The Background to our Background Image – Internal Diesel Flows & Sprays

The background image used in this website shows an optical measurement arrangement to measure the cavitation vapour volume fraction and the spray jet droplet size distribution from the internal flow and spray jets obtained in a 6-hole Diesel injector operating at various accumulator pressures and temperatures, into atmospheric pressure. A Nd:YLF laser is operating with a pulse frequency of 7.5kHz at 527nm wavelength, with a pulse energy of approximately 1mJ/pulse. The laser light is split into 2 laser beams using a 50:50 45deg AOI laser beam splitter. One of the pulsed laser beams is directed across one of the 6 spray jets in order to excite fluorescence in a Rhodamine-B doped Diesel fuel sample. The nearest HS video camera in the background image is set to collect elastic Lorentz-Mie scattered light and long wavelength fluorescence light (560nm – 580nm collection) obtained from one of the six spray jets emanating from the 6-hole Diesel injector. The HS video camera is synchronised with the Nd:YLF laser operating at 7.5kHz. Accurate combination of the fluorescence images with the 527nm images permits the determination of spray droplet size distribution in the spray jet that is being subjected to investigation. The investigation into the properties of the Diesel spray is able to provide quantitative data on Diesel sprays that are formed based on varying composition (conventional Diesel, CTL/GTL Diesel, Diesel with FAME additives etc), anti-corrosion additives, additives for modifying viscosity and/or surface tension etc., fuel temperature and accumulator pressure.

The second laser beam is directed into an optically accessible Diesel nozzle. The fluorescence images that are obtained from Rhodamine-B doped Diesel fuel inside the injector are imaged on the second HS video camera with the long aluminium tube in the background images. The fluorescence images resolve the line-integrated spatial distribution of Diesel fuel inside the injector minisac and nozzle holes with a synchronised frequency of 7.5kHz, thereby providing both time-resolved and 2-dimensional spatially-resolved data on line-integrated fuel concentration inside the Diesel injectors during operation. Once again, this investigation is able to provide quantitative data on the internal flow characteristics of various Diesel fuels with varying composition, anti-corrosion additives, additives for modifying viscosity and/or surface tension etc., fuel temperature and accumulator pressure.

It is known from other research that liquids passing through high pressure nozzles are able to produce large internal pressure gradients that result in local fluid pressure regions that fall below the saturated vapour pressure for the liquid. This causes local supersaturation, which is able to form local vapour bubbles, clouds or sheets when accompanied by nucleation particles or surface irregularities. This type of multi-phase fluid flow is called a cavitating flow. Once the flow pressure recovers, the vapour bubbles, cloud or sheet collapse. Large temperatures, pressures and shock waves occur in the neighbourhood of the collapsing vapour during the collapse, which are able to erode adjacent containment surfaces. This process is known as cavitation erosion, which can result in critical damage to a relevant component.

Cavitating flows obtained in Diesel injectors during fuel injection have a strong effect on the structure and stability of the spray jets that are formed inside the engine cylinder outside of the nozzle holes. They are also capable of choking the amount of fuel delivered to the engine cylinders during injection. Variation of injector geometry and non-Newtonian viscosity modifiers are employed to control nozzle cavitation and the flow and atomization properties of the emergent spray jets.