Browsing by Issue Date, starting with "2022-01-04"
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- A new insight into the degradation of anthocyanins: reversible versus the irreversible chemical processesPublication . Sousa, Diogo; Basílio, Nuno; Oliveira, Joana; Freitas, Victor De; Pina, FernandoThe kinetics and thermodynamics of the pH-dependent reversible and irreversible processes leading to color fading of pelargonidin-3-O-glucoside, peonidin-3-O-glucoside, malvidin-3-O-glucoside, and cyanidin-3-O-glucoside dyes in aqueous solutions are reported. Following the addition of base to the flavylium cation, the quinoidal bases disappear by three distinct steps: (i) in an acidic medium by a biexponential process, in which the faster step is controlled by the hydration reaction and the slower one by cistrans isomerization; the degradation process occurs essentially from the anionic quinoidal base; (ii) in a basic medium (pH > 9.5), in which the disappearance of the anionic bases is monoexponential, with the rate proportional to the hydroxyl concentration (hydroxyl attack), leading to anionic chalcones (cis and trans) at equilibrium-the slower degradation step occurs from the di- and trianionic chalcones; and (iii) in the pH region circa 7.7 < pH < 9.5, in which hydration and hydroxyl attacks are much slower than anionic quinoidal base degradation (which is the rate-controlling step) and the equilibrium cannot be attained.
- Fault-tolerant multilevel converter to feed a switched reluctance machinePublication . Fernao Pires, Vitor; Cordeiro, Armando; Foito, Daniel; Pires, Armando J.The switched reluctance machine (SRM) is one of the most interesting machines, being adopted for many applications. However, this machine requires a power electronic converter that usually is the most fragile element of the system. Thus, in order to ensure high reliability for this system, it is fundamental to design a power electronic converter with fault-tolerant capability. In this context, a new solution is proposed to give this capability to the system. This converter was designed with the purpose to ensure fault-tolerant capability to two types of switch faults, namely open- and short-circuit. Moreover, apart from this feature, the proposed topology is characterized by a multilevel operation that allows improvement of the performance of the SRM, taking into consideration a wide speed range. Although the proposed solution is presented for an 8/6 SRM, it can be used for other configurations. The operation of the proposed topology will be described for the two modes, fault-tolerant and normal operation. Another aspect that is addressed in this paper is the proposal of fault detection and diagnosis method for this fault-tolerant inverter. It was specifically developed for a multilevel SRM drive. The theoretical assumptions will be verified through two different types of tests, firstly by simulation and secondly by experiments with a laboratory prototype.
- Effect of shear/axial stress ratio on multiaxial non-proportional loading fatigue damage on AISI 303 steelPublication . Anes, Vitor; Reis, Luis; de Freitas, M.In this paper, we investigate the cyclic response of AISI 303 stainless steel subjected to non-proportional loads with different amplitude ratios between shear stresses and normal stresses. Based on the experiments, a relationship between the proportional reference load and a varied range of non-proportional loads was established. To achieve this objective, an experimental program was implemented to evaluate the non-proportional parameter Y. Then, the evolution of this parameter was analyzed with the number of cycles to failure and with the ratio between shear and normal stresses, finally, the evolution of the non-proportional parameter Y was mapped by two functions. The results show that the non-proportional response of the AISI 303 can be estimated using the two functions obtained. This allows the estimation of the relationship between non-proportional and proportional stresses as a function of the number of cycles to failure together with the relationship between shear and normal stresses. The results obtained have direct application in the evaluation of accumulated damage, assessed in real-time, resulting from variable amplitude loading spectra. This is of particular interest for the evaluation of structural health monitoring of structures and mechanical components.