5-1 Conclusions
This study analyzes heat transfer in practical VIPs, that is, VIPs with a broken cell ratio higher than 90%. The structure of these non-black-body VIP foams consists of struts, closed cells, and open cell residue membranes. Two parameters, namely, the broken cell ratio and the average cell size, are proposed to characterize the cell structure. The experimental samples are further grouped based on their solid volume fraction to reveal the influence of the solid material on heat transfer. Some conclusions derived from the experimental findings are summarized below.
1. Radiation heat transfer, as manifested by the mean extinction coefficient, is influenced predominantly by the broken cell ratio. The effects of cell size and solid volume fraction upon radiation are relatively insignificant in the samples investigated in this study.
2. Under a specific solid volume fraction, the best broken cell ratio (best cell size) leads to the lowest total thermal conductivity.
3. Solid volume could affect the absorption coefficient in radiation transfer, but the effects are not obvious because the solid volume fraction is extremely low in this study, and the extinction coefficient is dominated by scattering. However, the solid volume fraction has a
A rule of thumb to improve VIP performance can be derived from the findings in this study. Firstly, the solid volume fraction must be kept low to diminish the solid conduction.
Secondly, the cell size and broken cell ratio must be carefully controlled to an optimum value to produce the lowest total thermal conductivity. A high broken cell ratio may cause high radiation transfer, and does not necessarily imply low total thermal conductivity.
Experimental results of this study suggest a best cell size in the range of 100 to 300 µm for practical VIP with a high broken cell ratio.
The structure of these non-black-body VIP foams consists of struts, closed cells and open cell residue membranes. PE additive is used as a way to alter the foam structure and the heat transfer. Two parameters, namely, the broken cell ratio and the average cell size, are proposed to characterize the structure. The experimental samples are further grouped based on their solid volume fraction to reveal the influence of the solid material on heat transfer. Some conclusions derived from the experimental findings may help improve VIP performance, as summarized below.
4. Under a specific solid volume fraction, a best cell size (best broken cell ratio) leads to the lowest total thermal conductivity.
5. Radiation heat transfer, as manifested by the mean extinction coefficient, is influenced predominantly by broken cell ratio. The effects of solid volume fraction upon radiation are relatively insignificant in the samples investigated in this study. PE2 samples have smaller
cell size and therefore higher extinction than PE5 samples.
6. An appropriate amount of PE additive has proven to be effective in tuning the cell structure and improving the VIP performance. The best PE additive percentage found in this study was 2%.
7. Solid volume could affect the absorption coefficient in radiation transfer, but the effects are not obvious because the solid volume fraction is extremely low in this study, and the extinction coefficient is dominated by scattering. However, the solid volume fraction has a crucial effect on solid conduction, which is the dominant heat transfer mechanism in VIP.
A rule of thumb to improve VIP permeance can be derived from the findings in this study.
Firstly, the solid volume fraction must be kept low to diminish the solid conduction.
Secondly, the cell size and broken cell ratio must be carefully controlled to an optimum value to produce the lowest total thermal conductivity. A high broken cell ratio may cause high radiation transfer, and does not necessarily imply low total thermal conductivity. In contrast to conventional closed-cell foam, where a small cell size reduces the heat transfer of trapped gas, the best cell size in practical VIP with high broken cell ratio ranges from 100 to 300 . The lowest thermal conductivity obtained in this study reached m
4.4mWm1K1, and was among the best when previously compared to published VIP performance results.
5-2 Recommendations
In the future, a key factor in further reducing the thermal conductivity of VIP is the elimination of solid conduction, which accounts for 90% of VIP’s total heat transport according to the results of this study. One attractive approach is to reduce the mass or volume of the solid material as much as possible, thus diminishing the solid conduction readily.
Nevertheless, the extremely low solid content poses new challenges not only to manufacturing but also to maintaining low thermal radiation. The cell membranes are generally thin due to the low solid content and become more transparent to thermal radiation. The low solid content also implies weak structural support from the solid material, such that the cell size must be reduced to maintain structural integrity. Small struts and nodes associated with the small cell size may no longer be effective in scattering thermal radiation, thus the radiative heat transport is increased. By largely reducing solid content and balancing solid conduction and thermal radiation, one should be able to attain extremely low thermal conductivity at an optimal solid content, which was not pursued in this study mainly due to the manufacturability limitation inherent in the in-house facility employed in this study.
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