Hydrogen flame and detonation physics

Hydrogen is attracting significant interest as a carbon-free fuel for energy storage and propulsion systems. Despite its potential, hydrogen's utilization faces issues such as safe storage and transportation due to its high reactivity and flammability. Currently, hydrogen is being increasingly integrated into energy conversion and combustion technologies, including fuel cells, gas turbines, and internal combustion engines, which play an important role in decarbonizing various industries. Hydrogen's unique properties, such as its high diffusivity and low ignition energy, present both challenges and opportunities for advanced combustion technologies. This special collection thoroughly explores hydrogen's complex flame dynamics, detonative combustion, and interactions with turbulent flows and acoustics. It reflects significant advancements in our understanding of hydrogen combustion physics and propulsion technologies, thereby offering a comprehensive view of the current state of this dynamic field.

To enhance the application of hydrogen in gas turbines, Bai et al.¹ focus on improving hydrogen–air mixing in gas turbines to improve combustion performance. They introduced a rib in front of a hydrogen transverse jet, designed to prevent the formation of hot spots in the reactivity of hydrogen within a combustion chamber, which is a critical aspect for efficient and safe combustion. Nagao et al.² focus on reducing the noise in gas turbine combustors. They employed a hybrid LES/APE-RF approach to analyze the impact of a wall on combustion noise of a lean hydrogen/air jet flame. Their study reveals that the characteristics of far-field combustion noise are affected not only by the overall oscillation of the flame heat release rate but also by the anisotropic sound propagation, indicating that the presence of a wall can modulate combustion noise. Kundu et al.³ examine how different wall materials (glass, steel, and aluminum) in micro-combustors affect the stabilization and dynamics of lean hydrogen–air flames. Moreover, Xu and Chen⁴ explore the interactions between hydrogen diffusion flame and Taylor–Green vortex through direct numerical simulation (DNS). They analyzed how the turbulence intensity, Reynolds number, and molecular diffusion models influence the structure and propagation of flames, as well as how the flames, in turn, influence the turbulence. Ganti et al.⁵ study the behaviors of high-hydrogen content syngas in turbulent conditions. Their results show that the energy released by small-scale chemical reactions is transferred to large-scale, and the increase in turbulence intensity caused by chemical reactions is related to flame thickening.

The addition of hydrogen results in significant improvements to the combustion performance of ammonia-based engines. Guan and Zhao⁶ enhanced the ammonia combustion process in an open-ended combustor using a partial premixed fuel injection design: double-ring-shaped inlets. Their study focuses on the minimum hydrogen blend required for sustainable combustion and explores key thermodynamic parameters affecting thermal, combustion, and emission performances. Wang et al.⁷ numerically evaluated the ammonia/hydrogen/air rotating detonation engine (RDE) and concluded that the pressure gain performance is primarily influenced by the injection temperature of the mixture.

Efforts have also been made to explore the fundamental dynamics of hydrogen combustion under different conditions and configurations. For instance, Cai et al.⁸ study cellular transition and self-similar propagation in expanding spherical hydrogen flames at high pressures. They used a constant-pressure and dual-chamber vessel, which is carefully controlled to distinguish the separate and coupled effects of Darrieus–Landau instability and diffusive thermal instability. In addition, they used linear stability theory to understand the relationship between the critical radius, Peclet number, Markstein number, and flame instability observed in experiments. Dou et al.⁹ focus on the end-gas auto-ignition in a hydrogen spark-ignition engine, investigating the impact of temperature gradients on hydrogen auto-ignition in confined spaces, highlighting the significant influence of near-wall temperature gradients on autoignition combustion patterns and wall peak pressures. Moreover, Morii et al.¹⁰ used two-dimensional DNS to analyze the knocking phenomenon in engines, relating it to laminar premixed flame behaviors.

In pursuit of a deeper understanding and control of the deflagration-to-detonation transition (DDT) in hydrogen–air mixtures, a series of studies are available in this special collection, exploring various aspects of the DDT process. Chin et al.¹¹ and Hytovick et al.¹² investigate turbulence-induced DDT in hydrogen–air mixtures. They employed high-speed optical diagnostics to observe and analyze the structural changes of flames at different turbulent combustion speeds. Their research results show that when the turbulent flame speed exceeds the Chapman–Jouguet deflagration speed, the flame, thus, achieves a very-high velocity level and can generate strong shock waves, potentially leading to detonation. Additionally, these studies provide new experimental evidence for hot spot initiation during turbulence-induced DDT. Moreover, Ramachandran et al.¹³ used DNS with detailed chemistry to investigate flame acceleration and DDT in hydrogen combustion in microchannels. Their findings indicate that introducing water vapor as a diluent reduces the flame speed and decelerates the DDT, whereas the adiabatic walls enhance the likelihood of this transition. Wang et al.¹⁴ simulated the DDT in pulse detonation engines and emphasized that the vortex-flame interaction accelerates the flame propagation. Taileb et al.¹⁵ investigate how the collision angle of auto-ignition fronts affects detonation transition. Their findings show that the collision of two autoignition fronts can transform into detonation only when the collision angle is acute.

To explore the dynamics of hydrogen detonation, He et al.¹⁶ explore the longitudinal pulsed detonation (LPD) mechanism, investigating the effects of the injection pressure ratio (PR) on LPD stability and frequency in various combustor sizes. A low injection PR, approximately at 1.3, is conducive to the sustenance of LPD instability. When the frequency of LPD is nearly equal to the natural acoustic resonance frequency of the combustion chamber, LPD is more likely to be sustained. Iwata et al.¹⁷ used DNS to examine how detonation–turbulence interaction in hydrogen/oxygen/argon mixtures is influenced by the mixture's cell regularity and argon dilution rate. Iwata¹⁸ also conducted numerical simulations to examine oblique detonation waves in non-uniform hydrogen/air mixtures, focusing on their velocity and post-shock structures. Pan et al.¹⁹ investigate the response of detonation development in low-carbon fuels to temperature gradients, identifying different detonation regimes in hydrogen and syngas. Rong et al.²⁰ investigate the flow characteristics in a hollow RDE using statistical particle path tracking and numerical simulations, revealing significant insights into exhaust plume structure and detonation wave behavior. They²¹ also investigate the gas-solid two-phase detonation in RDE to optimize engine performance. Sheng et al.²² investigate the effect of detonation wave numbers on the performance and stability of rotating detonation combustors (RDC) using two-dimensional numerical simulations, revealing that an increase in the number of detonation waves improves the stability of the RDC flow field.

The comprehensive research covered in this special topic has significant implications for practical applications, particularly in advancing propulsion systems and enhancing hydrogen safety protocols. The insights gained from improving hydrogen–air mixing in gas turbines, understanding the dynamics of lean hydrogen–air flames, and exploring the fundamentals of hydrogen combustion under various conditions contributes to optimizing the efficiency and safety of fuel cells, gas turbines, and internal combustion engines. Moreover, the studies on deflagration-to-detonation transition and the dynamics of hydrogen detonation provide critical knowledge for designing safer hydrogen storage and transportation systems. These advancements are pivotal for the extensive use of hydrogen as a carbon-free fuel.

In conclusion, this special collection presents a diverse and comprehensive exploration of hydrogen flame and detonation physics with experimental, numerical, and theoretical approaches. Through a series of in-depth studies, it offers valuable insights into flame dynamics, detonation mechanisms, engine optimization, and the interaction of combustion with other flow dynamics, like acoustics, turbulence, and vortex phenomena. These contributions significantly advance our understanding of hydrogen-based systems, marking an important step toward developing next-generation carbon-neutral combustion technologies.