Macroscopic spray characteristics and internal structure studies of natural gas injection

Abstract

The macroscopic spray characteristics and internal structure of a highly expanded compressed natural gas (Methane; CH4) spray were experimentally and numerically investigated. The spray jet characteristics were analyzed under various pressure ratios (PR), defined as the ratio of the methane injection pressure to the ambient pressure. The spray characteristics and shockwave structures were investigated using Schlieren imaging. The novelty originates from comparing the methane spray development and shockwave parameters under low PR (4 < PR < 9) conditions. The investigation included an evaluation of the effect of choking phenomena on gas flow at different PRs by changing the methane injection pressure. The results showed that a high injection pressure did not improve the spray tip penetration but increased the choking phenomenon in methane flow in the near-field region. This phenomenon is a key factor that strongly influences the shockwave structure and leads to 3.5% longer injection duration and at least 13% increase in the spray volume and area before entering the fully developed region of the Mach disk. With respect to the Mach disk parameters, a PR change from 6 to 9 enhanced the Mack disk height by at least 13.5% and its width by at least 29%. Increasing the Mach disk width with PR increased the methane near-field angle of the spray by at least 29%, triple-point angle by at least 29%, and spray width at the triple point by at least 29% because of the larger radial gas spray development. Overall, the comparison of the macroscopic spray characteristics and Mach disk parameters showed that the macroscopic spray parameters were almost similar despite the changes in the PRs. In contrast, the Mach disk parameters (height, width, and triple point angle) and variations in the methane spray field were larger than those in the macroscopic spray parameters. In addition, the simulation results were used to analyze the energy conservation and transfer conversion efficiencies of the gaseous injection system. The same increase in PR led to greater energy transfer and conversion efficiencies by 0.05% and 6.7%, respectively, which adversely affects charge mixing in the intake manifold. Strong air turbulence is required to enhance the charge-mixing mechanism and achieve a homogeneous fuel-air mixture. Therefore, the choking phenomenon requires more attention when implementing gaseous injection systems for internal combustion engines.