Abel Rouboa

Optimizing the DMFC operating conditions using a response surface method

Abstract The power density of a Direct Methanol Fuel Cell (DMFC) as a function of temperature, methanol concentration, air flow rate, methanol flow rate and air relative humidity was studied using a Response Surface Methodology (RSM). For a DMFC equipped with a membrane of Nafion 112, it was observed that only the temperature, methanol concentration and air flow rate were relevant factors or operating variables. A new design of experiments was done for a narrower range of these variables and the operating values that optimise the power density were obtained using the software JMP 7.0 (SAS). The predicted power density values were in agreement with the experimental results obtained for the optimized operating conditions. Then, the RSM was applied to membranes with different thicknesses, Nafion 112, Nafion 1135 and Nafion 117, and as a function of the temperature and methanol concentration. The DMFC was characterized for the open circuit voltage (OCV), methanol crossover at the OC, power density and global efficiency. The membrane showing the best compromise between power density and efficiency was Nafion 117.

Computational fluid dynamics analysis of greenhouse microclimates by heated underground tubes

One of the main problems of Mediterranean climates is the large diurnal amplitude of temperature, with too low temperature during winter nights and too high temperatures during summer days. This is particularly felt in the north of Portugal, where the low temperature during winter nights can be compensated by the introduction of a heat source. The objective of this is work is to simulate the effects in the temperature and velocity fields by the introduction of hot water tubes along a greenhouse in night conditions. Three different situations are simulated: natural convective heating (case A), artificial heating tubes (case B), artificial heating tubes, and natural ventilation (case C). The commercial CFD package ANSYS® (FLOTRAN module) is used for this propose.The turbulence is modelled by the RNG turbulence model. The numerical results are compared with experimental values, the procedure for which is also presented.The average increase in air temperature for cases A, B and C was 2.2°C, 6.7°C and 3.5°C, respectively. Turbulence is lower in case A, increases slightly when the heating system is introduced (case B), and increases significantly in case C due to the effect of natural ventilation. A very good agreement between experimental and numerical temperature values was verified. This allows validating the RNG turbulence model as suitable to simulate arch-shaped greenhouse microclimates. Some improvements can be done to this work: introduction of night-time crop transpiration, 3D simulations, or optimizing the size of the element mesh in order to reduce the computation time.

Swimming simulation: a new tool for swimming research and practical applications

This chapter covers topics in swimming simulation from a computational fluid dynamics perspective. This perspective means emphasis on the fluid mechanics and CFD methodology applied in swimming research. We concentrated on numerical simulation results, considering the scientific simulation point-of-view and especially the practical implications with swimmers.

The feasibility of biomass pellets production in Portugal.

Abstract The production of pellets represents the possibility of using different biomass residues in a standardized fuel. In this article, the economic feasibility of pellets production is analyzed in the Portuguese scenario according to the key indicators of biomass availability, costs, and legal framework. The potential of biomass residues in Portugal is significant and mainly from forestry. However, several limitations to its utilization for pellets production may arise since they are already put to other uses, such as biomass power plants. The combination of the biomass power plants with pellet production plants seems to be the best option for pellet production in the actual Portuguese scenario. The main constrains for the pellets market has been to convince small-scale customers that pellets are a good alternative fuel, mainly due to the investment needed and the strong competition in terms of fuel price with natural gas. These market problems need incentives at a political level bringing the value added tax for the same level of natural gas.

The Hydrodynamic Study of the Swimming Gliding: a Two-Dimensional Computational Fluid Dynamics (CFD) Analysis

The Hydrodynamic Study of the Swimming Gliding: a Two-Dimensional Computational Fluid Dynamics (CFD) Analysis Nowadays the underwater gliding after the starts and the turns plays a major role in the overall swimming performance. Hence, minimizing hydrodynamic drag during the underwater phases should be a main aim during swimming. Indeed, there are several postures that swimmers can assume during the underwater gliding, although experimental results were not conclusive concerning the best body position to accomplish this aim. Therefore, the purpose of this study was to analyse the effect in hydrodynamic drag forces of using different body positions during gliding through computational fluid dynamics (CFD) methodology. For this purpose, two-dimensional models of the human body in steady flow conditions were studied. Two-dimensional virtual models had been created: (i) a prone position with the arms extended at the front of the body; (ii) a prone position with the arms placed alongside the trunk; (iii) a lateral position with the arms extended at the front and; (iv) a dorsal position with the arms extended at the front. The drag forces were computed between speeds of 1.6 m/s and 2 m/s in a two-dimensional Fluent® analysis. The positions with the arms extended at the front presented lower drag values than the position with the arms aside the trunk. The lateral position was the one in which the drag was lower and seems to be the one that should be adopted during the gliding after starts and turns.

Computational Fluid Dynamics Study of Swimmer's Hand Velocity, Orientation, and Shape: Contributions to Hydrodynamics

The aim of this paper is to determine the hydrodynamic characteristics of swimmers scanned hand models for various combinations of both the angle of attack and the sweepback angle and shape and velocity of swimmers hand, simulating separate underwater arm stroke phases of freestyle (front crawl) swimming. Four realistic 3D models of swimmers hand corresponding to different combinations of separated/closed fingers positions were used to simulate different underwater front crawl phases. The fluid flow was simulated using FLUENT (ANSYS, PA, USA). Drag force and drag coefficient were calculated using (computational fluid dynamics) CFD in steady state. Results showed that the drag force and coefficient varied at the different flow velocities on all shapes of the hand and variation was observed for different hand positions corresponding to different stroke phases. The models of the hand with thumb adducted and abducted generated the highest drag forces and drag coefficients. The current study suggests that the realistic variation of both the orientation angles influenced higher values of drag, lift, and resultant coefficients and forces. To augment resultant force, which affects swimmers propulsion, the swimmer should concentrate in effectively optimising achievable hand areas during crucial propulsive phases.

Numerical analysis of convective heat transfer in nanofluid

The main objective of this study is to analyze the heat convection through nanofluids, Al2O3 nanoparticle‐water mixture (7% of particle volume concentration), flowing inside an open system. This study was performed numerically in two dimensional geometry (box) combining fluid flow and heat transfer. The commercial code Ansys® was used to compute the fluid flow and heat transfer between entrance on the top left of the box and the exit on the bottom right. The finite volume method was used to discretize the whole domain in quadratic elements and to linearise the turbulence model. This model was governed by a Re‐Normalized Group Turbulence Model (RNG) turbulence differential equations system coupled with the heat convection. Velocity and temperature in the exit were calculated as function of initial conditions (velocity and temperature) in the entrance of the box (top left). Velocity about 10 m/s and temperatures between 100 K and 300 K were imposed in the entrance. Results showed that the heat transfer in n...

The Effect of Depth on Drag During the Streamlined Glide: A Three-Dimensional CFD Analysis

The Effect of Depth on Drag During the Streamlined Glide: A Three-Dimensional CFD Analysis The aim of this study was to analyze the effects of depth on drag during the streamlined glide in swimming using Computational Fluid Dynamics. The Computation Fluid Dynamic analysis consisted of using a three-dimensional mesh of cells that simulates the flow around the considered domain. We used the K-epsilon turbulent model implemented in the commercial code Fluent® and applied it to the flow around a three-dimensional model of an Olympic swimmer. The swimmer was modeled as if he were gliding underwater in a streamlined prone position, with hands overlapping, head between the extended arms, feet together and plantar flexed. Steady-state computational fluid dynamics analyses were performed using the Fluent® code and the drag coefficient and the drag force was calculated for velocities ranging from 1.5 to 2.5 m/s, in increments of 0.50m/s, which represents the velocity range used by club to elite level swimmers during the push-off and glide following a turn. The swimmer model middle line was placed at different water depths between 0 and 1.0 m underwater, in 0.25m increments. Hydrodynamic drag decreased with depth, although after 0.75m values remained almost constant. Water depth seems to have a positive effect on reducing hydrodynamic drag during the gliding. Although increasing depth position could contribute to decrease hydrodynamic drag, this reduction seems to be lower with depth, especially after 0.75 m depth, thus suggesting that possibly performing the underwater gliding more than 0.75 m depth could not be to the benefit of the swimmer.

Effect of wearing a swimsuit on hydrodynamic drag of swimmer

The purpose of this study was to analyse the effect of wearing a swimsuit on swimmers passive drag. A computational fluid dynamics analysis was carried out to determine the hydrodynamic drag of a female swimmers model (i) wearing a standard swimsuit; (ii) wearing a last generation swimsuit and; (iii) with no swimsuit, wearing light underwear. The three-dimensional surface geometry of a female swimmers model with different swimsuit/underwear was acquired through standard commercial laser scanner. Passive drag force and drag coefficient were computed with the swimmer in a prone position. Higher hydrodynamic drag values were determined when the swimmer was with no swimsuit in comparison with the situation when the swimmer was wearing a swimsuit. The last generation swimsuit presented lower hydrodynamic drag values, although very similar to standard swimsuit. In conclusion, wearing a swimsuit could positively influence the swimmers hydrodynamics, especially reducing the pressure drag component.

Design of a three-dimensional hand/forearm model to apply computational fluid dynamics

The purpose of this study was to develop a three-dimensional digital model of a human hand and forearm to apply Computational Fluid Dynamics to propulsion analysis in swimming. Computer tomography scans of the hand and forearm of an Olympic swimmer were applied. The data were converted, using image processing techniques, into relevant coordinate input, which could be used in Computational Fluid Dynamics software. From that analysis, it was possible to verify an almost perfect agreement between the true human segment and the digital model. This technique could be used as a means to overcome the difficulties in developing a true three-dimensional model of a specific segment of the human body. Additionally, it could be used to improve the use of Computational Fluid Dynamics generally in sports and specifically in swimming studies, decreasing the gap between the experimental and the computational data.