http://hdl.handle.net/10174/29534 2026-04-12T15:01:15Z http://hdl.handle.net/10174/41406 Title: On the Identification of Dissipative Phenomena in Fatigue-Loaded 2024 Aluminum by Means of Second Harmonic of Temperature Analysis Authors: Capello, Riccardo; Garção, José; Catalanotti, Giuseppe; Pitarresi, Giuseppe Abstract: This study explores the use of temperature harmonics to detect intrinsic dissipation during cyclic loading in aluminum alloys. Under sinusoidal loading, the temperature of a solid is modulated by thermomechanical heat sources. The primary source is the thermoelastic effect, which modulates the temperature at the load frequency and twice the load frequency (second harmonic). Thermoelastic stress analysis (TSA) signal processing is employed to extract the temperature harmonics and analyze their evolution when the stress amplitude increases. The detected second harmonic comprises three main contributions: a thermoelastic component, predicted by the second-order thermoelastic effect theory, a spurious contribution due to load components at twice the nominal frequency, and a dissipative second harmonic. The main aim of this work is to separate the thermoelastic and spurious contributions from the measured second harmonic to isolate and capture dissipation. AL 2024 alloy samples, which exhibits strong second-order thermoelastic response, are employed in the investigation. Aluminum has already been reported as a material where thermomechanical dissipation is difficult to quantify, or even qualitatively observe, with other more traditional thermographic methods. The results show interesting features of the second-harmonic decoupled components, providing insight into intrinsic dissipation of aluminum alloys under fatigue-loading conditions. 2025-09-30T23:00:00Z http://hdl.handle.net/10174/39249 Title: Performance Analysis of Multi Capillary Knudsen Heat Pumps Authors: Ajuda, Ana; Silva, Goncalo; Semiao, Viriato Abstract: The present work investigates the theoretical performance of the Knudsen heat pump (KHP), a novel heat pump concept in which the conventional mechanical compressor is replaced by a Knudsen compressor. This modification has the potential to reduce both maintenance requirements and energy consumption. The flow behavior within the Knudsen compressor, the core element of the KHP, is described using a simplified gas model derived from the formulation originally proposed by Muntz et al. The model predictions are initially validated against well-established data reported in the literature, and subsequently employed to analyze the performance of the KHP, with the final objective of enhancing its operational efficiency. To ensure the practical relevance of the performance assessment, the analysis is conducted using realistic geometrical and operational parameters derived from previously reported experimental studies of Knudsen compressors featuring rectangular or circular cross-sectional geometries. The results of this study suggest that, while the original KHP configuration exhibits limited performance, parametric analysis suggests the possibility to enhance its performance by more than 100% under optimal conditions, with additional factors identified that may enable further gains. 2025-09-02T23:00:00Z http://hdl.handle.net/10174/39006 Title: CFD analysis of twin turbulent plane jets confined by walls: Effects of geometry on flow dynamics and heat transfer Authors: Zdanowski, Francisco; Malico, Isabel Abstract: This study investigates the flow and heat transfer behavior of heated twin turbulent plane parallel jets confined between isothermal walls. The focus is on understanding the impact of key geometric parameters: the separation ratio and offset ratio. While dual plane offset jets have been studied in various unconfined or isothermal configurations, the combined impact of heating, confinement and jet geometry remains insufficiently addressed in the literature. This work fills the gap by conducting a systematic analysis using two-dimensional Computational Fluid Dynamics simulations. Validation against experimental data for three different jet configurations showed good agreement with experimental data, with the SST k-ω model providing the most balanced performance out of four turbulent models tested. The results revealed that despite geometrical symmetry, the jets consistently deflect towards one of the walls due to the Coanda effect, leading to flow asymmetry and influencing heat transfer. An increase in the separation and offset ratios shifts the reattachment point downstream, weakens the intensity of wall impingement, and reduces local peak wall shear stress and the maximum local Nusselt number at the walls. Increasing the offset ratio from 2 to 5 enhanced average heat transfer at the jet impingement wall by 20%. Conversely, increasing the separation ratios from 2 to 5 decreased the average heat transfer to the opposite wall by 11% and to the impingement wall by 1%. These findings contribute to a better understanding of complex wall-jet interactions and support the design of thermal systems involving confined turbulent dual offset jets. 2025-01-01T00:00:00Z http://hdl.handle.net/10174/38966 Title: Measurement of Battery Aging Using Impedance Spectroscopy with an Embedded Multisine Coherent Measurement System Authors: Lourenço, J.; Rosado, Luis S.; Ramos, Pedro M.; Janeiro, Fernando M. Abstract: This work describes the development of an embedded standalone measurement system that monitors the aging of batteries using impedance spectroscopy. The system generates a multisine stimulus that contains the frequency components at which the battery impedance is measured. Coherent generation and sampling is assured, and Goertzel filters, one for each measurement frequency, are updated with each new sample. This architecture reduces memory requirements because the current and voltage of the measured samples are discarded after processing. Aging is monitored, as the system is able to automatically perform complete or partial charge/discharge cycles as well as measurement cycles without requiring user interaction. 2025-05-31T23:00:00Z