Heat transfer analysis with nanofluids in a closed two-phase thermosyphon loop
Tutor / director / evaluatorKhodabandeh, Rahmatollah
Document typeMaster thesis (pre-Bologna period)
Rights accessRestricted access - confidentiality agreement
This report is concerned about the comparative analysis of the boiling heat transfer behavior of different refrigerants and nanofluids. The experimental apparatus was a thermosyphon loop with an evaporator composed of five vertical boiling channels heated by cartridge heaters. In the first part, experiments in boiling heat transfer were done for three different refrigerants: n-pentane, R-134a and isobutane. For all tests, saturation temperature was fixed at 16 ºC and a wide range of heat fluxes were applied in order to calculate the heat transfer coefficient for each case. Once the experimental results were analyzed, a comparative discussion was developed based on their thermal properties. In the second part, nanofluids with n-pentane and 1.0 vol. % of carbon nanotubes (CNT) were prepared following the two-step method. Although CNT stable suspension was not achieved, it was run in the rig with the same conditions as the pure n-pentane test. The results showed that non-dispersed CNT decreased nucleate boiling heat transfer coefficient down to 20.5% at 120W. The main predicted reasons of this decreasing were the low interaction between CNT and n-pentane and the proximity between CNT average size and surface roughness, which promoted the coverage of the nucleate cavities. However, an increase of approximately 25% in the critical heat flux was registered with the n-pentane nanofluid. Furthermore, fouling on the boiling wall surfaces was observed after the nanofluid test, which was monitored in SEM. An attempt to improve CNT dispersion was performed with the addition of oleylamine, although it was not successful. Optimum suspension of CNT should be further examined to get a noticeable enhancement in nucleate boiling heat transfer, and some suggested procedures are detailed to improve such CNT dispersion.
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