c. lu

The present study aims to describe characteristics of cavitation during the startup process of a condensate pump. The pump is featured by an impeller equipped with five splitter blades. A computational fluid dynamics (CFD) work was conducted to plumb the evolution of cavitation in the pump. Effect of the volumetric flow rate on instantaneous cavitation patterns as the rotational speed of the pump increased was analyzed. The results show that high resistance to cavitation of the pump depends greatly on large area of the impeller eye, which is related to the deployment of the splitter blades. The splitter blades are insignificantly affected by cavitation. During the startup process, both the pump head and the pump efficiency vary drastically, which is insensitive to the flow rate. At a net positive suction head (NPSH) of 2.0 m, high flow rates are responsible for intensified cavitation. High volume fraction of cavitation arises near the inlet of long blades. As the rotational speed increases, the evolution of cavitation is featured by intermittency and diversified cavity patterns. Furthermore, the sum of the volume fraction of cavitation fluctuates with continuously increasing rotational speed.

Past studies showed that a micron-sized surface roughness may cause the generation of a significant unstable, stationary wave in a crossflow boundary layer, and consequently promote or delay the laminar-turbulent transition. The crossflow boundary layer is usually driven by the favorable pressure gradient which is produced by accelerated inviscid velocity. Hence, for a fixed sweep angle, the magnitude of pressure gradient is the key parameter for the excitation and evolution of the stationary crossflow mode. In order to study the effect of pressure gradient on the excitation and subsequent linear development of stationary mode, a classical FalknerSkan-Cooke boundary layer is introduced so that the magnitude of pressure gradient can be easily parameterized by an acceleration coefficient. Numerical simulation is performed to induce the stationary perturbation by chordwise-isolated, spanwise-periodic roughness at the lower branch of neutral curve. Then the excited waves develop into Rayleigh modes in the downstream region. The stationary modes with different spanwise wavenumbers in various favorable-pressure-gradient boundary layers are simulated and analysed to determine the effect of pressure gradient. And the corresponding coupling coefficients are calculated to connect the initial amplitude and the eigenmode of linear stability theory for implementing the existing prediction method of laminar-turbulent transition.

The prediction and control of the laminar-turbulent transition is crucial to the designs of vehicles, turbines, etc. The initial condition of transition depends on the exciting process of boundary-layer instability, which is the key to implement its prediction and control. The current researches confirm that the exciting process of boundary-layer instability, namely receptivity, is affected not only by different types of free-stream disturbances and shape parameters of surface roughness elements, but also by the pressure gradient of mean flow. Hence, we study the effect of pressure-gradient on local excitation of boundary-layer instability under the interaction of the low-level, isotropic free-stream turbulence and micro surface roughness in this work. The numerical results reveal the pressure-gradient effect on the receptive process and the group speed of excited wave packets in the Falkner-Skan boundary layer. The favorable/adverse pressure gradients (FPG/APG) are found to be able to promote/suppress the excitation and subsequent evolution of Tollmien–Schlichting (T-S) waves. Then the relations of the pressure gradient with the amplitude, growth rate, wave number, phase speed and shape function of excited T-S waves are studied.