Jana Jablonská

Methods of experimental investigation of cavitation in a convergent - divergent nozzle of rectangular cross section

Abstract Cavitation is a phenomenon with both positive and negative effects and with dynamic manifestations in hydraulic, food, chemical and other machinery. This article deals with the detection and dynamic behavior of cavitation clouds in water flows through a rectangular cross-section convergent-divergent nozzle. Cavitation was measured by methods applicable in engineering practice. Pressure, flow rate, noise, vibration, and amount of air dissolved in the liquid were measured and cavitation region was recorded with a high-speed camera. Evaluation of acquired images in connection with measured pressure pulsations and mechanical vibrations was performed with the use of the FFT method. In certain cases, dimensionless parameters were used to generalize the measurements. The results will be used to specify multiphase mathematical cavitation model parameters.

Methodology of Cavitation Modelling in Diffusers

Cavitation is undesirable phenomenon occurring mainly in the flow of water in pumps and turbines, and therefore it is necessary to pay attention to it. The purpose is to explore the possibility of the mathematical modelling of the cavitation using Singhal cavitation model, which solves the multiphase flow of water and vapour. The issue can be solved taking into account the non-condensable gas (air). Singhal cavitation model was selected on the basis of good experience in the modelling of dynamic cavitation changes in the Laval nozzle [2], [10]. This article presents two alternatives. The first variant is testing a mathematical model for flow in a simple geometry of the cone. The second variant resolves the formation of cavitation rope behind vortex generator and the results are compared with experiment.

Diagnostics of Resistance Coefficients and Cavitation of Flow Control Valve

The paper deals with the determination of characteristic as dependence of the valve pressure drop on the flow rate, flow characteristic and cavitation conditions in case of water flow in the flow control valve. Emphasis is put on the utilization of simple, available relationships and measuring for identification of the basic valve coefficients, e.g. loss coefficient, flow rate coefficient and cavitation factor. These coefficients are used for designing of pipe circuits. In this paper there is defined methodology for determining those coefficients and is applied to the modified cone of flow control valve for verification the linear flow characteristic. It is necessary to consider the fact that in various countries the modifications of coefficients are preferred and it is therefore necessary to specify them.

The Investigation of the Cavitation Phenomenon in the Laval Nozzle with Full and Partial Surface Wetting

Abstract The article deals with the cavitation phenomenon affected by full and partial wetting of the wall. For the numerical computation of flow in the Laval nozzle the Schnerr-Sauer cavitation model was tested and was used for cavitation research of flow within the nozzle considering partial surface wetting. The coefficient of wetting for various materials was determined using experimental, theoretical and numerical methods of fluid flow due to partial surface wetting.

Spectral Analysis of Pressure, Noise and Vibration Velocity Measurement in Cavitation

Abstract The article deals with experimental investigation of water cavitation in the convergent-divergent nozzle of rectangular cross-section. In practice, a quick and simple determination of cavitation is essential, especially if it is basic cavitation or cavitation generated additionally by the air being sucked. Air influences the formation, development and size of the cavity area in hydraulic elements. Removal or reduction of the cavity area is possible by structural changes of the element. In case of the cavitation with the suction air, it is necessary to find the source of the air and seal it. The pressure gradient, the flow, the oxygen content in the tank, and hence the air dissolved in the water, the air flow rate, the noise intensity and the vibration velocity on the nozzle wall were measured on laboratory equipment. From the selected measurements the frequency spectrum of the variation of the water flow of the cavity with cavitation without air saturation and with air saturation was compared and evaluated.

Physical and mathematical fundamentals of cavitation

Increasing of operating parameters when using fittings in hydraulic circuit puts high demands on their durability and reliability. Under these operating conditions, the valves are mechanically worn mainly by cavitation. Therefore, it is necessary to identify the conditions of normal operation and operation with cavitation. One way of cavitation determining in addition to the practical test is mathematical modeling. Firstly, it is necessary to specify the mathematical models and verify their correctness by verification with experiment. The detailed analysis of the results obtained in a laboratory apparatus of the water flow through the convergent-divergent nozzle of rectangular cross section and the results of modeling, it was found that the cavitation is a term containing vapour and air cavitation, while both physical processes occur simultaneously. Experience has been applied to a practical problem of water flow through control valve.

Released air during vapor and air cavitation

Cavitation today is a very important problem that is solved by means of experimental and mathematical methods. The article deals with the generation of cavitation in convergent divergent nozzle of rectangular cross section. Measurement of pressure, flow rate, temperature, amount of dissolved air in the liquid and visualization of cavitation area using high-speed camera was performed for different flow rates. The measurement results were generalized by dimensionless analysis, which allows easy detection of cavitation in the nozzle. For numerical simulation the multiphase mathematical model of cavitation consisting of water and vapor was created. During verification the disagreement with the measurements for higher flow rates was proved, therefore the model was extended to multiphase mathematical model (water, vapor and air), due to release of dissolved air. For the mathematical modeling the multiphase turbulence RNG k-e model for low Reynolds number flow with vapor and air cavitation was used. Subsequently...

Mathematical Investigation of the Cavitation Phenomenon in the Nozzle with Partially Surface Wetting

Abstract Partially surface wetting has a great influence on friction losses in the fluid flow in both the pipeline system and the complex shape of hydraulic elements. In many hydraulic elements (valves, pump impellers), cavitation is generated, which significantly changes the hydraulic flow parameters, so the last part of the article is devoted to the mathematical solution of this phenomena and evaluates the impact of wall wetting on the size and shape of the cavitation area which appears in the nozzle and in small gaps at special conditions. If the cavitation appears e. g. near the wall of pipes, the blades of turbine or a pump, then it destroys the material surface. On the basis of this physical experiment (nozzle), a two-dimensional (2D) mathematical cavitation model of Schnerr-Sauer was made and calculated shape and size of the cavitation region was compared with the experiment. Later this verified model of cavitation was used for cavitation research flow with partial surface wetting. The pressure drop and the size of the cavitation area as it flows from partially surface wetting theory was tested depending on the adhesion coefficient.

Cavitation its Use and Mathematical Modelling

The problem of cavitation is highly discussed and published phenomenon not only in a negative sense, but in the positive use also. In hydraulic systems the cavitation is undesirable phenomena specific by cavitation wear and noise. These systems operate at ever-increasing operating conditions (pressure and flow). For this reason, this phenomenon occurs in many technical applications. The problem cavitation is very large, so it is necessary to define the area of scientific interest. Currently, there is an increasing interest in the fields of mathematical and experimental research in hydraulic elements and systems engineering practice. The paper is focused on modelling of shape of cavitation area and influence of air content primarily on the simple geometry of plane nozzle, where the water is flowing.