A. Razani

Metal hydride compacts of improved thermal conductivity

Abstract Metal hydrides begin as hydride-forming metal alloys with good thermal conductivity. However, undergoing hydriding/dehydriding reactions and subsequently causing large strain changes, they decrepitate and finally form a powder bed. Such a powder bed exhibits poor thermal conductivity (k eff ∼0.1 W / m K ) and reduces the heat transfer process to and from the bed that occurs with hydrogen absorption and desorption. A newly developed technique reported here (recompressed expanded graphite technique), that allows one to significantly improve the thermal conductivity of a metal hydride, LaNi5, is presented. The compacts with LaNi5 and recompressed expanded graphite were made and their thermal conductivity measurements were taken. Recompressed expanded graphite is used to allow good heat transfer while providing efficient mass transfer. This study reports that the manufactured metal hydride compact has the thermal conductivity in the range of keff∼3– 6 W / m K that shows a greater potential in developing high-power metal hydride devices. It should be pointed out that a minute amount of expanded graphite increases the thermal conductivity of metal hydride significantly.

Second law analysis and optimization of a combined triple power cycle

In this investigation, a combined triple (Brayton/Rankine/Rankine)/(gas/steam/ammonia) power cycle is analyzed. In the triple cycle, the exhaust of the Brayton gas topping cycle is used in a heat recovery steam generator (HRSG) to produce steam for a Rankine steam middle cycle followed by an ammonia Rankine bottoming cycle. The ammonia bottoming cycle provides a practical and more efficient hot and cold streams thermal matching for the triple cycle HRSG as compared to the HRSG of a conventional combined power cycle. Through exergy analysis of the cycle, the exergy of the exhaust streams and the irreversibility of each component in the cycle are determined, using reasonably practical constraints for the system components. These constraints are mainly due to the size of components and are conveniently parameterized and analyzed. The triple cycle was analyzed and optimized with respect to important system parameters, such as the gas topping cycle pressure ratio, gas turbine inlet temperature, HRSG pinch point, gas/steam approach temperature difference, rate of steam injection into combustion chamber and the effectiveness of the heat exchangers. One goal of the study is to find what configuration will achieve a thermal efficiency of 60% when reasonably practical constraints for system parameters are used.

Self-Consistent Open-Celled Metal Foam Model for Thermal Applications

Many engineering applications require thermal cycling of granular materials. Since these materials generally have poor effective thermal conductivity various techniques have been proposed to improve bed thermal transport. These include insertion of metal foam with the granular material residing in the interstitial space. The use of metal foam introduces a parasitic thermal capacitance, disrupts packing, and reduces the amount of active material. In order to optimize the combined high porosity metal foam-granular material matrix and study local thermal nonequilibrium, multiple energy equations are required. The interfacial conductance coefficients, specific interface area, and the effective thermal conductivities of the individual components, which are required for a multiple energy equation analysis, are functions of the foam geometry. An ideal three-dimensional geometric model of open-celled Duocell® foam is proposed. Computed tomography is used to acquire foam cell and ligament diameter distribution, ligament shape, and specific surface area for a range of foam parameters to address various shortcomings in the literature. These data are used to evaluate the geometric self-consistency of the proposed geometric model with respect to the intensive and extensive geometry parameters. Experimental thermal conductivity data for the same foam samples are acquired and are used to validate finite element analysis results of the proposed geometric model. A simple relation between density and thermal conductivity ratio is derived using the results. The foam samples tested exhibit a higher dependence on relative density and less dependence on interstitial fluid than data in the literature. The proposed metal foam geometric model is shown to be self-consistent with respect to both its geometric and thermal properties.

Performance of high power metal hydride reactors

Metal hydride reactors were built with porous powder metal hydride (PMH) compacts. An improved reactor built with copper coated PMH compacts of LaNi5 with a 1.27 cm diameter produced a nominal specific cooling power of 1.5 kW/kg hydride. A similar reactor, built with copper coated PMH compacts of Ca0.4Mm0.6Ni5, showed 2.2 kW/kg hydride. Results with copper coated PMH compacts showed improved thermal conductivity. The compacts are structurally strong and prevent migration of fine metal hydride particles. Life-cycle tests were performed on the reactor with LaNi5 for over 3000 cycles and the cooling power of the reactor gradually decreased by approximately 55%.

Development of LaNi5/Cu/Sn metal hydride powder composites

Abstract Metal hydride powder composites (MHPC) were manufactured employing the copper-encapsulation technique. Thermal conductivity and permeability of the unactivated MHPC were measured at k eff ∼ 5 w/mK and K H2 ∼ 5 × 10 −15 m 2 . Preliminary measurements of permeability of an activated MHPC appear to show permeability is increased; hysteretic behavior was observed. Since there are several transport phenomena occurring when reaction is going on, it is not easy to separate them. Nevertheless, the measurements obtained from the activated MHPC clearly behaved in a different fashion than the unactivated MHPC. The MHPC were used in two prototype fast reactors. Falling pressure characterization of their performance confirms that the obtained values of k eff and K H2 are useful in applications such as heat pumps.

Modeling Pulse Tube Cryocoolers with CFD

A commercial computational fluid dynamics (CFD) software package is used to model the oscillating flow inside a pulse tube cryocooler. Capabilities for modeling pulse tubes are demonstrated with preliminary case studies and the results presented. The 2D axi‐symmetric simulations demonstrate the time varying temperature and velocity fields in the tube along with computation of the heat fluxes at the hot and cold heat exchangers. The only externally imposed boundary conditions are a cyclically moving piston wall at one end of the tube and constant temperature or heat flux boundaries at the external walls of the hot and cold heat exchangers.

Compressor-driven metal-hydride heat pumps

Abstract Analysis and experiments are presented for a compressor-driven hydrogen metal-hydride heat-pump system. Such a system, utilizing fast hydride reactors, has the potential to achieve higher efficiency and competitive life-cycle costs with conventional refrigeration systems. Fast reactors utilizing copper-coated LaNi5 hydride compacts were designed with improved thermal conductivity. Reactors were built and tested and the results evaluated. The potential for hydride heat pumps is also described.

Thermal conductivity measurements of metal hydride compacts developed for high-power reactors

Thermal conductivity measurements on porous metal hybride compacts were performed to supplement the limited existing data. These materials are proposed for reactors that are designed for heat-pump applications requiring high specific powers. Previous computational studies have shown that the effective thermal conductivity k eff is a crucial optimization parameter. If it is too low, the reactors themselves limit the system performance. It is sufficiently high, external thermal resistances dominate and overdesign of the materials is unnecessary and unavoidably increases parasitic thermal losses. In this study nine samples were tested using the comparative method and careful attention was paid to ascertaining and propagating all errors into the final data reported.

Thermal analysis of the Ca0.4Mm0.6Ni5 metal–hydride reactor

Abstract Experimental results are presented for the coupled metal-hydride reactors of Ca 0.4 Mm 0.6 Ni 5 (Mm=misch metal) with hydrogen pumped by the compressor. In order to augment the heat transfer in the reactor, metal hydride powders were copper-coated and compressed into a porous metal hydride (PMH) compacts. The reactors, packaged with these PMH compacts, produced continuous cooling output of approximately 0.8 kW/kg of Ca 0.4 Mm 0.6 Ni 5 when the cycle time was set at 4 minutes. The experimental results indicate that optimized reactors of viable specific powers can be fabricated. In addition, comparisons between experimental and theoretical work are in a good agreement.

SOME OPTIMIZATION PROBLEMS RELATED TO COOLING FINS

Abstract The problems of minimizing the volume of purely conducting and conducting-convecting fins are solved. Exact solutions are obtained for the corresponding cross sectional areas and the temperature distributions. An approximate solution is also given for a converting-radiating fin. The results are plotted and discussed.