Abstract
The thermal conductivity of boron nitride/ethylene glycol (BN/EG) nanofluids was investigated by transient hot-wire method and two abnormal phenomena was reported. One is the abnormal higher thermal conductivity enhancement for BN/EG nanofluids at very low-volume fraction of particles, and the other is the thermal conductivity enhancement of BN/EG nanofluids synthesized with large BN nanoparticles (140 nm) which is higher than that synthesized with small BN nanoparticles (70 nm). The chain-like loose aggregation of nanoparticles is responsible for the abnormal increment of thermal conductivity enhancement for the BN/EG nanofluids at very low particles volume fraction. And the difference in specific surface area and aspect ratio of BN nanoparticles may be the main reasons for the abnormal difference between thermal conductivity enhancements for BN/EG nanofluids prepared with 140- and 70-nm BN nanoparticles, respectively.
Highlights
The concept “nanofluids” was proposed by Choi [1] in 1995
boron nitride/ethylene glycol (BN/EG) nanofluid (1.25 vol.%) with the BN nanoparticles of 140 nm was prepared and its thermal conductivity was measured after depositing for 216 days
In order to examine the effect of nanoparticle size on thermal conductivity enhancement, 70nm BN nanoparticles were used as additives to synthesize BN/EG nanofluids with the nanoparticles volume fraction of 1.0% to 5.5% and thermal conductivity of them was measured
Summary
The concept “nanofluids” was proposed by Choi [1] in 1995. Roughly speaking, nanofluids are solid-liquid composite materials consisting of solid nanoparticles or nanofibers with typically of 1-100 nm suspended in base liquid. The thermal conductivity of the BN/EG nanofluids was measured by transient hot-wire apparatus [40]. The uncertainty of this apparatus is between ± 2.0%. To improve the accuracy of the data, the thermal conductivity of BN/EG nanofluids with lower nanoparticles volume fraction was measured by an improved transient hot-wire apparatus [41]. This improved transient hotwire apparatus is simpler and more robust compared to previous ones besides the improvement on accuracy [42,43]. The uncertainty of the improved transient hotwire apparatus is between ± 0.51%
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