Jingyin Li

The phenomenon of liquid droplets colliding with wetted solid surfaces is widely found in nature and in industrial applications. In this study, we experimentally investigated the phenomenon of a droplet colliding with a droplet-carrying solid surface. The impact force on the solid surface was measured with a high-precision piezo-electric sensor and the morphological changes during droplet collisions were captured with a high-speed camera. Experimental results showed that in such collision, the spreading factor of the droplet and the impact force on the surface are substantially influenced by droplet Reynolds number and are slightly affected by droplet Weber number. At a high Reynolds number, an annular jet appears between the falling droplet and the deposited droplet during the initial stage of collision, and rapid droplet spreading and a multipeaked impact force curve were measured. On the contrary, at a low Reynolds number, the collision only results in a single force peak with the disappearance of the annular jet. Additionally, a considerable reduction in both the spreading factor and velocity is observed. This alteration is due to the quite different viscous forces in the two types of droplet collision. Detailed analysis suggests that the droplet spreading process at high Reynolds numbers can be classified into three distinct stages: the buffering stage, the high-speed spreading stage, and the maximum spreading stage. Furthermore, the volumetric ratio of the deposited droplets to the falling droplets has only a slight influence on the impact process.

In this study, the turbulent droplet-laden boundary layer over a flat plate was investigated using large eddy simulation. A two-way coupled Eulerian–Lagrange point particle method was used for droplet particles. The effect of droplet addition on flow loss and the underlying mechanism of droplet modulation on the boundary layer coherent structure in the near wall turbulence were explored. It was found that the droplet-laden boundary layer exhibits an increase in displacement and momentum thickness, resulting from an increased velocity gradient on the plate surface. As a result, the flow loss rises while the shape factor drops. The underlying mechanism of droplet modulation lies in the variations in turbulent structures and energy transfer budget. It was observed that the coherent structure changes from sparsely distributed larger vortices in the unladen flow to densely packed smaller vortices in the droplet-laden flow. The presence of droplets accounts for the intensified stretching and fragmentation of large-scale vortices into smaller scale vortices, which then leads to an increase in the streamwise vorticity and enhanced velocity fluctuations in the spanwise and vertical directions. Furthermore, in the streamwise direction, the fluctuating velocity decreases due to the obstruction caused by the low-velocity droplets. The total turbulent kinetic energy is suppressed by the presence of droplets, and the suppression effect is enhanced with the droplet size. Additionally, the current simulation results exhibit ambiguous vortex periodicity in the spanwise direction. Thus, for periodic boundary conditions, a large-span configuration is recommended.

The axial compressor is crucial for heavy-duty gas turbines, with its aerodynamic performance directly affecting efficiency. The current trend in the development of these compressors is to increase the stage load and efficiency, thereby achieving a higher pressure ratio with fewer stages. The aerodynamic characteristics of a 1.5-stage axial compressor from a 300 MW F-class heavy gas turbine at three different rotation speeds (100%, 90%, and 80%) were studied. Specifically, the distribution of the inlet Mach number, shock wave structures, isentropic Mach number of blade surface, and blade surface separation flow characteristics under three typical working conditions, at the near stall (NS) point, maximum efficiency (ME) point, and near choke point (NC), were discussed. The results indicate that at 80% rotational speed, 70~100% spanwise of the compressor rotor blade is operated under the transonic zone. Meanwhile, at 100% rotational speed, almost all the spanwise of the compressor rotor blade is operated under the transonic zone. Furthermore, compared to the detached shock wave observed under the NS condition, the normal passage shock wave observed under the NC condition exhibits more significant changes in shock intensity and shock pattern.

In this paper, large eddy simulation (LES) was performed to investigate the shock wave/boundary layer interaction (SBLI) phenomenon in transonic compressor cascades with a chord Reynolds number of 2.12 × 106. A comprehensive analysis was conducted on both the SBLI structures inherent to the transonic compressor cascade and the coherent vortex structures within the boundary layer. The underlying mechanisms of the shock-induced boundary layer transition and the shock low-frequency unsteadiness in the transonic compressor cascade were elucidated through spectral and dynamic mode decomposition (DMD) analysis. The results revealed that boundary layer separation induced by the SBLI cannot reattach, leading to the formation of large-scale coherent vortex structures. Spectral analysis revealed that the shock-induced boundary layer transition in the transonic compressor cascade was dominated by inviscid Kelvin–Helmholtz (K–H) and secondary instability mechanisms, characterized by a dimensionless Strouhal number of 0.06. Additionally, pressure signals showed the variations in sub-frequency from the separated shear layer to the main flow. The oscillation amplitude of the shock foot was significantly greater than that of the shock main body, and the oscillation frequency of the shock foot was consistent with the sub-frequency. The oscillation frequency of the shock main body coincided with that of the compression ramp and flat plate configurations. Finally, DMD modal analysis indicated that high-frequency modes were correlated with turbulent fluctuations in the boundary layer, while medium- and low-frequency modes corresponded to shedding motion in the separated shear layer and low-frequency motion of the shock. This work promotes the understanding of the complex flow mechanisms of SBLI in the transonic compressor cascades.

The effects of back pressure on the transonic cascade operating state are crucial and can determine the structure of internal shock waves and secondary flows. In this paper, numerical methods validated by experiments were employed to investigate the evolution mechanisms of the inlet flow field, shock structure, secondary flow structure, and cascade performance under different back pressures. Analysis revealed that transonic cascade exhibited unique incidence characteristics in the inlet flow field under both subsonic and supersonic regimes, although these two regimes involved different physical mechanisms. The results revealed that the operating state of the transonic compressor cascade under the unique incidence condition was influenced by the outlet back pressure, and there existed a critical static pressure ratio. The critical static pressure ratio shifted from 1.61 for two-dimensional flow to 1.37 for three-dimensional (3D) flow at M1 = 1.1, due to the corner separation and the characteristics of 3D shocks. The 3D shock structure exhibited a non-uniform distribution along the spanwise direction due to the influence of back pressure and the separated boundary layer. The vortex structures analysis revealed that the secondary flow structure on the sidewalls of the transonic compressor cascade was primarily dominated by corner vortices, whose formation mechanism was related to the interaction between the shock wave and the sidewall boundary layer. Additionally, this interaction also led to the formation of detached shock and lip shock vortex structures. Finally, loss analysis indicated that the wake region of the transonic cascade primarily includes six types of loss, and the total loss of the cascade decreased with the rise in back pressure.