Coolants Based on Thermoconductive Nanofluids
Ultrasonically synthesized nanofluids are efficient coolants and heat exchanger liquids. Thermoconductive nanomaterials increase heat transfer and heat dissipation capacity significantly. Sonication is well established in the synthesis and functionalization of thermoconductive nanoparticles as well as the production of stable high-performant nanofluids for cooling applications.
Nanofluidic Effects on Thermo-Hydraulic Performance
The thermal conductivity of a material is a measure of its ability to conduct heat. For coolants and heat transfer fluid (also called thermal fluid or thermal oil), a high thermal conductivity is desired. Numerous nanomaterials offer great thermo-conductive properties. In order to use the superior thermal conduciveness of nanomaterials, so-called nanofluids are used as cooling liquids. A nanofluids is a fluid, in which nanometer-sized particles are suspended in the base fluid like water, glycol or oil, where they form a colloidal solution. Nanofluids can significantly increases in thermal conductivity compared with liquids without nanoparticles or larger particles. Material, size, viscosity, surface charge, and fluid stability of the dispersed nanoparticles affect the thermal performance of nanofluids significantly. Nanofluids are rapidly gaining importance in heat transfer applications since they show superior heat transfer performance when compared to conventional base fluids.
Ultrasonic dispersion is a highly efficient, reliable and industrially-established technique to produce nanofluids with high-performance heat transfer capacities.
UP400St, a 400W powerful ultrasonic processor for the production of nanofluids with superior thermal conductivity.
- a high surface : volume ratio for significantly higher energy and mass transfer rates
- low mass for very good colloidal stability
- low inertia, which minimizes erosion
These nano-size related features give nanofluids their exceptional thermal conductivity. Ultrasonic dispersion is the most efficient technique to produce functionalized nanoparticles and nanofluids.
Ultrasonically Produced Nanofluids with Superior Thermal Conduciveness
Numerous nanomaterials – such as CNTs, silica, graphene, aluminium, silver, boron nitride, and many others – have been already proven to increase the thermal conduciveness of heat transfer fluids. Below, you can find exemplary research results for thermo-conductive nanofluids prepared under ultrasonication.
Alumiunium-based Nanofluid Production with Ultrasound
Buonomo et al. (2015) demonstrated the improved thermal conductivity of Al2O3 nanofluids, which were prepared under ultrasonication.
In order to disperse Al2O3 nanoparticles uniformly into water, the researchers used the Hielscher probe-type ultrasonicator UP400S. Ultrasonically deagglomerated and dispersed aluminium particles yielded in a particle size of approx. 120 nm for all nanofluids – independently from the particle concentration. The thermal conductivity of nanofluids was increasing at higher temperatures when compared to pure water. With 0.5% Al2O3 particle concentration at room temperature of 25°C the increase in thermal conductivity is only about 0.57%, but at 65°C this value increases to about 8%. For volume concentration of 4% the enhancement goes from 7.6% to 14.4% with temperature rising from 25°C to 65°C.
[cf. Buonomo et al., 2015]
Particle size distribution of water-based boron nitride nanofluids with various boron nitride concentration after ultrasonication with the UP400S (a) 0.1% hBN, (b) 0.5% hBN, (c) 2% hBN
(Study and graphs: © Ilhan et al., 2016)
Boron Nitride-based Nanofluid Production using Sonication
Ilhan et al. (2016) investigated the thermal conductivity of hexagonal boron nitride (hBN) based nanofluids. For this purpose a series of well dispersed, stable nanofluids, containing hBN nanoparticles with a mean diameter of 70 nm, are produced with a two-step method involving ultrasonication and surfactants such as sodium dodecyl sulfate (SDS) and polyvinyl pyrrolidone (PVP). The ultrasonically dispersed hBN–water nanofluid shows significant thermal conductivity increase even for very dilute particle concentrations. Sonication with the probe-type ultrasonicator UP400S reduced the average particle size of aggregates down to 40–60 nm range. The researchers conclude that large and dense boron nitride aggregates, which were observed in untreated dry state, are broken with ultrasonication process and surfactant addition. This makes ultrasonic dispersion an effective method for preparation of water-based nanofluids with various particle concentrations.
[cf. Ilhan et al., 2016]
STEM image showing morphology of an ultrasonically dispersed an ethylene glycol (EG)- based hBN nanofluid with 0.5% particle volume concentration.
(Study and graphs: © Ilhan et al., 2016)
“Ultrasonication is the most widely utilized process in the literature to increase the stability of nanofluids.” [Ilhan et al., 2016] And also in industrial production, sonication is nowadays the most effective, reliable and economical technique to obtain long-term stable nanofluids of outstanding performance.
Industrial Ultrasonicators for Coolant Production
Scientifically Proven, Industrially Established – Hielscher Ultrasonicators for Nanofluid Production
Ultrasonic high-shear dispersers are reliable machines for the continuous production of high-performance coolants and heat transfer fluids. Ultrasonically-driven mixing is known for its efficiency and reliability – even when demanding mixing conditions apply.
Hielscher Ultrasonics equipment allows to prepare non-toxic, non-hazardous, some even food-grade nanofluids. At the same time, all our ultrasonicators are highly efficient, reliable, safe-to-operate, and very robust. Built for 24/7 operation, even our bench-top and mid-size ultrasonicators are capable to produce remarkable volumes.
Read more about ultrasonic production of nanofluids or contact us right now to get in-depth consultation and a free proposal for an ultrasonic disperser!
The table below gives you an indication of the approximate processing capacity of our ultrasonicators:
| Batch Volume | Flow Rate | Recommended Devices |
|---|---|---|
| 1 to 500mL | 10 to 200mL/min | UP100H |
| 10 to 2000mL | 20 to 400mL/min | UP200Ht, UP400St |
| 0.1 to 20L | 0.2 to 4L/min | UIP2000hdT |
| 10 to 100L | 2 to 10L/min | UIP4000hdT |
| 15 to 150L | 3 to 15L/min | UIP6000hdT |
| n.a. | 10 to 100L/min | UIP16000 |
| n.a. | larger | cluster of UIP16000 |
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Hielscher Ultrasonics manufactures high-performance ultrasonic homogenizers for mixing applications, dispersion, emulsification and extraction on lab, pilot and industrial scale.
Literature / References
- B. Buonomo, O. Manca, L. Marinelli, S. Nardini (2015): Effect of temperature and sonication time on nanofluid thermal conductivity measurements by nano-flash method. Applied Thermal Engineering 2015.
- Beybin İlhan, Melike Kurt, Hakan Ertürk (2016): Experimental investigation of heat transfer enhancement and viscosity change of hBN nanofluids. Experimental Thermal and Fluid Science, Volume 77, 2016. 272-283.
- Oldenburg, S., Siekkinen, A., Darlington, T., Baldwin, R. (2007): Optimized Nanofluid Coolants for Spacecraft Thermal Control Systems. SAE Technical Paper, 2007.
- Mehdi Keyvani, Masoud Afrand, Davood Toghraie, Mahdi Reiszadeh (2018): An experimental study on the thermal conductivity of cerium oxide/ethylene glycol nanofluid: developing a new correlation. Journal of Molecular Liquids, Volume 266, 2018, 211-217.
Facts Worth Knowing
Why are Nanofluids Good for Cooling and Heat Transfer Applications?
A new class of coolants are nanofluids which consist of a base fluid (e.g., water), which acts as carrier liquid for nano-sized particles. Purpose-designed nanoparticles (e.g. nano-sized CuO, alumina titanium dioxide, carbon nanotubes, silica, or metals such as copper, silver nanorods) dispersed into the base fluid can enhance the heat transfer capacity of the resulting nanofluid significantly. This makes nanofluids extraordinary high-performance cooling liquids.
Using specifically manufactured nanofluids containing thermo-conductive nanoparticles allow for significant improvements in heat transfer and dissipation; e.g. silver nanorods of 55±12 nm diameter and 12.8 µm average length at 0.5 vol.% increased the thermal conductivity of water by 68%, and 0.5 vol.% of silver nanorods increased thermal conductivity of ethylene glycol based coolant by 98%. Alumina nanoparticles at 0.1% can increase the critical heat flux of water by as much as 70%; the particles form rough porous surface on the cooled object, which encourages formation of new bubbles, and their hydrophilic nature then helps pushing them away, hindering the formation of the steam layer. Nanofluid with the concentration more than 5% acts like non-Newtonian fluids. (cf. (Oldenburg et al., 2007)
The addition of metal nanoparticles to coolants used in thermal control systems can dramatically increase the thermal conductivity of the base fluid. Such metal nanoparticle-fluid composite materials are referred to as nanofluids and their use as coolants has the potential to reduce the weight and power requirements of spacecraft thermal control systems. The thermal conductivity of nanofluids is dependent on the concentration, size, shape, surface chemistry, and aggregation state of the constituent nanoparticles. The effects of nanoparticle loading concentration and the aspect ratio of the nanoparticles on the thermal conductivity and viscosity of water and ethylene glycol based coolants were investigated. Silver nanorods with a diameter of 55 ± 12 nm and an average length of 12.8 ± 8.5 μm at a concentration of 0.5% by volume increased the thermal conductivity of water by 68%. The thermal conductivity of an ethylene glycol based coolant was increased by 98% with a silver nanorod loading concentration of 0.5% by volume. Longer nanorods had a greater effect on the thermal conductivity than shorter nanorods at the same loading density. However, longer nanorods also increased the viscosity of the base fluid to a greater extent than shorter nanorods.
(Oldenburg et al., 2007)
Hielscher Ultrasonics manufactures high-performance ultrasonic homogenizers from lab to industrial size.