The internal flow of diesel fuel injector nozzles: A review

Abstract

Diesel fuel injector nozzles have a significant effect on the quality of spray and charge preparation. However, the mechanism and degree of this effect is unclear. The complexity of the internal nozzle flow has hindered the study of the fuel injection process. Diesel fuel injector nozzle flows are highly turbulent and usually two-phase. Several experiments have shown the presence of cavitation in the nozzles to be a dominating effect. Recent experimental work has revealed new qualitative details about the cavitation in fuel injector nozzles. The cavity tends to be smooth near the inlet, transitioning to a more ruffled appearance downstream. These flows are also strongly asymmetric in realistic geometries. Additionally, photographs have shown string cavitation inside the sac volume extending into the nozzles. The strings appear to be a form of stratified two-phase flow, like the cavities near the inlet corner. Like the cavities, the strings break down into bubbly flow near the exit of the nozzle. Future experiments in this field should address the exact nature of the two-phase flow at the exit. In spite of these difficulties, useful models for the flow in fuel injector nozzles have been developed. Analytical models work very well for the steady state behaviour of axisymmetric nozzles. Multi-dimensional models have proved to be useful for more general geometries and attempt to predict transient behaviour. There is currently no consensus on the basic physics behind the multi-dimensional models. Despite this controversy, there are several models available that have succeeded in predicting gross cavitation behaviour. The implications of the recent experimental investigations are that future cavitation models should include the ability to simulate more than one kind of cavitation. Typically, a model assumes that the cavitation is either a smooth cavity or a bubbly mixture. In general, the ideal cavitation model must capture either kind of behaviour.