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This article explores the physics of hydrogen flames and detonation, focusing on its potential as a carbon-free fuel for energy storage and propulsion systems. The collection covers a wide range of topics, including improving hydrogen-air mixing in gas turbines, reducing noise in gas turbine combustors, and examining the dynamics of lean hydrogen-air flames. It also delves into the fundamental dynamics of hydrogen combustion under different conditions and configurations, as well as the deflagration-to-detonation transition and the dynamics of hydrogen detonation. The research presented in this collection has important implications for advancing propulsion systems, enhancing hydrogen safety protocols, and optimizing the efficiency and safety of fuel cells, gas turbines, and internal combustion engines. [Extracted from the article]

Initiation of an oblique detonation wave (ODW) results in a near-field region of curved detonation wave surface, which cannot be predicted by the detonation polar theory and lacks in-depth work on its features. In this study, Euler equations coupled with a two-step kinetic model are used to simulate ODW dynamics, and then the flow structures are analyzed by examining the asymptotic decay of local surface angle. The flow/reaction coupling is quantified by a non-dimensional Damköhler number Das that is defined as the ratio of the characteristic flow and chemical reaction length along streamlines. The results demonstrate a spectrum of local strong solutions forms, which always happens when the ODW transition pattern is abrupt, but the strong solutions deviate from the isolated strong solution predicted by the polar theory. Furthermore, the abrupt transition leads to a strong peak of Das, while the smooth one leads to a bump. Various simulated cases indicate that the transition pattern becomes abrupt and the local strong solution arises when the maximum Das reaches a critical range. [ABSTRACT FROM AUTHOR]

Modes of detonation wave propagation in annular channels were investigated numerically by using the adaptive mesh refinement technique. Two-dimensional, reactive Euler equations with a detailed hydrogen/oxygen reaction model were adopted in the computations to simulate the detonation dynamics in the annular geometry. Considering both the decoupling of the detonation wave front and the development of the Mach-stem in reflection, the propagation is divided into unstable and stable propagation modes with different Mach-stem evolutions, namely, a growing, steady, or decaying type. The numerical observations indicate that in the unstable propagation mode, velocity loss and oscillation occur near the inner wall, while the wave front shape and velocity evolution are steadier for the stable propagation mode. The overdriven degree near the outer wall increases as the Mach-stem strength attenuates. The propagation mode diagrams demonstrate that an increase in the initial pressure and wall curvature radius can extend the range of the stable propagation mode, and the Mach-stem is transformed from a growing to steady, and finally a decaying type with the increase in the initial pressure or the decrease in the wall curvature radius to channel width ratio. The limit of wall curvature radius separating the unstable and stable modes is independent of the channel width for the Mach-stem steady and decaying types, while they are positively correlated for the Mach-stem growing type. Finally, a qualitative procedure is proposed to help distinguish different propagation modes based on the formation mechanism of each propagation dynamics. [ABSTRACT FROM AUTHOR]

Two-dimensional, wedge-induced oblique detonation waves (ODWs) subject to periodic inflow are simulated using the reactive Euler equations with a two-step induction–reaction kinetic model. The focus of this work is how the periodic unsteadiness of a sinusoidal density disturbance with varying frequency and amplitude influences an initially established ODW structure. Three fundamental ODW structures with different transition types and inflow Mach numbers are disturbed, resulting in two types of triple-point formations: the main triple point (MTP) and the train of triple points (TTP). The TTP features multi-triple points arising almost simultaneously and traveling together, which has never been observed before. A parametric study and frequency analysis reveal that the MTP derives from forced destabilization, while the TTP derives from the combined effect of surface instability and inflow disturbance. Furthermore, a new phenomenon of MTP degeneration is observed for a proper inflow Mach number and disturbance amplitude. Finally, the oscillation amplitudes of unsteady ODWs are analyzed with respect to the Mach number and inflow disturbance, demonstrating the effects of transition type on surface unsteadiness. [ABSTRACT FROM AUTHOR]

Edge-to-edge repair is a procedure introduced to overcome mitral valve regurgitation. However, it leads to an unusual flow in the left ventricle characterized by twin parallel pulsed jets. This type of flow has not been extensively investigated in the literature. We set up a basic experiment to better characterize this type of flow from a fundamental point of view. Planar time-resolved particle image velocimetry measurements were performed downstream of three configurations of mitral valves corresponding to healthy and repaired valves. The flow field is characterized using velocity profiles, viscous energy dissipation, and time-frequency spectra, and their potential clinical impact is highlighted. [ABSTRACT FROM AUTHOR]

We investigate experimentally the transitions between systems of two and three satellite vortices found within a shallow layer of water above a rotating disk at the bottom of an open stationary cylindrical tank. We found that these systems of two and three satellite vortices are associated with two fundamental multipolar modes, namely, the quadrupole and hexapole, respectively. The forward and backward transitions between the two systems of satellite vortices, which occur at critical disk speeds, involve growth and decay of the fundamental modes as well as the excitation of their common harmonic (the dodecapole) and symmetric radial oscillations. [ABSTRACT FROM AUTHOR]

A numerical study was performed to investigate the detailed effects of activation energy Ea on the oblique detonation wave surface instability. Numerical simulations were performed using an ideal reactive flow model given by the inviscid Euler equations with one-step irreversible Arrhenius reaction kinetics. The numerical results demonstrate two types of unstable structures following the initial smooth surface after detonation initiation. One exhibits by a "saw-tooth" reactive front and the other exhibits by a "keystone" feature. To quantify the destabilization processes, two characteristic length scales, L1 and L2, are defined statistically to be the length of the smooth detonation surface before the appearance of instabilities and the length of the unstable surface before the first cellular structure with the onset of right-running transverse waves, respectively. Their dependence on Ea was simulated and analyzed. In general, both lengths decrease with increasing Ea, making the surface more unstable. However, with increasing Ea, the high temperature sensitivity of the mixture causes an abrupt explosion in the initiation region, introducing a high overdriven surface and suppressing the instability. With the balance between the destabilizing effect of Ea and the stabilizing effect of increasing overdrive factor, both L1 and L2 are found to approach a near-constant value in the high Ea limit. [ABSTRACT FROM AUTHOR]