To obtain a smaller-scale ammonia-hydrogen hybrid mechanism with better predictive performance suitable for HCCI engine combustion, the stagni mechanism, which is closer to the combustion temperature, pressure and equivalence ratio conditions of HCCI engine, is selected as the original mechanism based on six ammonia-hydrogen detailed mechanisms, and the new detailed mechanism is obtained by adjusting the reaction rate constants related to HCCI combustion. Using three reduced methods, Directed Relation Graph with Error Propagation (DRGEP), Directed Relation Graph with Path Flux Analysis (DRGPFA) and Full Species Sensitivity Analysis (FSSA). Based on the allowable residual of 5%, the detailed mechanism of ammonia-hydrogen combustion is reduced. Based on sensitivity analysis to select key primitive responses, the pre-exponential factors of the reduced mechanism were designed by response surface method, and a series of optimization parameters were obtained. The optimization parameters are used to optimize the pre-exponential factor and verify the optimization mechanism. The results show that under the combination of R45=3.2×1011, R46=2.53862×1019, R103=338; R6=5.12×1022, R28=249 and R79=1.2×1012, the response value of predicting ignition delay time and laminar flame speed is the largest, and the error between the optimization mechanism and the detailed mechanism is the smallest, which proves the accuracy of the optimization parameters. The optimized mechanism has good prediction performance under HCCI conditions and can be used as an ammonia-hydrogen mixing mechanism suitable for HCCI engines.
The natural gas (NG) high pressure direct injection (HPDI) engine has the advantages of low emission and high thermal efficiency, which is a promising engine type at present. In order to ensure the engine work performance and reduce the emission at the same time, this study analyzes in detail the effect of reducing the diesel injection nozzle diameter on combustion and emission with a reduced diesel energy ratio (DER). The results show that with a reduced DER, reducing the diesel injection nozzle diameter extends the time of diesel ignition for NG but reduces the range of diesel ignition. At different DER, reducing the diesel injection nozzle diameter deteriorates the mixing uniformity of NG and air. The mixing and ignition of NG both affect the combustion of NG. As the diesel injection nozzle diameter is reduced, the phases of peak heat release rate (HRR) and peak cylinder pressure advance. The peak of HRR decreases and the maximum cylinder pressure increases. NOx and HC emissions decrease, but soot emission increases. The Indicated thermal efficiency (ITE) of the engine is maintained at 45% regardless of changing the diesel injection nozzle diameter or DER within the range studied in this paper.
To investigate the effects of inert-gas addition on the dynamic behavior and propagation characteristics of spherically expanding hydrogen-air flames, the experiments of premixed combustion were performed in a closed chamber. The dynamic behavior of premixed flames was caught by high-speed Schlieren imaging, and the flame radius and propagation velocity were measured by analyzing the Schlieren photography. When the flame radius was sufficiently small, smooth flame surface was observed, where the flame stretch affected strongly the propagation velocity. From the correlation between the propagation velocity and flame stretch rate, we estimated the propagation velocity of unstretched flame and the Markstein length including thermal-expansion effects. When the flame radius was large, on the other hand, cellular surface induced by intrinsic instability was observed, and then the flame acceleration was confirmed. As the results, the critical flame radius corresponding to the occurrence of flame acceleration and the increment coefficient of propagation velocity were obtained. It was found that the increment coefficient became larger at low equivalence ratios, which was because the diffusive-thermal instability became stronger. Under the conditions of high concentration of inert gas, the increment coefficient of propagation velocity became smaller. This was because the burning velocity became lower by increasing the inert-gas concentration. Moreover, we obtained the increment coefficient normalized by the propagation velocity of unstretched flame. The normalized increment coefficient increased as the inert-gas concentration became higher, which indicated that the addition of inert gas promoted the instability of hydrogen flames. Based on the dynamic behavior and propagation characteristics of premixed flames, the parameters of flame acceleration model depending on the inert-gas concentration were obtained, and then the flame propagation velocity was predicted.
Large-eddy simulation (LES) coupling with a non-adiabatic flamelet progress variable (NA-FPV) approach with reconstructed flamelet chemistry states is employed to simulate the hot coke oven gas (HCOG) reforming process. In the NA-FPV model, the chemistry states are first computed based on the correction factor for enthalpy defects and then modified by substituting the species statistics in the maximum heat loss state with those of less heat release to compensate for the unphysical results. The numerical results of LES coupling this NA-FPV model have been compared with the experimental measurement data in terms of temperature and yields, and reasonable agreements have been achieved. According to the LES results, it is seen O2 only participates in the combustion process in the upper stream and the combustion process which mainly consumes H2 and CO is to provide the other reforming process with heat and steam. In the upper and middle streams, the main HCOG jet is wrapped by the swirling high-temperature combustion products, and the reforming process primarily takes place by consuming CH4, polycyclic aromatic hydrocarbons (PAHs), and steam, while considerable H2, CO, and CO2 are produced. It is observed that accompanying the reforming process C2H2 is generated and it peaks in the middle stream, thus it is considered soot is formed in the complex reactions.