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Panao M.R.O.,University of Lisbon | Moreira A.L.N.,University of Lisbon | Durao D.G.,Lusiada University of Lisbon
Experiments in Fluids | Year: 2012

This paper analyzes the transient characteristics of intermittent sprays produced by the single-point impact of multiple cylindrical jets. The aim is to perform a transient analysis of the intermittent atomization process to study the effect of varying the number of impinging jets in the hydrodynamic mechanisms of droplet formation. The results evidence that hydrodynamic mechanisms underlying the physics of ligament fragmentation in 2-impinging jets sprays also apply to sprays produced with more than 2 jets during the main period of injection. Ligaments detaching from the liquid sheet, as well as from its bounding rim, have been identified and associated with distinct droplet clusters, which become more evident as the number of impinging jets increases. Droplets produced by detached ligaments constitute the main spray, and their axial velocity becomes more uniformly distributed with 4-impinging jets because of a delayed ligament fragmentation. Multijet spray dispersion patterns are geometric depending on the number of impinging jets. Finally, an analysis on the Weber number of droplets suggests that multijet sprays are more likely to deposit on interposed surfaces, thus becoming a promising and competitive atomization solution for improving spray cooling. © 2011 Springer-Verlag. Source

Barros H.F.M.,University of Coimbra | Martins R.A.F.,Lusiada University of Lisbon
Computers and Concrete | Year: 2012

This paper presents an algorithm for the evaluation of stresses in reinforced concrete sections under service loads. The algorithm is applicable to any section defined by polygonal contours and is based on an analytical integration of the stresses. The nonlinear behaviour of concrete is represented by the parabola-rectangle law used in the Eurocode-2 for the ultimate concrete design. An integrated definition of the strains in concrete and steel is possible by the use of Heaviside functions, similarly to what is done for ultimate section design in Barros et al. (2004). Other constitutive equations for the definition of the stresses in the concrete or steel can be easily incorporated into the code. The examples presented consist in the evaluation of resulting axial load and bending moment in an irregular section and in a section in L shape. The results, for service stresses, can also be plotted in terms of design abacus; a rectangular doubly reinforced section is presented as example. Source

Panao M.R.O.,University of Lisbon | Moreira A.L.N.,University of Lisbon | Durao D.F.G.,Lusiada University of Lisbon
Energy | Year: 2011

Thermal management is a particularly difficult challenge to the miniaturization of electronic components because it requires high performance cooling systems capable of removing large heat loads at fast rates in order to keep the operating temperature low and controlled. To meet this challenge, the Intermittent Spray Cooling (ISC) concept has been suggested as a promising technology which uses a proper match between the frequency and duration of consecutive injection cycles to control heat transfer. This concept also depends on: the atomization strategy; a homogeneous dispersion of droplets impinging on the hot surface; and the quantitative control of the liquid deposited, avoiding excessive secondary atomization or pre-impingement-evaporation. In this work, the use of liquid atomization by multiple jets impact, also referred as multijet atomization, is the subject of a thermal-fluid assessment using heat transfer correlations previously derived for intermittent sprays. Simultaneous measurements of droplet size and velocity are provided as input for the correlations and the analysis explores the influence of the number of impinging jets on the heat removal pattern and magnitude. Emphasis is put on the promising applicability of multijet atomization for promoting an intelligent use of energy in the thermal management of electronic devices. © 2010 Elsevier Ltd. Source

Panao M.R.O.,University of Lisbon | Moreira A.L.N.,University of Lisbon | Durao D.F.G.,Lusiada University of Lisbon
Fuel | Year: 2013

The use of multipoint injection systems for HCCI engines requires a mixture composition precisely controlled in order to obtain the optimum auto-ignition timing. The most favorable injection timing to achieve it is when the intake valve is open, in which case the fuel spray interacts with the air flow and impacts onto interposed surfaces where it may form a liquid film, especially at engine cold-start, and eventually generate smaller droplets by mechanisms of secondary atomization. An accurate description of the physics of spray-wall impact under cross flow conditions and of the effects of the latter on secondary atomization are key issues to devise appropriate strategies to optimize the injection system and to control mixture preparation. This is the aim of the work reported here. The experiments consider overlapping Mie and Shadowgraph visualization techniques and using a phase-Doppler interferometer to visualize and quantify the effects of a cross-flow on spray impact and secondary droplets. Furthermore, the experiments are conducted for well-defined boundary conditions, thus provide results useful for the development spray/wall interaction models. Analysis show that: the cross-flow decelerates the axial velocity of impinging droplets, thus decreasing the energy available at impact; because drag is more efficient on droplets more prone to adhere to the wall, the net effect is the formation of thinner films; this, in turn, enhances the generation of secondary drops which are then dragged away from re-impacting onto the wall. © 2012 Elsevier Ltd. All rights reserved. Source

Paiva T.,University of Lisbon | Gaspar T.,Lusiada University of Lisbon | Matos M.G.,University of Lisbon
Sleep Medicine | Year: 2015

Objectives: The present study aimed to evaluate the influences of sleep duration, sleep deprivation, and weekend variability of sleep upon other adolescents' features, namely those related to health and health-related quality of life. Methods: The Health Behaviour in School-Aged Children (HBSC) survey is based on a self-completed questionnaire. The participants in the present study were 3476 students (53.8% were girls) in the 8th and 10th grades at school; the mean age was 14.9 years (range 12.5-19.0). Subjective sleep duration during the weeknights and weekends was collected; sleep deprivation (SD) was considered whenever the difference was greater than 3 h. Health complaint frequency and health-related quality of life (with the Kidscreen 10) were collected. Results: Sleep deprivation was present in 18.9% of the students. It was negatively correlated with sleep duration on weeknights. There were no gender differences, but SD increased with age and grade. Higher school grades were mainly associated with fatigue. A considerable number of adolescents had sleep problems (37.2%); 25.5% had difficulties in sleep initiation, which was more prevalent in adolescents with SD. The sleep duration on weeknights was decreased in the SD group. The average health-related quality of life was reduced in adolescents with SD. The frequency of health complaint was higher is adolescents with SD. Girls had significantly more health complaints than boys, with special focus on headaches. Conclusions: Sleep deprivation is associated with the perception of health-related quality of life and perceived physical and mental health. © 2014. Source

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