Application of X-ray micro-computed tomography on high-speed cavitating diesel fuel flows

Abstract

efficiency of such engines. Among others, the fuel injection equipment (FIE) is a key component for improving the efficiency of environmentally friendly engines. Recent advances dictate the use of extra high injection pressures, of the order of 3000 bar and tiny flow passages (injection holes) of the order of 100-250 μm in diameter, for improved fuel atomisation and combustion efficiency. Under these circumstances, cavitation has been found to develop inside fuel injection nozzles at the early studies of Badock et al. (1999), Bergwerk (1959), Chaves et al. (1995) and Nurick (1976), followed by Arcoumanis et al. (2000), Blessing et al. (2003), Mitroglou et al. (2014) and Roth et al. (2002) in more realistic real-size nozzle geometries offering optical access; equally helpful studies performed in transparent enlarged nozzle replicas [selectively (Andriotis et al. 2008;Arcoumanis et al. 2000;Miranda et al. 2003; Mitroglou and Gavaises 2013;Powell et al. 2000)] also indicate that cavitation plays an increasingly significant role in the nozzle's internal flow structure and development. Cavitation inside an injection hole is believed to enhance spray atomisation, either directly through the implosion of cavitation bubbles or indirectly because it increases turbulence in the nozzle flow (Badock et al. 1999;Walther 2002); unfortunately, under certain circumstances induces erosion (Dular and Petkovšek 2015;Koukouvinis et al. 2016) that may lead to catastrophic failures. Moreover, cavitation inside the injection holes promotes shot-toshot spray instabilities (Mitroglou et al. 2011(Mitroglou et al. , 2012Suh and Lee 2008), which, in turn, are responsible for poor combustion efficiency and increased emissions (Hayashi et al. 2013). Two distinct macroscopic forms of cavitation have been identified, which are referred to as geometryinduced and vortex or 'string' cavitation. The former can be, partially or totally, suppressed by appropriate design of the inlet hole curvature and non-cylindrical injection hole Abstract The flow inside a purpose built enlarged singleorifice nozzle replica is quantified using time-averaged X-ray micro-computed tomography (micro-CT) and highspeed shadowgraphy. Results have been obtained at Reynolds and cavitation numbers similar to those of real-size injectors. Good agreement for the cavitation extent inside the orifice is found between the micro-CT and the corresponding temporal mean 2D cavitation image, as captured by the high-speed camera. However, the internal 3D structure of the developing cavitation cloud reveals a hollow vapour cloud ring formed at the hole entrance and extending only at the lower part of the hole due to the asymmetric flow entry. Moreover, the cavitation volume fraction exhibits a significant gradient along the orifice volume. The cavitation number and the needle valve lift seem to be the most influential operating parameters, while the Reynolds number seems to have only small effect for the range of values tested. Overall, the study demonstrates that use of micro-CT can ...

High-speed visualization of vortical cavitation using synchrotron radiation

“…Wang et al (2008) have elucidated the visualisation capabilities of both shadowgraphy and XPCI, in reference to spray flows. Besides, the failure of shadowgraphy to capture "internal" cavitation features in a nozzle flow has also been pointed out by (Mitroglou et al 2016), who observed that a thin cavitation film forming on the curved wall completely prevented the visualization of the morphology of cavitation emerging in the nozzle core.…”

“…On the contrary, high standard deviation is indicative of a fluctuating behaviour in reference to vapour formation and collapse. It constitutes a quantity suitable for the identification of transient cavitating structures, as also demonstrated by Mitroglou et al (2016).…”

“…Besides, a number of studies have employed x-ray imaging (attenuation or phase contrast) to visualize highly-turbulent, two phase nozzle and spray flows with relevance to the automotive industry and fuel injection equipment (Im et al 2013;Kastengren et al 2012;Moon 2016). For instance, Mitroglou et al (2016) utilized a cone-beam x-ray source and computed tomography to quantify, i.e. provide vapour fraction data for the timeaveraged extent of cavitation in an injector-replica orifice.…”

High-speed X-Ray Phase Contrast Imaging of String Cavitation in a Diesel Injector Orifice

“…It is a well-known fact that string cavitation exhibits a highly transient behaviour (e.g. refer to [10]). The string topology has been resolved in much more detail compared to conventional optical imaging and a far more complex topology has been revealed compared to typical shadowgraphy images (e.g.…”

“…In this study, an enlarged orifice similar to that employed in [10] is examined and focus is given on elucidating the topological and dynamical features of the highly-fluctuating vortical structures, conventionally termed as string cavitation. For this reason, high-speed XPCI has been selected for the flow visualization, which allows the acquisition of high-speed, high-resolution radiographies with a relatively wide field of view.…”