Experimental study and empirical modelling of direct-injection n-heptane sprays formed under flash-boiling conditions

Abstract

• Experiments of n -heptane injection under flash-boiling conditions were conducted. • A new simple spray angle enhancement model for flash-boiling spray was proposed. • The model was applied in numerical simulations under similar conditions. • The angle enhancement and adjusted bubble number density captured spray collapse. • The model didn't predict recondensation and the sharp tip of flare-flashing sprays. The aim of this study is to investigate the effect of flash boiling on global structures of n -heptane sprays formed by a multi-hole injector in a wide range of fuel temperatures and ambient pressures at different injection pressures to provide a solid foundation for a new sub-model capable of reproducing spray evolution under flash-boiling conditions using a Lagrangian Discrete Droplet Method (DDM). The experiments were focused on qualitative analysis of the spray structures and quantitative parameters such as axial spray penetration and spray angle. Based on the experimental and literature data, a new sub-model for half of the spray-cone angle (HOCA) was proposed, and the bubble number density was calibrated to simulate the evolution and collapse of flash-boiling sprays accurately. A commercial six-hole injector operating at three injection pressures (5, 10 and 15 MPa) was used to inject n -heptane at three initial temperatures (60, 90 and 120 °C) into a constant volume chamber filled with air at a pressure ranging from 0.01 to 0.1 MPa (absolute). It was observed that regardless of the injection pressure, the effect of flash boiling on the global parameters of the spray was similar. However, due to a stronger axial momentum, the global spray angle was lower for higher injection pressures. Observed changes in the spray parameters were used to create a new simple spray angle formula and calibrate the bubble number density submodel, which was implemented into the AVL FIRE™ solver. The model predicted the widening of the near-nozzle spray angle and accurately reproduced the spray collapse. However, it failed to capture an enhanced propagation of the centre part of the collapsed spray front, which was associated with the lack of vapour condensation, which wasn't included in the model.

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