Gas carburizing significantly enhances the surface properties of low-alloy gear steels, resulting in superior micro-hardness, layer thickness, carbon content, and overall mechanical properties. Unlike other thermochemical processes such as nitriding and carbonitriding, which have limitations in core properties and hardening depth, gas carburizing offers unmatched surface hardness, wear resistance, and mechanical strength. This makes it ideal for demanding applications in the automotive, aerospace, and manufacturing industries. In this research, samples were gas-carburized for 4, 6, or 8 h. The results showed significant improvements: micro-hardness increased from approximately 140 HV to over 819 HV, and the surface layer thickness grew by more than 41%, from 1166 μm to 1576 μm. Additionally, the carbon content in the surface layer increased by over 450%, reaching up to 0.94 wt%. Clear correlations were observed between the duration of heating and the mechanical properties. Longer heating times, particularly after 8 h, raised ultimate tensile strength from 427.29 MPa to 778.33 MPa, while simultaneously decreasing elongation from 26.07% to 2.88% and resilience from 180 J cm⁻² to 6.66 J cm⁻². This optimization not only enhances surface hardness and durability but also improves key mechanical properties such as tensile strength, stiffness, resilience, and overall mechanical performance.

The regenerator is one of the most important elements of a thermal machine operating in an alternating flow regime. The present study constituted an experiment on the dynamic characterization of different metallic regenerators crossed by alternating air flow. During an experiment, gas temperature, velocity, and pressure were measured at both ends of the regenerator. The acquisition frequency was set at 1 kHz for each temperature, pressure, and velocity signal. This enabled us to fully characterize an oscillating flow cycle. Experiments were performed for different regenerators (structures and porosities), frequencies, temperature gradients, and displaced air volumes. The experimental results showed that the density ratio is significant at high frequencies for all structures. The friction coefficient is determined based on a classical correlation at the time of maximum velocity. The friction factor$f$seems to decrease with the kinetic Reynolds number$R{e}_{\omega }$for a 30% porosity regenerator. For the other tested regenerators (35% and 40% porosity), we observed that it is almost constant with a little dispersion. To minimize the dispersion effects, another definition was proposed to calculate the friction factor at the time of maximum pressure drop. The results showed that for all regenerators a single clear trend is the function of the kinetic Reynolds number. A significant phase shift was observed between the velocity and pressure drop. It increases with the increasing kinetic Reynolds number. It was found that the phase shift only depends on frequency. Finally, a correlation equation was proposed to predict the phase shift of different regenerator structures. It was found that the effect of the kinetic Reynolds number$R{e}_{\omega }$on the phase shift is more dominant than that of the hydraulic diameter to length ratio $D_h/L$.

This study presents a numerical investigation of a parabolic trough absorber tube equipped with a novel Angularly Segmented Ring Turbulator (ASRT), designed to enhance heat transfer through periodic flow disturbance and improved wall–fluid interaction. The proposed ASRT geometry consists of segmented annular rings arranged along the tube length, characterized by two key parameters: the number of angular segments per ring (Nr = 4, 6, 8) and the angular spacing of each segment (α = 20° and 40°). Three dimensional simulations were performed using the finite volume method under turbulent flow conditions, with Reynolds numbers ranging from 3300 to 11,000. A non-uniform solar heat flux, obtained via Monte Carlo Ray Tracing (MCRT), was applied as a boundary condition at the outer wall to replicate realistic solar concentration. The results reveal that the ASRT significantly improves convective heat transfer, with the Nusselt number ratio $\left(Nu/{Nu}_s\right)$ reaching up to 3.7 for α = 20° and Nr = 8. This enhancement is accompanied by a moderate rise in the friction factor ratio $\left(f/f_s\right)$, reaching approximately 7.5 at Re = 3300, indicating efficient turbulence promotion with acceptable hydraulic penalties. The Performance Evaluation Criterion (PEC) ranges from 1.7 to 1.9, confirming the superiority of ASRT over the smooth tube.

One of the main challenges of solar parabolic trough collectors (PTCs) is their requirement for large land areas. To address this, a novel tri-pass receiver design has been experimentally developed, which is less sensitive to misalignments, easy to fabricate, maintain, and repair. First of all, the testbed thermal and optical efficiencies were evaluated. The PTC testbed comprises a polished stainless-steel reflector with a 44% optical efficiency integrated into a supervisory control and data acquisition system. No-load tests were initially performed, followed by an in-load test. These tests took place in North Africa, precisely in the northeast of Algeria. Initial experiments revealed a low PTC thermal efficiency of 20%. To address this issue, five different receiver configurations were manufactured, tested, and compared. the reference configuration is an uncoated copper tube. Therefore, a single black-coated copper tube receiver achieved a 27% increase in thermal efficiency. A bypass black-coated receiver showed a significant improvement, achieving 71% thermal efficiency. However, a three black-coated receiver with two centralized tubes experienced a 12% decrease in thermal efficiency. Yet, the three black-coated receiver with one central tube achieved the highest thermal efficiency of 85% due to the triangular pitch arrangement, which effectively focuses solar rays on the central tube while capturing deviated rays with the backside tubes. The no-load test determined the optimal fluid path for this configuration, ensuring smooth progression from preheating to heating as the fluid moved through increasingly hotter tubes. The in-load test confirmed the receiver's effectiveness.