Parallel perturbation analysis of combustion cycle-to-cycle variations and emissions characteristics in a natural gas spark ignition engine with comparison to consecutive cycle method.

Severe combustion cycle-to-cycle variations (CCVs) in spark ignition (SI) engines significantly increase partial or incomplete combustion cycles, which may result in combustion instability or even misfire under extreme conditions, thereby seriously affecting the engine performance and increasing the unburned hydrocarbon and carbon monoxide emissions. In this study, the consecutive cycle method (CCM) and parallel perturbation method (PPM) are utilized to simulate the CCVs in a natural-gas (NG) SI engine. Specifically, 25 consecutive and concurrent cycles of the SI engine are simulated, and simulation results are compared with the experimental data. Further, the factors affecting the CCVs and exhaust emissions in the NG SI engine are verified by analyzing the low-pressure (LP) and high-pressure (HP) cycles. The results indicate that the simulated in-cylinder pressures of the NG SI engine based on PPM are basically in agreement with the experimental in-cylinder pressure distribution range, which suggests that the PPM can effectively predict the CCVs in NG SI engines. Furthermore, the required wall clock time for the simulation of CCVs is greatly reduced from 1 to 2 months (using CCM) to 2-3 days by using the PPM, which makes it particularly suitable for the industrial applications. Besides, the velocity field of the HP cycle is obviously stronger than that of the LP cycle. During the early stage of flame development, the flame area and volume of LP and HP cycles do not show much difference. However, the flame front surface-volume ratio of the HP cycle is larger than that of the LP cycle at 15 CA after the spark timing. Furthermore, the emissions formation and oxidation of the NG SI engine are strongly depended on the HP and LP cycles due to the combustion rate and flame propagation in the cylinder.

Unraveling the geometry dependence of in-nozzle cavitation in high-pressure injectors.

Cavitation is an intricate multiphase phenomenon that interplays with turbulence in fluid flows. It exhibits clear duality in characteristics, being both destructive and beneficial in our daily lives and industrial processes. Despite the multitude of occurrences of this phenomenon, highly dynamic and multiphase cavitating flows have not been fundamentally well understood in guiding the effort to harness the transient and localized power generated by this process. In a microscale, multiphase flow liquid injection system, we synergistically combined experiments using time-resolved x-radiography and a novel simulation method to reveal the relationship between the injector geometry and the in-nozzle cavitation quantitatively. We demonstrate that a slight alteration of the geometry on the micrometer scale can induce distinct laminar-like or cavitating flows, validating the multiphase computational fluid dynamics simulation. Furthermore, the simulation identifies a critical geometric parameter with which the high-speed flow undergoes an intriguing transition from non-cavitating to cavitating.

Interaction between supersonic disintegrating liquid jets and their shock waves.

We used ultrafast x radiography and developed a novel multiphase numerical simulation to reveal the origin and the unique dynamics of the liquid-jet-generated shock waves and their interactions with the jets. Liquid-jet-generated shock waves are transiently correlated to the structural evolution of the disintegrating jets. The multiphase simulation revealed that the aerodynamic interaction between the liquid jet and the shock waves results in an intriguing ambient gas distribution in the vicinity of the shock front, as validated by the ultrafast x-radiography measurements. The excellent agreement between the data and the simulation suggests the combined experimental and computational approach should find broader applications in predicting and understanding dynamics of highly transient multiphase flows.