Kilohertz Mie Scattering and OH* Chemiluminescence Imaging of JP-8 Multiple Injections Using a 250 MPa Fuel Injector

The objective of the study was to investigate the spray and combustion characteristics of Jet Propellant-8 (JP-8) using a high-pressure fuel injector which is capable of up to 250-MPa fuel injection pressure. Experiments were performed in a constant-pressure flow-through combustion chamber at the ambient conditions of 825 K and 6 MPa for the oxygen concentration of 0 and 21%. JP-8 was injected over a range of fuel injection pressures from 50 to 250 MPa for single injection events to establish a baseline operation. Pilot and post injections were used to study the effect of multiple injections on spray and combustion of the high-pressure fuel injector. Both pilot and post injection separation times and quantities were systematically varied. JP-8 spray and combustion events were imaged at 75 kHz using a combination of Mie scattering and OH* chemiluminescence imaging.

The Mie scattering results showed that spray atomization and mixing was enhanced with increasing fuel injection pressure which was indicated as thinner and more uniform liquid columns. For non-reacting multiple injections, both pilot and post injections showed shorter liquid penetration lengths which were due to incomplete needle lift for these small injection quantities. Liquid penetration length of the main injection was slightly decreased as the pilot separation time was shortened. At higher fuel injection pressure, the main injection showed decreased hydraulic delay at the shortest pilot separation time. Furthermore, liquid penetration length of the post injection decreased at higher fuel injection pressure. Spray patterns became more uniform both within an injection and across injection events at the higher fuel pressures.

Ignition delay decreased almost linearly with increasing fuel injection pressure. The OH* intensity showed that pilot injection led to more uniform main combustion which may result in improved combustion stability. Ignition delay increased at higher fuel injection pressure for the double injection events due to leaner mixtures with faster combustion at higher fuel pressure. Higher fuel injection pressure retarded the post combustion and post combustion became insensitive to the post separation time.