Rotating flow of carbon nanotubes subject to prescribed heat flux condition

Quenching experiments were performed with hot stainless steel spheres in a pool of water-based nanofluids in the presence of carbon nanotubes (CNTs) of various sizes. In order to explore the size effect, a test matrix was developed by choosing multi-walled CNTs with lengths from 1 μm to 5 μm and outer diameters from 30 nm to 60 nm. The concentration was fixed at 0.5% by mass for all types of CNTs. The initial temperature was 400 °C and the transient temperature variations at the center of the sphere were recorded as quenching curves. By establishing a lumped capacitance model, the transient surface heat flux variations were obtained as boiling curves. The original and boiled surfaces were both subjected to a series of characterizations to determine the changes in morphology, roughness, and wettability to identify the effects of CNT size on the surface properties of the formed deposition layers as well as to elucidate the mechanisms for regulation of the boiling and quenching behaviors. The results suggested that the critical heat flux (CHF) and the Leidenfrost point (LFP) are enhanced to various degrees due to the discrepancy in the size of the CNTs in nanofluids. It was shown that the CNTs deposited on the surfaces create various morphologies depending on their size. The CNTs with a length of 5 μm and a diameter of 60 nm exhibited the most significant effect on the boiling behaviors. In comparison to CNTs with a shorter length of 1 μm, the 5 μm long CNTs were much easier to form porous layers. The results of the contact angle and roughness tests showed that the porous layers tend to affect the surface roughness instead of surface wettability. The increases of the nucleation site density and surface roughness due to the presence of porous layers were identified as the primary cause for the modified boiling behaviors during quenching.

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Flow Behavior and Flow Boiling Heat Transfer in Thin-Rectangular Mini-Channels

Volume 10: Heat Transfer, Fluid Flows, and Thermal Systems, Parts A, B, and C ◽

10.1115/imece2008-66928 ◽

2008 ◽

Author(s):

Yasuo Koizumi ◽

Hiroyasu Ohtake ◽

Tomonari Yamada

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Flow Pattern ◽

Critical Heat Flux ◽

Boiling Heat Transfer ◽

Annular Flow ◽

Flow Channel ◽

Heat Transfer Surface ◽

Liquid Slug ◽

Flux Condition

Boiling heat transfer of thin-rectangular channels of the width of 10 mm has been examined. The height of the flow channel was in a range from 0.6 mm to 0.4 mm. Experimental fluid was water. Bubbly flow, slug flow, semi annular flow and annular flow were observed. The flow pattern transition agreed well with the Baker flow pattern map for the usual sized flow path. The critical heat flux was lower than the value of the usual sized flow channel. The Koizumi and Ueda method predicted well the trend of the critical heat flux of the present experiments. At the critical heat flux condition, the heat transfer surface was covered by liquid slug, a large bubble pushed away the liquid slug, a dry area was formed on the heat transfer surface and then liquid slug came around to cover the heat transfer surface again. This process repeated rapidly. Following this observation, a heat transfer surface temperature calculation model at the critical heat flux condition was proposed. The calculated result re produced the experimental result.

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