Shell and tube heat exchangers are commonly used in a wide range of practical engineering. The key issue in such a system is the heat exchange between the hot and cold working media. An increased cost of production of these devices has forced all manufacturing companies to reduce the total amount of used materials by better optimizing their construction. Numerous studies on the heat exchanger design codes have been carried out, basically focusing on the use of fully time-dependent partial differential equations for mass, momentum, and energy balance. They are very complex and time-consuming, especially when the designers want to have full information in a full 3D system. The paper presents the 1D mathematical model for analysis of the thermal performance of the counter-current heat exchanger comprised of mixed time-dependent and time-independent equations, solved by the upwind numerical solution method, which allows for a reduction in the CPU time for obtaining the proper solution. The comparison of numerical results obtained from an in-house program called Upwind Heat Exchanger Solver written in a Fortran code, with those derived using commercial software package ASPEN, and those obtained experimentally, shows very good agreement in terms of the temperature and pressure distribution predictions. The proposed method for fast designing calculations appears beneficial for other tube shapes and types of heat exchangers.

The energy sector is a majorarea that is responsible for the country development. Almost 40% of the total energy requirement of an EU country is consumed by the building sector and 60% of which is only used for heating and cooling requirements. This is a prime concern as fossil fuel stocks are depleting and global warming is rising. This is where thermal energy storage can play a major role and reduce the dependence on the use of fossil fuels for energy requirements (heating and cooling) of the building sector. Thermal energy storage refers to the technology which is related to the transfer and storage of heat energy predominantly from solar radiation, alternatively to the transfer and storage of cold from the environment to maintain a comfortable temperature for the inhabitants in the buildings by providing cold in the summer and heat in the winter. This work is an extensive study on the use of thermal energy storage in buildings. It discusses different methods of implementing thermal energy storage into buildings, specifically the use of phase change materials, and also highlights the challenges and opportunities related to implementing this technology. Moreover, this work explains the principles of different types and methods involved in thermal energy storage.

The present study aims at investigating and simulating the hydrogen cycle production at low temperatures using thermochemical reactions. The cycle used in this work is based on the dissociation of water molecules depending on a copper chlorine couple. Furthermore, the proposed method uses mainly thermal energy provided by a solar thermal field. This proposed cycle differs from what is found in the literature. However, most of the thermochemical cycles for hydrogen production work at quite high temperatures which is a technical challenge. Therefore, the maximum temperature used in the present cycle is limited to 500°C. A thermodynamic analysis based on both the first and second laws is performed to evaluate the energy, exergy and efficiency of each reaction as well as the overall exergetic efficiency of the system. Furthermore, a parametric study is conducted to figure out the impact of the surrounding temperatures on the overall exergetic efficiency using commercial energy simulation software. The results show that the cycle can achieve an exergy efficiency of 30.5%.

In this study, a theoretical model is presented to investigate the performance of a thermoelectric (TE) radiant cooling system combined with photovoltaic (PV) modules as a power supply in a building with an ambient temperature reaching more than 45ºC. The combined system TE/PV performance is studied under different solar radiation by using the hourly analysis program and photovoltaic system software. The thermal and electric characteristics of TE are theoretically investigated under various supplied voltages using the multi-paradigm programming language and numerical computing environment. Also, a theoretical analysis of heat transfer between the TE radiant cooling system and an occupied zone from the side, and the other side between the TE radiant cooling system and duct zone is presented. The maximum power consumption by TE panels and building cooling load of 130 kW is predicted for May and June. The 145 units of PV panels could provide about 50% of the power required by TE panels. The thermal and electric characteristics of TE panels results show the minimum cold surface temperature of 15ºC at a supplied voltage between 6 V and 7 V, and the maximum hot surface temperature of 62ºC at a supplied voltage of 16 V. The surface temperature difference between supplied current and supplied power increases as supplied voltage increases. At a higher supplied voltage of 16 V, the maximum surface temperature difference between supplied current, and supplied power of 150ºC, 3.2 A, and 48 W, respectively. The cooling capacity increases as supplied voltage increases, at a surface temperature difference of –10ºC and supplied voltage of 16 V, the maximum cooling capacity is founded at about 60 W. As supplied voltage decreases the coefficient of performance increases. The maximum coefficient of performance is about 5 at the surface temperature difference of –10ºC and supplied voltage of 8 V.

An analysis of the methods used in Bulgaria for estimating CO2, SO2 and dust emissions has been conducted. The first methodology, which is officially used by all energy auditors at the Agency for Sustainable Energy Development targets the energy efficiency of combustion devices installed mainly at industrial enterprises. The second methodology, used by the Ministry of Environment and Water, is more comprehensive and can be applied to thermal power plants, small combustion plants as well as industrial systems. In recent years, many projects related to energy efficiency and renewable energy projects, including hydrogen technologies, which require an assessment of reduced greenhouse gas emissions, have been implemented as a priority. The use of reliable and accurate methods is essential in the assessment of greenhouse emissions. A novel methodology, based on stoichiometric equations of the combustion process for solid, liquid and gaseous fuels has been proposed and comprised. This novel methodology is characterized by higher precision compared to the methods currently in place and this is achieved through calculating emissions from the combustion of energy fuels accounting for the full elemental composition of the fuel and its heating value, whereas the current commonly applied methods use only the fuel type and the carbon content. A further benefit of the proposed methodology is the ability to estimate emissions of fuels for which there is no alternative method for calculating CO2, SO2 and dust. Results of emission calculations according to the analysed methods are presented. Finally, a comparative analysis between the presented methodologies including an assessment of their accuracy and universal applicability has been made.

The paper presents a theoretical analysis of thermal energy storage filled with phase change material (PCM) that is aimed at optimization of an adsorption chiller performance in an air-conditioning system. The equations describing a lumped parameter model were used to analyze internal heat transfer in the cooling installation. Those equations result from the energy balances of the chiller, PCM thermal storage unit and heat load. The influence of the control of the heat transfer fluid flow rate and heat capacity of the system components on the whole system operation was investigated. The model was used to validate the selection of Rubitherm RT62HC as a PCM for thermal storage. It also allowed us to assess the temperature levels that are likely to appear during the operation of the system before it will be constructed.

Mating electrodes made of copper alloys are commonly used for welding galvanized steel sheets used in the production of car bodies. These alloys are characterized by high mechanical properties, a high level of electrical and thermal conductivity as well as the stability of these properties under changing conditions of current, thermal and mechanical load. Much careful attention was paid to the essence of the ongoing structural changes as well as to the mechanical properties in the welding process (RSW – Resistant Spot Welding) of steel sheets, including high-strength ones. There is a lack of research on structural changes and the related mechanical properties occurring in welding electrodes made of copper alloys caused by the welding process.
This study is devoted to these issues and contains a critical review of the research results enabling a better understanding of the relationships between the structure and properties of welding electrodes caused by the cyclic welding process. In order to illustrate the phenomena occurring during the welding process, both in the material to be welded and in the tip electrodes, hardness and structural tests were carried out on electrode samples before and after their exploitation. The data collected in the article supplements a certain lack of information in the literature regarding the microstructural aspects of the welding process of galvanized steel sheets for the production of car bodies. The conducted research may be the starting point for the search for more effective materials for the tip electrodes.

This study examined the effects rheological properties of different composition kaolin and kaolin geo-filler in polypropylene composites. Polypropylene composites with varying composition of kaolin geo-filler 0 wt%, 2 wt%, 4 wt%, 6 wt%, 8 wt%, and 10 wt% was prepared and compared with polypropylene composite with raw kaolin. Kaolin is an aluminosilicate based mineral filler was used to prepare geopolymer paste by combining with alkaline activator solution. The polypropylene composite was compounded using a twin-screw extruder and the melt flow index was determined by a constant weight pressure of 2.16 kg at 230°C in 10 min. Knowing the melt flow index is necessary to predict and control the process, the study has demonstrated that the composition of kaolin filler and kaolin geo-filler affects the melt flow, melt density and surface morphology at varies composition. Composites with kaolin geo-filler have demonstrated high melt flow index process and having better distribution and flow.

The technology of producing castings of high-quality inoculated cast iron with flake graphite particles in the structure is a combination of the melting and inoculation process. Maintaining the stability of the strength and microstructure parameters of this cast iron is the goal of a series of studies on the control of graphitization and austenitic inoculation (increasing the number of primary austenite dendrites), and which affects the type of metal matrix in the structure. The ability to graphitize the molten alloy decreases with its holding in the melting furnace more than an hour. The tendency to crystallize large dendritic austenite grains and segregation of elements such as Si, Ni and Cu reduce the ductility properties of this cast iron. The austenite inoculation process may introduce a larger number of primary austenite grains into the structure, affecting the even distribution of graphite and metal matrix precipitation in the structure. Known inoculation effects the interaction (in low mass) of additives: Sr, Ca, Ba, Ce, La, produces MC2 carbide). Addition of Fe in the inoculant influences the number and shape of austenite dendrites. Hybrid modification combines the effects of these two factors. The introduction of nucleation sites for the graphite eutectics and primary austenite grains result in the stabilization of the cast iron microstructure and an increase in mechanical properties. The obtained test results set the direction for further research in this area in relation to the production of heavy plate castings in vertical and horizontal pouring.

The hydrogen embrittlement of metals is caused by the penetration and accumulation of hydrogen atoms inside the metal. The failure of the product due to hydrogen embrittlement is delayed in time and does not occur immediately after its manufacture, but several hours, days, or even weeks later. Therefore, the chances of detecting hydrogen embrittlement when checking the quality of the finished product are very slim. The use of high-strength bolts in industry is associated with the risk of hydrogen embrittlement. This phenomenon poses a threat to the safe use of devices by limiting or completely losing the functionality of the bolt joint. Even a low influence of moisture can trigger failure mechanisms.
The article proposes a method for assessing the risk of hydrogen embrittlement for high-strength bolts in class12.9. For this purpose, bolts made of material grade 32CrB4 were prepared and in a controlled manner the grain flow inconsistency was made, leading in extreme cases to the production of the forging lap. To perform the study, the device proposed by the European Assessment Document (EAD) was adapted to the testing of hydrogen embrittlement of threaded fasteners in concrete. The concrete substrate was replaced with metal spacers that were preloaded with a bolt. The use of the wedge distance under the bolt head led to the generation of two stress states – tensile and compressive, which translated into an increased risk of hydrogen embrittlement. After being tested, the bolts were visually and microscopically inspected to assess potential locations for cracks and hydrogen propagation. As a result of the conducted tests, it was found that the prepared test method allows to assess the resistance or susceptibility of the bolt to threats related to hydrogen embrittlement.

This paper presents a method of synthesizing copper powders by electrochemical method with the use of a rotating working electrode. The influence of the rotation speed of the working electrode, the current density, the concentration of copper ions, and the addition of ethylene glycol on the shape, size, and size distribution of the obtained powders were investigated. Properties of the synthesized powders were characterized by scanning electron microscopy (SEM) and X-ray powder diffractometry (XRD). It has been shown that it is possible to obtain copper powders with a size of 1 µm by an electrochemical method using the rotary cathode, in sulphate bath with addition of ethylene glycol as a surfactant. Increasing current density causes a decrease in the average size of the obtained powder particles. The addition of 2.5% of ethylene glycol prevents the formation of dendritic powders. The change in the concentration of copper ions in the range from 0.01 to 0.15 mol/dm3 in the electrolyte did not show any significant effect on the size of obtained particles. However, higher concentrations of copper limiting the presence of dendritic-shape particles. Changing the speed of rotation of the electrode affects both the size and the shape of synthesized copper powder. For the rotational speed of the electrode of 115 rpm, the obtained powders have a size distribution in the range of 0-3 µm and an average particle size of 1 µm. The particles had a polygonal shape with an agglomeration tendency.

This study aimed to prepare Zr55Cu30Al10Ni5 bulk amorphous alloys by spark plasma sintering of raw amorphous alloy powders and investigate their microstructure and micromechanical behaviors. When the sintering temperature ( T_s) was 675K, which was lower than the glass transition temperature ( T_g) of the material, the sintered sample was almost fully amorphous but the density was lower. However, when T_s was 705K, which was higher than T_g, partial crystallization occurred, but the density was higher. The hardness of the bonding zone of the sintered sample at 675K was 5.291 GPa due to the lower density, which was lower than that at 705K, and the hardness at 705K was 8.836 GPa. The generation of thermodynamically stable intermetallic phases, the hardness, and the elastic modulus of the samples sintered above T_g were higher due to the higher density.

Directed energy deposition (DED) is an additive manufacturing process wherein an energy source is focused on a substrate on which a feedstock material is simultaneously delivered, thereby forming a small melt pool. Melting, solidification, and subsequent cooling occur at high rates with considerable thermal gradients compared with traditional metallurgical processes. Hence, it is important to examine the effects of cooling rates on the microstructures and properties of the additive manufactured materials. In this study, after performing DED with various energy densities, we investigated the changes in the microstructures and Vickers hardness of cast Al-33 wt.% Cu alloy, which is widely used to estimate the cooling rate during processing by measuring the lamellar spacing of the microstructure after solidification. The effects of the energy density on the cooling rate and resultant mechanical properties are discussed, which suggests a simple way to estimate the cooling rate indirectly. This study corresponds to the basic stage of the current study, and will continue to apply DED in the future.

Surface coating technology, as the main technology to improve the fatigue life of mechanical systems, has been well applied in mechanical equipment. The present study aimed to explore low-cost surface coating preparation technology using inexpensive rice husk as the research object, and the pyrolysis process behavior of rice husk was analyzed. The Ni60/SiO ₂ coating was prepared on the surface of the 45# steel substrate using the pyrolysis product SiO ₂ fiber as the reinforcing phase and supersonic plasma-spraying equipment. The results showed no defects such as cracks, pores, and inclusions in the prepared coating. The nanohardness of the Ni60/SiO ₂ coating reached 6506 μN, and the average friction coefficient reached 0.42. In the friction-and-wear experiment, the Ni60/SiO ₂ coating was manifested as an abrasive wear mechanism.

To study the anti-seismic performance of steel structure under high temperature, the finite element analysis software ABAQUS was used to study the seismic performance of Q235 steel welded box section column at service stage under normal temperature and high temperature fire. The effects of welding residual stress, slenderness ratio, width thickness ratio and axial load level on the hysteretic behavior of columns were analyzed and the stable bearing capacity and hysteretic performance of the column under high temperature were investigated. The results show that the maximum bearing capacity of the column decreases with the increase of the residual stress peak value. With the increase of temperature, a decrease in the maximum bearing capacity of columns under constant axial force and horizontal cyclic load and an increase in the ductility occur.

The numerical solutions of stress and strain components on the critical plane of tungsten carbide coating were solved based on the critical plane method in three-dimensional coordinate system, and accordingly three strain energy density parameters (Smith-Watson-Topper, Nita-Ogatta-Kuwabara and Chen parameters) were determined to reveal the fretting fatigue characteristics of tungsten carbide coating. In order to predict the fretting fatigue life based on the strain energy density criterion, the expressions between the strain energy density parameter and the fretting fatigue life was obtained experimentally. After the comparison of the three strain energy parameters, it was found that all three parameters could accurately predict the crack initiation position, but only the Smith-Watson-Topper parameters could accurately predict the crack initiation angle. The effects of cyclic load, normal load and friction coefficient on fretting fatigue damage behaviors were discussed by using the Smith-Watson-Topper criterion. The results show that the fretting fatigue life decreases with the increase of cyclic load; an increase in the normal contact load will cause the Smith-Watson-Topper damage parameters more concentrated at the outer edge of the bridge foot; a decrease in the friction coefficient will increase the Smith-Watson-Topper damage parameters in the middle of the contact surface.

In the present research, we used molecular dynamics simulation to determine the effect of cutting parameters on micro-grain boundary structures and Burgers vector distribution in single crystal iron and polycrystalline iron materials. The result showed that the destruction of the lattice in polycrystalline iron caused by the cutting tool was restricted to the contact surface area. In addition, in the precision machining process, a higher refining grain was observed on the iron surface. During the cutting process of single crystal iron, large-scale slip occurred along the <111> crystal direction on the {110} crystal plane. And the slip presented an annular shape.

The paper presents a summary of research on the possibility of influencing the state of residual stresses in railway rails by changing the pass design of vertical and horizontal straightener rollers and optimising their distribution on the rail perimeter. The presented results are devoted to the influence of profiled rollers on the level of residual stresses. A wide range of theoretical considerations were carried out based on the use of the finite element method using the commercial Forge software package. In order to verify the results of the theoretical considerations most reliably, a series of “in situ” experiments were conducted in industrial conditions on an existing production line. The tests were carried out on 120 meters long 60E1 railway rails. A significant reduction in the level of residual stresses compared to the standard requirements was achieved.

The austenitic stability and strain-induced martensitic transformation behavior of a nanocrystalline FeNiCrMoC alloy were investigated. The alloy was fabricated by high-energy ball milling and spark plasma sintering. The phase fraction and grain size were measured using X-ray diffraction. The grain sizes of the milled powder and the sintered alloy were confirmed to be on the order of several nanometers. The variation in the austenite fraction according to compressive deformation was measured, and the austenite stability and strain-induced martensitic transformation behavior were calculated. The hardness was measured to evaluate the mechanical properties according to compression deformation, which confirmed that the hardness increased to 64.03 HRC when compressed up to 30%.

Cu-Sn alloys have been known as bronze since ancient times and widely used as electrode materials, ornaments, tableware and musical instruments. Cu-22Sn alloy fabrication by hot forging process is a Korean traditional forged high-tin bronze. The tin content is 22 percent, which is more than twice that of bronze ware traditionally used in China and the West. Copper and tin have a carbon solubility of several ppm at room temperature, making Cu-Sn-C alloys difficult to manufacture by conventional casting methods. Research on the production of carbon-added copper alloys has used a manufacturing method that is different from the conventional casting method. In this study, Cu-22Sn-xC alloy was fabricated by mechanical alloying and spark plasma sintering. The carbon solubility was confirmed in Cu-Sn alloy through mechanical alloying. The lattice parameter increased from A0 to C2, and then decreased from C4. The microstructural characteristics of sintered alloys were determined using X-ray diffraction and microscopic analysis. As a result of comparing the hardness of Cu-22Sn alloys manufactured by conventional rolling, casting, and forging and Cu-22Sn-xC alloy by sintered powder metallugy, B0 sintered alloy was the highest at about 110.9 HRB.

In this study, the effects of heat-treatment conditions of Fe powder compacts on densification, microstructure, strength and magnetic properties were investigated. The prepared Fe powder was compressed in a mold of diameter 20 mm at a pressure of 800 MPa for 30 sec. This Fe powder compact was heat-treated under different atmospheres (air and 90% Ar + 10% H2 and heat-treatment temperatures (300 and 700℃). The Fe powder compacts heat-treated in an Ar+H2 mixed gas atmosphere showed a denser microstructure and higher density than the Fe powder compacts heat-treated in an air atmosphere. Oxygen content in the heat-treatment conditions played a significant role in the improvement of the densification and magnetic properties.

Fusion welding of Ti-Cu is difficult because of big difference of melting points and formation of brittle intermetallic compounds. Friction stir welding is carried out by solid-state joining, thermo-mechanical stirring, and friction heat. Ti-Cu FSW dissimilar welding can supply a very sound joint area with a few intermetallic compounds. Optimized welding process conditions are essential to obtain suitable microstructure and mechanical properties of welded zones. Different welding speeds affect the evolution of microstructure and mechanical properties due to changes of input heat and internal stored deformation energy. The correlation of microstructure and mechanical properties of Ti-Cu welded zone according to welding speeds were investigated and analyzed. As the higher the welding speed, the lower the heat input and the lower the temperature rise. Ti-Cu 75 has the smallest grain size at 13.9 μm, but the optimum mechanical properties and the integrity of welding were shown in Ti-Cu 50.

Mechanical, electronic, thermodynamic phase diagram and optical properties of the FeVSb half-Heusler have been studied based on the density functional theory (DFT) framework. Studies have shown that this structure in the MgAgAs-type phase has static and dynamic mechanical stability with high thermodynamic phase consistency. Electronic calculations showed that this compound is a p-type semiconductor with an indirect energy gap of 0.39 eV. This compound’s optical response occurs in the infrared, visible regions, and at higher energies its dielectric sign is negative. The Plasmon oscillations have occurred in 20 eV, and its refraction index shifts to zero in 18 eV.