A.M.F.R. Pinto

Coalescence of two gas slugs rising in a vertical column of liquid

Abstract This work describes an experimental investigation about the coalescence of pairs of gas slugs rising in vertical columns of liquids covering a wide range of liquid viscosities. The experiments were performed in columns with 19, 32 and 52 mm of internal diameter and slug coalescence was followed by means of a new experimental technique, based on the signals of differential pressure transducers. Important data reported in this work are: • the minimum distance between slugs above which there is no interaction, l min ; • the velocity of approach of the trailing slug as a function of its distance to the leading one. Values of l min are shown to be related to the flow pattern in the wake of liquid behind gas slugs and correlations are provided for the determination of l min ; in all cases l min is about four times the wake length, l w .

Review on micro-direct methanol fuel cells

Fuel cells have unique technological attributes: efficiency, minimization of moving parts and low emissions. The Direct Methanol Fuel Cell (DMFC) has attracted much attention due to its potential applications as a power source for transportation and portable electronic devices. With the advance of micromachining technologies, miniaturization of power sources became one of the trends of evolution of research in this area. Based on the advantages of the scaling laws, miniaturization promises higher efficiency and performance of power generating devices, so, MicroDMFC is an emergent technology. There has been a growing interest in the development of this type of micro cells in the last years, resulting both in experimental studies (operating conditions, cell design and new materials) and in modeling studies. Despite the increase in the knowledge acquired, many challenges are still to be reached. This paper provides a detailed comprehensive review both on fundamental and technological aspects of micro-direct methanol fuel cells. Special attention is devoted to systematization of published results on experimental area since to date and also to a special section dedicated to modeling studies.

Flow in the negative wake of a Taylor bubble rising in viscoelastic carboxymethylcellulose solutions: particle image velocimetry measurements

A simultaneous technique employing particle image velocimetry (PIV) and shadowgraphy was used to study vertical slug flow in non-Newtonian fluids. Two aqueous solutions of 0.8 and 1.0 wt% carboxymethylcellulose (CMC) were studied and the flow field around individual Taylor bubbles fully characterized. The rheological fluid properties and pipe dimension yielded Reynolds numbers of 8 and 4 and Deborah numbers of 0.2 and 0.4. A negative wake was found downstream of the Taylor bubbles in both fluids. Below the bubble trailing edge, along the axis region, the fluid flows in the opposite direction to the bubble (negative wake), originating rotational liquid movements in adjacent regions. Even far downward from the bubble, rotational liquid movements are clearly seen and measured. In the 1.0 wt% CMC solution, the bubble trailing edge has the shape of a two-dimensional cusp. This two-dimensional cusp, of small dimensions, is seen in different orientations during the bubble rise-indicating a fast rotational movement. The asymmetrical shape of the trailing edge is responsible for small asymmetries in the flow in the wake region (three-dimensional flow). The asymmetrical shape associated with the rotational movement is responsible for an unsteady flow of small amplitude. In the 0.8 wt% CMC solution, the shape of the trailing edge changes during the bubble rise. An axisymmetric axial oscillation a continuous expansion and contraction of the trailing edge, is the origin of this behaviour. This oscillatory movement is responsible for an unsteady flow of small amplitude in the wake region.

GAS HOLD-UP IN AERATED SLUGGING COLUMNS

Gas hold-up is one of the most important parameters characterizing the hydrodynamics of bubble columns. A study is presented about the gas hold-up in gas-liquid slugging columns. Expansion of liquid columns is measured for a wide range of superficial velocities of bubbling gas, and the data are compared with available theory. The experiments were performed with liquids of different kinematic viscosities, in columns of 22 mm, 32 mm and 52 mm internal diameters and the initial liquid heights were greater than 2.5 m. A discussion is presented based on the effects of the flow pattern in the wakes of the Taylor bubbles and on coalescence of bubbles. When the flow regimes in the liquid and in the wake are both turbulent or both laminar, the theory predicts the gas hold-up. When the flow in the wake is turbulent and in the main liquid is laminar, the experimental gas hold-up is higher than predictions. This disagreement is explained by the shape of the liquid velocity profile emerging from the wake and by the long length, between bubbles, needed to restore the laminar profile in the liquid.

Measurement of mass transfer between the bubble and dense phases in a fluidized bed combustor

Abstract An experimental study is described on mass transfer between the bubble and dense phases in a fluidized bed, used as a coke combustor. The experimental technique allowed quantification of the mass transfer rate during bubble formation and during a bubble’s rise through the bed. The combustion experiments were performed at 1 atm and 1223 K, in a fluidized bed (i.d. 120 mm) of sand (average diam. 325 μm) with static heights of 0.10–0.21 m. The bubbling flow rate ranged from 2.5 to 5.0 times that at incipient fluidization. The coke particles were 3.0 or 3.5 mm in diameter. Results indicate that the equivalent bed height, L eq (the height a bubble must rise to transfer to the dense phase the same quantity of oxygen as during its formation) is independent of the bubbling air flow rate. The mean value L eq = 50 mm suggests that for shallow beds the mass transferred during bubble formation is a significant part of the total mass transferred. The measured mass transfer factor between phases during a bubble’s rise ( x ′ = X / L mf ) is independent of the bubbling air flow rate and substantially lower than the theoretical predictions of Kunii and Levenspiel [1] . This disagreement is explained by the fact that the theoretical model is for an isolated bubble and does not account for the strong interaction between consecutive bubbles; this increases a bubble’s velocity and induces their coalescence, leading to a decrease in mass transferred between phases.

Fuel Cells and On-Demand Hydrogen Production: Didactic Demonstration Prototype

Hydrogen is presently the most promising alternative to fossil fuels as an energy carrier. A demonstration prototype, for didactic purposes, is presented featuring a polymer electrolyte fuel cell (PEM) coupled to a reactor designed for simultaneous on-demand hydrogen production and storage. The operating principles are illustrated in a simple but comprehensive manner with the aid of an attached booklet emphasizing advantages and limitations to performance on the basis of the electrochemical and physical knowledge of the system. The cell generates electricity combining hydrogen and air in an electrochemical direct process, using a modular device, which is characterized by high efficiency in the conversion and low level of emissions and noise. Emphasis is given to the modularity of the cell and the role of the different components that makes the energy conversion possible, from chemical to electrical and from electrical to mechanical, driving a small motor associated to a bus-like shaped mobile platform. The cell is designed for an air breathing cathode. On-demand hydrogen using chemical hydrides is the option taken for production as well as storage of the fuel, based on the high volumetric and gravimetric efficiency exhibited. Data for comparison with other storage methods are provided in order to encourage a debate on the different storage systems and applications. The demonstration is intended for students at High School and University levels.

Fluidised-bed combustion of a charge of coke with a wide distribution of particle sizes

We describe a study of the combustion of charges of coke with a wide range of particle sizes in fluidised beds of sand. The coke particles ranged in size from 2.0 to 6.5 mm and experiments were performed with small and large charges of particles. In the latter case the experiments were performed only in conditions where the mass transfer resistance of O2 from the bubbles to the particulate phase had a negligible contribution to the overall O2 mass transfer resistance. The beds of sand were made of closely sized particles with average diameters of 0.23, 0.46 and 0.65 mm. The beds were operated at atmospheric pressure and at a temperature of 1223 K. A simple theory was developed, based on the findings of Pinto and Guedes de Carvalho (1990, Trans. Instn chem. Engrs68A, 503–509) and Guedes de Carvalho et al. (1991, Trans. Instn chem. Engrs69A, 63–70), to predict the mass transfer resistance of O2 from the particulate phase to the burning particles and the burn-out time. This theory shows excellent agreement with the experimental data.

Axial dispersion of particles in a slugging column—the role of the laminar wake of the bubbles

Axial solid dispersion promoted by Taylor bubbles in a batch liquid column was studied. A mechanistic model was developed to predict the axial solid dispersion. The model is based on the upward transport of particles inside closed wakes of non-interacting Taylor bubbles. The model predictions are compared with experimental data. The experimental data were obtained in a test tube of 32 mm internal diameter. The particle volumetric distribution was measured by several di:erential pressure transducers placed along the column. Two classes of glass beads, mean diameter 180 and 280 � m, were suspended in aqueous glycerol solutions, with glycerol percentage ranging from 40% (v/v) to 100% (v/v). The amount of particles in the column was such that the volumetric particle fractions were 0.1, 0.2 and 0.3, supposing homogeneous liquid–solid suspension. The air <ow rate ranged from 90 × 10 −6 to 250 × 10 −6 m 3 = sa t PTN conditions. The obtained experimental data are in good agreement with the model predictions for laminar wakes, i.e., closed wakes with internal recirculation and without vortex shedding. The experimental data show a higher upward particle transport for wakes in the transition laminar-turbulent regime; closed wakes with internal recirculation and vortex shedding. The upward particle transport is higher for increasing air <ow rate, decreasing particle diameter and increasing amount of particles in the column. ? 2003 Elsevier Ltd. All rights reserved.

Optimization of a single chamber microbial fuel cell using Lactobacillus pentosus: Influence of design and operating parameters

Microbial fuel cells (MFCs) have been receiving an increased attention over the last years due to their potential to combat two global problems: waste pollution and energy demand. Additionally, when a wastewater is used, MFCs can perform its treatment while recovering energy, leading to the possibility of energy-producing wastewater treatment plants, offsetting their operational costs. However, to overcome their current limitations (lower power outputs and higher costs), a clear understanding of the effect of operation and design parameters on its overall performance is mandatory. Therefore, the goal of this work was to evaluate the effect of operating conditions - batch cycle and yeast extract concentration, and design parameters - anode electrode area, membrane thickness and active area, on the overall performance of a single chamber MFC. The MFC operated with a pure culture of Lactobacillus pentosus and a synthetic wastewater based on a real dairy industry effluent. The overall performance was evaluated through the power output and the COD removal rate. Additionally, the biofilm formed at the anode electrode was characterized in terms of biomass, proteins and polysaccharides content. For the conditions used in this work, a maximum power density of 5.04 ± 0.39 mW/m2 was achieved with an anode electrode area of 61 cm2, a batch cycle of 48 h, 50 mg/L of yeast extract and a Nafion 212 membrane with an active area of 25 cm2. The different conditions tested had a clear effect on the MFC energy production and biofilm characteristics, but not on the ability of L. pentosus to treat the dairy wastewater. The COD removal rates were in the range between 42% and 58%, for all the conditions tested.

Liqid PIV measurements around a single gas slug rising through stagnant liquid in vertical pipes

In this work, the Particle Image Velocimetry (PIV) technique is used to characterise the flow around a single Taylor bubble rising through a vertical column of stagnant liquid. The experimental facility and its operating principle are described. The gas slug velocity is determined from the signals of two photocells that receive light from two laser diodes placed opposite them in the column. The use of fluorescent PIV particles together with an optical filter allows suppressing intense reflections at the liquid/gas interface. Details concerning the PCO CCD camera and Nd: YAG laser synchronisation are presented. The gas slug surface co-ordinates are not accurately determined from PIV images, due to spurious optical effects that occur at the bubble interface when cross illuminated by a laser sheet. These important optical effects are investigated in detail by means of a silicon model of the liquid around the bubble with the same fluorescent PIV particles embedded. An explanation for the spurious optical effects is formulated. It is argued that one of the methods that is suitable to reliably determine the bubble shape is the shadow detection technique. PIV results of the liquid flow in the rear and around the nose of the gas slug are presented. PIV measurements at the liquid film surrounding the gas slug, for laminar liquid film, are presented. The velocity profiles in the liquid film around the bubble are compared with theoretical values given by the falling film theory. The encountered differences are less than 5%. The shear stress profile in the liquid film around a gas slug is presented. Comparison between the theoretical and experimental results shows good agreement. The experimental and theoretical values match within 5 % in the liquid film at a distance from the wall of less than 2 mm; for higher distances the error grows until 15 %. Although optical problems are present near the gas/liquid interface, the results are encouraging.