Binary cavitation in a transparent three hole GDI nozzle

Abstract

A more or less real size three hole (0.1 mm diameter) transparent injection nozzle was made with 120° between the orifices and an inclination of 20° to the injector axis. The geometry is similar to that of a multi hole GDI injector. Experiments are performed using n-pentane and -methyl-naphtalene mixtures in varying composition. The flow in the orifices is observed under submerged injection conditions from downstream looking in direction of the injector using a beam splitter plate for illumination with the light from a Minilite NdYag laser that was made incoherent by fluorescence in a cuvette filled with a dilute rhodamine-ethanol mixture. The images show the appearance of cavitation depending on the cavitation number as well as on the composition of the mixture. The behaviour is not what can be expected from equilibrium thermodynamics. Due to the transient nature of the flow the n-pentane concentration cannot attain the equilibrium value one would expect and cavitation occurs at higher cavitation values. Diffusion appears to play a role in the onset appearance of cavitation for binary mixtures. KeywordsBinary cavitation, real size transparent nozzle, fuel injection. IntroductionThe internal flow of gasoline direct injection nozzles has been investigated in the last years intensively. X-ray phase contrast imaging [1] allows visualizing the density gradients in the nozzle orifices of aluminium nozzles using very short high energy x-ray pulses from the APS Electron Storage Ring, Argonne National Laboratory. These images have shown that the flow can show hydraulic flip and cavitation in real size geometries. These results corroborate the results obtained in large scaled up transparent acrylic models of GDI multi-hole injectors, [2]. Characteristic for multi-hole injectors is the small length to diameter ratio of the orifices. The counter bore with a larger diameter does not have a direct contact with the fluid, [1] and is therefore mainly important for reducing the L/D ratio. Already in the early work with cavitating nozzles Lichtarowicz [3] noted "The function of the orifice bore, which must be at least one diameter long, is to stabilize the cavitation bubble. If the bore length is insufficient and the orifice tends to be a plate orifice cavitation occurs in bursts." This result is found in new results for GDI nozzles [4] that show flapping in timescales "in the order of a hundredth of a millisecond ". In the case of longer orifices the length of the cavitation zone in the orifice also fluctuates [5] with frequencies of about 29 kHz. Therefore, when taking instantaneous images of cavitation there will be large scatter of the cavitation length within the nozzle. There have been some real size transparent nozzles for Diesel injector geometries, e.g. [6][7][8]. However, the geometry of multi-hole GDI injectors is more complex and what is more demanding is that gasoline is an aggressive medium for acrylics. This is more so when ethanol is contained in the fuel. Therefore, transparent nozzles for gasoline fue...

A study on the relationship between internal nozzle geometry and injected mass distribution of eight ECN Spray G nozzles.

“…A converging diameter profile and a sufficiently rounded hole inlet corner were found to decrease or eliminate geometric cavitation in comparison to a sharp-cornered, cylindrical geometry [4,5]. In addition, local asymmetries at the hole inlet have been shown to generate correspondingly asymmetric cavitation, forming at the location where the fuel flow experiences the largest change in direction [5].Research in the same vein has been carried out in collaboration with the Engine Combustion Network (ECN). The ECN coordinates the sharing of well-characterized gasoline and diesel injection nozzles and standardized hardware with experimental facilities worldwide.…”

“…Research has also focused on elucidating the relationship between nozzle shape and cavitation, whose presence can significantly perturb fuel spray characteristics such as the discharge coefficient, outlet velocity, spray angle, and atomization behavior [3]. A converging diameter profile and a sufficiently rounded hole inlet corner were found to decrease or eliminate geometric cavitation in comparison to a sharp-cornered, cylindrical geometry [4,5]. In addition, local asymmetries at the hole inlet have been shown to generate correspondingly asymmetric cavitation, forming at the location where the fuel flow experiences the largest change in direction [5].…”

Estudio del fenómeno de la cavitación en la inyección Diesel mediante la visualización del flujo interno en orificios transparentes.

“…Debido a las características del fenómeno (altas velocidades de flujo, diámetros pequeños y flujo bifásico) su visualización resulta considerablemente compleja. Por este motivo, es habitual la utilización de geometrías simplificadas, aunque existen estudios que han conseguido resultados satisfactorios utilizando geometrías más similares a las de una tobera real [69][70][71].…”

“…Los estudios previamente descritos son una muestra de cómo la naturaleza de la cavitación cambia en función del tamaño de los orificios. Por este motivo, y a pesar de las dificultades existentes en la visualización de este tipo de geometrías, son cada vez más los estudios que utilizan orificios de tamaños similares a los habituales en toberas de inyección [69,71,[87][88][89][90]. Sin embargo, si bien las condiciones de aparición de la cavitación y el desarrollo de la misma son diferentes de las observadas en modelos a escala ampliada, las conclusiones de la influencia de la cavitación sobre el comportamiento del flujo y del chorro parecen ser similares.…”