Abstract
Hydrodynamic turbulence and cavitation are known to play a significant role in high-pressure atomizers, but the small geometries and extreme operating conditions hinder the understanding of the flow’s characteristics. Diesel internal flow experiments are generally conducted using x-ray techniques or on transparent, and often enlarged, nozzles with different orifice geometries and surface roughness to those found in production injectors. In order to enable investigations of the fuel flow inside unmodified injectors, we have developed a new experimental approach to measure time-resolved vibration spectra of diesel nozzles using a 3D laser vibrometer. The technique we propose is based on the triangulation of the vibrometer and fuel pressure transducer signals, and enables the quantitative characterisation of quasi-cyclic internal flows without requiring modifications to the injector, the working fluid, or limiting the fuel injection pressure. The vibrometer, which uses the Doppler effect to measure the velocity of a vibrating object, was used to scan injector nozzle tips during the injection event. The data were processed using a discrete Fourier transform to provide time-resolved spectra for valve-closed-orifice, minisac and microsac nozzle geometries, and injection pressures ranging from 60 to 160 MPa, hence offering unprecedented insight into cyclic cavitation and internal mechanical dynamic processes. A peak was consistently found in the spectrograms between 6 and 7.5 kHz for all nozzles and injection pressures. Further evidence of a similar spectral peak was obtained from the fuel pressure transducer and a needle lift sensor mounted into the injector body. Evidence of propagation of the nozzle oscillations to the liquid sprays was obtained by recording high-speed videos of the near-nozzle diesel jet, and computing the fast Fourier transform for a number of pixel locations at the interface of the jets. This 6–7.5 kHz frequency peak is proposed to be the natural frequency for the injector’s main internal fuel line. Other spectral peaks were found between 35 and 45 kHz for certain nozzle geometries, suggesting that these particular frequencies may be linked to nozzle dependent cavitation phenomena.
Highlights
The importance of fuel atomisation is well recognized and has been extensively studied experimentally and theoretically [1,2,3,4,5,6,7,8,9,10,11]
We investigated the internal flow of diesel fuel injectors by simultaneously measuring the nozzle tip vibration, upstream fuel pressure and needle lift
The technique we propose, based on the triangulation of a laser Doppler vibrometer and fuel pressure transducer signals, enables the quantitative characterisation of internal flows for unmodified injectors with real fuels at elevated injection pressures
Summary
The importance of fuel atomisation is well recognized and has been extensively studied experimentally and theoretically [1,2,3,4,5,6,7,8,9,10,11]. Researchers often resort to applying non-intrusive optical diagnostic techniques onto specially modified [12] or enlarged nozzles, in order to study internal flows and cavitation processes [13, 14] This method is experimentally challenging and only provides qualitative data, hindering the validation of numerical models. There is some experimental and numerical evidence that high frequency pressure fluctuations exist inside modern diesel injectors, and that they influence the shape and stability of the sprays [23,24,25] These fluctuations are usually recorded using pressure sensors fitted to the common-rail or close to the injector. The authors believe that such non-intrusive, quantitative, and time-resolved measurements of high-frequency diesel nozzle vibrations are novel, and could lead to new insight into the internal dynamics of the injector’s mechanical components and fuel flow inside unmodified nozzles
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