# Thermal Conductivity of Nanofluids: A Review on Prediction Models, Controversies and Challenges
## 6 Backdraws and Future  
The backdraws, difficulties and general limitations of the NFs can be summarized in the following points: \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**NPs** & **BF** & **Size [nm]** & **Concentration** & **Keff Increase [\%]** & **Method** & **Ref.** \\ \hline \multirow{4}{*}{**Au**} & Toluene & 1.65 & 0.003 vol.\% & 8 & TSHW & [110] \\  & Toluene & 2 & 0.024 vol.\% & 1.4 & MSBD & [142] \\  & Ethanol & 4 & 0.018 vol.\% & 1.3 & MSBD & [142] \\  & & 96 & 1.7\(\times\)10\(-\)5 & 20.8 & TWRC & [143] \\  & Water & & vol.\% & & & \\  & & 96 & 3.5\(\times\)10\(-\)6 & 4 & TWRC & [143] \\  & DI & 5–25 & 0.5 vol.\% & 16 & THW & [144] \\  & & 20 & 4 vol.\% & 38.8 & Theory & [145] \\  & EG & 10 & 0.55 vol.\% & 18 & THW & [64] \\  & EG & 10 & 0.3 vol.\% & 16.5 & THW & [28] \\  & & 50 & 2 vol.\% & 15.5 & Theory & [145] \\
**Al** & EG & 80 & 5 vol.\% & 45 & THW & [127] \\ SiC & DI & 100 & 3 vol.\% & 7.2 & THW & [146] \\
**(sphere)** & DO & 30 & 0.8 vol.\% & 7.36 & THW & [147] \\  & & 40–50 & 3 vol.\% & 38.2 & THW & [148] \\
**SiO2** & Water & 12 & 1 vol.\% & 3.2 & THW & [132] \\  & & 12 & 1 vol.\% & 3 & THW & [40] \\  & & 50:50, 80:20, & & & & \\
**MgO/TiO2** & Water & 10–45 & 20:80, 60:40 and & 21.8 at 50 \({}^{\circ}\)C & THW & [149] \\
**Graphene** & Water & & 0.025 to 0.1 wt\% & 27 & THW & [150] \\
**(GFnanopl)** & EG & 0.7–1.3 & 0.05 vol.\% & 86 & TSHW & [151] \\
**Graphene** & EG & 0.7–1.4 & 0.05 vol.\% & 61 & TSHW & [151] \\
**Oxide** & EG & & 0.025, 0.05 and & & & \\
**GFnanopl** & Water & & 0.1 wt.\% & About 11.56 & Theory & [152] \\
**- SDBS** & Water & & 0.025, 0.05 and & & & \\
**GFnanopl** & Water & & 0.1 wt.\% & & & \\
**- COOH** & Water & & 0.1 wt.\% & & & \\
**Al2O3** & Water & 10–100 (\(Al_{2}O_{3}\)) & 0.01 vol.\% & 10.85 for MWCNT & THW & [153] \\
**MWCNT** & Water & & 0.2, 0.4, and0.6 & 30.59 with MWCNT & THW & [154] \\  & Solar glycol & 20–30 & & vol.\% & volume of 0.6\% & THW & [155] \\  & Jatropha & Length \(\times\) OD = & & & & \\  & seed oil & 2.5–20 \(\mu\)m \(\times\) & 0.2–0.8 wt.\% & 6.76 & THW & [155] \\
**MWCNT** & & 6–13 nm & & & & \\  & & \(\mathcal{O}\)40 & 0.49 vol.\% & 80 & THW & [156] \\  & Water & \(\mathcal{O}\)130 & 0.6 vol.\% & 34 & THW & [157] \\  & Water & \(\mathcal{O}\)10–30 & 1 vol.\% & 7 & THW & [132] \\  & & \(\mathcal{O}\)10–30 & 0.48 vol.\% & 5 & THW & [158] \\  & HTO & \(\mathcal{O}\)5–20 & 2 vol.\% & 15 & THW & [159] \\  & EG & \(\mathcal{O}\)20–50 & 1 vol.\% & 12.4 & THW & [160] \\  & EO & \(\mathcal{O}\)20–50 & 1 vol.\% & 8.5 & THW & [160] \\
**DWCNT** & Water & \(\mathcal{O}\)5 & 1 vol.\% & 8 & THW & [157] \\  & Water & \(\mathcal{O}\)1-2\(\times\)5000-30,000 & 0.48 vol.\% & 16.2 & THW & [157] \\
**SWCNT** & Water & \(\mathcal{O}\)1-2\(\times\)1000–30000 & 0.48 vol.\% & 8.1 & THW & [158] \\  & EG & 100–600 & 0.21 vol.\% & 15.5 & THW & [161] \\
**C60-C70** & Toluene & – & 0.378 vol.\% & 0.816 & MSBD & [143] \\
**fullerenes** & MO & 10 & 5 vol.\% & 6 & THW & [132] \\ \hline \hline \end{tabular}  
Legend: BF = Base Fluid; Keff = Effective thermal conductivity; THW = Transient Hot-Wire; SSCB = Steady state “cut bar”; TSHW = Transient Short Hot-Wire; TTS = Transient Plane-Source; MSBD = Microscale Beam Deflection Technique; TWRC = thermal-wave resonator cavity.  
\end{table} 
Table 3: _Cont._Complexity and high cost of NFs preparation;
* Long-term stable NFs are difficult to produce.