Table 2.

A summary of the recently published literature on the effects of using nanofluids on heat transfer performance and pressure drop of PHEs.

Reference	Nanofluid	Considered Conditions and Objectives	Type of HE	Findings
Tiwari et al. [196]	CeO2-water*	-Solid concentrations: 0.5 to 3 vol. % -Flow rates: 1 to 4 lpm -The effects of nanofluid on heat transfer and pressure drop.	Chevron corrugated PHE	They found that the optimum solid concentration (0.75 vol. %) in which the heat transfer reached its maximum enhancement by 39%. They reported that increasing the flow rate of the nanofluid and the hot water leads to enhancing the heat transfer coefficient. Moreover, the increase in the pressure drop at the optimum solid concentration is negligible while the heat transfer has been significantly improved.
Barzegarian et al. [190]	TiO2-water	-Solid concentrations: 0.3 to 1.5 wt. % -Flow regime: turbulent -Re numbers: 159 to 529 -Effects of solid concentration and Re number on heat transfer and pressure drop.	Brazed PHE	Their results revealed that increasing the Re number and solid concentration results in enhancing the convective heat transfer coefficient, and the maximum enhancement took place at the highest solid concentration by 23.7%. They also reported that the increase in pressure drop by increasing the solid concentration is negligible.
Kumar et al. [191]	ZnO-water	-Solid concentrations: 0.5 to 2.0 vol. % -The effects of nanofluid on heat transfer performance and finding the optimum solid concentration.	Chevron-type PHE	They reported that the solid concentration of 1.0 vol. % is the optimum solid concentration where the maximum heat transfer rate is achieved.
Unverdi and Islamoglu [197]	Al2O3-water	-Solid concentrations: 0.25 to 1 vol. % -Flow rates: 90 to 300 kg/h -Re number: 600 to 1900 -The effects of nanofluid on heat transfer and pressure drop.	Chevron-type PHE	They reported that increasing the solid concentration and flow rate results in enhancing the Nu number by the maximum of 42.4%. They also reported that the maximum increase in the heat transfer and pressure drop took place at the highest solid concentration and Re number by 6.4% and 8.4%, respectively.
Pourhoseini et al. [192]	Ag-water	-Nanofluid concentrations: 0 to 10 mg/L -Flow rate: 2 to 8 lpm -Nanofluid inlet temperature: 36, 46, and 56 °C -The effects of flow rate and solid concentration on heat transfer performance.	CR14-45 COMER PHE	They found that the effect of flow rate on heat transfer performance is more significant than the effect of solid concentration.
Wang et al. [193]	Graphene nanoplatelets-EG/water (50:50)	-Solid concentrations: 0.01 to 1.0 wt. % -Re number: 10 to 400 -The effects of using nanofluid on heat transfer and pressure drop.	Miniature PHE	They reported the maximum enhancement of 4% in heat transfer as the solid concentration increased. Moreover, they reported that the increase in Re number leads to enhancing the heat transfer performance in all the studied solid concentrations. The same trend as was observed for the pressure drop; increasing the solid concentration and Re number leads to increasing the pressure drop.
Mansoury et al. [194]	Al2O3-water	-Solid concentrations: 0.2 to 1 vol. % -Flow regime: turbulent -The effects of nanofluid on heat transfer and pressure drop in different HEs.	Different HEs; a Double-pipe, a Shell and tube, and a PHE	They reported that the maximum heat transfer of 60% is achieved in the double-pipe HE, while the minimum enhancement took place in the PHE by 11%. Moreover, the minimum increase in pressure drop has been experienced in the PHE.
Elias et al. [198]	Al2O3-water	-Solid concentrations: 0 to 0.5 vol. % -Temperatures: 25 to 55 °C -Re numbers: 180 to 350 -The effects of using nanofluid on heat transfer performance and pressure drop.	Chevron-type PHE	The results revealed the maximum enhancement of 7.8% in the heat transfer coefficient at the solid concentration of 0.5 vol. %. Moreover, increasing the solid concentration leads to increasing the pressure drop.
Tayyab at al. [199]	CuO-water	-Solid concentrations: 0.2 to 0.6 vol. % -Flow rates: 1 to 9 lpm -Different surface roughness -The effects of nanofluid on heat transfer performance in different HEs.	Different HEs: Shell and tube, concentric, spiral, and PHE	The results revealed that the heat transfer performance of the nanofluid in the PHE is better than the other studied HEs. The maximum enhancement in heat transfer for the PHE is 26% while for the other HEs, 21% is reported.
Attalla and Maghrabie [200]	Al2O3-water	-Solid concentrations: 1.2 to 2.6 vol. % -Re numbers: 500 to 5000 -The effects of using nanofluid on the Nu number, friction factor, and heat transfer enhancement.	PHE	The results revealed that the heat transfer performance and the pressure drop has been increased as the solid concentration and surface roughness increased. Moreover, it is found that the influence of the surface roughness is more noticeable than the solid concentration.
Talari et al. [195]	Al2O3-water	-Solid concentrations: 0 to 5 vol. % -Finding the optimum solid concentration for heat transfer intensification.	Corrugated PHE	They declared that since the heat transfer enhancement of the nanofluid showed a monotonic increase, it is not possible to find an optimum solid concentration.
Sözen et al. [201]	Kaolin-water	-Solid concentration: 2 wt. % -Temperatures: 40, 45, and 50 °C -The effect of using nanofluid on heat transfer performance.	Spiral PHE	It is revealed that using nanofluid instead of the based fluid leads to having 17.6% enhancement in heat transfer rate. Moreover, increasing the Re number leads to decreasing the effectiveness of the PHE.
Meisam et al. [202]	Al2O3-water TiO2-water SiO2-water	-Solid concentrations: 0.05, 0.1, and 0.2 wt. % -Temperatures: 30 to 50 °C -Re numbers: 35.9 to 160.6 -Flow rates: 0.4 to 2 L/m -The effects of using different nanofluids on heat transfer performance have been studied.	PHE	The results revealed that adding nanoparticles to the basefluid leads to considerable enhancement in heat transfer performance. The maximum enhancement in the heat transfer achieved by using SiO2-water nanofluid at the highest solid concentration and Re number of 37 by 2.82%, while the minimum enhancement has been experienced by using Al2O3-water nanofluid at the solid concentration of 0.1 wt. % and Re number 158 by 1.64%.
Soman et al. [203]	γ-Al2O3-water*	-Solid concentrations: 0.1, 0.2, and 0.3 wt. % -Mas flow rate: 0.016-0.082 kg/s -Re number: 200 to 1100	Dimpled PHE	It is revealed that increasing the mass flow rate leads to increasing the heat transfer rate in the PHE. Moreover, increasing the mass flow rate has a direct effect on the heat transfer performance. A new correlation for predicting the Nu number has also been proposed.

(*) Note: CeO₂, and γ-Al₂O₃ are referred to cerium dioxide, and the alpha phase of alumina, respectively