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As fossil fuels currently account for approximately 85 % of the world's energy demands, the rapid growth of the global population and the economy is projected to increase greenhouse gas emissions. Consequently, there is a growing interest in the concept of ultra-lean combustion. This advanced combustion technology has the potential to effectively reduce both fuel consumption and emissions, offering a more environmentally sustainable approach. This comprehensive review delves into hydrogen ultra-lean combustion, specifically exploring the flame ball phenomenon by specifically investigating computational solutions in uniform and non-uniform mixtures. Noteworthy is the attention given to the emergence of stable flame balls within quiescent premixtures, especially in microgravity conditions. The review elucidates how these flame balls achieve stability through thermal radiation loss. A thorough examination of the role of the Lewis number (Le) in flame instability is conducted, considering its integral connection with diffusive-thermal instability across various experimental setups. Beyond the theoretical aspects, this review encompasses the practical domain of stable spherical flames originating from ultra-lean premixed reactants based on hydrogen. The exploration extends to comprehending the intricate mechanisms governing lean flammability limits, particularly the interplay between low-speed counterflow premixed flames and flame balls. It sheds light on how the concept of flame balls contributes to a more profound understanding of emission characteristics in ultra-lean combustion. Lastly, the review provides valuable insights into potential avenues for further research. It outlines the challenges that lie ahead, which encapsulates experimental observations and in-depth investigations into flame balls and the propagation of hydrogen ultra-lean flames. • Focusing on the phenomenon and characteristics of hydrogen in ultra-lean combustions. • Exploration of the stability mechanisms of ultra-lean combustion flame ball. • Elucidating the role of the Lewis number in flame instability and emission characteristics. • Emphasizing the further investigations into hydrogen based ultra-lean flame propagation. [ABSTRACT FROM AUTHOR]

In this study, a three-dimensional numerical model of a hydrogen direct-injection engine was established, and the combustion model was verified by experimental data. The influence of the injection timing and nozzle diameter on ultra-lean combustion was evaluated. The results suggest that, with the delay in the injection timing, the mixture concentration near the spark plug and combustion speed gradually increase. The maximum thermal efficiency increased from 47.44% to 49.87%. The combustion duration and ignition lag are shortened from 19.15°CA to 11.15°CA to 16.13°CA and 5.92°CA, respectively. As the nozzle diameter increased, the injection duration was shortened, and the mixture distribution area became more concentrated. Furthermore, under ultra-lean combustion, the combustion rate is more sensitive to the distribution of the mixture. Appropriately increasing the equivalence ratio near the spark plug can significantly shorten the ignition lag and combustion duration and obtain a higher thermal efficiency. • The laminar flame speed model of hydrogen is calibrated by experimental results. • The G-equation is used to simulation the hydrogen combustion. • The in-cylinder mixture distribution, hydrogen jet and flow field were investigated. • The maximum indicated thermal efficiency reaches 50.05%. [ABSTRACT FROM AUTHOR]

• Liquid EtOH spray with H 2 enrichment combustion were conducted. • Combustion with global equivalence ratio 0.15(67% EtOH/33% H 2) was achieved. • The effects of ignition timing, injection pressure on lean burn were evaluated. • High jet pressure improves ignition stability but causes incomplete combustion. An experimental study was conducted to determine the effects of ignition timings, injection pressures, and hydrogen enrichment on ethanol stratified ultra-lean combustion. The combustion chamber was a constant volume vessel with an injection and ignition system. The ignition and combustion characteristics of ethanol spray with different ignition timings (0–50 ms after start of injection), injection pressures (5, 15, and 25 MPa), and hydrogen premixed equivalence ratios (0.05, 0.1, and 0.2) under ambient temperature 353 K and 1 atm. The direct injection of ethanol and hydrogen enrichment results in an increase in the stable ignition limit to an extremely lean condition: a global equivalence ratio of 0.15 (67% ethanol/33% H 2). Furthermore, the probability of ignition first increased then decreased with the delay in ignition timings; rose with an increase in the injection pressure and hydrogen ratio. However, the delay in ignition timings and high injection pressures resulting in extremely lean zone in the constant volume vessel, which led to incomplete combustion. The addition of hydrogen not only stabilized ignition process but also improved the combustion characteristics. Stable ignition and combustion were achieved at early ignition timing (2–3 ms), moderate injection pressure (15 MPa), and a minimum global equivalence ratio of 0.2 (50% ethanol/ 50% H 2). [ABSTRACT FROM AUTHOR]

Highlights • Ignition characteristics by impinging hot turbulent jet in a dual chamber system are proposed. • The ratio of impinging distance to the nozzle diameter, H / D and the impinging angle, θ are examined. • Using impinging jet ignition, a lower flammability limit of hydrogen/air is achieved. • Numerical simulation of jet impingement process is performed. Abstract The ignition characteristics of a hot turbulent jet impinging on a flat plate surrounded by an ultra-lean premixed H 2 /air was studied both experimentally and numerically. The hot turbulent jet was generated by burning a small quantity of stoichiometric H 2 /air mixture in a separate small volume called the pre-chamber. The higher pressure resulting from pre-chamber combustion pushed the combustion products into the main chamber through a small nozzle (0.75–4.5 mm in diameter) in the form of a hot turbulent jet, which then impinged on a flat plate. Six different plates with varying impinging heights and angles were used. Two important parameters controlling the impinging characteristics of the jet, the ratio of the impinging distance to the nozzle diameter, H / D and the impinging angle, θ were examined. Simultaneous high-speed Schlieren and OH∗ chemiluminescence imaging were applied to visualize the jet penetration/impinging and ignition process inside the main combustion chamber. Results illustrate the existence of two distinct types of ignition mechanisms. If the impinging distance is short and the hot turbulent jet hits the plate with high enough momentum, the temperature increases around the stagnation point and ignition starts from this impinging region. However, if the impinging distance is long, the hot turbulent jet mixes with the ambient unburned H 2 /air in the main chamber and ignites the mixture at the upstream from the plate. For such type of ignition, the impinging plate has a minimum role on main chamber ignition. Employing the stagnation point ignition, a leaner limit of H 2 /air in the main chamber was achieved. Numerical modeling of the turbulent hot jet impingement process was carried out to explain the impinging jet ignition mechanism. It was found that H / D ratio was the controlling parameter between the two ignition mechanisms. The limiting H / D ratio was found to be 21.6, below which ignition occurred via jet impingement. Unlike the H / D ratio, the impinging angle did not affect the ignition mechanism; however, it affected the main chamber burn time. [ABSTRACT FROM AUTHOR]

Abstract: It has been reasoned that the structures of strongly cellular flames in very lean mixtures approach an array of flame balls, each burning as if it were isolated, thereby indicating a connection between the critical conditions required for existence of steady flame balls and those necessary for occurrence of self-sustained premixed combustion. This is the starting assumption of the present study, in which structures of near-limit steady sphericosymmetrical flame balls are investigated with the objective of providing analytic expressions for critical combustion conditions in ultra-lean hydrogen-oxygen mixtures diluted with N2 and water vapor. If attention were restricted to planar premixed flames, then the lean-limit mole fraction of H2 would be found to be roughly ten percent, more than twice the observed flammability limits, thereby emphasizing the relevance of the flame-ball phenomena. Numerical integrations using detailed models for chemistry and radiation show that a one-step chemical-kinetic reduced mechanism based on steady-state assumptions for all chemical intermediates, together with a simple, optically thin approximation for water-vapor radiation, can be used to compute near-limit fuel-lean flame balls with excellent accuracy. The previously developed one-step reaction rate includes a crossover temperature that determines in the first approximation a chemical-kinetic lean limit below which combustion cannot occur, with critical conditions achieved when the diffusion-controlled radiation-free peak temperature, computed with account taken of hydrogen Soret diffusion, is equal to the crossover temperature. First-order corrections are found by activation-energy asymptotics in a solution that involves a near-field radiation-free zone surrounding a spherical flame sheet, together with a far-field radiation-conduction balance for the temperature profile. Different scalings are found depending on whether or not the surrounding atmosphere contains water vapor, leading to different analytic expressions for the critical conditions for flame-ball existence, which give results in very good agreement with those obtained by detailed numerical computations. The one-step chemistry employed in the present work, which involves a non-Arrhenius rate having a cutoff at the crossover temperature, applies with excellent accuracy to the description of lean premixed hydrogen-air combustion, i.e, for at atmospheric pressure, and could be used for instance in the numerical simulation of the propagation of curved or cellular flames in ultra-lean reactive atmospheres, of interest for safety analyses related to the storage, transport, and handling of hydrogen. [Copyright &y& Elsevier]