University of Beira Interior
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RANS Modeling of Transcritical and Supercritical Nitrogen Jets
No full textThe present work is dedicated to the study of cryogenic nitrogen jets under supercritical chamber conditions with the objective of simulating the process of fuel injection inside a combustion engine. In order to do so, a numerical simulation using a RANS model was performed over two case studies previously analyzed by other authors both in experimental and numerical studies. The result obtained by the present numerical approach were then compared with previous results and this way accessed the capabilities of RANS approach using a k-ε turbulence model in which the density is calculated through the mixture fraction value to correctly model cryogenic jets at supercritical conditions. The results show the ability to achieve good agreement with other studies for the axial density distribution however for other parameters like the jet spreading angle the same agreement was not found.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
A Trailblazing Flight for Portuguese Overseas Commercial Air Routes
No full textBetween September 5 and October 26, 1928, Capt. Pais Ramos, Lieut. João Esteves, Capt. Oliveira Viegas, and Sarg. Manuel Antonio, in the name of Portugal's prestige, devoted themselves to accomplishing a trailblazing exploratory flight sponsored by the Lisbon Commercial Association to demonstrate the feasibility of establishing commercial aviation interconnection among Portugal and its African territories (Guinea, St. Thomas & Prince, Angola, and Mozambique). In the 1920s, confronted with governmental riots and instabilities arising from the turbulent early years of Portugal's Republic and the accumulated repercussions of WWI, Portugal lacked long-term planning for the Overseas expansion of Commercial Aviation. This Journey, entirely planned and devised by prominent naval officers and aviation pioneers, beyond cohesion among the Portuguese Colonial Nation, also contributed to the strategic recovery of the Country's aviation progress and reinforced the reestablishment, recognition, and affirmation of Portugal as a competing power among other countries. The outcome had a significant impact on Mozambique's aviation applicability and development.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
The outset of the Portuguese Military Aeronautics (1912-1917)
No full textThe Outset of the Portuguese Military Aeronautics dates back to 1912 when Army and Navy aviation began to be constituted. António José de Almeida, a Portuguese republican politician, in the session of the Chamber of Deputies on June 26, proposed a Draft Law [Bill] and a Supportive Report, both aimed at creating Portuguese Military Aviation. Two months later, the War Ministry appointed a Military Aeronautics Commission to study the creation of a Military Aviation School. Months later, the Commission presented an extensive Official Report outlining the fundamental points for creating an Aeronautical School in Portugal, including its location. After further clarifications, the Minister of War presented a proposal approved in the Chamber of Deputies session on April 22, in the Senate on May 7, drafted a week later, under Law Nº. 162, May 14, 1914. The construction work of the Military Aviation School began on April 5, 1915, in Vila Nova da Rainha. According to the Military Aeronautics Commission report, the first Pilot Aviators should be trained abroad; on August 14, 1915, the Army's Order published a competition for Army and Navy Officers to serve in Aviation. After receiving their diplomas, 11 Officers returned to Portugal to form the initial core of instructors at the newly established School of Military Aeronautics. On September 1, 1916, the Government inaugurated the Military Aeronautics Service and the first Military Aviation School. From November 2, 1916, to May 1917, sixteen candidates were admitted to the first Portuguese Course for Military Aviation Pilots. On September 28, 1917, the School incorporated the Naval Aviation Service. As a result of the Escola de Aeronáutica Militar opening, the Portuguese Government issued Decree Nº 2907 of December 20, 1916, creating symbols and identifiers used by the aeronautical troops in their uniforms.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Influence of bubble growth and liquid film instabilities on droplet impact phenomena under saturated boiling regimes
No full textEvaporation and boiling are processes that occur in many industrial applications involving multiphase flows. For liquid films, however, studies are scarce regarding heat and mass transfer mechanisms and require further research. The main objective of this work is to evaluate bubble formation and detachment, followed by the impact phenomena. Therefore, an experimental setup was built and adapted for this purpose. A borosilicate glass impact surface is placed over a heat source, which consists of an aluminum block with four embedded cartridge heaters that heat the liquid film by conduction. Water and n-heptane are the fluids adopted for the experimental study, as the differences in thermophysical properties allow for a wider range of experiments. Study cases include dimensionless temperatures of θ > 0.6 for similar impact conditions. In terms of bubble formation, n-heptane displays smaller bubble diameters and higher release rates, whereas water exhibits larger bubbles and lower rates. Qualitatively, liquid film temperatures close to the saturation temperature do not reveal a direct influence on the crown development and posterior secondary atomization. For later stages of the impact, the central jet height and breakup are influenced by the film temperature, which is associated with the variation of thermophysical properties.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Optimal Operation of the NACA0012-IK30 Airfoil
No full textThe kinematics of oscillating airfoils are crucial to understanding subjects such as rotor dynamics and bio-inspired flows. Unsteady airfoils have been studied extensively, but there is an overall lack of knowledge regarding newer and more complex kinematics. The present paper builds upon previous studies of the NACA0012-IK30 airfoil by implementing a gradient-based method that searches for a leading-edge pitching amplitude that maximizes propulsive power. All of this is done numerically by solving the Reynolds-Averaged Navier-Stokes equations coupled with the Intermittency Transition model. Results indicate that for higher reduced frequencies, higher leading-edge pitching amplitudes are required to maximize the mean propulsive power. Additionally, propulsive power is achieved with near-optimal propulsive efficiency, which is a common limitation of traditional flapping airfoils.The present work was performed under the scope of the Aeronautics and Astronautics Research Center (AEROG) of the Laboratório Associado em Energia, Transportes e Aeronáutica (LAETA) activities, and it was supported by Fundação para a Ciência e Tecnologia (FCT) through the projects number UIDB/50022/2020, UIDP/50022/2020 and LA/P/0079/2020, and the grant sponsored by Fundação para a Ciência e a Tecnologia 2020.04648.BD. F. Marques acknowledges the financial support of the Brazilian National Council for Scientific and Technological Development (CNPq grant #306824/2019-1).info:eu-repo/semantics/publishedVersio
Unsteadiness of a Ground Vortex Flow
No full textThe impact zone of a wall jet with a boundary layer is studied computationally using a Reynolds averaged Navier-Stokes (RANS) approach with the "κ-ε" turbulence model. The computational domain corresponds to complete experimental rig and the measured boundary conditions were used. It was found that the gross features of the flow are well predicted, and the fluctuations of the flowfield around the ground vortex occur in a very small region near the wall where the impact between each flow occurs. The computational results showed a cyclic formation of two small secondary vortexes that appear and disappear cyclically around the separation and maximum penetration points of the ground vortex.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
A Comparison of Different Gas Turbine Engines Ground Vortex Flows
No full textA computational method was used to predict and compare the ground vortices forming between the ground and a 1/1th scale model intake of different gas turbine engines. Calculations have been performed for the case of an irrotational crossflow mode of formation. Two different kinds of vortices appear around the intake: an inlet vortex and a trailing vortex. Away from the inlet plane other types of complex vortical structures were also identified. The computational method was found to describe all the main features of the flow for full scale geometry.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Identification of Gas Turbine Ground Vortex Formation Regimes
No full textIn the present paper the preliminary work of Barata et al.11 on gas turbine ground vortex formation is extended to include eighteen different engines that are presently in service. The ground vortex flows produced by each engine are compared and discussed for the corresponding operational condition. The results have shown that more than one mode can occur for a particular engine-velocity ratio-clearance distance combination with 1, 2 or 3 vortexes that may merge before entering the inlet engine. Further improvement of gas turbine design is then required to prevent hazardous operation of aircrafts in the airports.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Experimental Study of Instabilities and Secondary Effects of a Ground Vortex Flow
No full textLaser-Doppler measurements of velocity characteristics of the flowfield resulting from the collision of a wall jet with a boundary layer are presented and discussed together with visualization of the flow. The experiments have been carried out for boundary layer to wall jet velocity ratios of 0.5 and 0.58. Direct photography, digital film imaging, and mean and turbulent laser Doppler measurements were obtained to characterize this type of flows. For the present conditions the results revealed the existence of a small counter-rotating recirculating zone located upstream the separation point not yet reported before for this type of flows. The results show that this secondary vortex has a similar oscillating behavior observed in some configurations leading to ground vortex flows, where they are also probably present, but due the geometric and characteristic parameters could not be identified separately. A particular ordered sequence was recognized for the small recirculation zone, and can be confirmed by the bimodal histogram of the horizontal velocity measurements obtained. In spite of the apparent organized sequence of the turbulent structure of the collision zone, the power spectra measurements did not exhibit any evident particular peaks.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio
Numerical study of cryogenic jets under supercritical conditions
No full textThe present work is devoted to the study of cryogenic nitrogen jets in sub- and supercritical conditions. A general trend to operate under an increasingly higher combustor pressures is observed in rockets, gas turbines, and diesel engines, primarily due to enhanced effects on thrust, power, or efficiency. In this conditions, the injected fluid(s) may experience ambient pressures exceeding the critical pressure(s) of the propellants, and recent experimental work of Chehroudi et al.1 showed a quantitative similarity to gas jet-like behavior This paper reports an investigation aimed to evaluate the capabilities of a computational method developed for incompressible but variable density flows when applied to supercritical conditions. The predicted initial jet growth rate was compared with available experimental data for liquid/gaseous jets and mixing layers, and showed a good agreement for different density ratios.Fundação para a Ciência e a Tecnologiainfo:eu-repo/semantics/publishedVersio