Machinery - Repository of the Faculty of Mechanical Engineering, University of Belgrade
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Error Estimators for a Krylov Subspace Iterative Method for Solving Linear Systems of Equations with a Symmetric Indefinite Matrix
No full textThis paper describes a Krylov subspace iterative method designed for solving linear systems of equations with a large, symmetric, nonsingular, and indefinite matrix. This method is tailored to enable the evaluation of error estimates for the computed iterates. The availability of error estimates makes it possible to terminate the iterative process when the estimated error is smaller than a user-specified tolerance. The error estimates are calculated by leveraging the relationship between the iterates and Gauss-type quadrature rules. Computed examples illustrate the performance of the iterative method and the error estimates
NiMn2O4 nano-cotton particles and nanofibers: Exploring structural, magnetic and electrochemical energy storage properties
No full textHerein, nickel manganite (NiMn2O4) was successfully synthesized via glycine-nitrate sol-gel combustion process to obtain nano-cotton particles (SG-800) and electrospinning method to obtain nanofibers (ES-400), both followed by calcination. Synthesized materials were characterized in order to evaluate structural, magnetic and energy storage properties. The X-ray diffraction (XRD) pattern revealed the formation of a cubic spinel structure in both cases. The crystallite size of SG-800 (53 nm) was higher than that of ES-400 (16 nm). X-ray photoelectron spectroscopy showed the presence of Ni2+, Mn2+, Mn3+ and Mn4+ valence states in both samples. Synthesized materials were paramagnetic at room temperature, turning to ferromagnetic ordering at the critical temperature of 104 and 95 K, while the appearance of the spin-glass-like state was observed at 65 and 80 K for SG-800 and ES-400, respectively. SG-800 and ES-400 were tested in different electrolytes on a glassy carbon electrode as a substrate and demonstrated potential for energy storage through diffusion-controlled, Faraday redox electrochemical reactions. Carbon aerogel (CA) produced by thermal carbonization of lyophilized sodium-alginate hydrogel exhibited EDLC capacitor-like behavior, as shown by electrochemical characterization. Hybrid supercapacitors were assembled from SG-800 or ES-400 and CA and their performance was evaluated. The SG-800 as an electrode material showed superior capacitance and stability, probably due to higher crystallinity and formation of active sites for electrochemical redox reactions.This is the peer-reviewed version of the article: Milena P. Dojcinović, Vladan Kusigerski, Ivana B. Stojković Simatović, Vera P. Pavlović, Janez Kovač, Matjaž Spreitzer, and Maria Vesna Nikolić, "NiMn2O4 Nano-Cotton Particles and Nanofibers: Exploring Structural, Magnetic and Electrochemical Energy Storage Properties" in Journal of Energy Storage, Volume 131, Part A (2025) [https://doi.org/10.1016/j.est.2025.117442
SURVEYING THE INFLUENTIAL FACTORS ON WHOLE-BODY VIBRATION AT EARTHMOVING MACHINERY WORKPLACES
No full textThe study aimed to measure whole-body vibrations exposure for operators of various earthmoving machines (bulldozers,
dumpers, excavators, bucket wheel excavators, and loaders) using the V31-A triaxial accelerometer and to survey its
influential factors. The study then compared the obtained results for the A(8) parameter which measures average exposure
over an eight-hour day across different types of earthmoving machines. The comparison focused on determining the
average values, value ranges, and deviations of A(8) m/s² values by machine type. Additionally, the study compared the
measured results with the prescribed whole-body vibrations limit values set by existing regulatory standards. Furthermore,
the dependence of the measured whole-body vibrations values on influential factors such as the age of the earthmoving
machines, the power of the machines, the operators’ age, experience, gender, and body mass index was analyzed. The
research results indicated that bulldozers have the highest mean value of daily average exposure to vibration while
excavators exhibit the widest range of daily average exposure to vibration. The findings also indicated that there is no
correlation between the daily average exposure to vibration and the age of the earthmoving machine, the power of
machines, or the operators’ age or body mass index. Daily average exposure to vibration exceedances occurred in both
older and newer earthmoving machines, regardless of their power, according to the ISO 2631-1 standard, with statistical
significance. The obtained research results were analyzed using correlation analysis and nonlinear models, but none of the
models showed a significant influence of the considered factors on the daily average exposure to vibration. The
recommendation for further research is to expand the range of influencing factors as well as the sample size and to make
additional efforts to identify significant predictors of whole-body vibrations
INVESTIGATION OF TRIBOLOGICAL BEHAVIOUR OF PA12 SLIDING BEARINGS PRODUCED BY SELECTIVE LASER SINTERING
No full textThis research examines dry sliding of journal bearings made of Polyamide 12 (PA12) using additive technologies, addressing the gap in knowledge related to tribological behaviour in real conditions. In contrast to traditional tribological tests on flat surfaces, in this paper, full-scale journal bearings are tested under radial load. Three different pressure-velocity (PV) regimes were used to determine their effect on the coefficient of friction (COF), contact temperature, and surface morphology change. The results show that after a period of running-in, all tested samples reached a steady state with friction coefficients around 0.1 with insignificant oscillations, which indicates a stable and repeatable friction behaviour. Contact temperatures vary with operating conditions, where they reach their maximum at the most loaded regime, around 90 degrees, but it is still below the PA12 Vicat softening temperature (175°C), which indicates thermal stability. Scanning electron microscopy (SEM) analysis of the surface before and after testing confirms microstructural changes at the contact zone, showing a transition from a particulate to a smoother structure. The intermediate mode used in the test shows an optimal balance between frictional performance and thermal stability. The obtained results demonstrate that SLS-printed PA12 bearings can be applied in conditions without external lubrication, thereby enabling maintenance-free systems with operational parameters suitable for use. This work provides significant insight into industrial applicability and provides a basis for future tests that will deal with long-term tests and comparative evaluation with bearings produced via other additive manufacturing techniques
Development of a static test platform for determining the loads on an Unmanned Aerial Vehicle (UAV)
No full textEnsuring the structural integrity of unmanned aerial vehicles (UAVs) before flight requires reliable experimental procedures that can replicate operational loading scenarios. In this paper, a static testing platform was developed to reproduce conditions characteristic of vertical lift and cruise motor activation, providing detailed insight into the distribution of forces, moments, and dynamic effects acting on the structure. The experimental data were employed to verify finite element simulations and to support the creation of an artificial neural network model capable of rapid load prediction in structural components. Beyond fast estimation, the ANN approach was used to identify sensor placement strategies that improve measurement quality within the testing setup. The platform integrates a reinforced frame with load and displacement sensors, while a dedicated acquisition system ensures accurate monitoring of responses under controlled loads. By combining experimental validation with advanced modeling techniques, the proposed methodology reduces structural uncertainty and contributes to the design of UAVs with higher reliability and efficiency, suitable for demanding applications in defense, surveillance, transport, and agriculture
Thermodynamic Evaluation and Working Fluid Selection for a Heat Pump Integrated into a Hydropower Plant HVAC System: A Case Study from Serbia
No full textThis paper presents a thermodynamic analysis of two types of heat pumps integrated into the heating, ventilation, and air conditioning (HVAC) system
of the “Bajina Bašta” hydropower plant located in Serbia. The study aims to replace existing electric boilers with more energy-efficient solutions by utilising
renewable heat sources. A comparative evaluation of water source heat pump (WSHP) and air source heat pump (ASHP) configurations was conducted using a
custom-developed MATLAB model based on CoolProp data of working fluids. The analysis was supported by real data obtained through in situ measurements
of air and water temperatures at the selected location, ensuring accurate input parameters for the simulations. Five refrigerants (R-410A, R-407C, R-134a,
R-32, and R-1270) were used, and R-32 was selected as the optimal working fluid because of its high efficiency, moderate flammability, and low environmental
impact. The lowest coefficient of performance (COP) for the WSHP was 3.27 in January, while the seasonal coefficient of performance (SCOP) reached 3.36,
approximately 15.5 % higher than the ASHP counterpart (SCOP = 2.91). The study confirms that, upon analyzing the entire heating period, WSHP systems are
technically and environmentally superior to ASHPs in the locations studied. The proposed configuration, based on real measured data and obtained results, can
significantly improve energy efficiency and reduce internal electricity consumption in hydropower plants, thereby supporting the decarbonisation of large-scale
renewable energy facilities. While most previous studies have focused on improving energy efficiency in buildings, this work demonstrates the substantial yet
underexplored potential for efficiency improvements in the electricity production sector in Serbia. The study specifically examines hydropower plants in Serbia,
where heating and air conditioning systems built in the 1960s remain highly energy inefficient. By quantifying the benefits of integrating WSHP and ASHP
systems, the research highlights a pathway toward significantly enhancing the energy efficiency of hydropower infrastructures.451-03-66/2024-03/20010
Neuro-fuzzy control of commercial vehicles braking
No full textIncreasing dynamic performance and the general level of automation of commercial vehicles emphasize the issue of safety. Modern braking systems focus on sustaining vehicle stability, often degrading the brake performance. The major downgrades of the braking performance are nearly impossible to model using a classical mathematical approach, making them not feasible to use in real braking system controllers. In this paper, the use of combined Neural Networks and Fuzzy logic for the control of the braking system of a commercial vehicle while maximizing performance and sustaining stability is proposed. The control system comprises adhesion estimation, an inverse brake model, and a fuzzy logic controller to keep the system giving optimal control signals in various brake conditions while sustaining vehicle stability and steerability. The results based on a semi-trailer system reveal the success of the proposed AI-based braking system algorithm while braking under varying conditions.No. 451-03-65/2024-03/200105, February 5, 2024
Nexus between geometry and thermal-hydraulic scaling of horizontal steam generator
No full textMature technology of large-scale nuclear power plants is a strong basis for the development of small modular and medium size reactors. Therefore, it is important to study the nexus between geometry and thermal-hydraulic scaling with the aim to take advantage of large-scale plants to develop safe and reliable scaled-down plants. The present study investigates the thermal-hydraulics of a large-scale Horizontal Steam Generator (HSG) built in the WWER 1000 type nuclear power plant, and a thermal-hydraulics of its replica with the 50% scaled-down geometry. Unlike most previous studies, which primarily relied on the similitude concept for scaling analysis, this work employs numerical simulations with an in-house computational code based on a three-dimensional two-fluid model approach and closure laws for the prediction of interfacial transport phenomena. Obtained results show that a uniform linear reduction of HSG dimensions in three-dimensional space leads to strong non-linear changes of thermal-hydraulic parameters. Nearly the same ranges of void fraction and two-phase flow velocity changes along vertical and horizontal directions of tube bundles are obtained in the full-size HSG and in the HSG with 50% scaled-down geometry if the tube bundle volumetric heat flux in the 50% scaled-down HSG geometry is two times greater than the full-size HSG value. It is shown that a significant increase of the volumetric heat flux is achievable with a reduced diameter of tubes in the bundle and a corresponding increase of the primary side reactor coolant flow rate, although the HSG primary and secondary fluid inlet and outlet temperatures and pressure levels are kept constant. Therefore, the scaled-down HSG geometry enables significant increase of heat power per unit of tube bundle volumes, while the preserved similarity of the thermal-hydraulic conditions ensures that the scaled-down HSG operates within safe and reliable limits comparable to the full-size HSG. The findings contribute to an understanding of HSG scaling effects and support the development of small modular and medium size reactors
EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE TURBULENT SWIRLING FLOW IN PIPE BEHIND THE AXIAL FAN IMPELLER
No full textExperimental and numerical research of the turbulent swirling flow in pipe behind the axial fan impeller, which generates solid body profile of the circumferential velocity, is presented in this paper. This is a phenomenon relevant to numerous industrial applications, where axial fans are still inbuilt without the guide vanes. The experimental investigation was carried out using stereo PIV, LDA, and original classical probes. However, in this research are discussed three velocity components measured subsequently by use of the one-component LDA system.
The applied numerical approach involved solving the Navier-Stokes equations under appropriate turbulence modeling conditions, ensuring a high level of accuracy in predicting the swirl intensity and velocity distribution in the pipe. The combination of the unstructured mesh in the inlet and the runner section and structured mesh in the outlet section with the application of the SST and k-ε turbulence models led to the final results. Average velocity profiles, determined in this numerical simulation, are compared with the experimentally obtained data and differences are quantified. These experimentally validated numerical results enable good physical interpretation of the development of the average velocity profiles of the generated turbulent swirling flow in the entire domain.no. 451-03-65/2024-03/20010