propagation characteristics of premixed H2/air in explosion venting

In premixed H₂/air explosion venting, an under-expansion jet may be caused by the pressure difference between the inside and outside of the explosion vent. Based upon the under-expansion jet, studying the structure of the under-expansion jet flame and the factors influencing its formation is essential to hydrogen safety in explosion venting. This study explored the basic characteristics of the under-expansion jet flame in premixed H₂/air explosion venting, and discussed the formation of two under-expansion structures (Mach disk and diamond shock wave) of such jet flames by conducting a premixed H₂/air explosion venting experiment. The influences of hydrogen fraction, explosion venting diameter, and duct length on the structure of under-expansion jet flames were evaluated. The results showed that after successful explosion venting, the under-expansion jet flame would be generated when the hydrogen fractions were 30–60 vol.%, and as the hydrogen fractions were 30–50 vol.%, the lengths of the venting duct were 30 and 50 cm. The duration of under-expansion jet flame was the longest when the hydrogen fraction was 40 vol.%. With the explosion venting diameter and hydrogen fraction increased, the spacing between under-expansion jet flame structures (S) increased. However, an increase in duct length led to the attenuation of the S. During the explosion venting, the under-expansion jet caused a pressure imbalance near the explosion vent and high-intensity convection forms on both sides of a jet, which can generate two or more explosions. Therefore, understanding the basic characteristics of under-expansion jet flame can aid the effective development of measures to prevent, mitigate, and protect against premixed H₂/air explosions.     

Numerical study of the effect of H2O diluents on NOx and CO formation in turbulent premixed methane-air flame

The interaction of turbulence–combustion inside the flame field is studied in a Launder-Sharma Low Reynolds Number (LS-LRN) k−ε/EDM framework, while suitable coefficients have been utilized in the code. A finite volume method (FVM) with staggered grids was applied to discrete set of governing equations. SIMPLE algorithm is applied with a fine grid resolution. The convective terms are discretized using Power Law Scheme (PLS). The system of governing equations is solved simultaneously using numerical or TDMA finite difference methods (Tri-Diagonal Matrix Algorithm). By implementation of the Zeldovich and Westbrook-Dryer mechanisms, NO_x and CO concentrations were obtained, respectively. It is illustrated that the implemented LS-LRN-k−ε/EDM method with the new coefficients by shorter runtimes has very good agreement with previously published experimental measurements. Increasing the H₂O diluent at the inlet leads to an increase in the temperature, which increases the NO_x and CO near the entrance, but gradually towards the outlet of the combustion chamber. The energy absorbed by H₂O leads to a severe decrease in temperature and subsequent reduction in the amount of NO_x and CO emissions. With increasing H₂O diluent, changes in temperature are not very significant, while changes in pollutants CO and especially NO_x, are remarkable. With the increase of H₂O diluent, the maximum amount of CO emission displaces towards the inlet of the combustion chamber. However, it should be noted that, at a specific value of H₂O diluent, the length of the combustion chamber should not be less than critical value, causing the exhaust of the pollutant with large volume to the environment. After the critical point, the increase in the length of the chamber has little effect on reduction of the pollutant exhaust. However, by increasing the H₂O diluent, enclosures with smaller length can be utilized.     

Experimental studies on the explosion indices in turbulent stoichiometric H2/CH4/air mixtures

Explosion characteristics of the stoichiometric hydrogen/methane/air mixtures with different hydrogen fractions (λ) and different turbulent intensities (u'_rms) in a fan-jet-stirred spherical explosion vessel. From the experimental results, it could be clearly found that both the maximum explosion overpressure (p_max) and the maximum rise rate of overpressure rose with the increase of u'_rms, but the major reasons to such rising were not totally the same. In turbulence, with the increase of λ, p_max declined but (dp/dt)_max rose, and such behaviours were mainly attributed to the completion on the variations between propagation speed and adiabatic explosion pressure. The explosion duration (t_c) was also measured, it rose with the increase of u'_rms and/or λ for the enhancement on propagation albeit such enhancement was attributed to different mechanism for different influence factors. The variations of deflagration index (K_G) indicated that the hazardous level of stoichiometric hydrogen/methane mixtures would become more hazardous in the presence of turbulence. Furthermore, the heat loss during the explosion also was calculated and analysed. The results reported in this article could provide more basic but important information to practical utilizations of hydrogen/methane blended fuels, especially on the safety protection strategies.