Visualization research of natural gas jet characteristics with ultra-high injection pressure

In this paper, based on a High-pressure direct injection (HPDI) dual-fuel valve, the schlieren imaging method combined with laser-induced fluorescence imaging method (PLIF) are used to investigate the macroscopic structure and concentration distribution characteristics of a high-pressure gas jet under different injection pressure ratio (NPR) and gas injection moment (Ti). The data processing method for macroscopic structure and concentration distribution of gas jet is optimized, and a new concentration partition method for the gas jet is proposed. The results show that comparing the gas jet penetration and volume correlation equations from Hamzehloo A and Vuorinen V, the correlation coefficients of ultra-high and small NPR conditions are quite different due to the different development processes of the shock wave structure. With the improvement of NPR, the influence on gas jet penetration is decreasing, which is the same as the conclusion of previous research. In this paper, the later stage of gas jet is divided into four regions: the Mach disk core region (A), the concentration rapidly decreasing region (B), the low concentration diffusion region (C), and the edge mixing region (D). The A and B regions are highly correlated with the shock wave structure of the under-expanded gas jet. The increase of NPR and Ti has a great promotion effect on the length and concentration distribution of regions A and B but has rarely effect on regions C and D. The transverse concentration gradient in regions A and B presents a Gaussian distribution structure. The average concentration descent gradient (G¯) in region A is about 4 times that in region B. The transverse concentrations of C and D gradually converge under the action of air turbulence. Under different Ti conditions, the length proportion of regions A and D rarely changed, while the length proportion of regions B and C presents a negative correlation. The G¯ in regions A and B are positively correlated with Ti, but the concentration increase rate keeps decreasing. The range of concentration fluctuation regions in regions C and D keeps expanding with the increase of Ti, indicating that the uniformity of gas jet concentration distribution keeps improving.