To investigate whether the excited CH* and OH* profiles are suitable for reaction mechanism validation, an electron-multiplying charge coupled device (EMCCD) was employed to capture the highly time-resolved spectral structure of free radicals during ignition events in a shock tube. Combined with traditional ignition delay measurements obtained using a photomultiplier tube (PMT) and specific optical filters, the results revealed that the CH* and OH* emission profiles in the post ignition stage comprised not only chemiluminescence from these excited radical emissions, but also continuum emission from strong radiators such as CO2 and H2O. Therefore, the entire post ignition CH* and OH* emission profiles are not suitable for mechanism validation, but the segments before the emission peak intensity can be used for this purpose. In addition, throughout the ignition event, the sustained emission duration of CH* was significantly shorter than that of OH*. The CH* emission detected by EMCCD lasted only about 100 µs while that of OH* persisted for approximately 20 ms within a single pulse period. This difference deserves consideration in mechanism analysis and validation studies. Meanwhile, mechanism validation is suggested to cover the time span from pre to post ignition stage to enhance the prediction performance.
It remains unclear whether roughness-induced differences across statistical orders merely reflect a redistribution of local statistical weight or instead indicate a reorganization of the underlying multiscale hierarchy of turbulence. Here, particle image velocimetry measurements of smooth- and rough-wall turbulent boundary layers at low-to-moderate Reynolds numbers are used to compare velocity structure functions across different flow conditions. Extended self-similarity, multifractal analysis, and characterization of the large-scale spatial organization are further employed to examine roughness effects across three complementary levels: scale, statistical order, and spatial structure. The results show that the lower-order structure functions exhibit strong similarity between the two wall conditions over a broad range of scales, whereas the smooth-rough differences become progressively more pronounced with increasing statistical order and emerge preferentially over scales that are more sensitive to intense velocity-increment events. Nevertheless, the relative-scaling relationships of the higher-order structure functions remain in close agreement between the two wall conditions, and the multifractal characteristics indicate similar cascade hierarchies. This suggests that roughness-induced modifications of the absolute velocity-increment statistics do not necessarily imply a corresponding reorganization of the underlying scale-cascade dynamics. Further analysis reveals that wall-attached large-scale motions over smooth and rough walls retain similar geometric scaling and spatial-coherence characteristics. In conclusion, these results indicate that roughness primarily modifies local statistics through the selective modulation of higher-order, strongly intermittent events over specific scales, while the relative cascade organization and large-scale spatial structure indicate substantially greater robustness to wall condition.
Although bubble growth is often investigated under quasi-static conditions, the surrounding liquid is frequently in motion in practical systems. Under such flow conditions, the hydrogen bubble growth exhibits a different trend. This study investigates the influence of externally imposed flow on the temporal evolution of hydrogen bubbles using a fine platinum wire electrode. Controlled liquid flow was introduced using a peristaltic pump, and the local flow field near the electrode was verified via particle image velocimetry. Bubble growth was recorded using a high-speed video camera. Under quasi-static conditions, the detachment radius progressively decreased over successive cycles from 131 to 52 μm, accompanied by a reduction in the fitted time coefficient (x) from 0.51 to 0.29. By contrast, under imposed flow, the detachment radius increased from 101 to 166 μm with increasing cycle number. This was accompanied by an increase in the value of x from 0.38 to 0.42 and a decrease in the growth coefficient (β). These results indicate that imposed flow can alter the temporal evolution of successive bubble-growth cycles, which does not fully align with the commonly reported flow-induced bubble size suppression. The findings of the study highlight the importance of near-wall transport effects and provide insights into the design of electrochemical systems involving gas-evolving interfaces.
Sprays of water, ethanol, and acetone of micrometre-scale droplets are investigated in a transonic flow in a flat converging–diverging nozzle with a shock wave structure downstream. High-magnification two-frame shadowgraphy with a short interframe delay is used to investigate the distributions of droplet sizes, velocities, and deformations. The investigation is focused on droplets with diameters less than 100μm and Weber numbers less than 100. Visualisation of the droplet shape evolution demonstrated that droplet break-up follows the bag and bag-and-stamen modes, which is consistent with other observations of this process for this range of Weber numbers. It is shown that droplet flattening before the shock wave in the direction of their movement increases when the Weber number increases if this number is less than the critical Weber number (Wecr=12). Droplet elongation is observed after the shock wave. In the accelerating pre-shock region, droplet velocity is only weakly dependent on droplet size for sufficiently large droplets.
Self-excited thermoacoustic instability in a methane–oxygen inverse diffusion flame stabilised over a shear-coaxial injector is investigated experimentally at three thermal power levels. Acoustic measurements, high-speed OH* and CH* chemiluminescence and schlieren imaging, and modal decomposition are applied as the oxygen flow rate is increased at fixed fuel flow rate. The flame passes from stable combustion through intermittency to limit-cycle oscillation, with onset at a consistent equivalence ratio and velocity ratio across the power levels. The instability occurs at the first longitudinal chamber mode, confirmed by eigenfrequency prediction and a shortened-chamber experiment. The heat release drives the oscillation, with a global Rayleigh index of ＋0.99, and is confined to the inner oxygen-methane shear layer, where it condenses into a single dominant coherent mode at the transition. Near the injector exit, vortex formation alternates between an active and a quiescent half-period once per acoustic cycle while the flame root remains anchored; this organisation is absent in the shortened chamber at the same injector conditions. These observations indicate that the instability is sustained by periodic vortex formation that depends on the chamber acoustics. Whether the acoustic oscillation triggers roll-up by modulating the methane injection velocity or by acting directly on the shear layer remains open.
