Study on transient structure characteristics of round liquid jet in supersonic crossflows

Excerpt

The research topic of liquid jet injected into a supersonic crossflow has been concerned since the 1960s when the concept of scramjet engine was put forward (Masutti et al. 2009; Horn et al. 1968). The liquid jet is injected into high-speed crossflows, experiencing the complex physical processes, including strong shear (Wang et al. 2014), strong convection (Wang et al. 2014), vortex motion (Behzad et al. 2015) and interaction of shock wave and jet (Yamauchi et al. 2012). Various physical processes are mutually coupled, bringing difficulty in the theoretical analysis of this problem and the establishment of simulation model. So far, the experiment method is the main method of research on this problem. The optical imaging method is the earliest test means used most widely, including ordinary camera imaging, high-speed photography, shadowgraph method, planar laser-induced fluorescence (PLIF), particle image velocimetry (PIV) and holography method. With these methods, massive researches have been carried out on breakup process (Lin et al. 2004; Sallam et al. 2004, 2006), droplet velocity distribution (Lin et al. 2004; Olinger et al. 2014) and spatial distribution of liquid jet (Sinha et al. 2015) in high-speed crossflows (including supersonic crossflows). The consistent conclusion has been reached that the jet penetration in supersonic crossflows could be predicted with empirical correlations of two forms (power law and logarithmic), which are mainly affected by three parameters x (distance from nozzle), q (jet-to-air momentum flux ratio) and d (diameter of nozzle) (Ghenai et al. 2009). Nevertheless, there are still a lot of unknown phenomena need to be researched on the liquid jet in high-speed crossflows, accompanying with the apparently formidable challenge to visualization. The liquid is injected into crossflows and accelerated to the speed close to the main flow after leaving the injection hole (Wang et al. 2014). The surface waves along column jet caused by R-T instability have sizes similar to nozzle diameters (Xiao et al. 2013). In particular, in supersonic crossflows, the enough spatial–temporal resolutions are necessary for imaging clear because of high speed of spray and small-scale structures. Flash lamp could provide an instantaneous illumination to achieve transient imaging (Hong et al. 2014). The disadvantage is unable to image continuously with small enough pulse separations. The decisive developments began with the introduction of pulsed shadowgraphy and holography method. Pulsed laser is used as plane lamp to illumination spray for frozen imaging. Sallam et al. (2004, 2006) revealed the primary breakup properties of round nonturbulent and aerated-liquid jets in uniform gaseous crossflows by using that. Pulsed photography was also exposed by Ng et al. (2008) to observe the column and surface waves along the liquid jet in subsonic crossflows. However, the interference effect of laser could lead to uneven distributions of gray level in the background of images shown in the articles referenced above. The uneven distributions bring troubles in the effective information extraction and have not been solved well in Sallam et al.’s (2004, 2006) and Ng et al.’s (2008) researches. The problem would be more obvious because of the density gradient in supersonic crossflows. In addition, laser-induced fluorescence method (LIF) presented by Theofanous (2011) has possibility to solve the imaging problem in supersonic crossflows. But it is a pity that the method is just used in studying breakup of one droplet in supersonic crossflows presently. …