## 3 Heat Transfer Enhancement
### Experimental Results
[11] conducted heat transfer experiments for nanofluids flowing inside flat aluminium tubes of a car radiator under turbulent flow conditions. They compared the heat transfer performances of three different alumina-based nanofluids: \(Al_{2}O_{3}\) in water, in EG, or in an EG/W mixture. They noted that the heat transfer behaviours of these nanofluids were highly dependent on the particle concentration and the flow conditions and weakly dependent on the temperature. The highest Nusselt number enhancement up to 40% was obtained by suspending only 1 vol.% of \(Al_{2}O_{3}\) nanoparticles into both water and ethylene-glycol. They assessed the effect of three concentrations of EG/W-based nanofluids including 5, 10, and 20 vol.% of ethylene-glycol on the heat transfer characteristics. They found that as the EG concentration in water increased, the Nusselt number decreased. For instance, at \(\varphi\) = 0.3 vol.% and an inlet temperature equal to 45 \({}^{\circ}\)C, the Nusselt number slightly reduced from 114.56-110.55 by increasing the EG concentration from 5-20 vol.%, respectively. However, for any given EG/W mixture concentration, the heat transfer rate was enhanced by adding alumina nanoparticles. As an example, suspending 0.3 vol.% of \(Al_{2}O_{3}\) nanoparticles into a 20:80 EG/W mixture increased the Nusselt number up to 22% compared to the pure base fluid. The heat dissipation performance of a motorcycle radiator filled with multi-walled carbon nanotubes (MWCNTs) dispersed into a 50:50 EG/W mixture has been investigated experimentally by [PERSON] and [PERSON] [13]. The experiments were carried under the temperature range [80-95 \({}^{\circ}\)C] and volumetric flow rates varying from 4.5-8.5 L/min. They prepared three nano-coolants with different concentrations of MWCNTs, namely 0.1, 0.2, and 0.4 wt.%. They found that the Nusselt number with any of these nano-coolants was lower than EG/W because of the larger enhancement of the thermal conductivity compared to that of the heat transfer coefficient. They evaluated the overall efficiency of the heat exchanger by calculating the ratio of heat exchange capacity and the pumping power defined as the efficiency factor. They reported that the nano-coolant with \(\varphi\) = 0.1 wt.% exhibited the highest heat exchange capacity and efficiency factor compared to the two other nano-coolants and base fluid. They affirmed that a high concentration of MWCNTs could not guarantee a better heat exchange capacity because the uneven density of nano-coolant in the flow state increased the thermal resistance of the solid-liquid interface, which effectively decreased the contact area between the MWCNTs and the EG/W mixture. For all the experimental conditions, the maximum enhancement ratios of heat exchange, pumping power and efficiency factor were approximately 12.8%, 4.9%, and 14.1%, respectively, in comparison with the base fluid. The performances of ethylene-glycol- and water-based \(TiO_{2}\) nanofluids as an automobile radiator coolant have been studied by [PERSON] et al. [18]. The nanofluids were prepared by dispersing different \(TiO_{2}\) nanoparticle concentrations (0.1, 0.3, and 0.5 vol.%) into a 40:60 EG/W mixture. The nanofluids flowed through the radiator tubes with an elliptical cross-section under turbulent flow conditions. The results demonstrated that the presence of titanium oxide nanoparticles enhanced considerably the heat transfer rate in the automobile radiator by up to 37% compared to the base fluid. They observed that increasing the fluid circulation rate could improve the heat transfer performance while the fluid inlet temperature had a small effect for both the pure base fluid and the nanofluids for  
Figure 6: (**a**) Photograph of a real car radiator setup; (**b**) heat transfer coefficient at different Reynolds numbers and volume fractions of MWCNT in a water-ethylene-glycol mixture (50:50). After [17], with permission from Elsevier. this set of parameters. The effective thermal conductivity was found to increase as the nanoparticle concentration increased.