Matevz Dular

Numerical Modelling of Cavitation Erosion

The goal of the work is to develop an expert system for monitoring and control of cavitation in hydraulic machines and to research the possibility of cavitation erosion prediction using CFD tools only. The geometry in question is a simple single hydrofoil, which is exposed to the developed cavitating flow at different flow conditions. The work was divided in more parts: numerical simulation of cavitating flow, experimental evaluation of the simulation, measurements of cavitation erosion, development of cavitation erosion model and finally the prediction of cavitation erosion using solely CFD. A study of erosion effects of cavitation on simple single hydrofoil configurations in a cavitation tunnel was made. A thin copper foil, applied to the surface of the hydrofoils, was used as an erosion sensor. A pit-count method was used to evaluate the damage. The cavitation phenomenon on hydrofoils at different flow conditions (system pressure, flow velocity) was observed. The erosion model is based on the physical description of different phenomena (cavitation cloud implosion, pressure wave emission and its attenuation, micro-jet formation and finally pit formation), which are involved in the process of pit formation. The cavitating flow was simulated using an “in house” CFD code which uses barotropic state law. The code was previously tested on numerous experiments. For the present case the predictions of velocity profiles and pressure evolutions in the vicinity of the hydrofoil were compared to experimentally measured data. In all cases a very good correlation was obtained. The erosion model was implemented into the code. It used values of local pressure, local void fraction and flow velocity to determine the magnitude of damage at a certain point. The results of prediction were compared to the experimentally measured damage on the hydrofoil and it was shown that it is possible, for this simple case, to use solely CFD tools to predict cavitation erosion evolution in time, final extent and final magnitude with a very good accuracy.Copyright

Rotating corrected based cavitation model for a centrifugal pump

Cavitation has bothered the hydraulic machinery for centuries, especially in pumps. It is essential to establish a solid way to predict the unsteady cavitation evolution with considerable accuracy. A novel cavitation model was proposed, considering the rotating motion characteristic of centrifugal pump. Comparisons were made with three other cavitation models and validated by experiments. Considerable agreements can be noticed between simulations and tests. All cavitation models employed have similar performance on predicting the pump head drop curve with proper empirical coefficients, and also the unsteady cavitation evolution was well solved. The proposed rotating corrected-based cavitation model (rotating based Zwart-Gerber-Belamri (RZGB)) obtained identical triangle cavity structure with the experiment visualizations, while the others also got triangle structure but with opposite direction. The maximum flow velocity in the impeller passage appears near the shroud, contributing to the typical triangle cavity structure. A preprocessed method for instant rotating images was carried out for evaluating the erosion risk area in centrifugal pump, based on the standard deviation of gray level. The results imply that the unsteady rear part of the attached cavity is vulnerable to be damaged, where the re-entrant flow was noticed. This work presented a suitable cavitation model and reliable numerical simulation approach for predicting cavitating flows in centrifugal pump. [DOI: 10.1115/1.4040068]

Influence on the Axial Flow Fan Aerodynamic Characteristic Due to an Introduction of Internal Secondary Flow

Axial fans often show adverse flow conditions at the fan hub and at the tip of the blades. Modification of conventional axial fan blades is presented. Hollow blades were manufactured from the hub to the trailing edge at the tip of the blades. Hollow blades enabled the formation of self-induced internal flow through internal passages. The internal flow enters the internal radial flow passages of the hollow blades through the openings near the fan hub and exits through the tip trailing edge slots. Study of the influence of internal flow on the flow field of axial fan and modifications of axial fan aerodynamic characteristics is presented. The characteristics of the axial fan with the internal flow were compared to characteristics of a geometrically equivalent fan without internal flow. The results show integral measurements of performance testing using standardized test rig, and the measurements of local characteristics. The measurements of local characteristics were performed with a hot-wire anemometry, five-hole probe and computer-aided visualization. We attained reduction of adverse flow conditions near the blade tip trailing edge, boundary-layer reduction on the blade suction side and reduction of flow separation. Introduction of the self-induced blowing led to the preservation of external flow direction, defined by blade geometry and enabled maximal local energy conversion. The integral characteristic reached higher degree of efficiency.Copyright

Method for Cavitation Erosion Prediction: Model Development

A study of visual and erosion effects of cavitation on simple single hydrofoil configurations in a cavitation tunnel was made. A two-dimensional hydrofoil with circular leading edge was used for the experiments. In addition, the hydrofoil geometry was modified to obtain some three-dimensional cavitation effects. A thin copper foil, applied to the surface of the hydrofoil, was used as an erosion sensor. The cavitation phenomenon on hydrofoils at different flow conditions (system pressure, water gas content, flow velocity) was observed. Images of vapour cavities from above and from a side view were taken. Plausible results that showed a significant relationship between cavitation erosion and the visual effects of cavitation made it possible to use these information to develop a cavitation erosion model. The model is based on the physical description of different phenomena, which are involved in the process of pit formation — pressure wave emission and its attenuation, micro-jet formation, the jet impact to the solid surface and pit creation. The model is capable to predict the influence of significant parameters as flow velocity and gas content of water. The model that was developed on the basis of measurements of cavitation on a single hydrofoil was later tested on an actual hydraulic machine in the form of a radial pump. The predicted magnitude and distribution of cavitation damage relates well to the experimentally measured one.Copyright