Constant Volume Combustion Bomb Research Articles

Development of a chemical mechanism for ammonia/hydrogen blends for engines

ABSTRACT Ammonia/hydrogen blend fuels have been extensively used as alternative fuel for engines. The combustion and emissions characteristics of ammonia/hydrogen engines have been investigated utilising a CFD model. Developing a robust and accurate chemical reaction mechanism for ammonia/hydrogen blend fuels is of utmost importance. In this paper, a chemical mechanism has been developed and validated for the combustion characteristics of ammonia/hydrogen, which includes 31 species and 223 reactions. Extensive validation has been conducted using experimental data as the basis. The obtained data indicate that the simulated values for the ignition delay times of the shock tube and the concentration of the main components in the stirred jet reactor align well with the measured values. The simulated laminar flame speed under turbulent flow conditions shows a remarkable agreement with the corresponding experimental data. The study compared simulated flame evolution values with experimental data from a constant volume combustion bomb. The results indicate that the proposed mechanism is able to produce favourable flame evolution and combustion characteristics. The presented detailed mechanism demonstrates credible overall performance in terms of combustion behaviours, indicating its suitability for effectively modelling ammonia/hydrogen combustion in real engines.

Study on combustion and emission characteristics of hydrogen/air mixtures in a constant volume combustion bomb

Hydrogen is an environmentally-friendly fuel with the characteristics of low carbon, high calorific value, and extensive combustible range, making it an optimal alternative fuel for internal combustion engines. In this study, an experiment using a spherical constant volume combustion bomb (CVCB) was conducted to investigate the combustion and emission characteristics of hydrogen/air mixtures. The results shows that the flame propagation velocity of hydrogen laminar flow initially increases and then decreases with the excess air coefficient (λ), reaching peak at λ = 0.6, and the flame becomes unstable on the side of lean burning (λ > 1.0). Both the NOx emissions and the average temperature in the CVCB initially rise before eventually declining with the λ, the peak values for both NOx emission and mean temperature are reached at λ = 1.0. The dilution of N2 inhibits the laminar combustion and emission of hydrogen. As the dilution rate of N2 increases, little change occurs in the Markstein length, while flame propagation velocity, NOx emission, and average temperature all decrease. An increase in initial pressure hinders the development of the initial flame, while promoting the formation of cellular structure within the flame. Consequently, unstable combustion leads to a rapid increase in flame propagation speed during later stages. The average temperature inside the CVCB and the NOx emission increase by 6.6 % and 25.9 %, respectively, with the initial pressure increasing from 0.1 MPa to 0.5 MPa. On the other hand, an increase in initial temperature can facilitate flame development and results in an increased flame propagation speed. Especially, Markstein length is less sensitive to changes in initial temperature than to changes in initial pressure. The average temperature and NOx emission present a pattern of increasing first and decreasing then with the increase in initial temperature. These results have provided empirical evidence for optimizing the combustion and emission control of hydrogen engines.

Turbulent flame propagation of C10 hydrocarbons/air expanding flames: Possible unified correlation based on the Markstein number

In this study, we experimentally analyzed the influence of turbulence and thermal-diffusional effect on the morphology and turbulent flame speed of premixed expanding flames. The turbulent flames of the n-decane/air and decalin/air mixtures were measured using a fan-stirred constant volume combustion bomb generating near-isotropic turbulence at elevated pressures (1, 2, and 5 bar), temperature (443 K), and a wide range of equivalence ratios (0.8–1.6). The results found that the wrinkling degree of the flame front was greater for Ma<0 compared to Ma>0. However, the distribution patterns of the curvature radius of the flame contours exhibited similarities across different equivalence ratios. Furthermore, an increase in turbulence intensity would aggravate the randomness of flame contour distribution. The turbulent expanding flames of n-decane/air and decalin/air mixtures were self-similar under different turbulence intensities and pressures, and this self-similar propagation followed a correlation between the normalized turbulent flame speed and the turbulent flame Reynolds number (ReT,f) to the one-half power. The similarity in normalized turbulent flame speeds was extended from normal alkanes (C4-C8) and isomeric alkanes (C8, C16) to longer straight-chain alkane (n-decane) and cycloalkane (decalin). As the increase of equivalence ratio, their normalized turbulent flame speeds increased nonlinearly due to the thermal-diffusional effect. Additionally, it was found that the classical, effective, and experimental Lewis numbers were confirmed as unsuitable parameters for characterizing the role of thermal-diffusional effect on the turbulent flame speed for hydrocarbon fuels with large molecular weight. Finally, two possible unified correlations were proposed based on the Markstein number, (d〈r〉/dt)/(σSL)=0.178e−0.231MaReT,f1/2 and (d〈r〉/dt)/(σSL)=(0.170−0.0447Ma+0.0067Ma2)ReT,f1/2, which could predict the turbulent flame speeds of hydrocarbon fuels with large molecular weight such as n-alkanes, iso-alkanes, and decalin.

Comprehensive analysis of laminar burning velocity in DEK/NH3-air premixed flames under varied pressure conditions

3-pentanone (DEK) is a biomass oxygenated fuel with high energy density and low emissions. It shows promising potential in increasing the combustion of NH3. This study examines the laminar burning velocity (LBV) of a fuel mixture composed of DEK/NH3-air. The LBV of DEK/NH3-air was determined with a constant-volume combustion bomb under the following conditions: initial temperature (T) of 448 K, pressure (P) of 1 to 3 atm, equivalence ratio (ϕ) between 0.7 and 1.3, and NH3 mole fraction (XNH3) varying from 0 % to 90 %. A combustion kinetic model for DEK/NH3 mixed fuel was developed and verified using experimental results. Overall, the model can capture the trend of LBV well. Subsequently, a comprehensive kinetic analysis was conducted utilizing this mechanism to elucidate the effect of NH3 mixing on DEK/NH3-air mixed fuel. The results indicate that LBV of DEK/NH3-air mixture declines as XNH3 increases, primarily attributed to the reduction in radical pool concentration. Conversely, the effect of the decrease in flame temperature is relatively minor. Furthermore, as XNH3 mole fraction decrease, there is a slight alteration in the consumption pathway of NH3. The elevation in DEK mole fraction leads to an increased production of radicals, thereby amplifying the oxidation process of NH3. The proportion of the oxidation of NHi pathway also increases. Finally, the change of NO content in DEK/NH3-air flame was observed. The rise in XNH3 leads to a gradual increase in the content of NO until it reaches a peak, after which it starts to decline. This phenomenon is dependent on the presence of O, OH, and NHi radicals. In addition, it has been discovered that augmenting ϕ can significantly reduce the concentration of NO in DEK/NH3-air mixture.

Experimental study on characteristics of methane-coal dust explosions and the spatiotemporal evolution of flow field in early flame

To understand the explosion characteristics of methane-lignite coal dust mixtures and the spatiotemporal evolution of the early flame flow field, explosion pressure, OH* emission spectra, and early flame schlieren images were captured in a 60L constant volume combustion bomb. Schlieren image velocimetry was used to obtain the instantaneous velocity distribution of the methane-coal dust flame front. The results indicate that adding coal dust causes a nonlinear increase in explosion pressure, particularly at low methane concentrations. At a 5 % methane concentration, adding coal dust can increase the maximum explosion pressure by up to 39 % (190 kPa). The trend of the OH* emission spectra corresponds with the maximum explosion pressure. A slight increase in coal dust significantly enhances the OH* emission spectra, especially at lower concentrations, potentially increasing it by nearly five times. However, at higher coal dust concentrations, the OH* emission spectra are significantly reduced. Additionally, within the considered concentration range (≤58.14 g/m³), larger turbulent integral length scales in initial conditions consistently result in the fastest flame acceleration. These findings provide experimental evidence for further exploration of the explosion mechanisms of methane-coal dust mixtures and the development of chemical reaction kinetic models.

Experimental study on the discharge characteristics of high-voltage nanosecond pulsed discharges and its effect on the ignition and combustion processes

Non-equilibrium plasma has been proven to expand ignition limits and enhance combustion. This paper uses high-voltage nanosecond pulsed discharge (NPD) to generate non-equilibrium plasma and experimentally studies the discharge characteristics at different parameters in a constant-volume combustion bomb. NPD is used to ignite the methane-air mixture, and the effects of pulse power output voltage and pulse interval (PI) on ignition and combustion processes are studied. In addition, the minimum ignition energy (MIE) at different ignition methods (Transistor coil ignition (TCI) and NPD) is also studied. The results show that the first pulse of NPD discharge has a higher breakdown voltage and discharge energy when using multi-pulse discharge. After the second pulse, the highest breakdown voltage and current tend to stabilize, and the discharge energy remains unchanged. As the PI increases, the breakdown peak voltage and the peak current increase slightly. The study on the MIE of NPD shows that at different mixture concentrations, the MIE of NPD is smaller than that of TCI, and NPD can expand the lean burn limit. An increase in initial temperature can significantly reduce MIE. In addition, when the discharge energy is near the MIE, the combustion stability begins to decrease, and the combustion cycle fluctuates wildly. When the ignition energy is increased, the stability improves, and the combustion fluctuation rate can be maintained within 5%.

Experimental and numerical study on the influence of N2/H2 on the laminar combustion characteristics of syngas premixed flames

This paper primarily analyzes the influence of N2 and H2 content on the laminar combustion characteristics of low calorific value syngas, using a constant-volume combustion bomb to measure the laminar combustion velocity (SL) of the fuel and compare it with five commonly used mechanisms. The study reveals that higher N2 content significantly reduces the SL of CO/CH4/CO2 mixed fuels, primarily due to a general decrease in the mole fractions of key intermediates and net reaction rates. Increasing H2 content leads to an increase in the SL of CO/CH4/CO2/N2 mixed fuels. The thermal effect becomes increasingly prominent with the elevation of H2 content, while the proportion of thermal effect initially decreases and subsequently increases as the equivalence ratio rises. With the increase in N2 content, the mole fraction of NO experiences a significant decrease, and the rate of decline in thermal NO production is even faster. Conversely, with the increase in H2 content, the rate of rise in non-thermal NO generation is faster, and the mole fraction of NO tends to increase.

Study on the combustion characteristics of NH3 and the inhibition characteristics of R134a on NH3

NH3 has been considered as an alternative to hydrofluorocarbon (HFC) refrigerants due to its environmentally friendly properties, while it is flammable. Therefore, it is necessary to investigate the flammability of NH3 (R717) and find its matching flame retardant to broaden its application. In this paper, the combustion behaviors of R717 and the inhibition characteristics of flame retardant R134a on R717 were studied experimentally and theoretically. Firstly, the flame propagation pattern, flammability limit, laminar burning velocity, and Markstein length of pure R717 were measured by a constant volume combustion bomb from 300 K to 400 K. Then the flammability characteristics of R134a/R717 mixture were further tested. The results indicated that R134a had a distinct flame retardancy on the flammability limits of R717. Further, the flame retarding mechanism of R134a on R717 was analyzed by density functional theory (DFT) on the 6–311++G(d,p) level. The flame retarding effects of R134a were classified to physical and chemical mechanisms. The physical force mainly depended on the dilution effect of additional R134a. The chemical mechanism was contributed to the newly flame retarding reactions introduced by R134a, which reflected in the dehydrogenation of R717 and the consumption of OH and H active radicals by fluorine-contained radicals (CF3, CF2, and F).

Experimental and kinetic modeling study on laminar flame speeds and emission characteristics of oxy-ammonia premixed flames

The low flame speed and NOx emissions of ammonia are the key to affecting combustion temperature, stability, flame structure, and pollutant control, limiting its application in power generation devices. This study proposes oxy-ammonia combustion (oxygen content in oxidant = 100%, NH3/O2 combustion) as a novel method for significantly enhancing ammonia combustion. A method for measuring the laminar flame speed using the NH* distribution of the Bunsen burner flame is proposed, and the measured data is close to the constant volume combustion bomb. 21 kinetic models were collected and optimized after comparison with the experimental data of constant volume combustion bombs, and results show that Han et al.'s model has higher prediction accuracy for NH3/O2 flames. The preheating temperature was relevant to gas turbine combustors (298.15–500 K), and the equivalence ratio ranged from 0.7 to 1.6. At this extreme condition (oxygen content in oxidant = 100%), the results show that the maximum laminar flame speed reaches 1.25 m/s at ambient pressure and temperature, which is approximately 18 times that of NH3/air combustion, mainly due to the increased reaction rates of OH, H, O and NH2 radicals in the reaction zone (primary ammonia decomposition and H/O radical pool). The NO emission of NH3/O2 combustion at stoichiometric conditions is about 7216 ppmv@15%O2, 1.5 and 13 times that of NH3/air and CH4/air combustion, respectively. NO production and consumption may follow the N2O and NNH mechanism, and HNO is an important precursor for NO formation. Increasing the preheating temperature has less effect on the reaction pathway directions in the reaction network of nitrogen conversion but affects the path flux significantly.

Analysis of the combustion characteristics of ammonia/air ignited by turbulent jet ignition with assisted hydrogen injection in pre-chamber

Turbulent jet ignition (TJI) is an advanced ignition strategy that can improve the ignition and combustion characteristics of low-reactivity mixtures. The utilization of TJI system may be reliable to achieve the application of ammonia (NH3) internal combustion engines. Hydrogen (H2) is a potential auxiliary fuel for the pre-chamber, and the injection of a small amount of H2 in the pre-chamber is beneficial for promoting the ignition and combustion of NH3/air in the main chamber. In this study, the ignition and combustion characteristics of NH3/air adopting the active TJI with assisted H2 injection in pre-chamber were investigated, and the relevant experiments were conducted in the constant volume combustion bomb system. The results show that the H2 pre-chamber can improve the flammability of NH3/air, and properly increasing H2 injection is conducive to the rapid ignition of NH3/air in the main chamber. The turbulence introduced into the main chamber by the hot jet enhances the combustion process, and the generation of turbulence weakens the sensitivity of the combustion rate to the reactivity of the unburned mixture. The turbulence intensity can be increased by decreasing the pre-chamber orifice diameter, which increases the ignition delay but significantly shortens the combustion duration.

Study on the Synergistic Suppression Effect and Mechanism of N2/Ultrafine Water Mist on Liquefied Petroleum Gas Explosion.

Liquefied petroleum gas (LPG) is widely used for its cleanliness and high efficiency in industry and city life. In order to improve the suppression effect on LPG explosion, a constant volume combustion bomb was used to investigate the synergistic influence of N2/ultrafine water mist on the explosion and combustion characteristics of 6% premixed LPG/air gas. The results showed that (1) the effect of a single ultrafine water mist on suppressing LPG explosion is unstable. When the concentration of ultrafine water mist is low, the flame acceleration in the initial stage of explosion is promoted, and when the ultrafine water mist with a mass fraction of 420 g/m3 is introduced, the maximum pressure rise rate increases. (2) The combination of N2/ultrafine water mist has a synergistic effect on LPG explosion. Compared to the individual suppression effects, the combination of N2/ultrafine water mist showed more effective suppression on the explosion pressure, flame propagation, and flame instability of LPG explosion. (3) Through the mechanism analysis, it is found that the combined action of N2/ ultrafine water mist can better reduce the mole fraction and ROP peak of active free radicals such as H, O, and OH by inhibiting the main reaction of generating H, O, and OH radicals during the explosion of LPG, resulting in the reduction of flame free radicals in the explosion system, thus effectively inhibiting the chain reaction of ignition and explosion of LPG. This research can provide guidance for a better understanding and implementation of gas-liquid two-phase suppression technology for LPG explosion.

Experimental study on the combustion of NH3/H2/air based on the passive turbulent jet ignition

The mixture of ammonia (NH3) and hydrogen (H2) is the potential alternative fuel for internal combustion engines (ICEs). The turbulent jet ignition (TJI) system can provide high ignition energy and turbulent disturbance to promote the combustion of the mixture for NH3/H2 ICEs. Therefore, the combustion characteristics of NH3/H2/air under passive TJI conditions were experimentally studied in the present study. The experiment was conducted in a constant volume combustion bomb, and the effect of fuel composition and equivalence ratio was investigated. The experimental results show that compared to spark ignition, the combustion of NH3/H2 can be effectively improved by using TJI, especially under high NH3 fraction conditions. The addition of H2 has a significant positive effect on the ignition and combustion performance. With the addition of H2, the ignition delay and combustion duration decrease evidently. And the H2 addition is beneficial for improving the ignition mechanism and achieving flame ignition. In addition, poor ignition performance may occur under rich conditions due to the fluid-dynamic quenching of the jet. However, the high flame propagation rate on the rich side still leads to lower combustion duration. Moreover, increasing the orifices number appropriately can enhance the ignition and combustion performance. The jet strength is weakened by the increased total orifice area and the ignition of NH3/H2 can be improved.

Experimental study on high pressure injection characteristics of gas fuel based on turbulent constant volume combustion bomb

In the face of strict carbon regulations, the internal combustion engine industry is turning to carbon-neutral and low-emission fuels. The experimental study of the high-pressure direct jet characteristics of gaseous fuels is still limited to the study of the high-pressure direct jet characteristics in stationary and non-uniform turbulent environments. The effect of isotropic turbulence on the high-pressure direct jet characteristics of gaseous fuels has not been thoroughly investigated. This paper investigates the high-pressure direct injection characteristics of gaseous fuels in an isotropic turbulent environment. Based on the experimental platform of turbulent constant volume combustion bomb, the effects of laminar flow environment and turbulence intensity on the high-pressure injection characteristics of gaseous fuels in turbulent environment are investigated in this paper. The results show that the turbulent environment has accelerated the axial development of high-pressure gaseous fuel injection compared to the laminar environment. The greater the velocity of the jet front, the greater the penetration distance. Under the turbulence intensity of 2.25 m/s, the penetration distance was 94.2 mm at 2.5 ms with a jet pressure of 6 MPa, which was 12.3% larger than that in the laminar flow environment (83.9 mm). Turbulence increases the radial development of the high-pressure gaseous fuel jet, facilitates radial diffusion and mixing at the jet boundary, accelerates the continuous merging and fragmentation process of the vortices at the jet boundary, enhances the entrainment of the surrounding medium gases, and contributes to the mixing of fuel and air in the jet. The stronger the turbulence intensity, the larger the near-field cone angle and far-field cone angle, and the larger the excess air coefficient. The stronger the turbulence intensity, the more the jet volume will be. This paper reveals the mechanism of background turbulence on jet development. The study is of great academic significance to enrich and deepen the theory of gaseous fuel jets.

Combustion performance and flame front morphology of producer gas from a biomass gasification-based cookstove

The present work investigates the last phase of the biomass gasification-based cookstove, which is the combustion process of biomass producer gas (BGP). A constant volume combustion bomb and kinetic simulation are used to characterize the behavior of the biomass producer gas and its pollutant emissions under a conventional premixed combustion process. The BGP combustion processes of two types of biomasses available in Colombia (Pinus Patula and Cordia Alliodora) are studied under certain conditions of pressure, temperature, and equivalence ratio. To characterize the combustible mixture of gases obtained after biomass gasification, a combustion chamber with cylindrical geometry equipped with a Schlieren optical diagnostic system to visualize the combustion process, and a piezoelectric pressure transducer to register the instantaneous pressure are used. By visualizing the flame front, the flame propagation speed at which the flame propagates through the combustion chamber and the morphology of the flame are studied. Instantaneous pressure is the input of a two-zone diagnostic model through which variables, such as burning velocity, temperature, mass burned fraction, etc., are obtained to characterize the combustion process of the mentioned gasification gases. Experimental results are compared and complemented with kinetic modeling results obtained with the Cantera package using the Gri-Mech 3.0 and Aramko 1.3 kinetic mechanisms, in terms of laminar burning velocity and NO, NO2, CO, and CO2 exhaust emissions. Results show that Cordia Alliodora presents higher burning velocities than Pinus Patula BP. However, lower CO2 emissions are obtained during the Cordia Alliodora combustion, and NOx emissions are not influenced by the type of biomass considered.

Study on spray characteristics of biodiesel alternative fuels for in-cylinder environment of diesel engine

This paper presents an experimental study of spray characterization of MD-NH (a blend of methyl decanoate and n-heptane in a molar ratio of 1:1 to characterize biodiesel) in an in-cylinder environment of diesel engines to provide a reference for the experimental study of engines in advanced combustion mode. Based on the Peng-Robinson state equation, the thermodynamic model of real fluid was established, and the influence law of ambient temperature and pressure on the diffusion characteristics of fuel flow was analyzed. Based on the constant volume combustion bomb experiments, the variations of spray penetration distance, spray cone angle, spray projection area, spray air entrainment mass, and spray fragmentation forms in different critical environment conditions were studied. The results show that the transport properties of the fuel change abruptly within the critical point neighborhood of the fuel. As the ambient temperature increases, near the critical temperature, the thermal conductivity of biodiesel decreases and then increases; density and kinematic viscosity decrease at an accelerated rate; and surface tension increases to a maximum and then disappears sharply. The diffusion coefficient decreases sharply when the ambient pressure reaches the critical pressure. The spray characteristics of biodiesel in sub/supercritical conditions were different. Compared with the subcritical environment, the biodiesel spray penetration distance decreases, the spray cone angle increases, the spray projection area decreases, and the air entrainment mass increases in the supercritical atmosphere, which induces a gas-liquid interface mixing layer and promotes the mixing of biodiesel with the atmosphere gas. The experimental data in this paper fill the gap in the study of biodiesel spray characteristics in supercritical environments and provide a basis for high-fidelity numerical simulation of the spray model.

Effect of Turbulence and Diffusional–Thermal Instability on Hydrogen‐Rich Syngas Turbulent Premixed Flame

As a kind of fuel with low‐carbon emissions, hydrogen‐rich syngas has a large resource potential and is a promising alternative energy resource. A series of experiments in a constant volume combustion bomb are carried out to investigate the effects of turbulence and diffusional–thermal (DT) instability on a hydrogen‐rich syngas turbulent premixed flame. The flow field in the constant volume combustion bomb is calibrated, and the turbulent flow field's homogeneity and isotropy were investigated. The effects of different speeds of fan (1366–4273 rpm), hydrogen fractions (55%–95%), pressures (1.0–3.0 bar), and equivalence ratios (0.6–1.0) on the hydrogen‐rich syngas turbulent premixed flame are studied. According to the analyses, the flow field in the experimental device exhibits good homogeneous and isotropic characteristics. With the increase of turbulence intensity, equivalence ratio, hydrogen fraction, or pressure, the turbulent flame propagation speed increases gradually. As the hydrogen fraction increases or the equivalence ratio decreases, the effective Lewis number decreases, and the effect of DT instability on the flame increases. The research in this article is crucial for the clean and efficient utilization of hydrogen‐rich syngas and the design of related burners.

Experimental and simulation study on the combustion characteristics of ammonia/n-dodecane mixtures

Laminar burning velocity is a crucial characterization parameter in fuel combustion progress. Investigating the combustion properties is significant to improve combustion efficiency. This paper employs a constant volume combustion bomb and high-speed ripple shadow camera technology to explore the influence of equivalence ratios (0.8–1.4) and ammonia energy shares (0%–40%) on the laminar burning velocity of n-dodecane/ammonia mixture at an ambient temperature of 400 K and a pressure of 1 bar. The results illuminated that the unstretched laminar burning velocity of n-dodecane increased from 46 cm/s at ϕ = 0.8 to 61 cm/s at ϕ = 1 and then decreased to 32 cm/s at ϕ = 1.4. When the ammonia energy ratio reaches 40%, the maximum laminar burning velocity experiences a reduction of 44% because the blended ammonia inhibited the formation of H and O radicals based on kinetic analysis in the fast reaction zone. With the increase of ammonia ratio, the Markstein length of mixed burned gas gradually increases. Particularly under a 1.4 equivalent ratio condition, the Markstein length increases from 0.2 mm to 0.8 mm, indicating that ammonia components significantly enhance flame stability. To gain a deeper understanding of the influence mechanisms of laminar burning velocity, a simplified Yao-Otomo (Y-O) model with 92 species and 355 reactions is proposed, coupling the mechanisms of n-dodecane and ammonia. The Y-O model has demonstrated a predictive trend that is consistent with more detailed WUT-NH3 model (176 species and 2839 reactions), and the Y-O model reduces the amount of computation to save calculation time, which has a higher engineering applicability. The turbulent kinetic energy (TKE) induced by premixed fuel in engine cylinders is estimated through comprehensively analyzing the laminar burning velocity and flame thickness characteristics. The findings indicate a 45% reduction in the peak value of TKE as the ammonia energy ratio increases.

Energy attenuation behavior on propane–air premixed combustion on account of nonmetallic spherical spacers in confined space with two perforated plates

AbstractEnergy attenuation behavior on propane premixed combustion on account of nonmetallic spherical spacers (NSSs) was investigated in a constant volume combustion bomb with two accelerative perforated plates. The flame evolution process is captured by a photography device with schlieren. Energy attenuation effects of NSSs were explored on flame propagation, overpressures, and pressure oscillation. Normal combustion accomplished with flame acceleration, jet autoignition, deflagration, and weak detonation is observed, respectively. NSSs show a good energy attenuation effect on propane normal combustion, jet autoignition, and deflagration evolution behavior, however, during the evolution of weak detonation, the attenuation effect is relatively weak. The mean velocities difference, as well as explosion overpressure decay rate caused by NSSs, decreases from (45.70 m/s and 26.48%) to (2.70 m/s and 18.15%) when the initial pressure increases from 0.1 to 0.5 MPa, namely, attenuation effect of NSSs on propane explosion decreases with increasing initial pressure.

Optical experiment study on Ammonia/Methanol mixture combustion performance induced by methanol jet ignition in a constant volume combustion bomb

Ammonia as an alternative fuel has shown great potential in reducing carbon dioxide emissions. However, the issues of high ignition temperature and unstable combustion of ammonia limit its extensive application. As an ignition energy enhanced method, the ignition chamber (pre-chamber) seems to be an effective solution to improve the combustion performance and extend the ammonia lean flammability limit. In this study, the methanol was provided as relatively high activity fuel to improve the mixture activity in the ignition chamber and main chamber. The mixture activity control method was introduced to accelerate ammonia premixed combustion. The visualization experiment compounded with shadow method was applied to investigate. Results showed that the methanol fueled ignition chamber as jet ignition model reduced the combustion duration by almost half compared to spark ignition method. The methanol jet flame propagation formatted multiple zones of ignition and combustion in the main chamber. The ammonia combustion speed shifted from being determined by laminar flame propagation speed to being determined by jet velocity. The methanol jet velocity is related to the methanol energy substitution ratio and nozzle diameter of the ignition chamber. A 2 % methanol energy substitution ratio is optimized for the lean condition of Φ = 0.8 compared to 1 % methanol energy substitution ratio achieve for the condition Φ = 1.0 in the main chamber. When 50 % methanol (energy) was added in the main chamber, the activity thermo-atmosphere improvement induced the combustion duration to decrease by 43.8 % and 51.9 % at equivalent ratios of 1.0 and 0.8, respectively, compared to pure ammonia. The small nozzle diameter of ignition chamber can improve the jet velocity, however it caused the methanol combustion flame quenching leading to unstable combustion phenomenon. A 3 mm nozzle diameter achieved a short combustion duration compared to 4.5 mm and 6 mm nozzle diameter.

An investigation on the laminar flame propagation and auto-ignition characteristics of a coal-derived rocket kerosene-Part I: Experimental study

The energy structure of coal-rich and oil-poor in China, makes the conversion of coal to liquid fuels an attractive option to reduce petroleum dependence. In this study, the combustion characteristics of a newly developed coal-derived rocket kerosene were investigated to provide fundamental parameters for evaluating the feasibility and compatibility of the new fuel in potential applications. The laminar flame propagation properties of the fuel were studied at pressures of 1, 2 and 5 bar over a temperature range of 423–483 K for equivalence ratio of 0.7–1.8 in a constant volume combustion bomb, and the auto-ignition characteristics of the target fuel were then experimentally studied in a heated shock tube. Flame propagation process was studied based on the recorded flame morphology firstly. The unsteady transition from spark assisted ignition kernel propagation to normal flame was analyzed and was found to become less conspicuous in flames with stronger reaction intensity (higher initial pressure or temperature) and smaller Lewis number (higher equivalence ratio). The Markstein length was extracted by extrapolating the flame trajectories and used to investigate the flame instability. The results shows that the increase of pressure, equivalence ratio enhance the flame instability, but the impact of initial temperature shows no prominent pattern. The laminar burning velocity and ignition delay times were reported over a wide range to strengthen the understanding of combustion characteristics of the new fuel. A kinetic model was proposed to simulate the combustion chemistry of the fuel and kinetic analysis was performed to identify the key reactions driving the flame propagation and auto-ignition. The present study is hoped to provide insights into the evaluation of fuel application in the future and database support for further combustion reaction kinetic mechanism development.