# Heat Transfer Enhancement of Nanofluids with Non-Spherical Nanoparticles: A Review
## 2 Thermal Conductivity of NSN Nanofluids
### Experimental Data Evaluation
#### 2.1.2 Nanorods/Ellipsoids  
Nanofluids containing cylindrical nanoparticles are another type of suspension commonly used in research (2; 10; 30; 31). Notably, nanoparticles in cylindrical shape are referred to as nanorods with stiffness and finite length. These nanomaterials are synthesized from metals or semiconducting materials and cannot bend. The cross-section of particles is circular and nanosized, with a moderate aspect ratio range of around 5 ([PERSON] and [PERSON] (2016). [PERSON] and [PERSON] (2016) documented that nanofluids containing cylindrical nanoparticles with such an aspect ratio could show different behaviors.  
In Figure 2, the findings reported by [PERSON] et al. (2015) and [PERSON] et al. (2016) are quite similar when particle loading ranges from 2% to 5%. The aspect ratios of used nanorods are approximately 4 and 6.25, and the materials include TiO\({}_{2}\) and CuO, respectively. [PERSON] et al. suspended aluminum oxide (Al\({}_{2}\)O\({}_{3}\)) nanoparticles of four different shapes ([PERSON] and [PERSON] (2016) in DW and EG mixed 50/50 by volume to assess the particle shape effect on thermal conductivity of nanofluids. The nanoparticles supported by a company have the shapes of rods, bricks, platelets, and blades, with different sizes and aspect ratios. The data measured in experiments illustrated that nanofluids containing cylindrical nanoparticles exhibit the best heat conductivity performance. They found that particle shape and interfacial layer contribute to the particle volume fraction thermal conductivity of nanoparticle suspensions. The predictions of the classical H-C model are higher than the measured TCE for particles with blade and platelet shapes.  
Other studies (10; 31; 35) measured similar values of TCE, which are approximately half of the TCE reported by [PERSON] et al. (2015) and [PERSON] et al. (2016). [PERSON] et al. (2016) prepared aqueous nanofluids containing CNTs with an aspect ratio of 10, closer to rod-like. [PERSON] et al. (2016) suspended rod-like Al\({}_{2}\)O\({}_{3}\) nanoparticles into molten salt to prepare nanofluids and discovered approximately 10% TCE in the liquid state. The particle shapes discussed above have an aspect ratio larger than 1. As shown in Figure 1, the nanofluid TCE is positively related to the particle concentration. To eliminate the effect of particle concentration to examine the shape effect, we defined the enhancement per concentration as the ratio of TCE and particle volume fraction. Figure 3 shows the enhancement per concentration as a function of aspect ratio at room temperature. Only a handful of studies have provided particle aspect ratios. Ag nanowire/CNT-based nanofluids made by [PERSON] et al. (2017) and [PERSON] et al. (2019) were not plotted due to the out-of-range values.  
Nanoparticles agglomerate and form aggregates with a small aspect ratio; this explains why many studies have found enhanced heat transfer except for the concentration increment. With increasing AR, the enhancement per concentration increases; this implies that a non-spherical shape helps TCE. The reported data are near the diagonal (enhancement per concentration/AR = 1). Interestingly, the increase induced by the aspect ratio appears to be more apparent in the small-aspect-ratio region (AR < 10), which represents rod-like particles. TCE per concentration no longer increased with increasing AR for a tubular/wire-shaped dispersed phase with a large aspect ratio. This means that the enhancement of the network effect on heat transfer is limited. [PERSON] et al. [37] discovered that the effect of particle AR on TCE was more apparent for small particles.