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Projects / Programmes source: ARIS

Study of thermodynamic effects of cavitation

Research activity

Code Science Field Subfield
2.05.05  Engineering sciences and technologies  Mechanics  Fluid mechanics 

Code Science Field
T210  Technological sciences  Mechanical engineering, hydraulics, vacuum technology, vibration and acoustic engineering 
Keywords
Cavitation, erosion, fluid dynamics, two-phase flow, numerical simulation
Evaluation (rules)
source: COBISS
Researchers (1)
no. Code Name and surname Research area Role Period No. of publicationsNo. of publications
1.  23471  PhD Matevž Dular  Energy engineering  Head  2007 - 2009  465 
Abstract
Different problems from cavitation are encountered in practice. Effects are mostly related to vibration, noise, erosion, changes in flow hydrodynamics, increase of hydrodynamic drag, light and thermal effects. Especially the former are still relatively unknown, but are of great significance due to high temperatures that occur on the micro scale level. On macroscopic level thermal effects become significant when dealing with liquids of temperature close to the critical temperatures – for example at pumping of cryogenic liquids. In the last years also a possibility of using of high temperatures for energy harvesting in nuclear fusion reactors has been proposed. Cavitation will be studied on more scale levels. Firstly we will consider acoustic cavitation, where we will study thermodynamic effects of single bubbles. Cavitation will be generated with ultrasound in a small vessel. With aid of high-speed thermo camera analyses of local thermal effects in boundary layer that occur prior and post bubble collapse will be studied. A transfer of knowledge from studies of single bubble effects to the cavitation cloud scale will follow. Cavitation in form of clouds will be generated hydro-dynamically in the vicinity of a submerged body in a cavitation tunnel. Again thermovision method will be used to study the thermodynamic effects in the boundary layer close to the channel wall, which will be made out of silicon glass. In the last years a significant increase of computer power could be seen. As a consequence different approaches to simulations of cavitating flow have gained on importance. But still none of them considers thermodynamic effects. There are two main reasons for this: 1. there are no experimental data to evaluate the simulation results and 2. the complexity of simulations is immense. With measurements, which will be performed within the scope of the project, we will first determine and evaluate the thermodynamic effects, which will lead to better and more general simulation methods. On the basis of results conclusions on events that preceded the moment of measurement could be made. There will be a possibility to evaluate extreme conditions in cavitation bubble (temperature), which are at the present time estimated only on a basis of theoretical studies. Further application of measurements and improved simulation techniques is development of better models for the prediction of cavitation erosion, which is probably the most ubiquitous problem of cavitating flow.
Significance for science
The project results are visible. In the past two years, papers received 20 citations, which is a high number in the field of mechanical engineering and for a post-doctoral project. Study of thermodynamic effects of cavitation is of a great importance since the field of fluid flow at temperatures near the critical point is still relatively unexplored at a basic level - only indirect studies of thermodynamic effects of cavitation exist. The research results will be used for the purpose of validation of existing theories and to develop new or refine the old hypotheses. In addition to studies of thermodynamic effects of cavitation we also developed a new model of cavitation erosion prediction, which was successfully implemented in the CFD numerical code. We showed for the first time that the numerical prediction of cavitation erosion is at all possible. We also showed that the cavitation damage tends to cluster, what accelerates the erosion and eplaines the physical background of the erosion rate trend. By an experimental and numerical study we gave an interpretation of the asymmetric cavitation phenomena on an isolated profile. The results of this study led to the construction of a better test section for the study of thermodynamic effects. Further study showed that cavitation in a small test section becomes stabile without cloud separation. The hypothesis on this phenomenon, was made together with researchers from LML ENSAM Lille (France). For its conformation we performed additional experiments in the six test sections. The results of this study will serve to improve model testing methods in turbo machinery. The paper is currently under review. Numerical simulation techniques were upgraded to take account the deformable nature of the walls of bodies and by the introduction of compressibility and of liquid and vapor phase. This enabled the prediction of several effects that could previously be estimated only indirectly.
Significance for the country
Research results can be applied in different Slovenian companies, which deal with the problem of fluid flow (Litostroj, Turboinštitut, Rotomatika ...). This mainly applies to the improved methods of simulation of cavitating flow, enabling better and faster performance projections of various hydraulic machines. We can optimize the operation of the machines - increase efficiency, reduce the need for experimental measurements and at the same time lower the cost of the prototype and maintenance costs. Further on the improvement in predictions of cavitation erosion, which is among the most complex problems of cavitation in the machine, leads to improved conditions in hydraulic machinery, increased efficiency, lowering costs of maintenance and enabled longer periods between required overhauls. Acoustic cavitation, which has been the subject of research in the beginning of the project is also used for different purposes in medicine (for example, at removal of the kidney stones - lithotripsy). Indirect research findings will could serve for improvements in the use of different techniques. In the scope of the project also the only cavitation tunnel in Slovenia was improved and a number of smaller test sections were constructed. Finally, more cooperation with foreign institutions was established. Many measurements and numerical simulations were carried out together with researchers from LML ENSAM Lille (France). Some additional experiments were performed at the TU Darmstadt (Germany). Opportunities for cooperation with TU Munich also opened. Since the project was related to the flow of fluids near the critical temperature we also received much interest from the European Space Agency (ESA).
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