Associate Laboratory of Energy, Transports and Aeronautics

Funder

Organizational Unit

Publications

Injection lap riveting of aluminum busbars — a thermo-electro-mechanical investigation

Publication . Pragana, J.P.M; Sampaio, Rui F. V.; Bragança, Ivo; Martins, P. A. F.

This paper presents a new mechanical joining process to assemble aluminum busbars in energy distribution systems. The process is based on the extension of injection lap riveting to the connection of busbars made from the same material as the rivets and requires redesigning the joints to ensure complete filling with good mechanical interlocking and appropriate contact pressures on the overlapping area. The experimental work was carried out in unit cells and involved the fabrication of the riveted joints and the evaluation of their electrical resistance at different service temperatures. Comparisons with the bolted joints that were fabricated and tested for reference purposes show that injection riveted joints provide lower values of electrical resistance and require much less space for assembly due to the absence of material protrusions above and below their surfaces. Numerical simulation with finite elements allows the relating of the reduction in electrical resistance with the changes in the electric current flow when the bolts are replaced by the new type of rivets. The experimental and numerical predictions revealed that the new type of rivets experience an increase in electrical resistance of up to 6 μΩ (30%) when the service temperature approaches 105 °C. Still, the resistance at this temperature (26.2 μΩ) is more than 3 times smaller than that of the bolted joints (80.5 μΩ).

2022-07-06Journal article

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ESIMPLE, a new pressure-velocity coupling algorithm for built-environment CFD simulations

Publication . Serra, Nuno; Semião, Viriato

Built environments are major energy consumers and, therefore, tools supporting their efficient design and guaranteeing thermal comfort and indoor air quality are a key factor for energy, environmental sustainability and healthiness. This has been particularly stressed recently by the need to understand the phenomenon of transport of pollutants and/or pathogens leading to exhalation of droplets and aerosols in built environments from people potentially contaminated with the new coronaviruses. In the pursuance of such objectives, CFD procedures have been widely used as prediction tools due to its ability and flexibility in capturing the main features of built environment flows. On the other hand, CFD methods are supported by complex and timeconsuming calculation procedures, especially when used to predict heat and mass transport phenomena in built environment. A possible strategy to reduce computational time is the optimization of the pressure-velocity coupling. An extension of the SIMPLE algorithm (ESIMPLE) is proposed, and its performance compared with the well stablished algorithms, SIMPLE, PISO, SIMPLEC and LIMPO. For that, three test case scenarios are simulated: i) a cubical room (1 m3) with heated floor; ii) a small-scale room, with an occupant, mimicking an office room; and iii) a real-scale office room with an occupant. For the worst scenario, ESIMPLE yielded similar CPU-time required for convergence, and for the best scenario, a three times faster convergence rate was attained. Simultaneously, this newly proposed coupling scheme algorithm, yielded a lower number of iteration steps required for convergence, in 6 of the 9 simulated cases.

2021-10-15Journal article

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Expansion of additive-manufactured tubes: deformation and metallurgical analysis

Publication . Pragana, João; Rosa, Luis G.; Bragança, Ivo; Silva, Carlos; Martins, Paulo

Herein, the mechanical and metallurgical feasibility of integrating tube end-forming operations with additive manufacturing is investigated. The work makes use of wire-arc additively manufactured AISI 316L stainless steel preforms that are subsequently machined into tubes and expanded with a tapered conical punch. Experimental measurements of force, surface strains, thickness, and microhardness combined with microstructure observations and fractography of the fractured surfaces are utilized to characterize plastic deformation and formability limits of the additively manufactured tubes and understand the main differences against the results obtained from commercial wrought tubes of the same material. Results show that the material deformation characteristics, namely, the evolution of microhardness along the expanded tube length, and the formability limits by necking and fracture, are strongly influenced by the columnar microstructure originated by a noncyclic dendritic growth aligned with the building direction. Still, results demonstrate that the additively manufactured AISI 316L tubes are ductile enough to be successfully included in hybrid additive manufacturing routes.

2021-09Journal article

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The importance of sleepers spacing in railways

Publication . Ortega, Roberto Sañudo; Pombo, João; Ricci, Stefano; Miranda, Marina

The railway tracks are composed of steel rails that are supported by the infrastructure through sleepers, which are generally assembled with a fixed separation between them. Sleepers and fastenings are the most numerous elements of the superstructure. The global cost of these elements is directly proportional to their number along the track. Therefore, the reduction of the number of these components can generate relevant reductions of track construction costs. Nonetheless, very few studies focused on the optimal sleeper spacing and its consequences. Moreover, the railway industry uses fixed separation values based on past experience, rather than on scientific knowledge. In line with this reasoning, the objective of this work is to analyse the optimal sleeper spacing in order to contribute to the minimization of track costs. The work starts from an overview of the sleepers spacing used over the world and analyses how this can affect the track performance. This study is valuable for design and construction of new tracks, as well as for the maintenance and renewal of existing ones, helping to achieve cost savings for the rail industry, positive environmental results and consequent contributions to the competitiveness of rail transport.

2021-09-20Journal article

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Injection lap riveting

Publication . Ribeiro Ferreira, Francisco; Pragana, João; Bragança, Ivo; Silva, Carlos; Martins, Paulo

This paper presents an injection lap riveting process to connect two sheets placed one on top of the other. The process is carried out at ambient temperature and differs from self-pierce riveting because its joining principle is based on plasticity and friction without fracture and formation of new surfaces. The working principle is based on two consecutive operations first, a dovetail ring hole is machined in the lower sheet and then a semi tubular rivet is injected through the upper sheet into the dovetail ring hole of the lower sheet, by compression with a punch. The presentation is based on a combined experimental and numerical investigation and special emphasis is put on the influence of the dovetail ring hole geometry in material flow, riveting force, and pull-out and shear destructive forces. The last past of the paper includes details of a prototype cutting tool that was developed by the authors for producing the dovetail ring holes in-site and fostering the portability and applicability of the process.

2021Journal article

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