Abel Hernandez-Guerrero

Numerical Analysis of Novel Micro Pin Fin Heat Sink With Variable Fin Density

Numerical analyses to characterize and design micro pin fin heat sinks for cooling the 2016s IC chip heat generation are carried out in this paper. A novel design with variable fin density is proposed to generate a more uniform temperature of the IC chip junctions. The variable-density feature allows the gradual increase of the heat transfer area as coolant passes through the system. Single-phase water in the laminar regime is employed. Four different fin shapes (circle, square, elliptical, and flat with two redounded sides) are analyzed. The junction temperature and pressure drop variations in the heat sink generated by these shapes are presented. The effects of varying the fin length and height are also studied. The best heat sink configuration has a thermal resistance ranging from 0.14 to 0.25 K/W with a pressure drop lower than 90 kPa and a junction temperature ~ 314 K under the conditions studied. The temperature gradient at the bottom wall of the heat sink is considered as a parameter for comparing various heat sink designs. The novel cooling device has an overall temperature gradient lower than 2°C/mm, which is significantly lower than the temperature gradients in other schemes reported in literature.

Performance of Online and Offset Micro Pin-Fin Heat Sinks With Variable Fin Density

A comparison of the performances of online and offset micro pin-fin heat sinks with variable fin density is given in this paper. The cooling systems generate uniform junction temperatures, which improves the integrated circuit (IC) chips performance. Water is used as a coolant in the single phase and laminar regime. 4748 micro flat fins with rounded sides, which are distributed in three different sections along the flow length, are used in these configurations. The bottom wall temperature profile along the flow length, overall thermal resistances, pressure drops, and pumping powers for both configurations are presented. The results indicate that the offset micro pin-fin heat sink is a good alternative for cooling the IC chips of 2016. The cooling system using this fin configuration is capable of achieving a thermal resistance as low as 0.1 K/W with a pumping power requirement of 0.45 W. Comparisons with other cooling devices reported in the technical literature are presented.

Experimental study and comparison of various designs of gas flow fields to PEM fuel cells and cell stack performance

In this study, a significant number of experimental tests to PEM fuel cells were conducted to investigate the effect of gas flow fields on fuel cell performance. Graphite plates with various flow field or flow channel designs, from literature survey and also novel designs by the authors, were used for the PEM fuel cell assembly. The fabricated fuel cells all have an effective membrane area of 23.5 cm2. The results showed that the serpentine flow channel design is still favorable, giving the best single fuel cell performance amongst all the studied flow channel designs. A novel symmetric serpentine flow field was proposed for relatively large size fuel cell application. Four fuel cell stacks each including four cells were assembled using different designs of serpentine flow channels. The output power performances of fuel cell stacks were compared and the novel symmetric serpentine flow field design is recommended for its very good performance.

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 Cell Geometry on the Freezing and Melting Processes inside a Thermal Energy Storage Cell

This paper presents an analysis of the charge and discharge processes in a latent thermal energy storage cell. An individual cell is analyzed to study how its behavior affects the performance of a thermal energy storage system. The analysis considers the exchange of thermal energy between a thermal energy storage cell and a source or sink of thermal energy. Two cases are considered, (i) a process in which the phase change material melts and freezes when a constant and uniform temperature is imposed at the lower surface of the cell, and (ii) a process in which the phase change material melts and freezes when a fluid with a constant inlet temperature flows under the cell. The effect of the aspect ratio of the energy storage cell is analyzed in detail as a possible method to enhance heat transfer and improve performance of the thermal energy storage system. The results include, for different aspect ratios of the storage cell, the evolution of the solid-liquid interface, the rates of melting and solidification, the rate of energy storage and the total amount of energy storage.

Natural Patterns Applied to the Design of Microchannel Heat Sinks

The present work shows a study developed of the thermal and hydrodynamic behaviors present in microchannel heat sinks formed by non-conventional arrangements. These arrangements are based on patterns that nature presents. There are two postulates that model natural forms in a mathematical way: the Allometric Law and the Biomimetic Tendency. Both theories have been applied in the last few years in different fields of science and technology. Using both theories, six models were analyzed (there are three cases proposed and both theories are applied to each case). Microchannel heat sinks with split channels are obtained as a result of applying these theories. Water is the cooling fluid of the system. The inlet hydraulic diameter is kept in each model in order to have a reference for comparison. The Reynolds number inside the heat sink remains below the transition Reynolds number value published by several researchers for this channel dimensions. The inlet Reynolds number of the fluid at the channel inlet is the same for each model. A heat flux is supplied to the bottom wall of the heat sink. The magnitude of this heat flux is 150 W/cm2 . The temperature fields and velocity profiles are obtained for each case and compared.Copyright

Performance Assessment Comparison of Variable Fin Density Microchannels With Offset Configurations

Current technologies for the cooling of integrated circuits (IC) chips employ single-phase liquid flow through microchannel heat sinks. The surface temperature in these devices increases along the flow direction, leading to low heat transfer coefficients toward the channel exit. Enhancement of the heat transfer coefficient while mitigating the temperature nonuniformity has been possible with the utilization of variable fin density microchannels. A microchannel cooling layer suitable for three-dimensional (3D) stacking of IC chips is studied in this paper. The effect of the variation of geometrical parameters and operating conditions on the overall performance of the cooling layer is reported. The overall performance of eight different configurations presenting offset strip fins with variable fin density has been evaluated and compared to two cases with smooth rectangular microchannels, one case for each channel height. The cooling layer, with a length of 8 mm and a width of 7.92 mm, dissipates a total thermal power of 200 W by means of pumping coolant through 12 flow channels. Results from this investigation demonstrate the capability of the proposed configurations to decrease the surface temperature nonuniformities and to maintain the surface temperature below the allowable limits for IC chips while achieving low pressure drops. A parameter for the overall performance assessment, named the overall performance index, has been proposed; by computing and properly comparing the values for this parameter, the best performing configuration is identified.

Evaluation of Nonintrusive Active Infrared Thermography Technique to Detect Hidden Solder Ball Defects on Plastic Ball Grid Array Components

It is essential for electronics reliability to develop effective methodologies to detect hidden solder joint defects. Active infrared thermography is an alternative to X-ray detection methodologies. The limits of active infrared thermography to detect solder ball defects on plastic ball grid arrays (PBGA) components (missing, open, cracked, and head on pillow defects) are investigated here. A FEM was used to simulate the thermal phenomena during the infrared thermography inspection of a PBGA component. The FEM was proven to be temporal and spatial grids size independent. The average temperature difference (DT) amid regions with and without defects was used as a detectability indicator. Defects detectability was found to decrease as the number of blocking objects increases. Missing solder balls were barely detected when blocked by the substrate and moulding compound with detectability numbers close to 1 � C. Head on pillow and cracked defects were impossible to detect with a maximum DT ¼0.6 � C. Open solder balls were not detected below two objects with a maximum DT ¼0.3 � C. These results clearly suggest that infrared thermography can be effectively used to detect hidden missing and open solder ball defects on PBGA components composed by a substrate and a die. [DOI: 10.1115/1.4027378]

Entropy Generation Analysis for a PEM Fuel-Cell With a Biomimetic Flow Field

The present paper shows an entropy generation analysis for a PEM fuel cell. The numerical model takes into account the complete solution of Navier-Stokes, energy conservation, species conservation and two potential fields for the transport of electrons and protons in the collectors and membrane respectively. The entropy generation equation is added to the governing equations using the solutions provided from the energy, momentum, species and potential flow equations. The entropy generation analysis reveals the locations where the main losses (irreversibilities) are produced. This in consequence shows the best flow field to minimize the entropy production. A new parameter is proposed in this work that will compare the entropy production due to the mass flow to the total entropy production.Copyright

Three Dimensional Analysis of a PEM Fuel Cell with the Shape of a Fermat Spiral for the Flow Channel Configuration

This work presents the analysis of a non-isothermal three-dimensional model in single phase of a PEM fuel cell with an innovative flow field path in the form of the Fermat spiral, i.e. two concentric spirals. The model is used to predict the current density contours and the water content in all of the zones of the fuel cell. The three-dimensional model includes: the gas flow channels with the shape of the new geometry proposed, the current collectors, gas diffusion layers, catalyst layers on both sides of the model, anode and cathode, and a proton exchange membrane in between. The model solves the energy equation, mass conservation, and species transport equations, including the source terms due the electrochemical effects occurring in the cell. The results show a higher average current density than the fuel cells with conventional flow paths, showing also that the current density attained is more uniform from the inlet to the outlet of the flow channels.Copyright