Page 1/8 Cavitation simulation and experimental verification using a new Diesel nozzle design concept N. Mitroglou , M. Gavaises , A. Theodorakakos 1: City University London, Northampton Square, EC1V 0HB, London Abstract: The onset and development of cavitation in a new Diesel injector nozzle design is investigated both computationally through use of CFD and experimentally using transparent nozzle replicas. The injector design eliminates the sac volume and isolates the flow path that links adjacent holes. It is proved that this results to elimination of vortex cavitation and profound spray stability. Unlike most existing nozzle designs, geometric cavitation becomes a controlled flow characteristic that can be used to determine fuel atomisation and near-nozzle spray angle. Modelling of cavitation is performed using various sub-models for nucleation and bubble formation, further bubble growth and collapse, as well as bubble break-up and transport are incorporated into the model. Simulations are performed both under fixed and transient needle lift conditions. Model validation is performed against experimental data performed in transparent nozzle replicas operating under steady-state flow conditions. Measurements include, in addition to nozzle discharge coefficient, images of the geometric hole cavitation at various combinations of needle lifts, Reynolds and cavitation number. Keywords: cavitation, string cavitation, vortex flow 1. Introduction The role of the fuel injection system in modern direct-injection Diesel engines is paramount and well recognised as a means of controlling their performance and meeting the ever more stringent emission regulations. Electronic common-rail injection systems employ a variety of nozzle designs and engine optimisation injection strategies to cover a wide range of operating conditions that modern Diesel engines are expected to perform. Increasing injection pressure, piezo-controlled mechanisms for achieving fast response of the needle valve and multiple injections are among the methods explored and known to improve combustion and engine performance. However, under such operating conditions, cavitation phenomena are present inside the nozzle and become the dominant and frequently uncontrolled flow characteristics that affect fuel system durability and the properties of the near- nozzle emerging spray. Success of modern Diesel direct-injection fuel equipment is based on their ability to control accurately timing, duration, rate and number of injections, as well as, shaping of the spray pattern to match piston-bowl geometry and enhanced cyclic variability behaviour employing a number of different nozzle designs. Investigations over the years have demonstrated that Diesel injector nozzles generate cavitation [1-3] under typical operating conditions, a fact that complicates further the already complex design of high-pressure Diesel injection systems. As demonstrated in [4-6], two distinct forms of cavitation have been identified inside injection nozzles, geometrically-induced and vortex or string cavitation. Geometric-induced cavitation is the most common form of cavitation and it has become gradually a well-understood phenomenon; it initiates at sharp hole inlet corners due to the abrupt acceleration of the fuel flow as it enters the nozzle holes. This increase of velocity creates a pressure drop which induces cavitation at the core of the recirculation zone formed at the hole inlet; this is more pronounced with sharper rather than rounded inlet hole geometries achieved through hydro-grinding. On the other hand, string or vortex cavitation structures have been observed in the bulk of the liquid inside sac, mini-sac and valve covering orifice (VCO) nozzles, where the formed internal volume allows for formation of relatively large-scale vortical structures [7-9]. Vortex cavitation is commonly found in propellers, hydraulic turbines and hydrofoils, as explained in [10-13]. However, recent studies have confirmed similar behaviour in multi- hole nozzles for high-pressure direct-injection gasoline engines and low-speed two-stroke Diesel engines [7, 14-17]. Cavitation is linked to undesirable effects such as sharp reduction in engine performance, increase in noise and vibrations, as well as surface erosion [16]. In most cases of practical interest, cavitation bubbles survive until the nozzle hole exit [4]. Therefore, it is generally accepted that cavitation promotes fuel atomisation, which is desirable for enhanced air fuel mixing. However, some studies [18] have indicated that cavitation may also be associated with hole-to- hole and cycle-to-cycle variations. Appropriate design of the inlet hole curvature and the non- cylindrical shape of the holes as [19, 20] showed, alter cavitation inception and development characteristics; at the same time, careful system