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The structure and thermoacoustic instability (TI) of premixed CH 4 /H 2 /air swirling flames were experimentally investigated for a range of hydrogen fraction (η H2) up to 80% under different equivalence ratio (Φ) and swirl number (S) conditions. It is shown that the onset of TI is enhanced when increasing either η H2 , S , or Φ. The dominant frequency of TI increases dramatically with η H2. The higher dominant frequency in the hydrogen-enriched flames can be attributed to a shorter flame length which results in a reduced flame convection time. It is observed that the unstable flames are always accompanied by the appearance of outer recirculation zone (ORZ) flame. Therefore, the flame kernel residing in the ORZ can be an indicator of the occurrence of TI. The flame front of thermoacoustic unstable flames was observed to be more wrinkled, e.g., with larger mean absolute curvature (κ abs) and local flame surface area ratio (δ Σ max). Importantly, the phase-locked analysis shows that κ abs and δ Σ max can be modified at different oscillation phases, and their maximum and minimum values are simultaneously achieved at phase angles θ of about 0° and 180°, respectively. Variations of κ abs and δ Σ max are in phase with the heat release rate, indicating a strong correlation between the TI and flame structure modification; however, they show a phase lag of about 72° behind the pressure in this work. These results are vital when understanding and predicting the TI based on the flame structure, especially when adopting a visual detection method of the instability. [Display omitted] • Thermoacoustic instability of swirling lean premixed CH 4 /H 2 /air flames is studied. • The dominant frequency increases significantly with hydrogen fraction. • Appearance of the ORZ flame is highly related to the instability. • The flame front is more wrinkled in unstable flames. • Flame curvature and surface area oscillate in phase with heat release rate. [ABSTRACT FROM AUTHOR]

This paper investigated methane/air flame characteristics with hydrogen addition in micro confined combustion space experimentally and computationally. The focus is on the effect of hydrogen addition on the methane/air flame stabilization, the onset of flame with repetitive extinction and ignition (FREI), and the global flame quenching in decreasing continuously combustion space. Furthermore, the effects of hydrogen addition on the flame temperature and the local equivalence ratio distribution were analyzed systematically using numerical simulations. In addition, the effects of hydrogen addition on the concentrations of OH and H radicals, and the critical scalar dissipation rate of local flame extinction were discussed. With a higher hydrogen ratio, the mixing is faster, and the flame is smaller. When the micro confined space is narrower, the heat loss to the combustor walls has a higher impact on the flames. The flames with higher hydrogen ratios have therefore lower peak flame temperatures and lower concentrations of H and OH radicals. The results show that hydrogen addition can effectively widen the stable combustion range of methane/air flames in the micro confined space by about 20% when the hydrogen addition ratio reaches 50%. The frequency and the maximum propagation velocity of FREI flames can be increased as well. The quenching distance of methane/hydrogen/air flames decreases nearly linearly with the increase of hydrogen ratio. This is attributed to the higher critical scalar dissipation rate of local flame extinction in flames with a higher hydrogen ratio. • Flame characteristics of CH 4 /Air with H 2 addition in the micro space were studied. • Adding H 2 can broaden the flame stabilization zone and reduce the quenching distance. • H 2 enhanced flame stabilization is due to the increased SDR of local flame extinction. • Hydrogen addition leads to a faster mixing of the fuel/air mixture. [ABSTRACT FROM AUTHOR]

Hydrogen fuel cell electric vehicle (FCEV) can reduce air pollution as well as achieve efficient use of hydrogen energy. Farther travel distance requires larger hydrogen storage pressure, thereby imposing more demanding working conditions on the pressure reducing system. In this paper, a multi-stage high pressure reducing valve (MSHPRV) for hydrogen decompression in FCEV is proposed, and the effects of different structural parameters on its internal flow characteristics are investigated to achieve a better hydrogen decompression process. Results show that compared with perforated plate, multi-stage perforated sleeves and valve core hold the dominant position in hydrogen throttling process. Larger multi-stage perforated sleeve diameter, perforated plate diameter and pressure ratio relate to larger hydrogen kinetic energy, turbulence vortex and energy consumption. However, with the increase of perforated plate stage and perforated plate radius, the turbulent intensity and energy consumption inside MSHPRV decreases correspondingly. This study can provide some technical supports for achieving hydrogen decompression in FCEV when facing harsh working conditions, or help with dealing energy conversion during decompression process. • Demanding conditions are imposed on the pressure reducing system of hydrogen fuel cell electric vehicle. • Multi-stage high pressure reducing valve for hydrogen decompression is proposed. • Effects of different structural parameters on the internal flow characteristics are investigated. • A better hydrogen decompression process is achieved with the parametric results. [ABSTRACT FROM AUTHOR]

Abstract A comprehensive coarse grain model (CGM) is applied to simulation of biomass steam gasification in bubbling fluidized bed reactor. The CGM was evaluated by comparing the hydrodynamic behavior and heat transfer prediction with the results predicted using the discrete element method (DEM) and experimental data in a lab-scale fluidized bed furnace. CGM shows good performance and the computational time is significantly shorter than the DEM approach. The CGM is used to study the effects of different operating temperature and steam/biomass (S/B) ratio on the gasification process and product gas composition. The results show that higher temperature enhances the production of CO, and higher S/B ratio improves the production of H 2 , while it suppresses the production of CO. For the main product H 2 , the minimum relative error of CGM in comparison with experiment is 1%, the maximum relative error is less than 4%. For the total gas yield and H 2 gas yield, the maximum relative errors are less than 7%. The predicted concentration of different product gases is in good agreement with experimental data. CGM is shown to provide reliable prediction of the gasification process in fluidized bed furnace with considerably reduced computational time. Highlights • Computational time of Coarse Grain Method significantly shorter than that of Discrete Element Method. • In simulation of biomass gasification, maximal relative error of H 2 predicted by CGM is less than 4%. • Higher gasification temperature in favor of production of CO. • Higher gasification steam/biomass ratio in favor of production of H 2. [ABSTRACT FROM AUTHOR]