Characteristics Of Heat Conduction In Nanoparticle Packed Beds

The development and efficient use of energy is a desperate need of mankind.With the rapid development of material science,there are kinds of nano-structure materials emerging in different application fields.Due to the unique structure,nanomaterials have obvious advantages over bulk materials in catalysis,adsorption,mechanical properties,electrical and thermal conductivities,which promote the development and innovation of technology in various fields.Metal-Organic-Frameworks(MOFs)is an inorganic/organic hybrid crystal material assembled from metal ions(or clusters)and organic linkers with ultra-high specific surface area and pore volume.The hybrid nature of MOFs provides almost an infinite set of building units that can be manipulated to target specific applications by combining different metal nodes and organic linkers in the structure,or by further modulating the properties by postsynthetic modification.MOF-based materials have promising futures in gas adsorption and storage related applications because of their state of art adsorption rate and capacity,along with its selectivity.MOFs adsorbents will release/absorb heat during gas adsorption/desorption processes.Such exothermic or endothermic process will significantly change the temperature of MOFs adsorbents,and sequentially affect the mass transfer rate and gas storage capacity.Therefore,thermal management of MOFss is crucial for rapid and efficient gas adsorption,and thermal conductivity of MOFs crystal may provide crucial insights into the thermal manipulation of MOF-based gas storage systems.However,experimental measurement of the thermal conductivity of MOFs crystals is very challenging because of the small size of MOFs crystals.As for the transient heat conduction process within nano-porous structures,non-Fourier heat conduction occurs when confronted to high heat flux density or during a short period of time.Numerous works have been dedicated in nano-porous materials trying to demonstrate the transient non-Fourier behavior and obtain the thermal relaxation time.However,the physical meaning of thermal relaxation time and the physical mechanism underneath the non-Fourier behavior remains to be further explored.Considering the above research background,this work focuses on the heat conduction characteristics of nanoparticle packed bed,and studies the thermophysical properties and nano-scale heat transfer mechanism in various nanoparticles packed beds.Specific content involves the following four aspects:(1)The thermal conductivities of MOFs crystal particle packed beds are tested by the transient hot wire method.Results show that the thermal conductivities of MOFs crystal particle packed beds are extremely low,comparable to the thermal conductivity of air,due to their high packing porosity and inter-particle thermal contact resistance.The packed bed thermal conductivity is mainly affected by particle size.Materials with smaller particle size tend to have lower thermal conductivity.(2)Thermal conductivities of different types and sizes of MOFs particle packed bed under various adsorption conditions are measured by the transient hot wire method.For all kinds of adsorbent materials and adsorbate gases,the packed bed thermal conductivity rises steadily with adsorption amounts with an increase rate around 30%.As the adsorption process proceeds,the gas molecules gradually adsorb and condense in the nano-pores inside the MOFs crystal,resulting in the change of thermal conductivity of MOFs particle,while the inter-particle contact between particles remains unaffected.The increase of thermal conductivity of packed bed is limited by contact thermal resistance,which ultimately leads to the limitation of packed bed thermal conductivity increase during adsorption process.(3)The MOFs particle packed bed is taken as a hierarchical system consisting of pristine MOFs crystals,macropores formed by MOFs particles in contact,and micropores inside MOFs crystals.A Biporous Effective Medium Approximation(BEMA)model was developed to derive the MOFs crystal thermal conductivity and interparticle thermal resistance from packed bed thermal conductivity.The experimentally measured crystal thermal conductivity is compared with Molecular Dynamic simulation result,which shows good consistency.(4)The transient non-Fourier heat conduction characteristics and relaxation time of carbon nanotubes deposits under the oscillation temperature boundary are studied.In order to explain the physical mechanism underneath the non-Fourier behavior,electrical analogy is taken into consideration.Considering the relativistic effects of heat flux,a thermally induced magnetism should exist.When a material is exposed to a fast changing heat flux,an reverse temperature gradient would be induced to resist the fast changing of the heat flux,defined as thermal inductance.Experimental evidences are observed in nanoparticle packed beds exposed to a nanosecond laser pulse.The experimental negative temperature change is identical to the theoretical prediction.