Machines. Technologies. Materials.
Vol. 19 (2025), Issue 9

Table of Contents

  • MACHINES

    • Effect of vortex generator shape and attack angle on thermal-hydraulic performance of a finned-tube heat exchanger

      pg(s) 310-313

      The objective of this study is to numerically investigate the fluid flow and heat transfer performance of a finned-tube heat exchanger (FTHEX). The analysis focuses on the implementation of three vortex generator (VG) configurations: rectangular winglet (RW), delta-winglet upstream (DWU), and delta-winglet downstream (DWD) — mounted on the fin surface in a “common-flow-up” orientation. Attack angles of 15°, 30°, and 45° are considered for each VG type to evaluate their impact on the heat exchanger’s heat transfer potential and friction losses. The air-side Reynolds number, based on the outside tube diameter, was varied within the range 684 ≤ Re ≤ 1532. The results indicate that among the tested configurations, the RWP setup with an attack angle of 45° achieves the highest enhancement in the airside Nusselt number, with improvements ranging from 20% to 45% compared to the reference configuration, but at the expense of a higher pressure drop. For attack angles αvg = 15° and αvg = 30°, the highest overall performance (TPF factor) is achieved with the rectangular winglet configuration across the entire Reynolds number range. At an attack angle of αvg = 45°, the heat exchanger with downstream delta winglets shows higher TPF values compared to the other configurations, except at Re = 1278.

  • TECHNOLOGIES

    • Effect of nano coating and nano fluid on photovoltaic module performance

      pg(s) 314-321

      Recent researches have shown that Nano-coating materials play a vital role in improving the performance of the PV cell operation, enhancing the life span and reducing its surface temperature. In addition to that, the Nano-coating can achieve many benefits such as making a smoother surface, stronger and less adhesive of externous on the surface of PV panel. In this work, the effect of nanomaterials coating using Titanium dioxide, silicon dioxide and Nano fluid Titanium dioxide on performance and temperature of PV cell when coated by these Nano particles separately with different thicknesses (0.5μm, 50μm,100μm and 300μm). To achieve these objectives ANSYS software technology (version.1) was used. The results showed that there is a significant effect specifically when using TiO2 Nano fluid. The maximum improvements were when using Nano coating TiO2 and SiO2 which are (0.62%) and (0.135%), respectively, at thickness 300μm and ambient temperature16 ͦ C in case without externous particles. But the minimum improvement was with TiO2 and SiO2 of coating thickness 0.5μm which are (0.0937%) and (0.0937%), respectively, at ambient temperature 23 ͦ C in presence of dust. The results of TiO2 Nano fluid with concentration and flow rate which are (5%and 0.01 kg/s), respectively, showed that the maximum improvement was (39.88%) in case without externous particles at ambient temperature 23 ͦ C, but the minimum improvement was (37.84%) in case with dust at ambient temperature 16 ͦ C.

    • Options of applying manual laser welding for thin sheets

      pg(s) 324-328

      Manual laser welding has become one of the progressive technologies in recent years for processing of metallic materials, showing particular advantages in the welding of different metals. By the high energy concentration and precise focus of the laser beam, it is possible to produce joints with minimal heat-affected zones, significantly reducing the risk of deformation, cracking and other types of defects. This technology enables high-quality and aesthetically welds without the need of post-processing, while also increasing productivity and process repeatability.
      This article examines the main principles, parameters and modes of laser beam welding itself, analyzes practical parameters of device LW-2000W, designed for manual laser welding. The final section summarizes practical part focusing on welding of sheet metals at different values of parameters such as power, welding speed, wire feed and beam oscillation.

    • Plasma technologies for the synthesis of wear-resistant multifunctional metal matrix composites of the Al–TiC System

      pg(s) 328-331

      A novel two-stage approach for obtaining wear-resistant multifunctional powder composites based on metal powders with highmodulus Ti–TiC system fillers is proposed. The method combines high-voltage electric discharge (HVED) treatment and spark plasma sintering (SPS), offering a promising alternative to conventional techniques for producing Al–Ti–C system composites. This approach enables the development of a unified route for material synthesis using high-energy-density processing. HVED treatment prevents oxidation of metal particles, reduces contamination by tool materials, and initiates the synthesis of additional dispersed strengthening phases. For example, HVED treatment of titanium powder in a hydrocarbon liquid promotes the in situ formation of titanium carbide (TiC) particles.
      The present work investigates the influence of adding Ti–TiC powder—synthesized via HVED in ethanol under reverse polarity mode with a specific energy input of 20 MJ/kg—on the structure, phase composition, and properties of Al–Ti–C metal matrix composites (MMCs). It was shown that the addition of 2 wt% of Ti–TiC powder synthesized via HVED in ethanol to aluminum powder results in an MMC with an electrical resistivity of 0.5 Ω·mm²/m and a hardness of 31 HRB. However, the heat resistance of this composite is 2.5 times lower than that of consolidated pure aluminum powder. Increasing the Ti–TiC content to 10 wt% leads to the formation of a wear-resistant Al–Ti–C composite, whose structure includes Al, Ti, TiC, the intermetallic compound Al₃ Ti, MAX phases Ti₂ AlC and Ti₃ AlC₂ , and free carbon. For the MMC sample with the addition of 10% Ti–TiC, the mass gain per cycle during the heat resistance test is 0.23%/cycle, whereas for samples made from consolidated Al powder it is 0.18%/cycle, indicating that their heat resistance is approximately the same. The wear resistance of this composite is more than three times higher than that of the consolidated base aluminum powder, with wear rates of 0.003 g/km and 0.010 g/km, respectively. This material also demonstrates a hardness of 43 HRB and relatively low electrical resistivity at the level of 0.3 Ω·mm²/m.

    • Measurements of electromagnetic fields emitted in urban environments

      pg(s) 329-341

      The paper presents an analysis of the results of measurements of electromagnetic radiation from a base station installed on the roof of a site located in an area with a higher degree of urbanization, an extended center of the capital of Bulgaria, Sofia. Using mobile measuring equipment Narda AMB-8057-03, the dynamics of electromagnetic radiation in the frequency range from 100 kHz to 7 GHz was monitored over a period of eight days (weekdays and weekends). The obtained values for the levels of electromagnetic fields comply with the current national legislation and European standards. They are ≤ 10 μW/cm2. However, there remains a need for monitoring, tracking and control of the emitting EMF sources, because the susceptibility of an individual person is individual and the possible health effects are different, which in some cases manifests itself as “hypersensitivity” to electromagnetic radiation.

    • Amortization systems for objects with discrete switching of parts of a viscтoelastic element in a douffing type oscillator

      pg(s) 332-338

      In order to enhance the efficiency of oscillation damping, the viscoelastic element in a Duffing-type system is divided into parts, with discrete commutation occurring four times per period between them. It has been demonstrated that the enhancement of oscillation energy dissipation is achieved through mass transfer between the parts. The influence on the equivalent coefficients of the solution parameters has been analyzed. 3D surfaces of the dissipated energy and equivalent coefficients have been constructed as functions of amplitude, mass ratio of the parts, and tension. It has been shown that discrete commutation allows for obtaining a hyperbolic type of frequency dependence of the aperiodicity coefficient, reaching values of 0,4..0,6 in the resonance zone.

    • Еffects of gamma irradiation technology on biodeteriorated paper materials determined by thermogravimetry

      pg(s) 342-345

      The present manuscript describes some of the results obtained as part of a research contract investigating the effects of gamma irradiation on leather and library materials. Six samples of books and magazines, produced in Germany, the USA, and the former USSR between 1896 and 1962, were selected. All paper materials showed visible signs of biodeterioration and environmental damage. The samples were gamma-irradiated with a dose of 20 kGy using BULGAMMA radiation facility, based on the JS-850 60Co gamma irradiator at Sopharma AD, Bulgaria. The applied dose rate was 0.47 kGy/h. The effects of gamma irradiation were studied using thermogravimetric analysis. The results showed a 13% reduction in residual mass at 700 °C in the 1962 Soviet Union Newsletter, accompanied by a 3.6% decrease in the temperature (Tmax) at maximum weight loss rate. An 8.7% increase in residual mass at 700 °C was observed in the 1952 Reference Journal, and a 6.6% increase was measured in a German monograph from 1923. The study revealed that the effects of gamma radiation on paper structure depend on both the degree of prior damage and the type and composition of the paper.

    • Analysis of shell-and-tube latent thermal energy storage tube diameter on charging and discharging performance

      pg(s) 346-349

      The study reports on a series of numerical simulations conducted to assess how tube diameter affects charging (melting) and discharging (solidification) performance in a shell-and-tube latent thermal energy storage (LTES) with longitudinal fins. In the investigated LTES, water flows through the tubes and serves as the heat transfer fluid (HTF), while paraffin is used as the phase change material (PCM) and fills the shell side. Employing an experimentally validated mathematical model and numerical procedure, LTES charging and discharging performances were investigated for three tube diameters: 28/24, 38/34 and 48/44 mm. LTES performance for different tube diameters was assessed by comparing melting and solidification times, as well as stored and released thermal energies in 8, 9 and 10 h of charging and 12, 13 and 14 h of discharging for each configuration. Results show that larger tube diameters accelerate melting and solidification processes due to increased conductive surface area, but also decrease LTES energy storing capacity as the amount of the PCM reduces as a result of increased tube diameter. The results indicate that tube diameter greatly influences LTES thermal performance and must be chosen carefully for the LTES to be effective.

  • MATERIALS

    • Solid State Contact Interaction Between Metal Matrix Composite Based on Ti64 with the Composite Based on B4C

      pg(s) 350-353

      The contact interaction between the metal matrix composite based on Ti64 and the B4C-based composite was investigated. It is shown that the interaction process is influenced by the annealing temperature and holding time. The phase that formed independently of the contact pair is TiB. However, the thickness of the products formed at the boundary depends on the contact pair. In case of the Ti64-B4C pair the thickness is 70 μm, while for (Ti64-40 wt% TiC)-B4C it becomes 10 μm. This significant difference in the thickness is due to the presence of refractory particles (TiC) in (Ti64-40 wt% TiC)-B4C couple, because the TiC phse reduce the diffusion of Ti into the contact zone.

    • Effect of electromagnetic radiation in space on aluminum alloy AA7075

      pg(s) 353-355

      Electromagnetic radiation in space presents a significant challenge to the durability of aluminum alloys used in spacecraft construction. This study analyzes the effects of ionizing radiation (gamma rays, cosmic rays, solar particles), ultraviolet and solar radiation, electromagnetic pulses (EMP), and extreme temperature fluctuations on a novel composite material based on AA7075 (B95) aluminum alloy. The results demonstrate that prolonged exposure of the material in outer space (28 months) leads to structural changes and alterations in mechanical properties. To ensure the reliability of the results, the space-exposed samples were compared with reference samples stored under terrestrial conditions.

    • Comparative study of hardness evolution in 5754 aluminum welds under AC and DC welding currents

      pg(s) 356-361

      This study examines how thermal and mechanical effects influence hardness distribution in 4, 6, and 8 mm AA5754 aluminum plates welded using TIG with AC and DC currents. Vickers hardness (HV0.01) was measured alongside tensile tests to evaluate weld performance. Results show that AC welding produces higher but more variable hardness, while DC welding yields more stable profiles. Notably, thinner plates (4 mm) showed minimal hardness differences between current types due to faster cooling. Strain hardening increased hardness up to 125 HV and reduced variation. The extent of hardness stabilization and heat-affected zone size depended on plate thickness. Although current type influenced hardness distribution, it had little effect on fracture toughness.