# Enhanced Heat Transfer Performance of the Tube Heat Exchangers Using Carbon-Based Nanofluids
## 1 Introduction  
Research on nanofluids in heat exchange, energy storage, and thermal collection systems has been very vigorous in recent years, and most studies show that nanofluids (NFs) can effectively improve system efficiency and operating performance in these systems [1, 2, 3, 4, 5]. In particular, the carbon-based nanofluids (CBNFs) prepared by adding carbon-based nanomaterials (CBNMs) have attracted more attention from researchers. CBNMs mainly include single-walled nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), multi-walled carbon nanotubes (MWCNTs), graphene (GN), graphene nanopllets (GNP), graphene oxide (GO), and reduced graphene oxide. Most of these materials have excellent stability and thermal properties. In particular, CNTs and graphene have received great attention because of their large aspect ratio, and special optical, mechanical, physical, and chemical properties. Therefore, CBNFs have been commonly used in heat exchange, thermal storage, and thermal collection systems to improve system performance [6, 7, 8, 9, 10]. In addition, high-performance CBNFs should have considerable potential in the application of relevant cutting-edge technologies in the future [11]. Many researchers have applied CBNFs to air-cooled heat exchangers (ACHEs) or radiators [12, 13, 14, 15], shell and tube heat exchangers [16, 17, 18], plate heat exchangers (PHE) [19, 20, 21, 22, 23], and double tube or tube heat exchangers [24, 25, 26] to improve heat exchange performance. In addition, different base fluids (BFs) such as water (W), ethylene glycol aqueous solution(EG-W) and lubricating oil are configured CBNFs for adapting to different use objects and temperature ranges. The application of CBNFs in heat exchangers can improve the convective heat transfer coefficient (_HTC_) or heat exchange capacity of the heat exchanger (HE), while also slightly increasing the power consumption of the pump that transports the fluid. However, very few studies have shown that CBNFs do not help the performance of heat exchangers [14; 27]. The results of the experimental research on the use of CBNFs in different HEs in recent years are summarized in Table 1. Table 1 includes adding materials, BFs, concentration, HE types, and main findings. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Materials/BFs** & **Concentrations** & **HE Types** & **Main Findings** & **Ref. ** \\ \hline MWCNTs/EG-W & 0.1, 0.25, and (1:1, \(v/v\)) & 0.5 vol\% & Vehicle radiator & HTC was proportional to flow rate (\(G\)), Re, and MWCNTs concentration. & \\  & & & Maximum enhancements of HTC was 196.3\% at 0.5 vol\% NF at Re of 1400. & \\  & & & HTC was proportional to GNP concentration, inlet temperature (\(T_{i}\)), and mass flow rate (\(m_{\text{off}}\)). & A maximum enhancement of HTC was 51\% for 0.5 vol\% NF at \(m_{\text{off}}\) of 100 g/s and \(T_{i}\) of 45 \({}^{\circ}\)C. & Maximum pressure drop (\(\Delta P\)) was 4.80 kPa for 0.5 vol\% NF at \(T_{i}\) of 35 \({}^{\circ}\)C and \(m_{\text{off}}\) of 100 g/s. & A maximum enhancement of heat exchange rate & \\  & & & was 3.3\% at 0.1 vol\% NF and \(T_{i}\) of 85 \({}^{\circ}\)C. & The heat exchange rate of 0.01 vol\% and 0.05 vol\% NF was lower than BF. & \\  & & & & A maximum improvement of heat exchange capacity was 8.17\% at 0.05 wt\% NF at \(G\) of 2.0 & LPM and \(T_{i}\) of 40 \({}^{\circ}\)C for R-ACHE. & \\  & & & ACHE & A maximum enhancement of heat exchange capacity was 4.88\% at 0.01 wt\% NF at \(G\) of 2.5 & LPM and \(T_{i}\) of 40 \({}^{\circ}\)C for C-ACHE. & \\  & & & & 0.01 wt\% NF had higher system efficiency. & \\  & & & & The _HTC_ increased at higher NF concentration and temperature. & \\  & & & & A maximum enhancement of _HTC_ was 35.6\% at 0.1 wt\% NF at fluid temperature of 38 \({}^{\circ}\)C. & \\  & & & & Exergy loss of 0.01 wt\% and 0.1 wt\% NF were & \\  & & & & 22\% and 109\% lower than water. & \\  & & & & Exergy loss was proportional to the \(T_{i}\) of the NF. & A maximum enhancement of _HTC_ was 29\% at 0.2 wt\% NF.