**Subscripts**

For 0.1 vol.% TiO2/water nanofluid flow, PT, STT, TTT, and QTT have Nusselt numbers of 1.3–30.4%, 46.1–83.2%, 53.8–97.5%, and 59.9–110.8%, respectively, higher than that in PT with water flow. This behavior is due to the fact that

nanoparticles presented in the base liquid increase the thermal conductivity, which

Heat transfer and fluid flow characteristics through tubes induced with multiple

plain twisted tape inserts using water and 0.1 vol.% TiO2/water nanofluid were investigated experimentally and numerically. The investigations were carried out for turbulent flow with Reynolds number ranging between 5000 and 20,000. The

• It is found that tube fitted with quintuple twisted tape inserts performed highest in augmenting heat transfer. Nusselt numbers obtained by using quintuple twisted tapes inserts are up to 110.8% for nanofluid flow and 100.0%

• As the number of twisted tapes increases, higher Nusselt number is obtained,

• The pressure drop and friction factor increase significantly by using twisted

• Using nanofluids with twisted tape inserts are resulting in more heat transfer

• Using twisted tape inserts need to increase fluid pumping power due to high pressure drop produced. When using nanofluid without inserts, pumping power is not affected as the increase in the pressure drop is negligible

Accordingly, the combined technique of twisted tape inserts and nanofluid is recommended for heat exchanger application, as the heat transfer is considerably

The authors acknowledge the supports provided by both, Al-Mustansiriyah University, Baghdad, Iraq for providing all the logistic and technical supports to produce this research using the relevant labs in the mechanical engineering department and Universiti Teknologi PETRONAS—Malaysia for providing the financial support to publish the research results using the budget under grant

for water flow higher than in plain tube with water flow.

leads to an increase in heat transfer performance.

*Inverse Heat Conduction and Heat Exchangers*

findings may be summarized as follows:

compared to plain tube.

**Acknowledgements**

**Abbreviations**

**42**

YUTP—FRG, CS: 015LC0-026.

*D* tube diameter (mm) *f* friction factor

tape inserts and slightly by nanofluid.

augmentation than using each one individually.

compared to the case of pure water flow.

enhancing, with reasonable penalty in pumping power.

**6. Conclusions**

