Energy efficiency and thermo-hydraulic performance of MWCNT/water nanofluid in flat and circular tubes

Abstract

In this manuscript, heat transfer and flow characteristics (HTFC) of multi-walled carbon nanotubes (MWCNT)/water nanofluids investigated in flat and circular tubes under constant heat flux (CHF) conditions, experimentally. The two-step method was used in preparing nanofluids at various weight fractions of 0.01, 0.05, 0.1 and 0.3 wt.% with the use of cetyl trimethyl ammonium bromide (CTAB) surfactant. The thermo-physical properties (TPP) such as thermal conductivity (TC) and viscosity were studied through instruments. Specific heat and density were calculated through standard theoretical equations. TC enhanced with temperature and particle concentration. The experiments were performed in test rig having both flat and circular tubes connected in parallel. Fluids were flow through tubes at different flow rates 0.15–0.5 L/min. Thermal performance was measured through two parameters such as heat transfer coefficient (HTC) and Nusselt number. Both parameters were increased with increase in nanoparticle concentration and flow rate of nanofluids. Highest enhancement in HTC of 0.3 wt.% nanofluid was 72.13% and 82.82% at flow rate 0.5 L/min compared to that of distilled water in circular and flat tubes, respectively. HTC enhancement of 17% was obtained in flat tube compared to circular tube at similar operating conditions. Pressure drop (PD) for 0.3 wt.% nanofluid was 2.35 times and 2.63 times at 0.5 L/min flow rate in circular and flat tubes, respectively. Energy efficiency ratio (\(\eta\)) was also calculated to be 1.57 and 1.39 for 0.3 wt.% and 0.5 L/min for flat and circular tubes, respectively. Flat tube showed good thermo-hydraulic performance compared to circular tube under similar operating conditions.