Hysteresis Characteristic in the Hump Region of a Pump-Turbine Model
The hump feature is one of the major instabilities in pump-turbines. When pump-turbines operate in the hump region, strong noise and serious fluctuations can be observed, which are harmful to their safe and stable operation and can even destroy the whole unit as well as water conveyance system. In this paper, a low specific speed ( n q = 36.1 min −1 ) pump-turbine model was experimentally investigated. Firstly, the hump characteristic was obtained under 19 mm guide vane opening conditions. More interestingly, when the hump characteristic was measured in two directions (increasing and decreasing the discharge), characteristic hysteresis was found in the hump region. The analysis of performance characteristics reveals that the hump instability is the result of Euler momentum and hydraulic losses, and different Euler momentum and hydraulic losses in the two development processes lead to the hysteresis phenomenon. Then, 12 pressure sensors were mounted in the different parts of the pump-turbine model to obtain the time and frequency characteristics. The analysis of the corresponding fast Fourier transform confirms that the hump characteristic is related to low-frequency (0.04–0.15 times rotational frequency) vortices. The occurrence and cessation of vortices depend on the operating condition and measurement direction, which contribute to the hysteresis feature. Finally, the type of the low-frequency vortices was analyzed through the cross power spectrum.
NUMERICAL ANALYSIS OF FLOW PHENOMENA RELATED TO THE UNSTABLE ENERGY-DISCHARGE CHARACTERISTIC OF A PUMP-TURBINE IN PUMP MODE
Regions of positive slope in the pressure-discharge characteristics are one of the major concerns in design and operation of centrifugal pumps, as they are limiting the admissible operating range to values above the critical discharge. The industrial pump turbine of specific speed ν=0.42 (nq=66 min-1) proposed as QNET-CFD test case TA6-04 shows a marked saddle in the energy-discharge characteristic associated to a sudden drop of efficiency versus discharge at part load. The pump-turbine consists of a shrouded impeller with five blades, a diffuser with 22 guide and stay vanes and a spiral casing. CFD flow simulations on a reduced model were carried out with a finite volume Navier-Stokes code (CFX-5.7) using block- tructured hexahedral meshes and the Menter-SST Turbulence model. Control of numerical quality has been performed. Reduced models with relatively low computational effort (mixing plane interface) already permit to capture the drop in efficiency and energy coefficient to analyze the flow phenomena inducing the drop of the energy coefficient Ψ that occurs at partial discharge. Analysis of local flow patterns and energy and velocity distributions at the rotor-stator interface that are related to the onset of recirculation are presented.
Detection of Fractures Based on Directional Filtering in Image Logging
According to the feature of the direction of fractures in image logging, a method of fracture detection in image logging based on directional filtering is presented in this paper. The fracture image is filtered by the corresponding filter based on the local direction of fractures, which enhances the fracture traces in the direction of fractures in image logging. The final fracture traces are extracted by the combination of the results of both directional filtering and the threshold segmentation. Experimental results show this method effectively detect the fractures in image logging.