The influence of cavitation on the internal flow and the spray characteristics in diesel injection nozzles F. Payri, V. Bermu ´dez, R. Payri * , F.J. Salvador CMT-Motores Te ´rmicos, Universidad Polite ´cnica de Valencia, Camino de Vera s/n, E-46022 Spain Received 10 June 2003; revised 23 September 2003; accepted 25 September 2003; available online 14 October 2003 Abstract A study was carried out on the influence of cavitation on the internal flow and the macroscopic behaviour of the spray in Diesel injection nozzles. For this study, two bi-orifice nozzles were used, one cylindrical, and the other convergent (conical). From the point of view of cavitation, the two chosen nozzles are very different, as the first nozzle is much more inclined to cavitate, whereas the second nozzle inhibits the cavitation phenomenon. First, in order to find the exact internal geometry of the two nozzles, a non-destructive characterisation method is used, which is based on the creation of silicone moulds. Once the nozzles are characterised dimensionally, a hydraulic characterisation is made. The results of this hydraulic characterisation, together with the predetermined dimensional characterisation, enable the discharge coefficient and the critical cavitation conditions to be determined. By identifying the critical cavitation conditions, it is possible to complete a study of the macroscopic parameters of the spray, with cavitating and non-cavitating conditions, and therefore a study can be carried out examining the influence of cavitation on the macroscopic spray behaviour. From the point of view of the spray macroscopic behaviour, the main conclusion of the paper is that cavitation leads to an increment of the spray cone angle. On the other hand, from the point of view of the internal flow, the hole outlet velocity increases when cavitation appears. This phenomenon can be explained by the reduction in the cross section of the liquid phase in the outlet section of the hole. q 2004 Elsevier Ltd. All rights reserved. Keywords: Cavitation; Diesel; Injection; Nozzle Geometry; Spray 1. Introduction The injector nozzle is one of the most important parts of a Diesel engine. Nozzle geometry affects spray characteristics and therefore atomisation behaviour, which is decisive for engine performance and pollutant formation. A major objective of the research carried out in this area in the past few years has been to improve nozzle design in order to obtain better air fuel mixing. A thorough understanding of the internal flow physics inside the nozzle is fundamental for predicting spray development. Although a number of studies have provided evidence on the existence of cavitation inside the nozzle depending on injection pressure, the detailed nature of nozzle flow has remained unknown until recently. There is experimental evidence to show that cavitation within the nozzle modifies the characteristics of the nozzle exit spray and probably favours atomisation of the spray [1,2]. It may, however, also affect the internal flow in other ways that are not yet clear. Real size production nozzles have very small dimensions and operate at very high injection pressure over very short time periods. It is therefore very difficult to visualise the internal flow. Hence, most of these studies were performed on large- scale transparent models [1–9] with the aim of visualising the cavitation structure within the nozzles. Different types of nozzles have been investigated, from one orifice nozzles in Refs. [1,5–8] to different multi-hole VCO and Sac-nozzles. Some authors [1,4,5,7 – 8] have also studied the velocity field using the LDV technique. Real size nozzle studies were also performed by several authors [10–14]. In particular, Arcoumanis et al. [11] observed that the cavitation structure was different in large scale and real size experiments. Throughout the past few years, studies have been carried out looking at the influence of nozzle geometries on emissions and performance features of the engine, independently from 0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2003.09.010 Fuel 83 (2004) 419–431 www.fuelfirst.com * Corresponding author. Tel.: þ 34-963879658; fax: þ 34-963877659. E-mail address: rpayri@mot.upv.es (R. Payri).