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- Mass transfer models for oxygen-water co-current flow in vertical bubble columnsPublication . Garcia, Valdemar; Sobrinho Teixeira, João AlbertoThe present work reports a theoretical and experimental study of mass transfer for oxygen-water co-current flow in vertical bubble columns. The axial dispersion of liquid phase was also studied. Experiments were carried out in a 32 mm internal diameter and 5.35 and 5.37 m height columns. The superficial liquid velocity ranged from 0.3 to 0.8 m/s and volumetric flow rate ratio of gas to liquid ranged from 0.015 to 0.25. Mathematical models were developed to predict concentration of gas dissolved in the liquid as function of different physical and dynamic variables for two-phase cocurrent downflow and upflow. We obtained for the ratio of the liquid side mass transfer coefficient to initial bubbles radius, kL/r0=0.12 s-1.
- Convergent-diffuser for small horizontal wind turbinesPublication . Ribeiro, Luís Frölén; Paulo, Jorge Alexandre Rente; Garcia, ValdemarA small horizontal axis wind turbine (HAWT) equipped with an annular Convergent-Diffuser (C-D) is under development. Small to medium wind turbines equipped solely with a diffuser are becoming quite common: the annular structure around the rotor decreases the pressure downstream the rotor, augmenting the air velocity and power output. It also may replace the wind vane. Previous studies for such designs, for instance Ohya, Y. 2010, report a maximum increase in power output up to 150%. We present the preliminary results of an alternative design for a convergent-divergent annular structure that involves the rotor. This structure was optimized in CFD-simulations and 2 models where built for wind tunnel measurements. The maximum power output obtained was 187% higher than the case for a rotor without the C-D structure. Field tests are undergoing at this moment for a C-D adapted in model Rutland 913 Windcharger of 90 W.
- Two-phase bubble flow: experimental and numerical challengesPublication . Matoba, Gláucio Kenji; Ribeiro, Luís Frölén; Garcia, Valdemar; Suguimoto, Fábio KenjiA water and air mixture model is validated via experimental data in a newly rebuild laboratory apparatus where the main variable compared was the pressure gradient along the pipe in a bubble flow pattern. The experimental apparatus presents a constant circular crosssection of 0.032m of diameter and 3.815m of length. The flow set-up was ascendant co-current and 30 pairs of superficial velocities of liquidgas were measured. A numerical solution, using the mixture model, was implemented to predict the pressure gradient and void fraction of the two-phase flow for a one dimensional, steady-state, isothermal, no phase transition, no mass transfer and constant specific mass and viscosity conditions. The pressure gradient numerical results of the mixture model agreed with experimental data within an relative error envelope inferior to 2%.
- Blood flow in a bifurcation and confluence microchannel: effect of the cell-free layer in velocity profilesPublication . Pinho, Diana; Bento, David; Rodrigues, Raquel Oliveira; Fernandes, Carla S.; Garcia, Valdemar; Lima, Rui A.A few detailed studies have been performed in complex in vitro microvascular networks composed by bifurcations and confluences. The main purpose of the present work is to numerically simulate the flow of two distinct fluids through bifurcation and confluence geometries, i. e red blood cells (RBCs)suspended in Dextran40 with about 14% of heamatocrit and pure water. The simulations of pure water and RBCs flows were performed resorting to the commercial finite volume software package FLUENT. A well known hemodynamic phenomenon, known as Fahraeus-Lindqvist effect, observed in both in vivo and in vitro studies, results in the formation of a marginal cell-free layer (CFL) at regions adjacent to the wall. Recently, studies have shown that the formation of the CFL is affected by the geometry of the microchannel and for the case of the confluences a CFL tend to appear in the middle of the microchannel after the apex of the confluence. By using the CFL experimental data, the main objective of this work is to implement a CFL in the numerical simulations in order to obtain a better understanding of the effect of this layer on the velocity profiles.
- Cell-free layer (CFL) measurements in complex geometries: contractions and bifurcationsPublication . Novais, Susana; Pinho, Diana; Bento, David; Pinto, Elmano; Yaginuma, Tomoko; Fernandes, Carla S.; Garcia, Valdemar; Pereira, Ana I.; Lima, José; Mujika, Maite; Oliveira, Mónica S.N.; Dias, Ricardo P.; Arana, Sergio; Lima, Rui A.In this chapter we discuss the cell-free layer (CFL) developed adjacent to the wall of microgeometries containing complex features representative of the microcirculation, such as contractions, expansions, bifurcations and confluences. The microchannels with the different geometries were made of polydimethylsiloxane (PDMS) and we use optical techniques to evaluate the cell-free layer for red blood cells (RBC) suspensions with different hematocrit (Hct). The images are captured using a high-speed video microscopy system and the thickness of the cell free layer was measured using both manual and automatic image analysis techniques. The results show that in in vitro microcirculation, the hematocrit and the geometrical configuration have a major impact on the CFL thickness. In particular, the thickness of the cell-free layer increases as the fluid flows through a contraction-expansion sequence and that this increase is enhanced for lower hematocrit. In contrast, the flow rates tested in this studies did not show a clear influence on the CFL thickness.
- Técnicas de separação biomimética em microfluídicaPublication . Faustino, Vera; Pinho, Diana; Rodriques, Raquel; Garcia, Valdemar; Lima, Rui A.A separação e a identificação de células são essenciais em várias aplicações biomédicas, incluindo a biologia celular e os métodos de diagnóstico e terapêuticos. O sangue é um fluido não-Newtoniano contendo inúmeras informações preciosas sobre o estado fisiológico e patológico do corpo humano. No entanto, devido à sua complexidade, existem actualmente poucos métodos de análise precisos. A maioria das técnicas convencionais usadas na separação e contagem de células são dispendiosas e normalmente é necessário usar agentes externos adicionais para identificar as células. As técnicas biomiméticas de separação em microfluídica usam fenómenos microfluídicos, que ocorrem à microescala, para realizar a separação de determinadas células sanguíneas. Estes fenómenos incluem a separação do plasma, a camada livre de células (CLC), a migração dos glóbulos brancos (GBs), também designados por leucócitos, e a lei da bifurcação. Recentemente, vários investigadores têm replicado estes efeitos, que acontecem em ambientes in vivo, em sistemas microfluídicos, ou seja, em ambientes in vitro. Nos microcanais, os glóbulos vermelhos (GVs), devido à sua deformabilidade e ao gradiente de velocidades (às forças de sustentação), tendem a concentrar-se na zona central dos microcanais, enquanto os GBs e os GVs rígidos (tal como, os GVs infectados com malária) tendem a migrar para a CLC que se forma nas zonas próximas das paredes. A lei da bifurcação estabelece, relativamente ao comportamento dos GVs, que em microcanais com bifurcações eles tendem a escolher o microcanal de secção maior.
- Química física da vidaPublication . Tavares, João; Vieira, Luís; Mosqueira, Diogo; Amaral, Isabel; Granja, Pedro; Pinto-do-Ó, Perpétua; Lima, Rui A.; Garcia, Valdemar; Mendes, Adélio; Almeida, SusanaEste capítulo aborda a Química Física da Vida, incluindo a fotossíntese, a energia, a mecanobiologia, a microfluídica e as membranas, com especial foco nos seres vivos, em particular no Homem. A fotossíntese e a energia são abordadas no primeiro subcapítulo. Assim, a problemática da geração de energia, formas de conversão da mesma, bem como a produção de trabalho são introduzidas. No segundo subcapítulo é abordada a mecânica da vida, sendo introduzidas: a mecanobiologia, isto é, a aplicação da engenharia mecânica no estudo dos princípios da vida; a mecânica da medicina regenerativa e construção de órgãos; as células estaminais tendo em conta o conceito de “nicho” como paradigma unificador biologia-engenharia; a mecanotransdução, isto é o “cross-talk” célula-microambiente; e finalmente, os análogos da matriz extracelular com aplicação em engenharia de tecidos, com relevo para a importância das propriedades mecânicas. O terceiro subcapítulo é dedicado à microfluídica, sendo apresentada uma resenha histórica, indicados exemplos de aplicações, introduzidos os seus princípios físicos, indicadas as técnicas principais de fabrico, descritas técnicas de visualização dos escoamentos, terminando com exemplos envolvendo escoamento sanguíneo e com a apresentação dos atuais desafios. O último subcapítulo é dedicado à separação por membranas; assim, após a introdução ao tema, são indicados exemplos de aplicações, incluindo no corpo humano.
- A rapid and low-cost nonlithographic method to fabricate biomedical microdevices for blood flow analysisPublication . Pinto, Elmano; Faustino, Vera; Rodrigues, Raquel Oliveira; Pinho, Diana; Garcia, Valdemar; Miranda, João Mário; Lima, Rui A.Microfluidic devices are electrical/mechanical systems that offer the ability to work with minimal sample volumes, short reactions times, and have the possibility to perform massive parallel operations. An important application of microfluidics is blood rheology in microdevices, which has played a key role in recent developments of lab-on-chip devices for blood sampling and analysis. The most popular and traditional method to fabricate these types of devices is the polydimethylsiloxane (PDMS) soft lithography technique, which requires molds, usually produced by photolithography. Although the research results are extremely encouraging, the high costs and time involved in the production of molds by photolithography is currently slowing down the development cycle of these types of devices. Here we present a simple, rapid, and low-cost nonlithographic technique to create microfluidic systems for biomedical applications. The results demonstrate the ability of the proposed method to perform cell free layer (CFL) measurements and the formation of microbubbles in continuous blood flow.
- Oxigenação de águas em colunas de borbulhamento a operar em escoamento co-corrente descendentePublication . Sobrinho Teixeira, João Alberto; Balsa, Carlos; Garcia, ValdemarApresenta-se um estudo teórico e experimental sobre transferência de massa num escoamento co-corrente descendente de oxigénio-água em colunas de borbulhamento verticais. As experiências foram efectuadas num tubo de 32 mm de diâmetro interno e 5.37 m de altura. A velocidade superficial do líquido variou entre 0.3 e 0.8 m/s e a razão de caudais volumétricos gás/líquido entre 0.015 e 0.30. Desenvolve-se um modelo teórico de transferência de massa para este tipo de escoamento. Obteve-se para a razão entre o coeficiente de transferência de massa e o raio inicial das bolhas, kL/r0=0.12 s-1.
- Flow of physiological fluids in microchannels: the sedimentation effectPublication . Garcia, Valdemar; Correia, Teresa Montenegro; Dias, Ricardo P.; Lima, Rui A.Microfluidic devices are becoming one of the most promising new tools for diagnostic applications and treatment of several chronic diseases. Hence, it is increasingly important to investigate the rheological behaviour of physiological fluids in microchannels. The main purpose of the present experimental work is to investigate the flow of two different physiological fluids frequently used in microfluidic devices. The working fluids were physiological saline (PS) and dextran 40 (Dx40) containing about 6% of sheep red blood cells (RBCs), respectively. The capillaries were placed horizontally on a slide glass and the flow rate of the working fluids was kept constant by using a syringe pump. By means of a camera the images were taken and transferred to the computer to be analysed. Generally, the results show that PS and Dx40 have different flow behaviour due to the sedimentation of the RBCs.