Thermal performance of a counter-current double pipe heat exchanger working with COOH-CNT/water nanofluids

• CNT/water nano-fluid is employed as a coolant inside a double pipe heat exchanger.
• Enhancement in convective heat transfer coefficient is confirmed.
• Thermal conductivity was 56% enhanced over the base fluid.
• A small penalty was registered for pressure drop for CNT/water nano-fluids.

The main aim of this work is to investigate the heat transfer coefficient and pressure drop characteristics of carbon nanotube water-based nanofluids inside the double pipe heat exchanger. Diameters of inner and outer copper tubes (ID and OD) were 6.35 and 12.7 mm, respectively (in accordance with ANSI/ASME/API 5L). Nanofluids were prepared using two-step method at mass concentrations of 0.1–0.3% by dispersing the COOH functionalized multi-walled carbon nanotubes, (FCNTs as purchased) into the deionized water. Since this work can technically be important, thermal conductivity of nanofluids were experimentally measured using KD2 Decagon instrument at different mass concentrations and temperatures. To assess the thermal performance of nanofluids, forced convection experiments were conducted at laminar and turbulent flow regimes (900 < Re < 10,500). Influence of different operating parameters including: flow rate, mass concentration of nanofluid, inlet temperature of nanofluid on the heat transfer coefficient and pressure drop is studied. Results demonstrated that presence of carbon nanotube can enhance the thermal conductivity up to 56% at wt.% = 0.3. Likewise, CNT/water nanofluids have higher convective heat transfer coefficient in comparison with water, which is due to internal thermal conduction of CNTs. Longer stability was seen due to the COOH group attached to the CNTs. Small penalty is reported for pressure drop and friction factor as well, which is due to the presence of carbon nanotube inside the bulk of base fluid. Considering the influence of CNTs on heat transfer and pressure drop, it was found that carbon nanotube nanofluids can drastically enhance the thermal performance of heat exchanger in comparison with water up to 44% at maximum mass concentration (wt.% = 0.3).