Strojniški vestnik - Journal of Mechanical Engineering

Phase Change Materials for Performance Enhancement in Household Refrigeration: A Review

Urban Tomc — 2026-09-23

This paper reviews the use of phase change materials (PCMs) to improve the performance of household vapour-compression refrigerators. It first summarizes PCM selection criteria (phase-change temperature matching the thermostat range, high latent heat, adequate thermal conductivity, low supercooling/segregation, limited volume change, and chemical stability/safety). Furthermore, it outlines the main PCM classes (organic, inorganic, eutectic) together with practical measures such as encapsulation and conductivity enhancement via composites. The review also compiles examples of commercially available PCMs with melting temperatures relevant for refrigeration applications (approximately from −21 °C to +7 °C), highlighting that suitable market-ready solutions already exist. The core of the review compares PCM integration concepts at key locations: (i) at the evaporator, where improved heat transfer can raise evaporating temperature, extend compressor off-time, reduce temperature fluctuations, and support demand-side management; (ii) at the condenser, where lower condensing temperature and shorter on-time are possible but may be offset by more frequent compressor cycling; and (iii) inside the refrigerated compartment, where added thermal inertia dampens temperature swings, mitigates door-opening disturbances, and improves resilience during power outages. Finally, combined placements (e.g., evaporator + condenser) are discussed as a route to synergistic benefits, provided phase-change temperatures and geometry are selected appropriately.

Vapor Thermodynamics and Fluid Merit for Pulsating Heat Pipe

Vadim Nikolayev — 2026-09-23

In this communication, we discuss a theoretical description of the vapor-phase thermodynamics in the pulsating heat pipe (PHP), to be used in numerical simulations. We advance a theory based on simulation results that allows us to derive a theoretical expression for a dimensionless quantity describing the vapor properties of a given fluid. One can use this quantity to evaluate the fluid merit for use in the PHP. This theory is compared with the simulation results obtained using the PHP simulation code CASCO. We compare the merits of water, ethanol, and FC-72 and show that water possesses better properties for use in PHPs.

Design-Based Assessment of Continuous Coaxial DED Nozzles Considering Substrate-Induced Gas Flow Effects

Mehmet Ermurat — 2026-09-23

Continuous coaxial nozzles are widely used in directed energy deposition (DED) processes. However, many numerical studies rely on substrate-free free-jet models that fail to capture critical gas–surface interactions. This study presents a design-oriented investigation of continuous coaxial DED nozzles by examining the influence of nozzle geometry and gas flow interactions under realistic deposition conditions. A three-dimensional CFD framework incorporating particle tracing was applied, in which a physical substrate was explicitly included at the intended standoff distance to account for stagnation pressure formation, gas rebound, and near-wall flow effects. Two nozzle geometries, a narrow nozzle (β = 24°) and a wide nozzle (β = 35°), were evaluated under varying carrier and shaping gas velocities relative to a fixed optical shielding gas velocity. Powder convergence behavior was quantified using a layered particle counting strategy and analyzed through number density distributions in the focal region. The results show that nozzle geometry strongly governs powder focusing behavior. The narrow nozzle consistently produced a compact and axisymmetric powder stream with higher catchment efficiency, whereas the wide nozzle exhibited diffuse flow patterns and reduced focal coherence. Excessive shaping gas velocities were found to degrade convergence by expanding the substrate-induced stagnation pressure zone. These findings provide a physics-based rationale for geometry-dependent nozzle selection in continuous coaxial DED applications.

Thermally Induced Dynamics of an Aero-Engine Active Magnetic Bearing–Rotor System

Peixun Tang — 2026-09-23

Under high-temperature operating conditions in aero-engines, the combined effects of nonuniform heat generation in active magnetic bearing (AMB) coils and ambient thermal loads can lead to nonuniform temperature distributions within the rotor system, thereby generating thermal bending loads and degrading the system’s dynamic stability. To address this issue, a one-way sequential thermo-mechanical analysis framework for an AMB-supported rotor system is developed. The model incorporates thermally induced loads derived from the temperature field, disk unbalance, gravity, and the PID-dependent equivalent stiffness and damping of the AMBs. The governing equations are solved using the Newmark-${\beta}$ method. Based on the proposed model, the dynamic response characteristics of the system under varying bias currents, rotational speeds, and ambient temperatures are systematically investigated. The results show that, under a given thermal condition, increasing the bias current enhances the effective support capability of the AMB, thereby reducing the vibration response. In contrast, increasing the rotational speed progressively amplifies the synchronous vibration response, with a more pronounced increase near the upper end of the investigated speed range. Furthermore, increasing the ambient temperature significantly increases the thermally induced bending excitation, resulting in larger vibration amplitudes. This suggests that, within the investigated operating range, thermal effects strongly affect the system’s dynamic behavior. This study provides a theoretical basis for thermal stability assessment and vibration control of AMB-supported rotor systems operating under high-temperature conditions.

Enhancing PEM Fuel Cell Efficiency with an Integrated Organic Rankine Cycle Using Low-GWP Working Fluids

Bagus Putra Pratama — 2026-09-23

Proton exchange membrane fuel cells (PEMFCs) offer high-efficiency, zero-emission electricity, yet 45 % to 60 % of their input chemical energy is lost as low-grade waste heat. In response to this problem, this study presents a thermodynamic analysis of a 50 kW PEMFC thermally integrated with a sub-critical organic rankine cycle (ORC) to convert this waste heat into additional power. Using a high-fidelity model validated with errors below 5 %, this study compares low-global warming potential (GWP) working fluids (R1233zd(E), R1234yf, R1234ze(Z)) against the legacy R245fa under a practical fixed-state-point control strategy. The results demonstrate up to 3 % increase in total electrical efficiency, driven by the successful conversion of the PEMFC’s low-grade waste heat into additional net power output by the ORC bottoming cycle. The main novelty of this research lies in the first comprehensive demonstration that a 3 % system efficiency enhancement can be sustainably achieved by integrating low-GWP working fluids and a practical fixed-state-point control strategy to ensure stable operation. The analysis confirms that the thermodynamically limited 10 % ORC thermal efficiency can be realized without compromising energy recovery potential, proving that the pursuit of sustainability is compatible with enhanced system performance. R1234yf is identified as the superior low-GWP fluid for high-load operations, while R1233zd(E) proves optimal for low-load conditions, validating a robust pathway for designing next-generation, eco-friendly fuel cell systems.