Research On The Pulverization, Expansion And Heat Transfer Characteristics For A New Type Of Rare Earth-Based Hydrogen Absorbing Alloys

Rare earth-based hydrogen absorbing alloys have great potentials in industries such as energy storage/conversion and hydrogen production. However, the fundamental researches on the their characteristics are still not adequate, which is obstructing their applications. Under the support of National Natural Science Foundation of China'Researches on several basic problems for metal-hydride cooling/heating system'(No. 50276063), and High-Tech Research and Development Program of China'The technology for continuously outputting more than 35 MPa high pressure, high purity hydrogen based on metal hydrides'(No. 2006AA05Z135), this thesis focuses on the research of alloy basic characteristics during cycling. A new type of rare earth-based alloy pairs are selected and prepared for the applications in the industries as heat pump/refrigeration, hydrogen compression, hydrogen stroegy and transport, and then a new type of thin wall, high-reliability reactors are designed according to the studies on alloy pulverization characteristics and the response characteristics of thin wall reactors to alloy expansion. The heat transfer properties of a cycling reactor and the packed alloy are studied theoretically and experimentally, and the design principles for large power, high efficiency metal hydride reaction beds are discussed with the application background of heat driven metal hydride automobile air-conditioners.(1) The development guidelines for metal hydrides are summarized. The new type of rare earth-based alloys LaNi4.61Mn0.26Al0.13/La0.6Y0.4Ni4.8Mn0.2 are prepared, their P-C-T curves, absorption/desorption hysteresis and hydrogen absorption dynamics are measured, and the theoretical performance is predicted for the heat driven air-conditioning systems adopting this alloy pair.(2) According to the laser diffraction and microscopic granularity analyses, the effects of heat treatment, initial particle sizes and cycle numbers are quantitatively studied. The distributions of pulverized alloy sizes are formulized, which provides a mathematical method for precisely describing alloy pulverization properties and pulverized products. Analysis results show that the distribution density functions of the pulverized alloys approximately follow tri-modal lognormal distribution. According to analyzing the forming mechanism and distribution characteristics, the pulverized alloys are proven to be a multi-distribution system composed of ultra coarse particles, coarse particles, fine particles and tiny particles, which are respectively the product of alloy segregation, bulk crushing, the cooperation of bulk crushing and surface grinding, and surface grinding. The tiny particles are the stable elements depending on alloy inherence; the coarse and fine particles are determined not only by alloy itself, but also by heat treatment and cycling conditions. Above work provide the detailed cognition for alloy pulverization and pulverized products.(3) With the help of strain test, the influences of cycle numbers, test point locations, hydrogen contents, initial packing densities and reactor placements in the stain on cycling cylindrical thin-wall reactors are studied, and the response principles of horizontally and vertically placed reactors are investigated. According to the experiment results, the phenomenon of'cycling compression effect'is discovered governed by the friction force between reactor inner wall and alloy power. The reactor slenderness ratio, cycle numbers, mass transfer rate, volume expansion rate and wall friction coefficient are proven to promote this cycling compression effect. The reactor stress accumulation mechanism is proposed, and three major courses and their detailed measures are put forward for upgrading reactor reliability. The optimal packing density and work conditions for thin wall reactors are induced. According the proposed stress accumulation mechanism, a new type of offset strip fin core structure is designed and tested, which is appropriate for the basic configuration and the function unit of metal hydrogen reactors.(4) A thermal conductivity test bench is set up, and the thermal transfer property is quantitatively studied for the offset strip fin reactor packed with La0.6Y0.4Ni4.8Mn0.2. Integrated with the test data and the numeric method, the effective thermal conductivity for the alloy powder is indirectly calculated. The numerical results indicate that under the normal conditions, the alloy powder thermal conductivity could satisfy system requirement, and the small heat transfer area is actually the bottleneck. Particles contact states inside reactors are divided into three stages by the variation of flatting coefficient. Based on the unit cell model for powder heat transfer, considering the influence of alloy expansion and pulverization, a semi-empirical formula is deducted for precisely describing the effective thermal conductivity of the cycling alloy powder. On this basis, according to some preliminary experiments on the heat-driven metal hydride air-conditioners, the design principle of'heat transfer enhancement primary, mass transfer enhancement secondary'is proposed and verified for large power, high efficiency metal hydride reactors.To sum up, according to the systematical researches on the pulverization, expansion and heat transfer characteristics for the new type of rare earth-based alloys, this thesis intends to detailedly clarify the cycling performance of the alloys and the design principles for high performance reactors, consequently provides the theoretical bases and technical supports for equipment development of heat pump/refrigeration, oilless hydrogen compression, high density hydrogen storegy and transport, with independent intellectual property rights. Moreover, the research could be applied to wide fields as fuel cells, heat recovery and solar energy upgrade, and consequently promote the development of related domains.