Effect of ammonia injection pressure and injector nozzle holes on flash-boiling free sprays

Le résumé fourni par la source

Ammonia sprays undergo significant flash boiling due to its high vapour pressure. Towards effective mixture making critical for ammonia combustion in engines, a fundamental understanding of flash-boiling ammonia sprays is required to which optical diagnostic research could provide useful information. The present study implements high-speed shadowgraph imaging in an optical constant-volume pressure chamber firstly for varied superheat indexes in the range of 0.086 – 0.43, which covers expected ambient gas pressure conditions of 100 – 500 kPa inside the cylinder of the engine. Considering ammonia direct injection, a high-pressure liquid pump is used for 5 – 15 MPa injection pressure and two different multi-hole nozzles with 3-hole and 6-hole configurations are investigated. From the results, it was found that increased flash-boiling leads to faster spray area growth and penetration. Also, the mid-point spreading angle decreases as the spray plumes merge and collapse to form a single-plume spray with narrower spreading angle. However, the near-nozzle spreading angle shows a monotonic increase as the flash-boiling occurs immediately after being issued to the ambient gas and thus vapour ammonia contributes to the volume expansion. Flash-boiling ammonia sprays show direct correspondence to increased injection momentum, exhibiting longer penetration, higher spreading angle and larger area. Regarding the nozzle hole configuration, the 6-hole nozzle shows higher penetration, smaller spreading angle and larger spray area due to decreased hole spacing and thus accelerated plume-to-plume interaction of flash-boiling ammonia and resulting spray collapse. These new findings suggest ammonia sprays possess unique characteristics that are different to conventional petrol sprays due to high flash-boiling tendency, and thus dedicated ammonia spray studies are required for injector optimisation.

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