Physics of liquid jet breakup in atomisers
Creators: name: Wang, Tianyi
Publisher: name: Imperial College London
Year: 2022

description: The present thesis focuses on the physical mechanisms behind the primary breakup stage of the atomisation process, which determine the performance of spray applications, including liquid fuel combustion, medical spray and chemical coating. State-of-the-art laser diagnostic techniques were applied to study the interfacial characteristics of atomising liquid jets which are exposed either to a quiescent or high-speed air stream. The interfacial characteristics of atomising liquid jets include the interfacial velocity and patterns of surface structure, which are essential to control the downstream spatial distribution and size of the droplets. Most of this interface characteristics are still unknown due to the lack of appropriate experimental techniques. The interfacial characterisation therefore can only be derived from theoretical and computational studies based on assumptions till now. The current study combined novel laser diagnostic techniques, including the Optical Connectivity (OC), Optical Flow Velocimetry (OFV), Planar Laser induced fluorescence (PLIF) and Two-Photon Excitation (TPE), to experimentally quantify, for the first time, the detailed distribution of the interfacial velocity on the surface of an injected liquid jet. This included the observation of the instantaneous circumferential geometry of the liquid jet during the primary breakup process. These developed techniques were applied to pressure and crossflow atomiser designs. The quantified interfacial velocity of the pressure atomiser confirmed the relaxation of the liquid boundary layer, induced by viscous force, after the liquid jet leaves the nozzle and may be the dominant mechanism of interfacial instabilities. The wavelength and amplification of interfacial waves could be determined by the viscous-driven T-S mode. Besides, the impact of the gas flow interaction with the liquid jet surface was also revealed by quantifying the interfacial velocity, reflecting the effect of aerodynamics on the liquid jet from pressure atomiser. Proper Orthogonal Decomposition (POD) algorithm was applied to the acquired liquid jet images to extract the dominant geometrical patterns on the liquid jet surface. The circumferential interfacial geometries were observed, and their temporal development was tracked to indicate the physics of the atomising liquid jet. Based on the optical experiments and POD analysis, new sights were obtained to explain remaining unknowns of the atomisation process for pressure atomiser. The appearance and development of asymmetric interfacial waves may be induced by the fluctuating axial interfacial velocity which randomly emerges on the jet surface during the turbulent transition process inside the liquid shear layer. From the quantified interfacial velocity, a new explanation of the generation of droplets during the primary breakup was also proposed. Moreover, the pressure liquid jet under controllable artificial perturbation was studied. The disturbed liquid jet intended to simulate the development of a disturbance that may be initiated in the liquid jet flow inside the nozzle, for example due to cavitation inside the nozzle. This allows the study of the effect from flow disturbance on liquid jet interfacial characteristics, including the interfacial velocity and geometry. For the liquid jet in crossflow (LJIC) atomiser, the interfacial characteristics were also analysed by optical measurement and mathematical algorithm. The dynamic features of LJIC, including the flapping motion and lengthscales of surface wave, were extracted by POD for variable flow conditions. The importance of different dynamic features was discussed based on the corresponding mode energy, and the dominant feature was found to change with liquid jet trajectory and gas flow velocity. Same as the dynamic features, the detailed interfacial velocity distribution on the circumferential surface of LJIC was also connected to the liquid jet deflection and surrounding gas flow patterns. In comparison to the pressure atomiser, the gas/liquid flow interaction was more intense on the surface of LJIC, as expected, and can be quantitively analysed from the interfacial velocity distribution. A new breakup mechanism of the liquid jet was proposed based on the velocity difference between interfacial waves with different wavelength, in order to explain the lack of coherence of the breakup process that has been observed by many studies. Finally, the cross-sectional shape of the LJIC, which is still unknown, was recorded by applying the novel optical technique, combining TPE, OC and PLIF. The relevant information will be useful to explain the formation of the droplet on the azimuthal side of LJIC.