# Heat Transfer Enhancement of Nanofluids with Non-Spherical Nanoparticles: A Review
## 3 Convective Heat Transfer of NSN Nanofluids
### Experimental Data Evaluation
#### 3.1.2 Thermohydraulic Performance Evaluation  
Thermohydraulic performance is evaluated via a different criterion. As a heat transfer medium, a nanofluid should meet the appropriate guidelines before it can be applied. [PERSON] et al. [93] evaluated their experimental data by different forms of figures of merit (FOMs) as follows:  
* The ratio of CHT coefficient, \(\text{FOM}_{h}=h_{nf}/h_{b}\);
* The ratio of the maximum overall temperature difference, \(\text{FOM}_{T}=\left(T_{\text{\tiny{w}},\rho}-T_{\text{\tiny{in}}}\right)_{nf} /\left(T_{\text{\tiny{w}},\rho}-T_{\text{\tiny{in}}}\right)_{b}\);
* Cooling with minimum pumping power, \(\text{FOM}_{P}=\left(c_{p}h/P\right)_{nf}/\left(c_{p}h/P\right)_{b}\);
* The relative ratio of heat transfer rate and pumping power, \(\text{FOM}_{Q}=\left(q/P\right)_{nf}/\left(q/P\right)_{b}\). The following criteria should be met: \(\text{FOM}_{h}>1\), \(\text{FOM}_{T}<1\), \(\text{FOM}_{P}>1\), and \(\text{FOM}_{Q}>1\). Research [94; 95] has shown that the criteria for nanofluids with spherical particles are hardly met in tests. We sampled the CHT data of NSN nanofluids reported in the literature (Table 3). [PERSON] et al. [96] used the relative thermal resistance to estimate the CHT performance of the nanofluid in an oscillating heat pipe; hence, the data are not shown. [PERSON] et al. [97] reported about 110 times viscosity of nanofluid with EG of 0.6% by weight in PAO, leading to a quite small \(\text{FOM}_{P}\) compared to others, which was excluded. The pumping power for other studies can be estimated based on [PERSON] et al. [98] for laminar and turbulent flows. Most of the studies found an enhancement of the CHT coefficient if the \(\text{FOM}_{h}\) or \(\text{FOM}_{T}\) were used to evaluate the CHT varying with the Reynolds number. Nevertheless, suppose other performance evaluation criteria (PEC) are used. In that case, it can be found from the experimental data in Figure 5 that CHT was not far beyond the base fluid, with the majority of thermohydraulic FOM values being between 0.8 and 1.2. This is because the \(\text{FOM}_{P}\) and \(\text{FOM}_{Q}\) criteria were employed in most research, which combines heat transfer enhancement with an increase in pumping power. The heat capacity of nanofluids with low particle loading is close to that of the base fluid. The \(\text{FOM}_{P}\) for CNT nanofluids at Re = 1200 was a maximum of 1.536, which possibly meant that the data collection location was not fully developed. [PERSON] et al. [99; 100] tested the hydrodynamic performance of graphene nanoplatelet (GNP) nanofluids and discovered that the pumping power of nanofluids (caused by increased viscosity) was over twice that of the base fluid. Consequently, their results had the lowest \(\text{FOM}_{P}\) and \(\text{FOM}_{Q}\) values. [PERSON] et al. [93] also found that different FOMs presented similar PEC for the same nanofluid. [PERSON] et al. [101] investigated the particle properties of CHT of nanofluids in the turbulent regime. Their experimental results based on oval-shaped \(\text{Al}_{2}\text{O}_{3}\) nanoparticles demonstrated an increase of less than 5% in Nusselt number. Additionally, the CHT coefficient of nanofluids was roughly equal to that of base fluid when the volume concentration was lower than 1%. They used near-spherical particles. Furthermore, [PERSON] et al. [42] and [PERSON] et al. [102] investigated the CHT performance of nanofluids in turbulent regimes, and the PEC values were smaller than 1. Although the increased pumping power in turbulent flow is less than that of laminar flow, the thermohydraulic performance for NSN nanofluid in turbulent regimes is uneconomical. This also confirms why most recent studies are based on the laminar flow [89]. [PERSON] et al. [23] and [PERSON] et al. [103] measured CHT coefficients of several types of nanofluids containing GNPs at different temperatures. The effect of operating temperature on CHT was unclear. In contrast, nanoparticle additives have a more significant impact on PEC.