Machinery - Repository of the Faculty of Mechanical Engineering, University of Belgrade
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The role of numerical simulation of fatigue crack growth in failure analysis of a turbine shaft
No full textFailure analysis of a hydropower plant turbine shaft was performed by using the Finite Element Method (FEM) to assess its structural integrity and remaining life. Static and dynamic loading was applied to assess relevant fracture mechanics parameters using FEM for stress analysis and its extended version (XFEM) for simulation of fatigue crack growth. Application of XFEM to turbine shaft crack growth problem is in focus of this paper, in combination with material properties and its expected behavior under amplitude loading. The goal of the research was to determine the remaining life of a turbine shaft that has failed in service. Such an approach provided clear answer to why the cracked shaft failed in a short period of time. Based on that, suggestions to prevent such a failure are given
Multi-Objective Hybrid Sailfish Optimization Algorithm for Planetary Gearbox and Mechanical Engineering Design Optimization Problems
No full textThis paper introduces a hybrid multi-objective optimization algorithm, designated HMODESFO, which amalgamates the exploratory prowess of Differential Evolution (DE) with the rapid convergence attributes of the Sailfish Optimization (SFO) algorithm. The primary objective is to address multi-objective optimization challenges within mechanical engineering, with a specific emphasis on planetary gearbox optimization. The algorithm is equipped with the ability to dynamically select the optimal mutation operator, contingent upon an adaptive normalized population spacing parameter. The efficacy of HMODESFO has been substantiated through rigorous validation against established industry benchmarks, including a suite of Zitzler-Deb-Thiele (ZDT) and Zeb-Thiele-Laumanns-Zitzler (DTLZ) problems, where it exhibited superior performance. The outcomes underscore the algorithm’s markedly enhanced optimization capabilities relative to existing methods, particularly in tackling highly intricate multi-objective planetary gearbox optimization problems. Additionally, the performance of HMODESFO is evaluated against selected well-known mechanical engineering test problems, further accentuating its adeptness in resolving complex optimization challenges within this domain
New error estimates for the conjugate gradient method
No full textThe conjugate gradient method is the default iterative method for the solution of linear systems of equations with a large symmetric positive definite matrix
. The development of techniques for estimating the norm of the error in iterates computed by this method has received considerable attention. Available methods for bracketing the
-norm of the error evaluate pairs of Gauss and Gauss–Radau quadrature rules to determine lower and upper bounds. The latter rule requires a user to allocate a node (the Radau node) between the origin and the smallest eigenvalue of the system matrix. The determination of such a node generally demands further computations to estimate the location of the smallest eigenvalue; see, e.g., Golub and Meurant (1997), Golub and Meurant (2010), Golub and Strakoš (1994), Meurant (1997), Meurant (1999). An approach that avoids the need to know a lower bound for the smallest eigenvalue is to replace the Gauss–Radau quadrature rule by an anti-Gauss rule as described by Calvetti et al. (2000). However, this approach may sometimes yield inaccurate error norm estimates. This paper proposes the use of pairs of Gauss and associated optimal averaged Gauss quadrature rules to estimate the
-norm of the error in iterates determined by the conjugate gradient method
Top and middle management organizational level resilience modelling in mining companies
No full textTo assess organizational resilience at different organizational levels in mining companies in Serbia, a questionnaire was created based on previous research. Total of 148 respondents, with different qualifications, whose jobs are at different organizational levels (Top/Strategic Manager Level, Middle Manager Level, First Line Managers, Transport and Mining Machine Operators and
Other Workers) were surveyed. Descriptive statistics, Statistical tests Kolmogorov and U* Mann Whitney are used to test the differences between organizational levels (operative, tactical and strategic). Aspects of organizational resilience later on have been modelled by Analytic Hierarchy Process (AHP) method. The weight coefficients obtained with the AHP model, were used to calculate the resilience index. In that aim a new software tool based on the AHP method was designed - the ResOrg software. Finally, results and conclusions are presented
Techno-Economic Comparison of a Large-Scale Nuclear Power Plant, Small Modular Reactors, and Wind and Solar Power Plant Deployment
No full textA comparison of the net present value, the payback period, and the levelized
cost of electricity for three different projects of construction and exploitation of plants for
electricity production with the aim of decarbonizing the energy sector is conducted. The
first project is the building of a large-scale nuclear power plant with a light-water reactor,
the second one is the deployment of several identical small modular reactors, and the
third project is based on solar and wind power plants. Given that the sun and wind are
intermittent renewable energy sources, it is inevitable to take into account the construction
of an energy storage facility in the last project. The results show that the most profitable
are the small modular reactors, while the investment into solar and wind power plants
is burdened with the necessary electricity storage plant costs. Another drawback of an
investment in solar and wind power plants is their shorter exploitation lifetime of 25 years
compared to the long-term operation of nuclear power plants of 60 years or even more
Energy and exergy analysis of a hybrid ORC power plant
No full textOrganic Rankine cycles (ORCs) are highly recommended technology to generate electricity from low-temperature sources such as geothermal sources. The
performance of the cycle mostly depends on operation conditions, installed equipments and selection of working fluid. This paper presents a comparative energy
and exergy analysis of a geothermal-biomass Organic Rankine cycle (ORC) for
three working fluids: R113, R245fa, and R600a. In addition, analyzed ORC is
compared with the Rankine cycle that runs on Serbian lignite from the aspect of
equivalent CO2 emission and necessary primary energy. The analyzed system is
an ORC power plant with two heat sources – geothermal water from Vranjska
Banja and wood biomass. The results revealed the highest thermal and exergy efficiency with R113, 14.53%, and 41.64%, respectively, as well as the turbine power
output of 4.21 MW with 73.20 kW pump power input. Exergy analysis showed the
highest exergy destruction rate in the heat exchanger with flue gases made by wood
combustion (80-89%) and the highest obtained exergy efficiency of 41.64% in the
case of R113. If the power plant used Serbian lignite as a fuel for the same thermal
power input, it would need between 133-169 kt/year of lignite, which would result
in CO2 emission of around 63-80 kt/yea
Temperature Response, Time Delay, and Hysteresis Loop of Heat Loss Flowmeter Thermistors
No full textA heat loss flowmeter for water was made of two
negative temperature coefficient (NTC) thick film thermistors
Th1 and Th2 and placed in a metal housing. The first thermistor
Th1 is cold and measures input water temperature Tw,
and the second thermistor Th2 is self-heated at a constant
voltage and measures water flow rate Q. At first temperature
responses T(Q), heat loss and time delay td were
measured and analyzed; then a temperature hysteresis loops
of self-heating thermistor at switching waterflow regime
was measured. The electrical responses I(Q) were measured
using the flowmeter and digital acquisition card (DAC).
An independent ultrasonic flowmeter was used in series
with a novel heat loss flowmeter. The current values of
temperature T(Q, t) of the water flow Q(t) were obtained
using Steinhart–Hart equation for thermistors. Input water
temperature Tw was used as a parameter (in the range of 5 ◦C–25 ◦C). The maximum thermistor power dissipation
was less than 1 W. The inaccuracy of the water flow rate measurement was estimated to be less than 3%. The measuring
flowmeter range of 0.003–0.3 L/s (1/2 in pipe) is suitable for measuring water consumption in households
EFFECTS OF EMPLOYEES’ ATTITUDES TOWARDS DIGITAL TRANSFORMATION FACTORS ON SMES’ SUSTAINABILITY AND FINANCIAL PERFORMANCE1
No full textDigital transformation is the most frequently analysed from the technological perspective. The following research investigates the social elements of organisational digital transformation, by placing employees in SMEs in focus. After extensive literature review the research questions and related hypotheses were defined. To test the proposed hypotheses, a questionnaire was eveloped, and 635 responses were received from individual owners, managers, and employees in organizations from Serbia, Bulgaria, and Visegrad Four countries (V4). A structural equation model has been used to explore the mediating effect on the relationship between the employee’s attitudes and their perceived opinions on the organisations’ sustainability and financial performance. All relationships within stated hypotheses were confirmed. It was revealed that employees’ positive attitude towards digitalisation is improving their self-efficacy in the digitalized work environment and, at the same time, decreasing their level of anxiety regarding such workplaces. Increased self-efficiency and lower anxiety strengthen the organization’s sustainability, which, in turn, positively influences financial performance. According to the obtained results, from the group of questions describing the attitude of the respondents towards the digitalisation of their workplaces, it
was revealed that most of the respondents consider that the digital transformation of their firms is a great idea in general (with a mean value of 3.883 on a scale of 5). Despite the lower mean value
of 3.321, a large majority of them indicated that working in a digitalised environment is exciting. When it comes to self-efficacy in a digitalised work environment, most respondents advise that
they need support and adequate task time available when completing their tasks. Importantly, the support they would find beneficial is someone they can contact if they require assistance
and/or a help assistant built into the application. Regarding the anxiety resulting from the digital transformation of their working environment, it can be determined that most of the respondents do
not experience a high level of it, considering the low mean values ranging from 1.869 to 1.937, on a scale of 5.contract no. 451-03-137/2025-03/ 20010
Estimation of residual operability and energy efficiency optimization of oil purification column
No full textThis paper examines the process of oil distillation in upstream oil and gas facilities, along with the
main equipment involved. A vertical column, a key component of the stabilization system that has
been in operation for almost 20 years, was tested. The column was temporarily taken out of
service to conduct a detailed, non-destructive inspection. Due to its long-term operation, aging,
and exposure to corrosive environments—especially oil corrosion—it represents high-risk
equipment with potential environmental and financial consequences in the event of failure.
This study aims to determine the optimal operating mode to improve output product quality.
Ultrasonic thickness testing (UTT) was performed, revealing wall thinning in critical areas. The
maximum calculated corrosion rate was 0.0722 mm/year, and the remaining service life was
estimated at 26 years. Based on the risk-based inspection (RBI) analysis, the column falls into the
high-risk category, emphasizing the importance of optimized inspection planning. An RBI analysis
was conducted using API 581 methodology, classifying the column as high-risk equipment with
potential financial consequences exceeding €2.7 million per year. In parallel, four different
operating regimes were tested to optimize energy utilization and product quality. The optimal
regime achieved a heat transfer of 100.07 MW, minimal CO2 and H2O concentrations, and
maximum recovery of C3–C5 hydrocarbons. This paper highlights the importance of condition
monitoring, targeted inspection strategies, and energy optimization in extending equipment life
and reducing operational risks. This approach enhances reliability and cost-effectiveness in oil
and gas facilities while prioritizing environmental protection.451-03-136/2025-03/200105, 451-03-136/2025-03/200051, 451-03-136/2025-03/200066, 451-03-136/2025-03/ 200213, 451-03-137/2025-03/20010
Fixed points results in S-left complete topological spaces and quasi b-metric spaces
No full textIn this paper, we presented the results for extended nonlinear contraction defined on
S-left complete topological spaces. Our results generalize earlier results in literature.
Also, we obtain the fixed point theorem for extended nonlinear contraction defined
on quasi b-metric spaces. This result extended recent results of Pasicki, obtained for
dislocated metric spaces.Grant number 451-03-137/2025-03/200105–4.2.2025