Ranji Vaidyanathan

Toughness in synthetic and biological multilayered systems

Toughness in hard biological tissues is associated with fibrous or lamellar structures that deflect or stop growing cracks. In some cases, such as nacreous shell, protein interlayers absorb much of the crack energy. In other tissues, such as tooth enamel, the toughness derives from the mineral microstructure, and the small amount of residual protein apparently has little effect. There have been a number of efforts to make tough synthetic materials using layered structures. In this work, freeform fabrication has been used to make layered structures with a view to introducing similar toughness into brittle materials. Results are presented for epoxy–glass composites with glass fabric interlayers, porous alumina back–filled with aluminium metal, and layered glass–ceramic/silver materials.

Modeling and Optimization of Multilayer Minichannel Heat Sinks in Single-Phase Flow

Liquid-cooled small channel heat sinks are a promising heat dissipation method for high power electronic devices. Traditional mini and microchannel heat sinks consist of a single layer of high aspect ratio rectangular channels. An alternative approach investigated in this paper is to stack multiple layers of low aspect ratio (circular or square cross-section) channels together to create multiple layer minichannel heat sinks. These multilayer heat sinks can achieve high heat flux due to the high heat transfer coefficients from small channels coupled with the large surface areas from the multilayer structure. In this research, multilayer copper and silicon carbide (SiC) minichannel heat sinks were experimentally and computationally characterized in single-phase flow over various flow rates. The experimental data indicated that in many cases, multilayer heat sinks have significant advantages over single-layer equivalents with reductions in thermal resistance and pressure drop. In order to investigate the optimal design of such structures, a detailed 3-D resistance network model was developed and used to predict the heat sink surface temperature and fluid pressure drop. The model uses an uncoupled approach and was validated by compared with conjugate CFD simulations and the experimental data. An extensive parametric study was performed on copper and SiC heat sinks with respect to channel geometry, number of layers, and thermal conductivity. The simulations indicated that for a fixed overall heat sink flow rate, an optimum number of channel layers exists for copper and SiC because of the competing trends of increasing surface area and decreasing per channel flow rate as the number of layers increases. In addition, the heat sink “effectiveness” decreases with increasing number of layers as the thermal resistance from the top surface, where heat is applied, to the lower layers of the heat sink becomes excessive. In the simulation the optimized number of layers is highly dependent on material, channel width, channel aspect ratio, and wall thickness. If the pumping power is an important issue for the optimization, the heat sink with medium channel width is a wise choice, which achieves small thermal resistance with reasonable pressure drop.Copyright

Heat Transfer in Water-Cooled Silicon Carbide Milli-Channel Heat Sinks for High Power Electronic Applications

Heat transfer and fluid flow in a novel class of water-cooled milli-channel heat sinks are investigated. The heat sinks are manufactured using an extrusion freeform fabrication (EFF) rapid prototyping technology and a water-soluble polymer material. EFF permits the fabrication of geometrically complex, three-dimensional structures in non-traditional materials. Silicon carbide, SiC, is TEC-matched to silicon and is an ideal material for heat exchangers that will be mounted directly to heat dissipating electronic packages. This paper presents experimental results on the heat transfer and flow in small SiC heat exchangers with multiple rows of parallel channels oriented in the flow direction. Rectangular heat exchangers with 3.2 cm × 2.2 cm planform area and varying thickness, porosity, number of channels, and channel diameter were fabricated and tested. Overall heat transfer and pressure drop coefficients in single-phase flow regimes are presented and analyzed. The per channel Reynolds number places the friction coefficients in the developing to developed hydrodynamic regime, and showed excellent agreement with laminar theory. The overall heat transfer coefficients for a single row SiC heat exchanger compared favorably with a validation heat exchanger fabricated from copper, however the heat transfer coefficient in multiple row heat sinks did not agree well with the laminar theory.Copyright

Fluorinated Single Wall Nanotube/Polyethylene Composites for Multifunctional Radiation Protection

Abstract : Fluorinated Single Wall Nanotubes (f-SWNTs) have been processed in polyethylene by an incipient wetting technique to achieve a well dispersed nanocomposite for radiation protection. In some cases, samples were further processed using the rapid prototyping method of extrusion freeform fabrication. Composites were exposed to 40 MeV proton radiation with a flux of about 1.7x10(exp 7) protons/sq cm/sec to a total fluence of 3x10(exp 10) protons/sq cm. This exposure is consistent with a long-term space mission in low earth orbit. The samples were evaluated by means of Raman spectroscopy and thermogravimetric analysis (TGA). These results were compared to the unexposed composite and unfilled polymer samples. This study has focused on the stability of the nanotube composites when exposed to radiation and prior to hydrogen exposure. It was shown that the stability of the functional group is not constant with SWNTs produced by different processes and that radiation exposure is capable of defluorinating SWNTs in polyethylene.

Anomalous Thermoelectric Behavior of BiSeTe Doped with SiGe:As

BiSeTe and BiSbTe have been two of the most efficient thermoelectric materials near room temperature applications for many years. In spite of recent progress in enhancement of the efficiency of BiSbTe thermoelectric materials, there has been little progress in developing efficient BiSeTe alloys. BiSeTe is an n-type thermoelectric material with negative Seebeck value and BiSbTe is p-type with positive Seebeck. We observed BiSeTe changes to p-type with the addition of 5% arsenic doped SiGe. After annealing process the Seebeck value changed sign again resulting in n-type BiSeTe. The electrical conductivity and thermal conductivity also changed during the course of annealing. Interestingly the minimum thermal conductivity corresponded to the maximum electrical conductivity and power factor of the p-type mode. This effect may prove to be a cornerstone in the enhancement and fabrication of thermoelectric devices based on bismuth telluride based alloys.

Experiments and Modeling of Two-Phase Heat Transfer in Single-Layer and Multilayer Minichannel Heat Sinks

Boiling in small channels has been studied by many investigators due to its promising applications in high power electrical devices. Most of research has been conducted on single channel or single-layer channel heat sinks. An alternative approach investigated in this paper is to stack multiple layers of square channels together to create multiple layer heat sinks. The thermal and hydraulic characteristics of single-layer and multilayer copper heat sinks were compared in this study. It was found the multilayer copper heat sinks had smaller average surface temperature than their single-layer counterpart at low heat flux. However multilayer copper heat sinks may lose stability at high heat flux, which results in surface overheating presumably due to dryout in the hottest channels. The boiling heat transfer coefficient correlations for conventional size channels have been thoroughly studied and docmumented by many researchers. The boiling correlations for small channels are sparse and mostly based on data from unique experiments, which are limited to certain working fluids and channel dimensions. This paper presents a systematic approach to validate and choose the best boiling correlations for modeling minichannel heat sinks. Combining a 3-D numerical model and a three zone flow model, several heat transfer coefficient correlations for conventional size channels and for small channels were compared. The traditional macro channels boiling correlations were found to overestimate the saturated boiling heat transfer coefficient in two-phase flow and consequently show large error in temperature predictions. The temperature predictions based on boiling correlations for small channels were more consistent with experimental measurements. The correlation of Yu et al. provided the closest agreement to the experimental data.Copyright

Dynamic Compression of Aluminum Foam Processed by a Freeform Fabrication Technique

The compressive deformation behavior of a new type of aluminum foam was assessed under static and dynamic loading conditions. The aluminum foam investigated was processed by Advanced Ceramics Research using an extrusion freeform fabrication technique. The foam contained approximately 50 to 60 % porosity. The dynamic compression response was evaluated in air using a split Hopkinson pressure bar (SHPB) system with aluminum bars, and strain rates ranging from 600 s−1 to 2000 s−1. Compression tests were also conducted at lower strain rates (10−3 s−1 to 4 s−1) to determine the extent of strain rate strengthening. The low strain rate tests were performed with a servo‐controlled hydraulic test machine. The results were analyzed as a function of foam density, structure, and process conditions.