IntrodutionThe collapsing fluidized-bed experiment, first described by Ž . Rietema 1967 , has been used to understand the dense-phase regime of fluidization, and is still commonly used today ŽGeldart et al., 1984;Grace, 1992;Barreto et al., 1988;Tianxiong et al., 1982;Yang et al., 1997; . Formisani et al., 2002, for example . This experiment consists of fluidizing a bed to the quasi-steady-state condition and then suddenly stopping the gas flow. Observations of the bed are made as the gas escapes and the particulate phase settles and consolidates. This technique gives general properties of the dense-phase regime, including the average void fraction and Ž . superficial gas velocity Weimer and Quarderer, 1984 . This technique has also been used to gain insight as to the effects of other variables such as high pressure andror high temper-Ž . ature Weimer and Quarderer, 1984;Lettieri, 1999 . ApplicaCorrespondence concerning this article should be addressed to R. W. Lyczkowski. tion of this technique is not limited to predicting the behavior of fluidized beds, but is also valuable in the understanding and design of the standpipes and hoppers.This investigation originated from an industry need to better understand the current computational fluid dynamics Ž . CFD hydrodynamics of fluidization capabilities. The collapsing fluidized-bed experiment is a small, well-defined experiment that involves many different hydrodynamic phenomena. The bed expansion, bubbling, sedimentation, and consolidation all play important roles in the collapsing fluidized-bed experiment. The capability of CFD to correctly model this experiment would increase the confidence of industrial users. The modeling of the collapsing fluidized-bed experiment can also serve to verify key particle characteristics and refine the hydrodynamic model before attempting to simulate more complex reacting systems. The following section gives an overview of the experiment.
Thermal interface materials (TIM) play a very important role in effectively dissipating unwanted heat generated in electronic devices. This requires that the TIM should have a high bulk thermal conductivity, intimate contact with the substrate surfaces, and the capability to form a thin bond line. In designing new TIMs to meet these industry needs, alkyl methyl siloxane (AMS) waxes have been studied as phase change matrices. AMS waxes are synthesized by grafting long chain alpha-olefins on siloxane polymers. The melting point range of the silicone wax is determined by the hydrocarbon chain length and the siloxane structure. When the AMS wax is mixed with thermally conductive fillers such as alumina, a phase change compound is created. The bulk thermal conductivities of the phase change material (PCM) are reduced as they go through the phase change transition from solid to liquid. By coating the PCM onto an aluminum mesh, both the mechanical strength and the thermal conductivity are drastically improved. The thermal conductivity increases from 4.5 W/mK for the PCM without aluminum support to 7.5 W/mK with the supporting mesh. The thermal resistance of the aluminum-supported sheet at a bond line thickness of 115 microns has been found to be ∼0.24 cm2-C/W. Applying pressure at the time of application has a positive effect on the thermal performance of the PCM. Between contact pressures of 5–80 psi, the thermal resistance decreases as the pressure increases. The weak mechanical strength of the phase change material turns out to be a benefit when ease of rework and the effects of shock and vibration during shipping and handling are considered. A stud pull test of the aluminum mesh-supported PCM shows an average of 13 psi stress at the peak of the break.
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