Abstract
The main challenge in the use of multi-wall carbon nanotube (MWCNT) as key components of nanofluids is to transfer excellent thermal properties from individual nanotubes into the bulk systems. We present studies on the performance of heat transfer nanofluids based on ultra-long (~2 mm), curly MWCNTs – in the background of various other nanoC-sp2, i.e. oxidized MWCNTs, commercially available Nanocyl™ MWCNTs and spherical carbon nanoparticles (SCNs). The nanofluids prepared via ultrasonication from water and propylene glycol were studied in terms of heat conductivity and heat transfer in a scaled up thermal circuit containing a copper helical heat exchanger. Ultra-long curly MWCNT (1 wt.%) nanofluids (stabilized with Gum Arabic in water) emerged as the most thermally conducting ones with a 23–30%- and 39%-enhancement as compared to the base-fluids for water and propylene glycol, respectively. For turbulent flows (Re = 8000–11,000), the increase of heat transfer coefficient for the over-months stable 1 wt.% ultra-long MWCNT nanofluid was found as high as >100%. The findings allow to confirm that longer MWCNTs are promising solid components in nanofluids and hence to predict their broader application in heat transfer media.
Highlights
With a continuous development of power plants, solar collectors, machines, machineries, devices and advancing miniaturization of electronics as well as increasing number of supercomputers, heat transfer intensification becomes a critical phenomenon [1, 2]
The main challenges in the heat transfer processing based on nanofluids are: (a) high thermal conductivity and high convective heat transfer coefficient in thermal systems enabling enhanced energy harvesting, (b) high energy conversion efficiency, (c) physicochemical stability over storage and working, (d) prevention of clogging in microchannels, (e) minimized biological and chemical corrosion of the construction materials caused e.g. by bacteria or acids formed via oxidation of base fluids like glycols, and (f) low abrasion of piping by dispersed nanoparticles [3]
As length and waviness of multi-wall carbon nanotube (MWCNT) can be controlled by duration time and pressure in the catalytic vapour deposition (c-CVD) furnace, ~2 mm and curly MWCNTs were synthesized (Fig. 2a, e)
Summary
With a continuous development of power plants, solar collectors, machines, machineries, devices and advancing miniaturization of electronics as well as increasing number of supercomputers, heat transfer intensification becomes a critical phenomenon [1, 2]. The main challenges in the heat transfer processing based on nanofluids (base-fluids containing uniformly dispersed nanoparticles) are: (a) high thermal conductivity and high convective heat transfer coefficient in thermal systems enabling enhanced energy harvesting, (b) high energy conversion efficiency (e.g. between radiation energy of sunlight into the heating medium), (c) physicochemical stability over storage and working (the latter frequently under harsh conditions), (d) prevention of clogging in microchannels, (e) minimized biological and chemical corrosion of the construction materials caused e.g. by bacteria or acids formed via oxidation of base fluids like glycols, and (f) low abrasion of piping by dispersed nanoparticles [3] Those problems have already met response from various nanomaterials (metal, metal oxides and other nanoparticles) with partial successes [4, 5]. The critical challenge in the use of MWCNTs is ‘translation’ of their excellent properties from the individual nanotubes (thermal conductivity ca. 3000 W m−1 K−1 for MWCNT [12, 13]) into the nanofluids
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