Milos S. Nedeljkovic

Frequency in Shedding/Discharging Cavitation Clouds Determined by Visualization of a Submerged Cavitating Jet

Visualization of a highly submerged cavitating water jet was done by high-speed camera photography in order to study and understand the jet structure and the behavior of cloud cavitation within time and space. The influencing parameters, such as injection pressure, nozzle diameter and geometry, and nozzle direction (convergent and divergent), were experimentally proven to be very significant. Periodical shedding and discharging of cavitation clouds have been also analyzed and the corresponding frequency was determined by cloud shape analysis. Additionally, the dependence of this frequency on injection pressure and nozzle geometry has been analyzed and a simple formula of correspondence has been proposed. The formula has been tested on self-measured and literature data. The recordings of sonoluminescence phenomenon proved the bubble collapse everywhere along the jet.

Predictive Fast DSP-Based Current Controller for Thyristor Converters

The predictive current controller presented in this paper provides fast current control for thyristor converters without relying on the accurate load model. Unlike load-model-based predictive methods of current control, this method does not use load parameters in the calculations of the firing angle. Instead, thyristors are fired when the sum of the measured instantaneous load current and the load current rise, which is predicted from the voltage-time area on the dc-side inductor, is equal to the current reference. This way, accurate current control is achieved just by knowing the dc-side inductor parameters and by measuring line voltages, the load current, and the load voltage. The proposed predictive current controller exhibits a practically negligible steady-state error, the fastest possible transient response, and the fastest possible and smooth transitions between continuous- and discontinuous-current conduction modes, for both the rectification and inversion modes of operation of the converter. It shows great stability and robustness. The controller also compensates for sudden changes in both line and load voltages, as well as for voltage drops due to the commutation process. Based on the proposed algorithm, the computer simulation model and the laboratory prototype of the system were developed. Both the simulation and experimental results demonstrated an excellent dynamic performance of the proposed controller.

Fast and Robust Predictive Current Controller for Flicker Reduction in DC Arc Furnaces

A fast and robust predictive current controller for reduction of flicker caused by dc electric arc furnaces (EAFs) supplied by thyristor converters is presented in this paper. During normal EAF operation, large active- and reactive-power variations can occur at the point of furnace connection, causing unwanted variations of network voltage and correspondingly a flicker phenomenon. The main cause of this phenomenon are electrode short-circuits with large current swings, which occur during the so-called bore-in period of the furnace cycle. The proposed controller reduces the flicker caused mainly by reactive-power variations during the electrode short-circuits by providing fast and robust predictive control of the arc current without relying on the accurate arc model. The obtained simulation and experimental results demonstrate the ability of the presented current controller to effectively compensate for the electrode short-circuits, thereby enabling a reduction of the unwanted fluctuations of the reactive power, which are the main cause of the induced flicker. The simulation results show that the proposed controller can reduce the induced flicker by up to 50%, compared to the traditionally used system with PI current controller. Also, the controller is robust to the asymmetries and the abrupt changes of the network voltage.

An Experimental Investigation of Cavitating Jet Dynamic Power and Cavitation Intensity

The principal focus of this investigation were the effect of hydrodynamic and geometrical parameters on cavitating jet dynamic power and cavitation intensity under different flow conditions. Jet performance indicators were taken to be the erosion rate of the target surface and the size of the erosion area.. In addition, flow visualization was used as an additional tool to demonstrate the effect of dynamic power on the cavitating jet characteristics. The results show that the erosion rate can be used an indicator of the strength of the impinging jet to establish a correlation between the jet power and the pertinent experimental parameters. In addition, the importance of non-dimensional standoff distance was investigated and an analytical relationship established between it and the jet intensity. These expressions are proposed for calculating the efficiency of cavitating jets.Copyright

APPEARANCE OF HIGH SUBMERGED CAVITATING JET: THE CAVITATION PHENOMENON AND SONO-LUMINESCENCE

In order to study jet structure and behaviour of cloud cavitation within time and space, visualization of highly submerged cavitating water jet has been done using Stanford Optics 4 Quick 05 equipment, through endoscopes and other lenses with Drello3244 and Strobex Flash Chadwick as flashlight stroboscope. This included obligatory synchronization with several types of techniques and lenses. Images of the flow regime have been taken, allowing calculation of the non-dimensional cavitation cloud length under working conditions. Consequently a certain correlation has been proposed. The influencing parameters, such as; injection pressure, downstream pressure and cavitation number were experimentally proved to be very significant. The recordings of sono-luminescence phenomenon proved the collapsing of bubbles everywhere along the jet trajectory. In addition, the effect of temperature on sono-luminescence recordings was also a point of investigation. [Projekat Ministarstva nauke Republike Srbije, br. TR35046]

Remote labs and problem oriented engineering education

This paper describes a study case of student production of laboratory model for engineering education and its integration in the local remote laboratory. Students guided by teaching staff are passing through all phases of development and testing of engineering product. In our case, student should produce and test a fan. This part of case study concerning with courses in Hydraulic and Pneumatic Machinery, which incorporates, besides fundamental courses, subjects related to the theory, construction, design and testing of the hydraulic machines (pumps, fans, turbines) and turbo-compressors. After this, students should produce fan and plate system, which is well-known a laboratory model for Control Engineering education. In this paper, are also described procedures and benefits of integration of a laboratory model in Go-Lab Portal as a remote controlled experimental setup with materials for teaching and learning.

Poster: Remote Engineering Education Set-Up of Hydraulic Pump and System

Educational setup for demonstration of a pump operation and determination of full performance curves is presented. The upgrade of the setup has been done in order to implement electronic measurements, data acquisition, computer control, and for preparation of all necessary hardware elements for remote operation via Internet. Used equipment, rationale behind, characteristics and possibilities of the current status of the setup is discussed in detail.

Activities of Euro-CASE Engineering Education Platform

A platform dedicated to the problems of engineering education (EngEdu) has been established by European Council of Academies of Applied Sciences, Technologies and Engineering (Euro-CASE). The renewed working group (WG) of this EngEdu Platform started to work in Sept. 2017 and now tries to establish a direct link with REV conference participants and organizers, and disseminate and exchange ideas and results. Current topics of interest are presented as well as one of the projects going on.

Virtual Instrumentation Used in Engineering Education Set-Up of Hydraulic Pump and System

Numerous students attend courses in hydraulic turbo pumps at the Faculty of Mechanical Engineering University of Belgrade, as well as from other faculties. They have oral lectures and exercises with numerical tasks, but also laboratory exercises. This easily driven upgraded installation is very convenient for laboratory exercises for various groups of students. In this paper is demonstrated upgrade of the present installation with transducers instead of U-tube analogue manometers, acquisition system and LabVIEW application for hydraulic pump diagnostics and flow meter calibration. Pressure transducers are also previously calibrated. This installation has three regulating valves and two for tanks emptying. In this way students could form two simple and one complex pipeline. Serious and parallel combinations of pipe lines could be demonstrated, as well as pump by-pass regulation. Hydraulic pipe curve could be formed for one position of the valve on the pump pressure side and various pump frequencies. This could be repeated for other valve positions. In addition pump head characteristic for one rotation number is formed by changing the valve position. Good overlapping with manufacturer’s curve is demonstrated. Cavitation and turbulent swirling flow phenomena could be also visualized and demonstrated on this installation with transparent pump housing, as well as partly suction and pressure pipes.