q. zheng

The performance of a Pelton turbine was investigated by means of an integrated numerical and experimental study that focused on the fluid dynamics associated with the bucket suction surface during the jet–bucket interactions. The transient pressure distributions and torque-generation mechanisms were characterized under a variety of conditions. The results revealed a distinct two-phase transition process: an initial high-pressure zone (θ = 48.7°) that generated a resistance torque was followed by a negative-pressure region (θ = 57.2°) that produced a significant torque enhancement. The impingement angle was quantitatively established as a primary control parameter since a direct correlation was observed between angle decreases and torque improvements. A thickness-based regulation method for the suction surface was developed to precisely control both the impingement angle and the subsequent jet trajectory. These results provide fundamental insights into the transient flow phenomena that affect the turbine efficiency and offer practical design guidelines for performance optimization in high-head hydropower applications. Critical knowledge gaps regarding the fluid mechanics of Pelton turbines were addressed using measurable benchmarks for bucket design improvements.

The boundary layer's separation loss in compressor cascades constitutes a significant portion of profile loss, critically influencing aerodynamic performance optimization and control. This study employs Large Eddy Simulation (LES) to examine separation losses at varying attack angles, focusing on a rectangular compressor cascade. Specifically, it explores the long separation bubble at a 45% blade height cross-section under designed incidence. Analysis of the separation bubble's transition process revealed a notable surge in total pressure loss rate prior to transition, which stabilized following reattachment. The study thoroughly investigates the evolution of long bubbles, employing quadrant analysis of Reynolds stress, critical point theory, and an in-depth examination of individual vortex dynamics. The findings indicate that the peak of cross-flow within the separation bubble acts as the primary mechanism initiating the transition. This insight is corroborated by DNS calculations of natural transitions on flat plates. Building upon these findings, the study discusses the effects of varying attack angles on transition processes. Notably, increased incidence prompted the upstream migration of the long separation bubble, transforming it into a short bubble at the leading edge. This shift led to a fivefold increase in separation loss and doubled the frequency of transverse flow fluctuations.

Casing treatment is an efficient technique that is used to increase the compressor stall margin with a minor reduction in efficiency. The interaction between the blade passage and the groove is the main cause of the stall margin improvement by various researches. A numerical study is conducted on a single circumferential casing groove using NASA rotor 37 in the current study. The performance of the two circumferential groove models is studied by discretizing RANS 3D equations using a finite volume technique. The inception of the stall is predicted according to the criteria of convergence. Two models of the circumferential groove have been suggested and numerically tested. A single passage simulation is selected for the two models. The performance of the smooth casing and the two models are analyzed. Moreover, the stall margin, total pressure ratio, and peak adiabatic efficiency of the normal casing, and the two models are analyzed to determine the influence of the groove on the axial compressor performance and stability. Models 1 and 2 stall margins are enhanced by 6.62 % and 4.45% respectively. The adiabatic efficiency of model 1 and model 2 are decreased by 0.79 % and 1.08 % respectively.