Alexander Stanilov

The present study investigates how bed roughness affects the velocity field in a laboratory water channel designed for testing hydrokinetic turbines. The main aim is to evaluate the impact of bed morphology on flow hydrodynamics and, consequently, on the turbines’ operating conditions. Experimental studies were carried out in two hydraulic regimes—smooth channel bed and bed with artificially created irregularities—at flow velocities of 0.3 and 0.4 m/s, with a depth of 180 mm. The results indicate that bed roughness significantly affects the velocity field, leading to increased turbulent fluctuations, the formation of vortex structures, and momentum redistribution. There is also localized velocity acceleration within the measurement region caused by local acceleration between the bed irregularities, which is influenced by the geometry of the water channel. A clear vertical velocity distribution is established, with larger fluctuations being registered in the surface layer, while near the channel bed, the flow is more stable. The results obtained emphasize the importance of bed roughness as a key factor in laboratory modeling and analysis of hydrokinetic turbine performance, with a direct impact on their efficiency and load.

This study presents a comparative analysis of the potential for combined use of wind and solar energy in nine key coastal settlements on the Bulgarian Black Sea coast—Shabla, Balchik, Varna, Byala, Obzor, Nesebar, Burgas, Primorsko, and Tsarevo—selected for their diverse geographical and meteorological characteristics. The study evaluates the feasibility of implementing hybrid renewable energy systems by analyzing the average annual solar radiation and wind velocity for each location. A methodology based on physical and technical parameters is applied to determine the required installed capacity of photovoltaic systems to meet the average annual household electricity consumption of 6000 kWh. Concurrently, wind energy potential is assessed through theoretical and practical models using two turbine sizes (3 m and 6 m in diameter), which represent small-scale residential wind applications.

In recent years, a growing global effort has been underway to reduce the Earth’s carbon footprint. One of the main strategies to achieve this goal is the utilization of available renewable energy resources. Among the largest and most inexhaustible is hydro-power. This paper presents an experimental study of three hydrokinetic turbines tested under real river conditions, aiming to evaluate their effectiveness in harnessing the kinetic energy of flowing water. The experiment is described in detail, including velocity field measurements conducted within the river section used for testing. Based on the experimental data, the main performance characteristics of the three turbines are presented, specifically their power output and efficiency. The importance of selecting an optimal riverbed site and customizing turbine runners to local flow conditions is highlighted, as even slight velocity fluctuations can significantly impact performance. Among the tested designs, the K1–6 turbine runner showed the highest power and efficiency, while the K2–4 runner provided superior rotational stability, making it promising for consistent energy output in variable flow environments

The centrifugal fans with radial blades are used in various industrial energy systems, associated with high dustiness, due to their self-cleaning ability and their reliable operation. Radial fans are used in cement, asphalt industries and various dust collection facilities. The uniqueness of the different working environment and the specifics of each system require the use of a fan, designed for the particular application. Due to the dimensions of the machines, before the fan is produced, it normally goes through model tests. In model studies of rotodynamic fluid machines, differences are sought between the expected parameters from the application of the similarity theory and the obtained energy parameters. These differences are commonly referred to as the scale effect. Estimating the scale effect in model tests leads to the use of energy efficient machines for a given system. These are the one, that give the desired system parameters when operating in the region of maximum efficiency. Thus, the purpose of modeling is to transfer the efficiency obtained for a given mode of operation of the model machine to the prototype. The operation in the optimal mode of the prototype fan leads to energy savings and a reduction in energy costs for a given technological system. In the presented paper are evaluated the differences between expected and obtained parameters that help to realize a model for a centrifugal fan with radial blades. The results of model tests, conducted on two geometrically similar centrifugal fans with radial blades, where the inlet and outlet angles of the impeller are the same (β₁=β₂=90⁰ ), are given. The fan impellers are semi-open type with 12 blades. Each of the impellers is tested in a geometrically similar helical chamber. Model tests have been conducted on a laboratory test-bench, meeting the requirements of AMCA 210 or ISO5801. The test-bench is equipped with the complete measuring equipment, according to the standards used. During the model tests, both the energy parameters of the fans and the parameters of the surrounding environment are measured. The results of the energy tests of the fans are given in the paper, as well as the coefficients concerning the scale effect are determined and analyzed.

The paper presents the results of a study of the error of a central water meter at different modes of water consumption. A stand has been designed for the purpose of the study and tests of the error of the water meter have been performed, in compliance with the requirements of Measuring Instruments (MID) Directive (2014/32/EU). The errors of the water meter have been determined in the area close to its start mode, which is not included in the area of inspection. The measuring errors have been also determined in specific operation modes with frequent starts and stops, associated with the flow of smaller water quantities. The study contains an analysis of the obtained differences in the accuracy of measurement during metrological verification of the water meter and reading on a real site. A quantitative estimate of the established losses in water metering is given.