Abstract
The flow inside a purpose built enlarged single-orifice nozzle replica is quantified using time-averaged X-ray micro-computed tomography (micro-CT) and high-speed 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 be a reliable tool for cavitation in nozzle orifices operating under nominal steady-state conditions.
Highlights
The ever more stringent emission regulations of directinjection diesel engines, imposed by the regulatory boards worldwide, have triggered the race of improving the 175 Page 2 of 14Exp Fluids (2016) 57:175 shape (Soteriou et al 2006), for sufficiently high positions of the needle valve; this is the standard practice in manufacturing automotive diesel injector nozzles (Potz et al 2000); still, some cavitation may be needed as it is thought to make nozzles more resistant against deposit formation (Tang et al 2009)
Many investigations have been performed on transparent, enlarged full nozzle replicas; due to the significantly lower injection pressures utilised in such enlarged models (Gavaises et al 2009; Mitroglou and Gavaises 2013; Soteriou et al 2006), dimensionless similarity based on the cavitation (CN) and Reynolds (Re) numbers has been adopted
Statistical information extracted from the shadowgraphy images, as mean intensity and standard deviation maps, illustrates a red-coloured area in the mean image, where cavitation is found to be present for the total duration of acquisition; on the contrary, the standard deviation image, having discarded the locations that exhibit zero variations, demonstrates the regions with high variability in intensity values throughout the test case
Summary
The ever more stringent emission regulations of directinjection diesel engines, imposed by the regulatory boards worldwide, have triggered the race of improving the 175 Page 2 of 14Exp Fluids (2016) 57:175 shape (Soteriou et al 2006), for sufficiently high positions of the needle valve (i.e. open); this is the standard practice in manufacturing automotive diesel injector nozzles (Potz et al 2000); still, some cavitation may be needed as it is thought to make nozzles more resistant against deposit formation (Tang et al 2009). Many investigations have been performed on transparent, enlarged full nozzle replicas; due to the significantly lower injection pressures utilised in such enlarged models (Gavaises et al 2009; Mitroglou and Gavaises 2013; Soteriou et al 2006), dimensionless similarity based on the cavitation (CN) and Reynolds (Re) numbers has been adopted. Parallel to those activities, attempts were made to manufacture real-size exact replicas of diesel nozzles from transparent materials that are able of withstanding pressures in the range of 500–2000 bar, which are similar to those in real injections. All observations so far have confirmed that the developed multi-phase flow field is complex and exhibits a transient behaviour, even at simplified single-orifice geometries
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