Combustion Characteristics Of Hydrogen Research Articles

The Influence of Gas Fuel Enrichment with Hydrogen on the Combustion Characteristics of Combustors: A Review

Hydrogen is a promising fuel because it has good capabilities to operate gas turbines. Due to its ignition speed, which exceeds the ignition of traditional fuel, it achieves a higher thermal efficiency while the resulting emissions are low. So, it was used as a clean and sustainable energy source. This paper reviews the most important research that was concerned with studying the characteristics of hydrogen combustion within incinerators and power generation equipment, where hydrogen was used as a fuel mixed with traditional fuel in the combustion chambers of gas turbines. It also includes an evaluation of the combustion processes and flame formation resulting from the enrichment of gaseous fuels with hydrogen and partial oxidation. A large amount of theoretical and experimental work in this field has been reviewed. This review summarizes the predictive and experimental results of various research interests in the field of hydrogen combustion and also production.

Parametric Study of Flow and Combustion Characteristic in a Cavitied Scramjet with Multi-Position Injection

This study focuses on the three-dimensional flow and combustion characteristics of a cavitied scramjet engine with multi-position injection. A single-equation large eddy simulation (LES) turbulence model is employed, with a detailed reaction mechanism for hydrogen combustion, as described by Jachimowski. The combustion characteristics of hydrogen in the scramjet combustion chamber are analyzed. Based on the combustion chamber model, the influence of different equivalence ratios, injection timing, injection positions, and injection pressures on the flame formation and propagation process are compared. The results indicate that within a certain range, an increase in the equivalence ratio enhances the combustion intensity and chamber pressure. In the case of multi-position injection, the order of injection from different nozzles has little effect on the final flame stabilization mode and pressure distribution. The opposite-side distribution of nozzles can effectively improve the fuel efficiency and the internal pressure. Furthermore, when the nozzles are closely placed in the opposite-side distribution, the combustion efficiency increases, although this leads to a higher total pressure loss. In scenarios where the fuel injection duration is short, an increase in the injection pressure at the upstream nozzles of the cavity results in a higher local equivalence ratio, as well as reduced fuel mixing and ignition time.

Chemical Kinetic Analysis of High-Pressure Hydrogen Ignition and Combustion toward Green Aviation

In the framework of the “Multidisciplinary Optimization and Regulations for Low-boom and Environmentally Sustainable Supersonic aviation” project, pursued by a consortium of European government and academic institutions, coordinated by Politecnico di Torino under the European Commission Horizon 2020 financial support, the Italian Aerospace Research Centre is computationally investigating the high-pressure hydrogen/air kinetic combustion in the operative conditions typically encountered in supersonic aeronautic ramjet engines. This task is being carried out starting from the zero-dimensional and one-dimensional chemical kinetic assessment of the complex and strongly pressure-sensitive ignition behavior and flame propagation characteristics of hydrogen combustion through the validation against experimental shock tube and laminar flame speed measurements. The 0D results indicate that the kinetic mechanism by Politecnico di Milano and the scheme formulated by Kéromnès et al. provide the best matching with the experimental ignition delay time measurements carried out in high-pressure shock tube strongly argon-diluted reaction conditions. Otherwise, the best behavior in terms of laminar flame propagation is achieved by the Mueller scheme, while the other investigated kinetic mechanisms fail to predict the flame speeds at elevated pressures. This confirms the non-linear and intensive pressure-sensitive behavior of hydrogen combustion especially in the critical high-pressure and low-temperature region which is hard to be described by a single all-encompassing chemical model.

Development and testing of a 100 kW fuel-flexible micro gas turbine running on 100% hydrogen

Hydrogen, as a carbon-free energy carrier, has emerged as a crucial component in the decarbonization of the energy system, serving as both an energy storage option and fuel for dispatchable power generation to mitigate the intermittent nature of renewable energy sources. However, the unique physical and combustion characteristics of hydrogen, which differ from conventional gaseous fuels such as biogas and natural gas, present new challenges that must be addressed.To fully integrate hydrogen as an energy carrier in the energy system, the development of low-emission and highly reliable technologies capable of handling hydrogen combustion is imperative. This study presents a ground-breaking achievement - the first successful test of a micro gas turbine running on 100% hydrogen with NOx emissions below the standard limits. Furthermore, the combustor of the micro gas turbine demonstrates exceptional fuel flexibility, allowing for the use of various blends of hydrogen, biogas, and natural gas, covering a wide range of heating values. In addition to a comprehensive presentation of the test rig and its instrumentation, this paper illuminates the challenges of hydrogen combustion and offers real-world operational data from engine operation with 100% hydrogen and its blends with methane.

Application of hydrogen mechanisms in combustion simulation of DLR scramjet combustor and their effect on combustion performance

In view of the promising application of hydrogen in scramjet combustor and the rapid development of hydrogen kinetic mechanisms, present work applies representative mechanisms with good kinetic performance to the combustion simulation of DLR scramjet combustor with a customized solver based on OpenFOAM platform. Different Reynolds-Averaged Navier Stokes (RANS) turbulence models including standard k-ε, SST k-ω and their corresponding modified versions for turbulent round jets, are utilized to simulate the non-reactive and reactive flow fields. Compared with the detailed experimental measurements of schlieren image, streamwise velocity, pressure etc., the modified k-ω SST turbulence model shows good performance and improves the simulation results near hydrogen jet. In combination with partially stirred reactor (PaSR) combustion model, three representative detailed hydrogen mechanisms (marked with Ó Conaire-2004, Varga-2015 and Konnov-2019) are adopted to evaluate their effect on combustion performance under scramjet combustion condition. And their effects on the combustion performance are quantified based on combustion efficiency, net thrust, total pressure loss, etc. All of them can well characterize the thermal combustion characteristics of hydrogen in the combustor. Among them, Ó Conaire-2004 slightly overestimates flame temperature, combustion efficiency and net thrust, and underestimates total pressure loss. Whereas Varga-2015 and Konnov-2019 provide nearly identical results on these performance parameters and give temperature profiles coinciding well with experimental data. Although the newly added excited states in Varga-2015 and Konnov-2019 deepen the understanding of hydrogen combustion processes and can served for optical combustion diagnostics, their effects on thermal combustion characteristics are negligible under present combustion condition, judging from their low content and the analysis on the performance parameters. Present work is conducive to the selection of hydrogen mechanisms for the combustion simulation of hydrogen-fueled scramjet combustor, and facilitates to obtain reliable simulation results for scramjet with turbulent round jets by employing the modified SST k-ω turbulence model.

Experimental study on the load control strategy of ammonia-hydrogen dual-fuel internal combustion engine for hybrid power system

The development of hybrid technology and the use of zero-carbon fuels are effective means to improve energy efficiency and reduce carbon emissions. In this study, the Miller cycle spark ignition engine was used, and four load control strategies of throttle, ammonia-hydrogen ratio, air–fuel ratio, and variable valve timing were used to conduct experimental research on the ammonia-hydrogen engine. The experimental results show that the throttle strategy provides the widest output range, and the BMEP is raised from a minimum of 1 bar to 7.4 bar. The ammonia-hydrogen mixing ratio strategy is suitable for cold start conditions, fully utilizing the good combustion characteristics of hydrogen, and at the same time reducing the amount of hydrogen used in the heat engine process as much as possible. The air–fuel ratio strategy provides the highest thermal efficiency with a maximum BTE approaching 40%. It satisfies the energy-saving demand under the single-point working condition of the engine in the series mode. The variable valve timing strategy can realize switching between BMEP between 3.5 bar and 6.5 bar, and keep BTE above 33%. The valve timing strategy can not only provide a wider load adjustment range but also maintain high thermal efficiency. Finally, combining the effects of the four control strategies and the typical operating modes of the hybrid system, this study suggests that the throttle strategy is suitable for the feed mode, the VVT strategy is suitable for the parallel mode, the ammonia-hydrogen mixture ratio strategy is suitable for the cold start condition, and the air–fuel ratio control is suitable for the series mode.

The characteristics of waste-cooking palm biodiesel-fueled CRDI diesel engines: Effect hydrogen enrichment and nanoparticle addition

The influence of iron nanoparticle (INP) addition (75 ppm) and hydrogen enrichment (10 lpm) with waste cooking palm biodiesel blend (WCB) on a CRDI diesel engine is evaluated. A blend of 20% WCB and 80% diesel is used, and the dosing level of INP has been kept at 75 ppm, which has been decided based on the oxygen content of biodiesel. Results indicate that the combination of H2 enrichment and INP addition improves the BTE and BSFC of biodiesel blends as that of diesel. A maximum improvement of BTE of 7.1% than that of diesel is obtained at 90% loading. The combined impact of better hydrogen combustion characteristics and improved air-fuel mixing with nanoparticles reduces CO and HC emissions by 37.5% and 41.8%, respectively, for the WCB fuel sample. However, NOX emission shows an elevation of 27.4% compared to diesel. Combustion parameters, namely ICP (80.1 bar) and HRR (89.5 J/˚CA) indicate an improvement of 5.3% and 6.7% compared to diesel for WCB + INP + H2. This is owing to the combination of hydrogen's rapid flame speed and INP-added biodiesel's increased thermal conductivity.

Study on fermentation gas combustion with hydrogen addition under various throttle openings

Regional energy systems are designed to contribute to a green and “carbon neutral” economy of localities. In this system, the engine combustion is significant for power generation. Therefore, this study mainly investigated the effect of throttle openings on the combustion characteristics of hydrogen (H2) and methane (CH4) mixtures to achieve high efficiency. Throttle opening has a strong relationship with combustion performance, particularly for power output and efficiency. Therefore, 10%, 20%, 40%, and 100% throttle openings were tested to obtain higher efficiency for power generation. Combustion characteristics of CH4 and CH4+H2 were also compared. With H2 addition, the volume percentage of H2 varied between 10%, 30%, and 50%. The ratio of air to gas fuel was controlled to determine λ varying from 1.0 to 1.4. Subsequently, the effect of throttle openings under different λ with H2 addition was examined. Finally, brake thermal efficiency (BTE), power output, brake mean effective pressure (BMEP) and brake specific fuel consumption (BSFC) were compared. Moreover, the maximum value of the cylinder pressure in cycles (Pmax) and the coefficient of variation (COV) in Pmax were discussed. The results showed that the torque and power output decreased slightly from full throttle opening to 40% throttle opening. The 30% H2 addition in Case # 5 (full opening under lean-burn conditions) is the best working condition to satisfy both the power generation and energy-saving requirements in this study. Furthermore, Pmax decreased with smaller throttle opening, and H2 addition increased Pmax. In addition, H2 addition decreased COV, but throttle opening had less effect on COV.

Flame acceleration process of premixed hydrogen in confined space with different obstacle shapes

Obstacles can greatly stimulate the detonation power of gas, and have great research value in industrial safety and energy utilization. Thus, in this work, the combustion characteristics of hydrogen under different shape obstacles are investigated. The important parameters for the evaluation of the gas combustion characteristics are analyzed, including the flame structure, flame propagation velocity and explosion overpressure. The results show that it is difficult for the flame front to recover to the regular front after passing through grid-type obstacles with many voids. To further understand the influence of the obstacle shape on the flame structure evolution, the change in the flow field before and after the flame approaches the obstacle is analyzed. The results show that the flow field formed by the combustion products can be seriously unstable after the flame front passes through grid-type obstacles, resulting in a severe distortion of the flame front. Moreover, the influence of the obstacle shape on the flame propagation velocity under different blockage ratios is compared. The results show that the single-type obstacle has a better effect on the acceleration of the flame, and it is more sensitive to the increase in the blockage ratio. The grid-type obstacle can improve the sensitivity to the blocking rate by reducing the void area. Furthermore, by analyzing the pressure growth rate, it is found that the influence of the obstacle shape on the explosion overpressure under different blockage ratios is the same as that of the flame propagation velocity. This work can provide some guidance for industrial explosion prevention and control and energy power system design.

Future HD Diesel and Hydrogen-Fueled Concepts: Emissions Challenges and System Solutions

<div class="section abstract"><div class="htmlview paragraph">Future heavy-duty (HD) concepts should fulfill very tight tail-pipe NO<sub>x</sub> emissions and simultaneously fulfill the fuel efficiency targets. In current HD Euro VII discussions, real working cycles become key to ensure emission conformity. For instance, cold start and cold ambient conditions during testing with low load profiles starting from 0% payload, require external heating measures. Knowing the trade-off between fuel consumption and tail-pipe NO<sub>x</sub> emissions a holistic engine and EAT system optimization with innovative thermal management is required.</div><div class="htmlview paragraph">Towards a carbon neutral mobility, Hydrogen combustion engines are one of the key solutions. Advanced combustion system development enables maximal usage of lean burning as the major advantage of the Hydrogen fuel for efficiency improvement and NO<sub>x</sub> reduction. Considering future NO<sub>x</sub> regulations in real-world working cycles, an efficient exhaust after-treatment (EAT) system which is well optimized to the air-path and the Hydrogen combustion characteristics is required.</div><div class="htmlview paragraph">In this paper, a systematic investigation is carried out by means of the model-based holistic approach. The target is an optimization of both engine and EAT layout, including advanced thermal management for future HD concepts. Both Diesel and Hydrogen HD concepts are investigated.</div><div class="htmlview paragraph">The model-based emission conception is based on both the conventional WHTC cycle as well as on selected real field cycles. Close-coupled dual-stage SCR with twin dosing is considered with multiple system layouts in order to minimize the Diesel engine fuel penalty during the heat up phases. A comparison between the emission behavior and after-treatment systems of Diesel and Hydrogen engines is performed.</div><div class="htmlview paragraph">In addition, external EAT heating such as electric heated catalyst is investigated for Diesel applications and the potentials regarding CO<sub>2</sub> emissions compared to conventional engine-based thermal management measures are shown.</div><div class="htmlview paragraph">The optimized system layout for both Diesel and Hydrogen HD concepts are compared, especially for the EAT technology and layout. As outlook, future potentials and optimization possibilities for combustion systems and future emission reduction technologies are described.</div></div>

Effects of Compression Ratios on Combustion and Emission Characteristics of SI Engine Fueled with Hydrogen-Enriched Biogas Mixture

The effects of hydrogen-enriched biogas on combustion and emissions of a dual-fuel spark-ignition engine with different hydrogen concentration ratios were studied numerically. A 1-cylinder spark ignition was used to perform a numerical simulation. To reveal the influence of the compression ratios on combustion and emissions of a gaseous engine, the crankshaft of the engine was modified to generate different compression ratios of 8.5, 9.0, 9.4, 10.0, and 10.4. The biogas contained 60 and 40% methane (CH4) and carbon dioxide (CO2), respectively, while the hydrogen fractions used to enrich biogas were 10, 20, and 30% of the mixture by volume. The ignition timing is fixed at 350 CA°. The results indicate that the in-cylinder pressure, combustion temperature, and combustion burning speed increase gradually with increasing hydrogen concentration due to the combustion characteristics of hydrogen in blends. As increasing the compression ratio, NOx emissions increase proportionally, while CO2 emissions decrease gradually. Almost no combustion process occurs as operating the compression ratio below 8.5 when using pure biogas. However, adding 20% of hydrogen fraction could improve the combustion process significantly even at a low compression ratio.

Analysis of the combustion characteristics of hydrogen and hydrocarbon fuels based on the results of numerical simulation

At present, an upward trend in the field of studying the processes of hydrogen combustion in the combustion chambers of the ground-based gas turbine power plants is obvious. The use of pure hydrogen as a fuel gas would solve the problem of environmental decarbonization. One of the emerging problems is to ensure the stable combustion of such fuels in combustion chambers of various applications. The information-analytical review of studies showed that there is a large number of theoretical and experimental results on the diffusion and homogeneous combustion of hydrogen and hydrogen-containing fuels in various burners and combustion chambers, which are not part of the existing gas turbine power plants. The purpose of this work is a comparative analysis of the gas-dynamic and emission characteristics of the combustion of the hydrogen-air and methane-air components based on the results of numerical simulation of a convertible combustion chamber of a 75 kW microgas turbine power plant. This goal is achieved by numerical simulation of the diffusion combustion of hydrogen and methane with air in a convertible combustion chamber. The most significant result of the work is obtaining the isosurface of the flame, which made it possible to obtain the conditions for stable combustion in the form of the Damköhler criterion and the ratio of the midsection velocity to the velocity of turbulent combustion. The significance of the results obtained lies in the further development of the methodology for the conversion of megawatt-class gas turbine plants to hydrogen and hydrogencontaining fuels.

Effect of different volume fractions of ammonia on the combustion and emission characteristics of the hydrogen-fueled engine

Hydrogen internal combustion engines (ICE) will play an important role in reducing carbon emissions, but low power density and abnormal combustion problems are the main obstacles restricting the promotion of hydrogen ICE. Ammonia is a low-reactivity renewable fuel. The purpose of this study is to study the effect of different ammonia-added volume fractions on hydrogen ICE. In this experimental study, the combustion and emission characteristics of an engine fueled by a hydrogen/ammonia mixture were evaluated at part-load operating conditions. The experiment was carried out on a modified engine, the engine speed was 1300 rpm, the absolute pressure of the manifold was 61 kPa, and the volume fraction of ammonia added was 5.2%, 7.96%, and 10.68%, respectively. The test results show that the addition of ammonia changes the combustion characteristics of hydrogen. As the volume fraction of ammonia added increases, the flame development period and flame propagation period are both prolonged, and the peak heat release rate decreases. The addition of ammonia increases the power of the engine and reduces the indicated thermal efficiency. At the ignition timing of the maximum braking torque, as the volume fraction of ammonia added increases, the indicated mean effective pressure and indicated thermal efficiency increase. Adding ammonia volume fraction has little effect on Nitrogen oxides (NOx) emissions, and NOx emissions gradually increase with the delay of ignition timing.

A novel approach for experimental study and numerical modeling of combustion characteristics of a hydrogen fuelled spark ignition engine

This work is aimed for experimental study on effects of spark timings, compression ratios (4.5:1 to 7.2:1) and exhaust gas recirculation (EGR) on combustion characteristics of hydrogen fuelled spark ignition (SI) engine operating at 3000 rpm. The results indicate spark timing should be reduced for hydrogen fuelled engine as compared to the gasoline fuelled engine. Both ignition lag and DOC decreased with an increase in equivalence ratio and compression ratio, while the opposite trend was observed with an increase in EGR. The EGR was limited up to 23.5 % by volume, as higher EGR lead to cyclic variation. The standard Wiebe function constants were modified for the calculation of mass fraction burnt in a hydrogen fuelled SI engine. In addition to this, the empirical correlations were developed for ignition lag (IL) and duration of combustion (DOC) as a function of compression ratio, equivalence ratio, and EGR. The notable findings that emerged from this study are that hydrogen-fuelled SI engines should be operated at varying spark timing with respect to varying equivalence ratios (0.5 to 0.8) and EGR up to 23.5% by volume.

Numerical investigation of the combustion characteristics of hydrogen and oxygen in a microthruster

Recent efforts have developed hydrogen and oxygen propellant-based (HOP) microthruster systems. In this study, a novel microthruster was investigated numerically. The combustion and mixing characteristics of hydrogen and oxygen were investigated numerically according to a variety of inlet flow rates and equivalence ratios in a 3-dimensional microthruster. Results demonstrated that when the equivalence ratio equalled 2, the temperature was the highest. When the equivalence ratio equalled 4, the specific impulse peaked. With identical equivalence ratios, as the flow rate increases, the temperature and specific impulse increased, and as the flow rates increases up to a certain value, the specific impulse changed more slowly. Computational results showed that the proposed microthruster could achieve steady combustion at various mass flow rates in all tested conditions. This study describes the relationship of the mixing performance and combustion characteristics with the equivalence ratio and flow rate, which provides a guideline for the design of novel microthrusters.

Combustion characteristics of hydrogen in a noble gas compression ignition engine

Hydrogen eliminates carbon emissions from compression ignition (CI) engines, while noble gases eliminate nitrogen oxide (NOx) emissions by replacing nitrogen. Noble gases can increase the in-cylinder temperature during the compression stroke due to their high specific heat ratio. This paper aims to find the optimum parameters for hydrogen combustion in an argon–oxygen atmosphere and to study hydrogen combustion in all noble gases, providing hydrogen combustion data with suitable engine parameters to predict hydrogen ignitability under different conditions. Simulations are performed with Converge CFD software based on the Yanmar NF19SK direct injection CI (DICI) engine parameters. The results are validated with the experimental results of hydrogen combustion in an argon–oxygen atmosphere with a rapid compression expansion machine (RCEM), and modifications of the hydrogen injection timing and initial temperature are proposed. Hydrogen ignition in an argon atmosphere is dependent on a minimum initial temperature of 340 K, but the combustion is slightly unstable. Helium and neon are found to be suitable for hydrogen combustion in low compression ratio (CR) engines. However, krypton and xenon require temperature modification and a high CR for stable ignition. Detailed parameter recommendations are needed to improve hydrogen ignitability in conventional diesel engines with the least engine modification.

Euro VII and Beyond with Hydrogen Combustion for Commercial Vehicle Applications: From Concept to Series Development

<div class="section abstract"><div class="htmlview paragraph">One challenge for the development of commercial vehicles is the reduction of CO<sub>2</sub> greenhouse, where hydrogen can help to reduce the fleet CO<sub>2</sub>. For instance, in Europe a drop in fleet consumption of 15% and 30% is set as target by the regulation until 2025 and 2030. Another challenge is EURO VII in EU or even already approved CARB HD Low NO<sub>x</sub> Regulation in USA, not only for Diesel but also for hydrogen combustion engines.</div><div class="htmlview paragraph">In this study, first the requirements for the combustion and after-treatment system of a hydrogen engine are defined based on future emission regulations. The major advantages regarded to hydrogen combustion are due to the wide range of flammability and very high flame speed numbers compared to other fossil based fuels. Thus, it can be well used for lean burn combustion with much better fuel efficiency and very low NO<sub>x</sub> emissions with an ultra lean combustion.</div><div class="htmlview paragraph">A comprehensive experimental investigation is performed on a HD 2 L single-cylinder engine. The hydrogen combustion characteristics are studied with variation of multiple operating parameters like EGR, air-fuel ratio, etc. A predictive hydrogen combustion and NO<sub>x</sub> model is then developed and validated using the test results.</div><div class="htmlview paragraph">As baseline for the numerical investigations of engine transient behavior in the cold cycle, an in-line six cylinder 12L HD diesel engine is developed. Cold WHTC and FTP cycles are simulated and the combustion, exhaust gas temperature and emission behavior are evaluated.</div><div class="htmlview paragraph">The effects of lean-burning combustion and exhaust after-treatment for engine NO<sub>x</sub> reduction as well as thermal management in transient cycles on exhaust after-treatment (EAT) system to fulfil future regulations are discussed. Multiple EAT architectures are investigated and the trade-off between fuel consumption and the end-of-pipe NO<sub>x</sub> is assessed. Challenges and potentials of hydrogen combustion for heavy-duty applications considering future regulations are addressed. Variation of engine operating parameters and the potentials of engine calibration for series development is demonstrated using predictive engine and EAT models.</div></div>

Recombination of hydrogen-air catalyzed by Pt/C catalyst in a confined vessel at ambient temperature

Most of the previous studies have investigated the detonation and combustion characteristics of hydrogen and air mixtures, but the change process of hydrogen-air recombination reaction at ambient temperature is unclear. In this study, the variation of temperature and hydrogen conversion during the H2-air mixture reaction catalyzed by Pt/C catalyst was examined. Experiments were carried out in a small-scale cylindrical vessel to investigate the effects of hydrogen volume fraction, catalyst mass, and inlet gas flow rate on the reaction process. The results revealed that the reaction temperature climbed as the hydrogen volume fraction increased, with a peak value achieved when the hydrogen volume fraction reached 35 vol%, and subsequently reduced as the hydrogen volume fraction increased further. The increase of the inlet gas flow rate also promoted the growth of reaction temperature. However, there was no significant linear relationship between the rise of catalyst mass and the change of reaction temperature.

The influence of the hydrogen injection timing on the internal combustion engine working cycle

From an ecological aspect, the hydrogen has all properties to be a very good fuel for internal combustion engines. However the high combustion speed, as well as the possibility of backfire, is inconvenient properties of port injection. In this paper, the influence of the injection timing on the internal combustion engine working cycle parameters (pressure and temperature) was investigated deeply. The investigation, of the injection timing influence on the internal combustion engine working cycle parameters, was performed numerically by application of ANSYS software. It was observed the geometry of the real engine with added pre chamber, in order of layer mixture formation and pressure damping, because of high combustion speed. The results are presented for four cases with different injection timing and the same spark timing. By earlier injection, the time for mixing rise as well as the possibility of homogenization and uniform mixture creation, in pre chamber and cylinder. This claim it is confirmed on the basis of obtaining pressure and pressure rise gradient, which are growing with earlier injection, because of hydrogen combustion characteristics in stoichiometric mixture. The higher pressures as well as the surface under the diagram are positive from the aspect of the engine efficiency. However, with the earlier injection, the values of the pressure rise gradient are higher than for the classic Diesel engine. This means that this phenomena can cause brutal engine work from the aspect of mechanical stresses. However the value of the maximum pressure is smaller than this in a Diesel engine, this is due to added pre chamber, which has decreased the compression ratio.

Numerical study of combustion characteristics and oscillating behaviors of hydrogen–air combustion in converging–diverging microtubes

Combustion characteristics of hydrogen–air premixed mixture are investigated in 1 mm converging–diverging microtubes with heated wall. Numerical study performed by considering three-dimensional and transient forms of the governing equations and detailed chemical kinetics. Effects of inlet velocity, equivalence ratio and converging–diverging angle are studied on the combustion characteristics. It was founded that the flame position has decisive role in the temperature, and inlet velocity has more impact on the flame position, flame extension and luminous zone at a constant converging–diverging angle. According to the results, when the flame places in the converging–diverging section, increase of the convection heat transfer rate due to the flow velocity increasement and increase of the heat loss due to the surface to volume ratio increasement cause to decrease the flame temperature. It leads to form oscillating flames at some conditions. On the other hand, converging–diverging section plays positive role in flame stability. For cases that the flame places in the outlet section, it removes flame instability near the wall and increases upper flammability limit by increasing heat recirculation in the converging–diverging section. The results showed that converging–diverging microtubes have higher upper flammability limit than the constant cross-sectional one. Also, upper flammability limit increases with the converging–diverging angle.