Experimental investigation on the near detonation limits of propane/hydrogen/oxygen mixtures in a rectangular tube

In this paper, an experimental study on the near detonation limits for propane-hydrogen-oxygen is performed. Three mixtures (i.e., 8H₂–C₃H₈–9O₂, 4H₂–C₃H₈–7O₂ and 12H₂–C₃H₈–11O₂) are tested in a rectangular tube (52 mm × 32 mm). Photodiodes with regular intervals are mounted on the tube wall to measure the time of arrival of detonation waves, from which the detonation velocity is determined. Smoked foils are inserted into the tube to obtain the detonation cell pattern. The results indicate that well within the detonation limits, the detonation can propagate at a steady velocity. By reducing the initial pressure, the detonation velocity decreases gradually. Subsequently, the detonation fails as the initial pressure is below a critical pressure. The critical pressures for 8H₂–C₃H₈–9O₂, 4H₂–C₃H₈–7O₂ and 12H₂–C₃H₈–11O₂ mixtures are 4 kPa, 5 kPa and 6 kPa, and the corresponding detonation velocity deficits are 10%, 9%, 10%, respectively. The cellular detonation structures show that the cell size decreases with the decrease of the hydrogen concentration, and the cell structures are very irregular near the detonation limits.     

The instability of laminar methane/hydrogen/air flames: Correlation between small and large-scale explosions

Darrieus–Landau (D-L) instability can cause significant acceleration in freely expanding spherical flames, which can lead to accidental large-scale gas explosions. To evaluate the potential of using high-pressure lab-scale experiments to predict the onset of cellular instabilities in large-scale atmospheric explosions, experimental measurements of the cellular instabilities for hydrogen and methane mixtures are conducted, in laboratory spherical explosions at elevated pressures. These measurements are compared with those from several large-scale atmospheric experiments. Comprehensive correlations of the pressure effect on a critical Karlovitz number, $K_{cl}$, together with those of strain rate Markstein number, $M{a}_{sr}$, are developed for hydrogen/air mixtures. The regime of stability reduces for all mixtures, as $M{a}_{sr}$ becomes negative. Values derived from large-scale experiments closely follow the same correlation of $K_{cl}$ with $M{a}_{sr}$. As a result, the extent of the regime where the laminar explosion flames become unstable can be predicted as a function of $M{a}_{sr}$ and pressure.     

Structure of laminar premixed flames of methane near the auto-ignition limit

Auto-ignition and flame propagation are the two different controlling mechanisms for stabilizing the flame in secondary stage combustion in hot vitiated air environment and at elevated pressure. The present work aims at the investigation of the flame stabilization mechanism of flames developing in such an environment. In order to better understand the structure of turbulent flames at inlet temperature well above the auto-ignition temperature, the behavior of laminar flames at those conditions needs to be analyzed. As an alternative to challenging and expensive measurements at high temperature and pressure, the behavior of laminar flames at such conditions can be predicted from theory using mathematical simulation. In the present work, the laminar burning velocities and flame structures of premixed stoichiometric methane/air mixtures for inlet temperatures from 300 to 1450 K and absolute pressures from 1 to 8 bar have been calculated using a freely propagating laminar, one dimensional, planar flame model. The prediction shows that at inlet temperatures below the auto-ignition temperature, the predicted laminar burning velocity which corresponds to the unburned mixture velocity in order to create a steady laminar flame decreases with increase in pressure. When the inlet temperature of the mixture goes well beyond the auto-ignition temperature of the mixture, however, the unburned mixture velocity increases steeply at higher pressure level, because of a complete transition of the flame structure.     

Experimental evaluation of aluminum powder fuel in a hydrogen/oxygen detonation tube

Solid powder detonation technology was investigated in this study. Several problems related to the controllable and uniform injection of solid powder fuels were examined. This study demonstrated the feasibility of solid powder detonation technology by designing fluidization devices and evaluating the solid powder supply system. A stable supply of powder was achieved, and ignition tests were conducted using Al/H₂/O₂ and H₂/O₂ fuels. The maximum detonation wave velocity obtained using Al/H₂/O₂ was 2950 m/s, which was 40% higher than that obtained using H₂/O₂. We observed a mixed two-phase double-front detonation structure, with hydrogen/oxygen being detonated in the first front and aluminum powder/oxygen being detonated in the second wave. At a relatively low equivalence ratio, the aluminum powder/oxygen wave surface pressure was higher, whereas the hydrogen/oxygen wave surface pressure was higher at a higher equivalence.     

Flame front evolution and laminar flame parameter evaluation of buoyancy-affected ammonia/air flames

For flames with very low burning speed, the flame propagation is affected by buoyancy. Flame front evolution and laminar flame parameter evaluation methods of buoyancy-affected flame have been proposed. The evolution and propagation process of a center ignited expanding ammonia/air flame has been analyzed by using the methods. The laminar flame parameters of ammonia/air mixture under different equivalence ratio (ER) and initial pressure have been studied. At barometric pressure, with the increase of ER, the laminar burning velocity (LBV) of ammonia/air mixture undergoes a first increase and then decrease process and reaches its maximum value of 7.17 cm/s at the ER of 1.1, while the Markstein length increases monotonously. For ammonia/air flames with ER less than unity, the flame velocity shows a decreasing trend with stretch rate, resulting in the propensity to flame instability, but no cellular structure was observed in the process of flame propagation. As the initial pressure increases, the LBV decreases monotonously as well as the Markstein length. The flame thicknesses of ammonia/air mixtures decrease with initial pressure and are much thicker than those of hydrogen flames, which makes a stronger stabilizing effect of curvature on the flame front. The most enhancement of LBV is contributed by the dehydrogenation reaction of NH₃ with OH. The NO concentration decreases significantly with the increase of ER.     

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.     

Experimental and numerical investigation on the effect of hydrogen addition and N2/CO2 dilution on laminar burning velocity of methane/oxygen mixtures

An experimental and numerical study on the combined effect of N₂/CO₂ dilution and hydrogen addition on the laminar burning velocity (LBV) of methane-oxygen mixtures was conducted. The experiments were performed at atmospheric conditions using the heat flux method for effective equivalence ratios (ϕ_F) varying from 0.7 to 1.3. The results show that the hydrogen addition causes an increase in LBV for all the mixture conditions. The variation in LBV based on hydrogen addition parameter (R_H) for all N₂ dilution conditions were following a linearly increasing trend. The strong effect of hydrogen addition on LBV is observed at lean and rich mixtures compared to that at near stoichiometric mixture conditions. The experimental results show that the percentage variation in LBV with R_H at rich mixture is more substantial at 75% N₂ dilution compared to that at 65% N₂ dilution.     

Numerical study on CH4 laminar premixed flames for combustion characteristics in the oxidant atmospheres of N2/CO2/H2O/Ar-O2

The freely-propagating laminar premixed flames of CH₄–N₂/CO₂/H₂O/Ar-O₂ mixtures were conducted with the PREMIX code. The effects of the equivalence ratio and various oxidant atmospheres on the basic combustion characteristics were analyzed with the initial pressure and temperature of 1 atm and 398 K, respectively, O₂ content in the oxidant of 21%. The chemical reaction mechanism GRI-Mech 3.0 was chosen to determine the effects of the oxidant atmospheres of N₂/O₂, CO₂/O₂, H₂O/O₂, and Ar/O₂ on the adiabatic flame temperature, laminar burning velocity, flame structure, free radicals, intermediate species, net heat release rate and specific heat of the fuel/oxidant mixtures. The numerical results show that the maximum adiabatic flame temperatures and laminar burning velocities are at Ar/O₂ atmosphere. The mole fractions of CO and H₂ increased fastest at CO₂/O₂ atmosphere and H₂O/O₂, respectively. The mole fractions of CH₃ and H follow the order Ar/O₂> N₂/O₂>H₂O/O₂>CO₂/O₂. In addition, for 4 oxidant atmospheres, the peak mole fraction of C₂H₂ is following the order H₂O/O₂>Ar/O₂>N₂/O₂>CO₂/O₂ and the net heat release rate is following the order Ar/O₂>N₂/O₂>H₂O/O₂>CO₂/O₂ for all equivalence ratios.     

Laminar burning velocities and flame stability analysis of hydrogen/air premixed flames at low pressure

An experimental and numerical study on laminar burning velocities of hydrogen/air flames was performed at low pressure, room temperature, and different equivalence ratios. Flames were generated using a small contoured slot-type nozzle burner (5 mm × 13.8  mm). Measurements of laminar burning velocity were conducted using the angle method combined with Schlieren photography. Numerical calculations were also conducted using existing detailed reaction mechanisms and transport properties. Additionally, an analysis of the intrinsic flame instabilities of hydrogen/air flames at low pressure was performed. Results show that the behavior of the laminar burning velocity is not regular when decreasing pressure and that it depends on the equivalence ratio range. The behavior of the laminar burning velocity with decreasing pressure can be reasonably predicted using existing reaction mechanisms; however changes in the magnitude of the laminar burning velocity are underestimated. Finally, it has been found experimentally and proved analytically that the intrinsic flame instabilities are reduced when decreasing the pressure at sub-atmospheric conditions.     

Experimental and numerical study of the laminar burning velocity of NH3/H2/air premixed flames at elevated pressure and temperature

Ammonia (NH₃) is a carbon-free fuel that shows great research prospects due to its ideal production and storage systems. The experimental data of the laminar burning velocity of NH₃/H₂/air flame at different hydrogen ratios (X_H2 = 0.1–0.5), equivalent ratios (φ = 0.8–1.3), initial pressures (P = 0.1–0.7 MPa), and initial temperatures (T = 298–493 K) were measured. The laminar burning velocity of the NH₃/H₂/air flame increased upon increasing the hydrogen ratios and temperature, but it decreased upon increasing the pressure. The equivalent ratio of the maximum laminar burning velocity was only affected by the proportion of reactants. The equivalence ratio value of the maximum laminar burning velocity was between 1.1 and 1.2 when X_H2 = 0.3. The chemical reaction kinetics of NH₃/H₂/air flame under four different initial conditions was analyzed. The less NO maximum mole fraction was produced during rich combustion (φ > 1). The results provide a new reference for ammonia as an alternative fuel for internal combustion engines.     

Effects of hydrogen peroxide on combustion enhancement of premixed methane/air flames

Hydrogen peroxide is generally considered to be an effective combustion promoter for different fuels. The effects of hydrogen peroxide on the combustion enhancement of premixed methane/air flames are investigated numerically using the PREMIX code of Chemkin collection 3.5 with the GRI-Mech 3.0 chemical kinetic mechanisms and detailed transport properties. To study into the enhancement behavior, hydrogen peroxide is used for two different conditions: (1) as the oxidizer substituent by partial replacement of air and (2) as the oxidizer supplier by using different concentrations of H₂O₂. Results show that the laminar burning velocity and adiabatic flame temperature of methane flame are significantly enhanced with H₂O₂ addition. Besides, the addition of H₂O₂ increases the CH₄ consumption rate and CO production rate, but reduces CO₂ productions. Nevertheless, using a lower volumetric concentration of H₂O₂ as an oxidizer is prone to reduce CO formation. The OH concentration is increased with increasing H₂O₂ addition due to apparent shifting of major reaction pathways. The increase of OH concentration significantly enhances the reaction rate leading to enhanced laminar burning velocity and combustion. As to NO emission, using H₂O₂ as an oxidizer will never produce NO, but NO emission will increase due to enhanced flame temperature when air is partially replaced by H₂O_2.     

A comprehensive study of light hydrocarbon mechanisms performance in predicting methane/hydrogen/air laminar burning velocities

In order to obtain the precise predicted values of methane/hydrogen/air burning velocities from simulations, the performances of GRI mech 3.0, Aramco mech 1.3, USC mech 2.0 and San diego mech mechanisms were systematically studied under various conditions by PREMIX code and compared with experimental data from literature. The conditions where each mechanism gave their good performance are obtained and concluded. The flowrate sensitivity and rate constants of key elementary reactions were analyzed to insight the different behavior of each mechanism. The results showed that all these widely used small hydrocarbon mechanisms could gave reasonable predictions for pure methane and methane hydrogen blends. Nevertheless, they lack sensitivity for rich hydrogen at elevated pressures due to their complex reactions competitions controlled by hydrogen sub model. USC mech 2.0 was found more suitable for being used at low hydrogen contents while San diego mech gained better results at high hydrogen contents. GRI 3.0 gave good predictions for methane hydrogen blends except for high initial pressures. Generally, Aramco mech 1.3 showed the best performance for all testing conditions. Moreover, there was relatively large deviation from the predicted results and experimental data in the transition regime where the hydrogen fractions were between 60% and 80%, it may could be optimized by tuning the rate constants of reactions.     

The flame propagation characteristics and detonation parameters of ammonia/oxygen in a large-scale horizontal tube: As a carbon-free fuel and hydrogen-energy carrier

As a carbon-free fuel and a hydrogen-energy carrier, ammonia is a potential candidate for future energy utilization. Therefore, in order to promote the application of ammonia in detonation engines and to evaluate the safety of ammonia related industrial process, DDT experiments for ammonia/oxygen mixtures with different ERs were carried out in a large-scale horizontal tube. Moreover, pressure transducers and self-developed temperature sensors were applied to record the overpressure and the instantaneous flame temperature during DDT process. The results show that the DDT process in ammonia/oxygen mixtures contains four stages: Slow propagation stage, Flame and pressure wave acceleration stage, Fast propagation and detonation wave formation stage, Detonation wave self-sustained propagation stage. For stoichiometric ammonia/oxygen mixtures, flame front and the leading shock wave propagate one after another with different velocity, until they closely coupled and propagated together with one steady velocity. At the same time, it is found that an interesting retonation wave propagates backward. The peak overpressure, detonation velocity, and flame temperature of the self-sustained detonation are 2 MPa, 2000 m/s and 3500 K, respectively. With the ER increased from 0.6 to 1.6, the detonation velocities and peak overpressures ranged from 2310 m/s to 2480 m/s and 25.6 bar–28.7 bar, respectively. In addition, the detonation parameters of ammonia were compared with those of methane and hydrogen to evaluate the detonation performance and destructiveness of ammonia.     

An experimental and numerical study on characteristics of laminar premixed H2/CO/CH4/air flames

The effects of variations in the fuel composition on the characteristics of H₂/CO/CH₄/air flames of gasified biomass are investigated experimentally and numerically. Experimental measurements and numerical simulations of the flame front position and temperature are performed in the premixed stoichiometric H₂/CO/CH₄/air opposed-jet flames with various H₂ and CO contents in the fuel. The adiabatic flame temperatures and laminar burning velocities are calculated using the EQUIL and PREMIX codes of Chemkin collection 3.5, respectively. Whereas the flame structures of the laminar premixed stoichiometric H₂/CO/CH₄/air opposed-jet flames are simulated using the OPPDIF package with the GRI-Mech 3.0 chemical kinetic mechanisms and detailed transport properties. The measured flame front position and temperature of the stoichiometric H₂/CO/CH₄/air opposed-jet flames are closely predicted by the numerical calculations. Detailed analysis of the calculated chemical kinetic structures reveals that the reaction rate of reactions (R38), (R46), and (R84) increase with increasing H₂ content in the fuel mixture. It is also found that the increase in the laminar flame speed with H₂ addition is most likely due to an increase in active radicals during combustion (chemical effect), rather than from changes in the adiabatic flame temperature (thermal effect). Chemical kinetic structure and sensitivity analyses indicate that for the stoichiometric H₂/CO/CH₄/air flames with fixed H₂ concentration in the fuel mixture, the reactions (R99) and (R46) play a dominant role in affecting the laminar burning velocity as the CO content in the fuel is increased.     

Laminar burning velocity of hydrogen–methane/air premixed flames

The laminar burning velocities of hydrogen–methane/air mixtures at NTP conditions were calculated using the CHEMKIN PREMIX code with the GRI kinetic mechanism. The equivalence ratio and the fuel composition were varied from lean to rich and from pure methane to pure hydrogen, respectively.     

Study of ozone-enhanced combustion in H2/CO/N2/air premixed flames by laminar burning velocity measurements and kinetic modeling

The enhancement effect of ozone addition for H₂/CO/N₂/Air premixed flames at ambient condition is investigated both experimentally and computationally. Adiabatic laminar velocities under different amount of O₃ addition were directly measured using the Heat Flux Method. The ozone concentration in the oxidizer is monitored online to ensure the precise control and stability of ozone injection. Experimental data shows significant enhancement of the burning velocities due to O₃ addition. With 8500 ppm ozone seeded, maximum 18.74% of burning velocity enhancement is observed at equivalence ratio Φ = 0.7. Kinetic modeling works were conducted by integrating ozone sub-mechanism with three kinetic mechanisms: GRI-Mech 3.0, Davis mechanism and USC Mech II. The modeling results were compared with experimental data. GRI-Mech 3.0 + Ozone mechanism demonstrated the ability to reproduce the experimental data. Extra OH radicals promoted by ozone was found in the pre-heat regime which initiates the chain-branching reaction and results in the combustion enhancement.     

Experimental and numerical study on the laminar burning velocity of hydrogen enriched biogas mixture

The laminar burning velocities of biogas-hydrogen-air mixture at different fuel compositions and equivalence ratios were determined and studied using the spherical flame method. The combined effects of H₂ and CO₂ on the laminar burning velocity were investigated quantitatively based on the kinetic effects and the thermal effects. The results show that the laminar burning velocities of the BG40, BG50 and BG60 are increased almost linearly with the H₂ addition owing to the improved fuel kinetics and the increased adiabatic flame temperature. The dropping trend of laminar burning velocity from the BG60-hydrogen to the BG40-hydrogen is primarily attributed to the decreased adiabatic flame temperature (thermal effects). The GRI 3.0 mechanism can predict the laminar burning velocity of biogas-hydrogen mixture better than the San Diego mechanism in this study. Whereas, the GRI mechanism still needs to be modified properly for the hydrogen-enriched biogas as the CO₂ proportion exceeds 50% in the biogas at the fuel-rich condition. The increased CO₂ exerts the stronger suppression on the net reaction rate of H + O₂=OH + O than that of H + CH₃(+M) = CH₄(+M), which contributes to that the rich-shift of peak laminar burning velocity of biogas-hydrogen mixture requires higher H₂ addition as the CO₂ content is enhanced. For the biogas-hydrogen fuel, the H₂ addition decreases the flame stability of biogas fuel effectively due to the increased diffusive-thermal instability and hydrodynamic instability. The improved flame stability of biogas-hydrogen fuel with the increased CO₂ content is resulted from the combined effects of diffusive-thermal instability and hydrodynamic instability.     

Experimental study of detonation propagation in a square tube filled with orifice plates

DDT experiments were conducted in a 6000 mm long square cross-section (112 mm × 112 mm) tube with various obstacle configurations with hydrogen-air mixtures and ethylene-air mixtures at ambient pressure (101 kPa) and room temperature (298 K). Square orifice plates with inner side 86.8 mm and 70.8 mm (BR = 0.4 and 0.6) and round orifice plates with inner diameter 80.0 mm (BR = 0.6) were used to assemble the obstacle configurations. The plates were installed at 1, 2 and 3 times the tube inner side. Soot foils were placed between the two orifice plates at the end of the tube for $S=3D$, where $S$ is the obstacle spacing and $D$ is the tube inner side. The DDT limits were determined based on the flame velocity above the isobaric sound speed of the burnt products. The results show that at the DDT limits, the criterion $d_{eff}/\lambda \approx 1$ is not pervasive, i.e., $d_{eff}/\lambda$ decreases with the obstacle spacing increase, in which $d_{eff}$ and $\lambda$ are the effective diameter of the orifice and the detonation cell size. Within the limits, the measured velocity for BR = 0.6 square orifice plates is higher than that for round orifice plates. On the other hand, no obvious difference in the limits can be observed for the BR = 0.6 obstacle configurations. Soot foils provide insights into the detonation propagation mechanism in the orifice plate section. It is shown that hot spots formed via the interactions between the decoupled shock wave and the tube wall can be responsible for the re-initiation of detonation. In addition, overdriven detonations induced by shock focus at the corners, followed by a band of fine cells. For less sensitive mixture and smaller orifice, the re-initiation distance is longer. Near the limits, no cellular structure can be observed, indicating longer cycle period for detonation re-initiation. This also accounts for the significant velocity fluctuation for larger spacing ($S=2D$ and $S=3D$) when the limits are approached.