| 其他摘要 | Three-dimensional network silicon carbide (SiC) and carbon (C) foams have found a large spectrum of applications due to their high porosity, high specific surface area, high strength-to-weight ratio and promising microwave absorbing, thermal, fluid dissipating properties. They are used as filter and catalyst carrier elements, as heat exchanger core, microwave/acoustic absorbent and many others. In this work, the microwave absorbing, thermal, mechanical and fluid dynamic characteristics of such foams were intensively studied by using computational simulation technique. The microwave absorbing and thermal properties were validated through experiments. Conclusion can be drawn as follow.
The microwave absorbability of SiC and C foams is much higher than their corresponding bulk materials for that they have better impedance match with free space for their low effective permittivity. In addition, their network structure can couple to the incident microwave and induce currents on the struts; therefore the foams have artificial magnetism, which can absorb the magnetic energy so enhance the absorbability. The fraction, electric conductivity and dimensions of specimens exert compound effects on the absorbability. The optimized parameters of such factors were found in the simulation. The absorbability of silica composites significantly decreases compares with that of the SiC-foam because the effective permittivity of a composite is much larger than that of a monolithic SiC-foam, which leads to impedance mismatch much.
Based on the calculation of four types of model, it is found that the effective thermal conductivity of a foam increases nonlinearly with the fraction. The correlation between effective thermal conductivity and fraction can be deduced as Keff / Kreal=0.0005+0.35 f+0.69 f 2+0.055 f 3. The cell size and number have little effect on the calculation of the effective thermal conductivity when the tetrakaidecahedral model is employed. The temperature field and heat flux distribution on the struts are coordinated, and the heat conducts mainly in the struts paralleling to the temperature gradient, however the heat transfer in the struts perpendicular to the temperature is very low.
The effective elastic modulus of SiC-foam as a function of fraction can be expressed as parabola formula Eeff /Es= f 2. The effective Poisson’s ratio decreases with the increase of the fraction till it reachs to a constant as same as the intrinsic poisson’s ratio of SiC when the fraction is 0.6. As shown in the calculation results, the stress concentration occurs at the joints of the struts, so it is assumed that cracks and failure take place in these areas.
The velocity is uneven when fluid flows in foams, and there are some paths with low resistance. The fluid is apt to flow in such paths. Unlike flowing in tube or fined tube, the flow directions in foams are tortuous, which can coordinate with the temperature gradient that parallel to the wall of the tube. Therefore the heat transfer is enhanced when the tube is filled with foams. The pressure drop depends on the cell size and fraction. When the cell size keeps constant, the pressure drop increases with the fraction. However, it decreases with the increase of the cell size at a fixed fraction. The permeability decreases with the fraction at a fixed cell size, and increases with cell size at a fixed fraction. The inertial coefficient increases with the tortuosity and the fraction of the foam. As for the friction factor, it depends weakly on cell size at a fixed fraciton, and strongly on the fraction, i.e. the larger the fraction is, the higher the friction factor is. |
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