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Item SYNTHESIS AND PHOTOSTABILITY OF SOME NEW 1,8-NAPHTHALIMIDE DERIVATIVES AS COLOUR MONOMERS FOR FLUORESCENT POLYMERS(2025-09-03) Miladinova P.M.Two new polymerizable 1,8-naphthalimide fluorophores containing a residue of a glycine or β-alanine in C-4 position of the naphthalimide ring have been synthesized. Their copolymers with methyl methacrylate were obtained. The colour coordinates of the prepared polymer have been determined. The photostability of the dyes and copolymers in dimethylformamide has been investigated and an increase in the photostability of dyes included in polymer with about 25-30 % was observed. The photostability of the polymer films obtained with copolymers and polymethyl methacrylate dyed “in mass” with the synthesized dyes has been determined by a change of colour coordinates and reflectance during the radiation with UV light. Results obtained lead to the conclusion that the dyes have good photostability and can be applied both as fluorescent dyes for dyeing of polymer materials “in mass” and as colour fluorescent monomers for obtaining of fluorescent polymers.Item FEASIBILITY STUDY INTO THE POSSIBILITY OF MANGANESE ORE ENRICHMENT WASTE USE FOR SORBENT MATERIAL DEVELOPMENT(2026-01-01) Kalymbet A.; Kubekova S.; Kapralova V.; Lavrova S.This study explores the synthesis of sorption-active phosphate materials from manganese ore enrichment tailings of the Zhairem deposit. The initial tailings, predominantly composed of calcite (76.4 %), quartz (16.4 %), and braunite, were characterized by XRD and EPMA. Acid-thermal treatment with phosphoric acid followed by calcination at 200-800°C yielded calcium - manganese phosphate materials. Phase transformations were monitored via XRD, showing formation of crystalline phosphates at 200-600°C and a glassy phase at 800°C. The product synthesized at 600°C demonstrated the lowest water solubility (9.91 %), highest pore volume (0.175 cm3 g-1), and optimal sorption capacity for Ni2+ (0.2934 mg-eq g-1), which increased to 0.4697 mg-eq g-1 after 0.4 wt. % of HCl activation. The enhanced performance is attributed to the formation of low-solubility polyphosphates and well-developed porous structures. SEM confirmed porous morphology at 600°C and denser, glassy structure at 800°C. The synthesized material showed no toxic elements such as Pb, Cd, or As, making it suitable for environmental applications. The findings indicate that Zhairem tailings are a promising raw material for producing effective sorbents for heavy metal removal, especially after acid activation. The optimal product is the calcium-manganese phosphate synthesized at 600°C.Item Modelling and numerical simulation of fire in a coffee storage hall(2021-11-22) Krumov K.; Penkova N.; Mladenov B.; Stoyanov Y.An approach for modelling of fire in halls at solid materials burning, based on gas phase combustion models in ANSYS/CFX, is proposed. The models are suitable for quick prediction of the flue gases distribution and thermal loads on the building envelopes at a design stage. They was applied for numerical simulation of such processes in a coffee storage hall at different fire positions and forced anti-smoke ventilation. The thermal, fluid flow and concentration field were obtained at the modelled scenarios. Maximal thermal loads on the building envelopes are established at the case of fire near their non-streamlined parts.Item Modelling and numerical simulation of the heat transfer and natural ventilation in storage halls(2020-11-18) Penkova N.; Krumov K.; Mladenov B.; Stoyanov Y.An algorithm for modelling of the heat transfer in storage halls at different combinations of open doors and smoke vents is developed. The models allow numerical simulations of velocity, pressure and temperature fields in the halls in cases of transient and steady state conditions, taking into account the indoor buoyancy, heat sources, heat transfer through the building envelopes and outdoor wind pressures. The models are solved numerically via finite volume method in order to estimate the necessity of additional heating at a design stage of a courier logistic storage hall, situated in the temperate climate zone, at the hardest winter conditions of the region. Recommendations about the organization of the heating and the cargo input are done based on the analyses of the fluid flow and thermal fields at the modelled scenarios.Item Performance of diffusion absorption refrigerators at variable power input(2024-01-01) Stoyanov Y.; Penkova N.; Krumov K.Absorption refrigerators have been used in recent decades for utilization of waste or solar energy to produce heat or cold for industrial purposes or to condition the microclimate in buildings. Although these units are designed at nominal power input in the generator, they can also operate at variable parameters of the heat source, which is possible in the real situations. An experimental determination of the operating range and the performance of a diffusion absorption refrigerator at heat pump and refrigeration mode were carried out in the present study. The results are useful for predicting the efficiency and the effectiveness of such systems at variable heat source capacities.Item ABSORPTION REFRIGERATION SYSTEMS FOR WASTE HEAT RECOVERY AT THE SILICATE INDUSTRY(2024-01-01) Stoyanov Y.; Penkova N.; Krumov K.; Kassabov I.The glass, ceramic and cement plants are energy intensive industrial systems, releasing high amounts of greenhouse gases. The dissipation of the waste heat reduces their profitability, energy and ecological efficiency. The absorption refrigeration systems are successful solutions for the recovery of thermal energy at different temperature levels to obtain useful cooling and heating powers for technological needs, and for air conditioning of administrative and industrial buildings. Although the absorption units in a heat pump and refrigeration cycle are currently used in building and industrial systems, they have not yet penetrated widely into the silicate factories. This paper discusses possibilities for the applications of basic types of absorption cycles at heat pump and refrigeration modes to utilize waste energy at different temperature levels in glass, ceramic and cement plants. The examined variants are demonstrated via examples, diagrams and analyses of the possible energy saving.Item INTEGRATED ALGORITHM FOR TOXICITY ASSESSMENT OF MINE TAILINGS AND SURROUNDING SOILS BASED ON CHEMICAL AND GERMINATION ASSAYS(2026-01-01) Angelova L.; Ilieva D.The evaluation of the environmental impact arising from mine tailing toxicity can be accomplished by chemical and biological assays. Several research groups and environmental agencies established working protocols for heavy metal determination and risk assessment. Nevertheless, there is no standardized methodology that integrates chemical and biological approaches for the toxicity assessment of soils and waters contaminated with heavy metals and metalloids. This study presents a comprehensive algorithm designed to assess the environmental toxicity of mine tailings and surrounding soils by applying a germination assay. Therefore, wheat germination bioassays and evaluation of bioconcentration of As, Pb, Cu, Zn, Cd, Ag, Fe, Mn, Ni, and Cr in contaminated environmental samples are carried out. The germination index showed results greater than 80 % classified as “excellent” in soil samples P1 and P2, and less than 40 % in soil sample P3 due to high Pb toxicity. The capability of plant species to remediate soils was evaluated by the Bioconcentration factor (BCF) and the Translocation factor (TF). The average results for BCF, based on Pb, As, Zn, and Cu, were 0.25, and a low TF was obtained, with values ranging between 0.05 and 0.2.Item New Glass-Ceramics in the System Ca2SiO4-Ca3(PO4)2—Phase Composition, Microstructure, and Effect on the Cell Viability(2025-08-01) Mihailova I.; Dimitrova P.; Avdeev G.; Ivanova R.; Georgiev H.; Nedkova-Shtipska M.; Teodosieva R.; Radev L.The CaO-SiO2-P2O5 system is one of the main systems studied aiming for the synthesis of new bioactive materials for bone regeneration. The interest in materials containing calcium-phosphate-silicate phases is determined by their biocompatibility, biodegradability, bioactivity, and osseointegration. The object of the present study is the synthesis by the sol-gel method of biocompatible glass-ceramics in the Ca2SiO4-Ca3(PO4)2 subsystem with the composition 6Ca2SiO4·Ca3(PO4)2 = Ca15(PO4)2(SiO4)6. The phase-structural evolution of the samples was monitored using X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and surface area analysis. A powder (20–30 µm) glass-ceramic material containing fine crystalline aggregates of dicalcium silicate and plates of silicon-substituted hydroxyapatite was obtained after heat treatment at 700 °C. After heat treatment at 1200 °C, Ca15(PO4)2(SiO4)6, silicocarnotite Ca5(PO4)2(SiO4), and pseudowollastonite CaSiO3 were identified by XRD, and the particle size varied between 20 and 70 µm. The compact glass-ceramic obtained at 1400 °C contained Ca2SiO4-Ca3(PO4)2 solid solutions with an α-Ca2SiO4 structure as a main crystalline phase. SEM showed the specific morphology of the crystalline phases and illustrated the trend of increasing particle size depending on the synthesis temperature. Effects of the glass-ceramic materials on cell viability of HL-60-derived osteoclast-like cells and on the expression of apoptotic and osteoclast-driven marker suggested that all materials at low concentrations, above 1 µg mL−1, are biocompatible, and S-1400 might have a potential application as a scaffold material for bone regeneration.Item Deep Learning-Driven Insights into Hardness and Electrical Conductivity of Low-Alloyed Copper Alloys(2025-12-01) Kolev M.; Javorova J.; Simeonova T.; Hadjitodorov Y.; Krastev B.Understanding the intricate relationship between composition, processing conditions, and material properties is essential for optimizing Cu-based alloys. Machine learning offers a powerful tool for decoding these complex interactions, enabling more efficient alloy design. This work introduces a comprehensive machine learning framework aimed at accurately predicting key properties such as hardness and electrical conductivity of low-alloyed Cu-based alloys. By integrating various input parameters, including chemical composition and thermo-mechanical processing parameters, the study develops and validates multiple machine learning models, including Multi-Layer Perceptron with Production-Aware Deep Architecture (MLP-PADA), Deep Feedforward Network with Multi-Regularization Framework (DFF-MRF), Feedforward Network with Self-Adaptive Optimization (FFN-SAO), and Feedforward Network with Materials Mapping (FFN-TMM). On a held-out test set, DFF-MRF achieved the best generalization (R2_test = 0.9066; RMSE_test = 5.3644), followed by MLP-PADA (R2_test = 0.8953; RMSE_test = 5.7080) and FFN-TMM (R2_test = 0.8914; RMSE_test = 5.8126), with FFN-SAO slightly lower (R2_test = 0.8709). Additionally, a computational performance analysis was conducted to evaluate inference time, memory usage, energy consumption, and batch scalability across all models. Feature importance analysis was conducted, revealing that aging temperature, Cr, and aging duration were the most influential factors for hardness. In contrast, aging duration, aging temperature, solution treatment temperature, and Cu played key roles in electrical conductivity. The results demonstrate the effectiveness of these advanced machine learning models in predicting critical material properties, offering insightful advancements for materials science research. This study introduces the first controlled, statistically validated, multi-model benchmark that integrates composition and thermo-mechanical processing with deployment-grade profiling for property prediction of low-alloyed Cu alloys.Item Anti-Inflammatory and Immunomodulatory Effects of 2-(3-Acetyl-5-(4-Chlorophenyl)-2-Methyl-1H-Pyrrol-1-yl)-3-Phenylpropanoic Acid(2025-08-01) Zlatanova-Tenisheva H.; Vladimirova S.Background: The pursuit of novel anti-inflammatory agents with enhanced efficacy and safety is crucial. Pyrrole-containing compounds, integral to many NSAIDs, exhibit promising anti-inflammatory properties. Compound 3f (2-(3-acetyl-5-(4-chlorophenyl)-2-methyl-1H-pyrrol-1-yl)-3-phenylpropanoic acid), a pyrrole derivative structurally inspired by the COX-2 selective inhibitor celecoxib, was evaluated for its anti-inflammatory and immunomodulatory effects. Methods: Anti-inflammatory activity was assessed in a carrageenan-induced paw edema model in Wistar rats. Compound 3f was administered intraperitoneally at 10, 20, and 40 mg/kg, either as a single dose or daily for 14 days. Diclofenac (25 mg/kg) served as the reference. Edema volume was measured by plethysmometry. Systemic inflammation was induced by lipopolysaccharide (LPS), and serum levels of the pro-inflammatory cytokine TNF-α and anti-inflammatory cytokines IL-10 and TGF-β1 were quantified by ELISA following single and repeated administration of compound 3f. Results: Single-dose administration of compound 3f at 20 mg/kg significantly reduced paw edema at 2 h (p = 0.001). After 14 days, all tested doses significantly inhibited paw edema at all time points (p < 0.001). In the LPS-induced systemic inflammation model, repeated treatment with 40 mg/kg of compound 3f significantly decreased serum TNF-α (p = 0.032). TGF-β1 levels increased significantly after both single and repeated doses (p = 0.002 and p = 0.045, respectively), while IL-10 levels remained unaffected. Conclusions: Compound 3f exhibits potent anti-inflammatory activity, particularly after repeated dosing, reflected by reduced local edema and systemic TNF-α suppression. The marked elevation of TGF-β1 indicates a potential immunomodulatory mechanism, selectively modulating cytokine profiles without altering IL-10. These findings support compound 3f as a promising candidate for targeted anti-inflammatory therapy involving cytokine regulation.Item Study on Encapsulation of Lavender Oil Essence in Zeolite and Bentonite Matrices(2025-01-01) Mateeva J.; Djambazov S.; Yoleva A.; Musa O.Abstract: In the present study, four matrices for encapsulation of essential oils were investigated: based on natural zeolite, based on alkali-activated bentonite, based on combination of 60 mass % alkali-activated bentonite and 40 mass % natural zeolite and based on 60 mass % alkali-activated bentonite and 40 mass % limestone. All natural raw materials are from deposits in the Eastern Rhodopes, Bulgaria. These matrices were loaded with lavender oil essence in amounts of 5 mass %, 15 mass % and 30 mass % respectively. The change in weight was monitored after the matrices loaded with lavender oil essence and had been exposed to air for a certain period of time at room temperature (for 1, 2, 7, 9, 14 and 60 days). Using XRD and FT-IR, the preservation of lavender oil essence in the matrices was established after 1, 2, 7, 9, 14 and 60 days of exposure to air at room temperature. It can be concluded that after the granulation into spherical particles with a size of 1 to 2.5 mm, alkaline-activated bentonite can be successfully used as a matrix alone or with the addition of other natural mineral raw materials such as zeolite and limestone for encapsulation of essential oils.Item Accounting for passive solar gains at energy certification of buildings(2025-01-01) Penkova N.; Panova P.; Krumov K.Passive solar gains via accumulation and heat transfer through the building envelopes are inherent to all buildings. The amounts of the solar energy utilized in this way, depend on the geographical locations, the structural designs of the building elements, their spatial orientation and the used materials. European and national regulations contain methodologies for determining solar gains through transparent and opaque building envelopes. They are used in the thermal balances of buildings at determining the required energies for heating and cooling. However, passively utilized solar gains through the building envelopes are often neglected in the public space. For example, they are not included as required renewable energy (RE) at the established practice of energy audit and energy certification of the buildings, which is generally incorrect. This work proposes guidelines for easy computation and counting of the passive solar gains in the energy consumption mix of the buildings. Applications of the guidelines for various buildings in the Seventh climate zone of Bulgaria have been demonstrated. It has been proven that the accounting for passive solar gains as energy consumption results in increasing of RE proportion in the supplied energies in the buildings.Item Arduino-Based Sensor System Prototype for Microclimate Monitoring of an Experimental Greenhouse †(2025-01-01) Belovski I.; Mihalev T.; Koleva E.; Mandadzhiev A.Arduino-based sensor systems are gaining widespread adoption in modern technological applications due to their accessibility, low-cost components, diverse sensor compatibility, high reliability, and user-friendly programming. Because of these advantages, such a system was selected to monitor and control microclimate parameters in a small-scale experimental greenhouse. The greenhouse will cultivate several vegetable species in soils with varying zeolite concentrations. The aim of this paper is to present the design and prototype development of a sensor system capable of tracking key environmental parameters, including temperature, humidity, atmospheric pressure, and soil moisture, while also enabling automated irrigation.Item HYDROLYTIC STABILITY OF ADAMANTANE HYBRID MOLECULES(2025-11-02) Stoymirska A.; Chayrov R.; Chuchkov K.; Danalev D.; Stankova I.Herein, the hydrolytic stability of new hybrid adamantane molecules modified with amino acid cysteine (Cys) at different pH is reported. Cys is a rare proteinogenic amino acid but it is a key structural unit in proteins. Cys is the only amino acid containing thiol group in the lateral chain which make it important source of sulphur for human organism. In addition, Cys has many biological functions such as antioxidant properties, immunomodulation activity by influence of the levels of the glutathione hormone, support liver function to eliminate toxins, help the breakdown of mucus in the lungs and improve breathing, etc. Adamantane derivatives are organic compounds largely used as antiviral therapeutics for treatment of influenza virus type A as well as neurodegenerative illnesses such as Parkinson’s and Alzheimer’s diseases. The adamantane motif assures high thermic stability and resistance. The modification of many adamantane derivatives such as amantadine, rimantadine and memantine with proteinogenic amino acid Cys could lead to increasing of activity and bioavailability of newly designed molecules. It is well known that hydrolytic stability is important feature for prodrug molecules related to the ability to penetrate cell membranes and to reach the specific receptors. A series of prodrugs based on adamantane motif including Cys-S-tert.-butylamantadine, Cys-S-tert.-butylrimantadine and Cys-S-tert.-butylmemantine was studied. The hydrolytic stability was determined at two different pH 1.0 and 7.4 at 37°C, similar to these in the human stomach and blood plasma. Kinetic of hydrolysis is monitored spectrophotometrically by specifically created UV-VIS method following the concentration of non-hydrolyzed part of the compounds. The most stable compound at pH 7.4 was Cys-S-tert.-butylamantadine with t1/2 = 8.5 h. The compound Cys-S-tert.-butylmemantine also has good hydrolytic stability with t1/2 = 6.7 h and Cys-S-tert.-butylrimantadine has t1/2 = 6.2 h. Almost identical are t1/2 values at acid pH 1.0: the most stable is Cys-S-tert.-butylamantadine with t1/2 = 4.7 h, followed by Cys-S-tert.-butylrimantadine with t1/2 = 3.9 h and Cys-S-tert.butylmemantine with t1/2 = 3.5 h. However, it was revealed that hydrolytic stability of tested compounds in the two model systems at acid pH is relatively lower than those in neutral conditions.Item Development and Characterization of Sustainable Biocomposites from Wood Fibers, Spent Coffee Grounds, and Ammonium Lignosulfonate(2025-10-01) Savov V.; Antov P.; Kostadinova-Slaveva A.; Yusein J.; Dudeva V.; Todorova E.; Petrin S.Coffee processing generates large volumes of spent coffee grounds (SCGs), which contain 30–40% hemicellulose, 8.6–13.3% cellulose, and 25–33% lignin, making them a promising lignin-rich filler for biocomposites. Conventional wood composites rely on urea-formaldehyde (UF), melamine–urea–formaldehyde (MUF), and phenol–formaldehyde resins (PF), which dominate 95% of the market. Although formaldehyde emissions from these resins can be mitigated through strict hygiene standards and technological measures, concerns remain due to their classification as category 1B carcinogens under EU regulations. In this study, fiber-based biocomposites were fabricated from thermomechanical wood fibers, SCGs, and ammonium lignosulfonate (ALS). SCGs and ALS were mixed in a 1:1 ratio and incorporated at 40–75% of the oven-dry fiber mass. Hot pressing was performed at 150 °C under 1.1–1.8 MPa to produce panels with a nominal density of 750 kg m−3, and we subsequently tested them for their physical properties (density, water absorption (WA), and thickness swelling (TS)), mechanical properties (modulus of elasticity (MOE), modulus of rupture (MOR), and internal bond (IB) strength), and thermal behavior and biodegradation performance. A binder content of 50% yielded MOE ≈ 2707 N mm−2 and MOR ≈ 22.6 N mm−2, comparable to UF-bonded medium-density fiberboards (MDFs) for dry-use applications. Higher binder contents resulted in reduced strength and increased WA values. Thermogravimetric analysis (TGA/DTG) revealed an inorganic residue of 2.9–8.5% and slower burning compared to the UF-bonded panels. These results demonstrate that SCGs and ALS can be co-utilized as a renewable, formaldehyde-free adhesive system for manufacturing wood fiber composites, achieving adequate performance for value-added practical applications while advancing sustainable material development.Item MODELLING CONFINED ZONE EFFECT IN REINFORCED CONCRETE CORBEL STRENGTHENED BY CFRP(2025-11-02) Stankov V.; Ivanova I.; Assih J.; Dontchev D.Confined zone effect, which is primarily observed in reinforced concrete (RC) columns subjected to compression, is crucial for the load-bearing capacity of (RC) structures. This occurs when, despite an increase in the load, the resulting deformations are nearly zero. The same effect can be observed in structures strengthened by different types of fabrics (carbon, glass, Kevlar, etc.). It is well known that experimental studies are a high-resources and time-consuming process. Therefore, a combination of experimental study and non-linear finite element (FE) simulation strategy is considered. The structure investigated in this paper is an RC corbel strengthened by bonding carbon fibre reinforced polymer (CFRP). Two RC corbel structures were developed, one with strengthening by three layers of CFRP and one without strengthening. The experimental results were used to validate the FE simulation. The model was successfully validated and provides opportunities for future parametric investigations. The aim of this study is to model he confined zone effect in reinforced concrete corbels strengthened by CFRP. Modelling confined zone effect is a significant challenge for engineers. This study shows a new approach to FE simulation. The resulting model successfully simulates the mechanical behaviour of the structure.Item Influence of Na Additives on the Characteristics of Titania-Based Humidity Sensing Elements, Prepared via a Sol–Gel Method(2025-10-01) Nenova Z.; Kozhukharov S.; Nedev N.; Nenov T.Humidity sensing elements based on sodium-doped titanium dioxide (Na-doped TiO2) were prepared using a sol–gel method in the presence of cerium ions and sintered at 400 °C and 800 °C. Titanium (IV) n-butoxide and a saturated solution of diammonium hexanitratocerate in isobutanol served as starting materials. Sodium hydroxide and sodium tert-butoxide were used as inorganic and organometallic sodium sources, respectively. The influence of sodium additives on the properties of the humidity sensing elements was systematically investigated. The surface morphologies of the obtained layers were examined by scanning electron microscopy (SEM). Elemental mapping was conducted by energy-dispersive X-ray (EDX) spectroscopy, and structural characterization was performed using X-ray diffractometry (XRD). Electrical properties were studied for samples sintered at different temperatures over a relative humidity range of 15% to 95% at 20 Hz and 25 °C. Experimental results indicate that sodium doping enhances humidity sensitivity compared to undoped reference samples. Incorporation of sodium additives increases the resistance variation range of the sensing elements, reaching over five orders of magnitude for samples sintered at 400 °C and four orders of magnitude for those sintered at 800 °C.Item ANTIOXIDANT CAPACITY OF (KLAKLAK)2 BIOCONJUGATES ASSESSED BY THE ELECTRON-TRANSFER METHODS FRAP AND CUPRAC(2025-11-02) Stoyanova Y.; Jaber S.; Naydenova E.; Georgieva N.; Danalev D.Oxidative stress and metal-driven redox processes are key contributors to the pathogenesis of chronic diseases and cancer, motivating the search for novel antioxidant molecules. In this study, the antioxidant potential of a series of synthetic peptides previously reported to possess antitumor and antibacterial properties was evaluated using two complementary electron-transfer assays: ferric reducing antioxidant power (FRAP) and cupric ion reducing antioxidant capacity (CUPRAC). Both assays were calibrated against caffeic acid, and results were expressed as caffeic acid equivalents (CAE). The FRAP assay revealed substantial differences in reducing activity, with Si8 exhibiting the highest value (0.558 ± 0.132), followed by Si12 (0.478 ± 0.0240), Si10 (0.293 ± 0.0220), and Si15 (0.250 ± 0.0200), whereas Si1 (0.00439 ± 0.00240) and Si11 (0.00260 ± 0.000500) showed negligible responses. A comparable pattern was observed in the CUPRAC assay, where Si8 again displayed the strongest reducing capacity (0.381 ± 0.0948), with Si12 (0.290 ± 0.0225), Si15 (0.262 ± 0.0223), and Si10 (0.224 ± 0.0290) also demonstrating appreciable activity, while Si1 (0.001800 ± 0.000400) and Si11 (0.0132 ± 0.000500) remained inactive. The combined application of FRAP and CUPRAC provided complementary and reproducible measures of peptide antioxidant capacity, establishing a framework for systematic characterization of redox-active peptides in relation to oxidative stress.Item Investigation of the Possibilities for Infrared Diagnosis of Peirce–Smith Converters in Non-Ferrous Metallurgy(2025-09-01) Mihailov E.; Choshnova D.; Ivanova M.; Asenova M.To implement predictive maintenance of units in the practice of metallurgical manufacturers, computer information and diagnostic systems are being developed to assess the current state of individual units throughout their entire life cycle. This publication presents the results of a study on developing an infrared diagnostic system for predictive maintenance of converter units in the non-ferrous metallurgy industry. A 3D mathematical model of the transient heat transfer in the wall of a real operating unit has been developed and numerically implemented to study, analyze, and diagnose surface temperature fields resulting from wear and local damage. To adjust the operation of the mathematical model, the design parameters and the results for operating and technological parameters from an industrial experiment are taken into consideration. Using the model, a full-factor experiment was simulated to study the surface temperature fields resulting from the erosion wear of the wall and the presence of local damage. Based on the simulation results, the optimal time range for thermographic monitoring is determined. A regression dependence was derived to predict the refractory wall wear as a function of the outer surface temperature of the converter unit. The results are part of a comprehensive investigation aimed at developing thermal imaging techniques for converter units in non-ferrous metallurgy.Item TRENDS IN DISTANCE LEARNING IN AN ELECTRONIC ENVIRONMENT(2025-11-02) Petrunova M.; Atanassov A.; Pilev D.This paper introduces an analysis in the forms of education and learning in the past and nowadays and especially the tendencies in distance learning in an electronic environment in COVID-19 pandemic conditions. After the brief introduction of the types of training before and during the pandemic, a comparative analysis of the problems related to the distance learning in an electronic environment identified by Institute for Educational Research/IER/in 2021 and our private surveys in 2020 and 2023 has been done. The results in both researches show many common tendencies commented in the following parts of the paper.