Francisco Elizalde-Blancas

Procedure to Estimate Thermophysical and Geometrical Parameters of Embedded Cancerous Lesions Using Thermography

Based on the fact that malignant cancerous lesions (neoplasms) develop high metabolism and use more blood supply than normal tissue, infrared thermography (IR) has become a reliable clinical technique used to indicate noninvasively the presence of cancerous diseases, e.g., skin and breast cancer. However, to diagnose cancerous diseases by IR, the technique requires procedures that explore the relationship between the neoplasm characteristics (size, blood perfusion rate and heat generated) and the resulting temperature distribution on the skin surface. In this research work the dual reciprocity boundary element method (DRBEM) has been coupled with the simulated annealing technique (SA) in a new inverse procedure, which coupled to the IR technique, is capable of estimating simultaneously geometrical and thermophysical parameters of the neoplasm. The method is of an evolutionary type, requiring random initial values for the unknown parameters and no calculations of sensitivities or search directions. In addition, the DRBEM does not require any re-meshing at each proposed solution to solve the bioheat model. The inverse procedure has been tested considering input data for simulated neoplasms of different sizes and positions in relation to the skin surface. The successful estimation of unknown neoplasm parameters validates the idea of using the SA technique and the DRBEM in the estimation of parameters. Other estimation techniques, based on genetic algorithms or sensitivity coefficients, have not been capable of obtaining a solution because the skin surface temperature difference is very small.

Effect of Channel Aspect Ratio on Planar SOFC Performance

The optimization process is in general an important issue to show the viability of solid oxide fuel cells (SOFCs) compared to traditional power sources. This optimization process can be done in a faster and cheaper way by making use of numerical simulations. In this study, three-dimensional, non-isothermal, steady state numerical simulations of planar solid oxide fuel cells (SOFC) are performed using the commercial FLUENT software. First, a detailed analysis of grid and iteration-dependent simulations is performed. This analysis predicts a 20% difference between a coarse and fine grid in the velocity magnitude in both anode and cathode gas flow channels, and in the y-component of current density. Then, the performance of a planar SOFC with changing channel aspect ratio is analyzed comparing their V-I curves and critical parameters like temperature, concentration, and current density distributions. The predictions show a 12 degrees difference in temperature at the fuel exhaust between low and high aspect ratio channel simulations. These results suggest that the channel aspect ratio is a significant parameter, worthwhile to be investigated.Copyright

Transient behavior of CO2 in the internal heat exchanger during the start-up of the transcritical refrigeration system

Transient behavior of internal heat exchanger during start-up of transcritical refrigeration system is investigated in this article. The work is focused on the local transient analysis of CO2 thermophysical properties, in order to see how these transient changes affect the heat transfer rate and the effectiveness of internal heat exchanger, as well as the coefficient of performance during the start-up of the system. The study is conducted through a numerical simulation using the commercial computational fluid dynamics (CFD) software Ansys Fluent. Validation of numerical results is carried out by using seven different empirical correlations applied for the Nusselt number. It is observed that the thermophysical properties of the hot CO2 stream experience large changes during the transient period. This instability is accompanied by a decrease in the heat transfer rate. Finally, the change in the internal heat exchanger effectiveness during the start-up results in a loss of about 12% of the coefficient of performance.

3D Analysis of a New Radial Channel for PEMFCs and Comparison With a Traditional Channeled System

The present study shows a 3D analysis of a new radial geometry for a PEM fuel cell flow field. The objective of this study is to understand the effects of this configuration on the fuel cell performance. The geometry is proposed with the objective to increase the fuel cell performance while reducing the pressure drop. In order to compare the results, a conventional commercial geometry was also analyzed and then both geometries were compared via polarization and power curves. The geometry proposed in this work shows quite a good improvement with respect to conventional geometries reducing the pressure drop and generating also a good power.Copyright

Improving the Performance of a PEMFC by Means of Achieving Uniform Flow Distribution

A performance analysis of a proton exchange membrane fuel cell is reported in this work. Two different flow patterns are modeled as gas distributors and current collectors of a PEM fuel cell. Both flow patterns have the same active area with similar channel distribution over the membrane electrode assembly. Three dimensional models are used in order to simulate the performance of the fuel cells. The Navier-Stokes equations as well as potential fields (potentiostatic and galvanostatic) are solved using computational fluid dynamics techniques. Two dimensionless parameters were computed to quantify and compare the uniformity of the flow over the reaction area. The present analysis shows that achieving a good flow distribution is a key parameter in the PEMFC performance. The reduction of the concentration losses is the main result when a parallel channel configuration operates with uniform reactants distribution. In this study is demonstrated that the conventional parallel channels flow pattern does not achieve similar flow conditions in each sub-stream and therefore, irregular energy generation is obtained.Copyright

Experimental Analysis of PEM Fuel Cells With Biomimetical Mixed Flows as Gas Distributors

A proton exchange membrane fuel cell (PEMFC) is an electrochemical device that converts the chemical energy from the gases into electrical energy. The PEMFCs consist of many parts, and the current collector plate is one of the key components among them. Channels in the bipolar plate distribute air on the cathode side and hydrogen on the anode side. Theoretically a fuel cell produces more current as more fuel is supplied. However the way in which the gases are supplied affects dramatically the performance of the cell. The present paper shows how the mixed flows improve the current density produced by fuel cells. Polarization and power density curves are presented. The results suggest that a flow with two levels of bifurcations is preferred for the anode side. This behavior is expected due to the similitude with the performance of the natural world in which geometries with this type of bifurcations transport the nutrients inside the tree leaves and plants.Copyright

Numerical Evaluation and Comparison of Different Reduced Mechanisms for Predicting the Performance of a SOFC Operating on Coal Syngas

Three-dimensional numerical simulations of an anode supported button solid oxide fuel cell were performed using the code developed in house DREAM SOFC. The cell operates on coal syngas at atmospheric pressure and 1073 K. A gas phase mechanism and a heterogeneous mechanism are studied in this work to assess their influence on the performance of the button cell. Both mechanisms take into account the steam methane reforming reaction and water gas shift reaction. The implemented electrochemistry model allows the cell to simultaneously electrochemically oxidize H2 and CO. Results show that methane reforming from the bulk reactions is negligible compared to the catalyzed reactions. Also with a higher reformation the power delivered by the cell is improved. A small temperature difference of one degree is observed when both mechanisms are compared. The electrochemistry model does not require the ratio between current produced from H2 and CO to be prescribed a priori as an input. Under the operating conditions used in this study the model predicts the ratio to be around 4 for both mechanisms.Copyright

Uncertainty Estimation for Numerical Solutions of Wall Bounded Turbulent Flows

In this study numerical solutions are presented for a steady state, incompressible, 2-D turbulent flow near a wall. For this specific problem a manufactured (exact) solution was provided by the organizers of the 2006 Lisbon Workshop [6]. With the help of manufactured solution, assessment of the true error and other relevant uncertainty measures are possible. The calculations were performed using the commercial flow solver FLUENT along with some user defined functions to define source terms and velocity profiles at boundaries. Though the flow regime is turbulent; the numerical solution is carried out for pseudo-laminar flow. This was done in order to avoid the errors implicit in turbulence models. The transformation from turbulent to laminar flow was done by defining a momentum source term which precludes the pressure gradient term. A detailed grid convergence analysis was performed. Using three-grid triplets the limiting values of the variables solved as the grid size tends to zero were calculated using different extrapolations. The L2 norms of the true error obtained from various extrapolations are assessed. These results exhibit solution convergence as the grid size decreases. It was also shown that cubic spline extrapolation perform the best among the methods considered.© 2006 ASME

Numerical and Experimental Study of Lead-Acid Battery

Lead-Acid battery was the earliest secondary battery to be developed. It is the battery that is most widely used in applications ranging from automotive to industrial storage. Nowadays it is often used to store energy from renewable energy sources. There is a growing interest to continue using Lead-Acid batteries in the energy systems due to the recyclability and the manufacturing infrastructure which is already in place. Due to this rising interest, there is also a need to improve the efficiency and extend the life cycle of Lead-Acid batteries. To achieve these objectives, it is necessary to gain a better understanding of the physics taking place within individual batteries. A physics based computational model can be used to simulate the mechanisms of the battery accurately and describe all the processes that are happening inside; including the interactions between the battery elements, based upon the physical processes that the model takes into account.In the present paper, we present a discharge/charge experimental study that has been carried out with small Lead-Acid batteries (with a capacity of 7 Ah). The experiments were performed with a constant current rate of 0.1C [A]1 for two different battery arrangements.An in-house zero dimensional model was developed to perform simulations of Lead-Acid batteries under different operating conditions. A validation analysis of the model was executed to confirm the accuracy of the results obtained by the model compared to the aforementioned experiments. Additional simulations of the battery were carried out under different current rates and geometry modifications in order to study how the performance of the battery may change under these conditions.Copyright

A Comparative 1D Analysis of PEM, SO and DB Fuel Cells

Although fuel cells represent an attractive alternative for electricity generation, different technical problems, such as the hydrogen storage, have not been solved, as yet. Nowadays direct sodium borohydride fuel cells are considered as a promising technology since NaBH4 (fuel) is a stable, nonflammable and nontoxic liquid solution. In the present study a one-dimensional numerical study of a proton exchange membrane, a solid oxide, and a direct sodium borohydride fuel cell is performed. The objective of this work is to compare qualitatively the fuel cell performance between these technologies. For proton exchange membrane and solid oxide fuel cells there are already established useful models and correlations widely known, and used, to predict the current density and the power generated. Direct Borohydride fuel cells, on the other hand, are still in their early developments; in the present paper DBFCs are analyzed using a novel model. This proposed model for DBFCs includes the prediction of the NaBH4 oxidation in the anode side, the H2O2 reduction in the cathode side and the effect of the solution concentration and temperature on the membrane. It is noteworthy mentioning that this last effect has not been integrated in any of the established models in the current technical literature.© 2014 ASME