On the fouling formation of functionalized and non-functionalized carbon nanotube nano-fluids under pool boiling condition

•Pool boiling heat transfer characteristics of CNT/water nano-fluid is studied.•Functionalized and non-functionalized CNT nano-fluids are investigated.•Fouling formation of both nano-fluid is experimentally studied.•Non-functionalized nano-fluid are found to enhance the CHF.•Functionalize nano-fluids are found to enhance the heat transfer coefficient. Pool boiling heat transfer coefficient (HTC) of functionalized carbon nanotube (FCNT) and non-functionalized carbon nanotube (CNT) is experimentally quantified at high-heat flux conditions up to the critical heat flux (CHF) point. Nano-fluids are prepared using two-step method and are well dispersed within the deionized water, followed by functionalizing (by carboxyl group), sonication and adding the nonylphenolethoxylates nonionic surfactant as a dispersant. In terms of stability, results showed that FCNT could be stable up to three successive weeks, while seven days of stability was registered for non-FCNT. In addition, FCNT considerably enhance the heat transfer coefficient and CHF point. Since this nano-fluid does not change the roughness of surface considerably and has great wettability in comparison with CNT nano-fluid. Results also demonstrated although fouling formation of functionalized carbon nanotube is insignificant in comparison with CNT, the lower contact angle between liquid droplet and the surface caused more liquid to be absorbed by the porous fouling layer. Therefore, more liquid are available on the surface, which leads the dry-out regions to be re-wetted. Consequently, significant enhancement in CHF point was registered for FCNT nano-fluids in comparison with CNT. In terms of fouling behavior, asymptotic behavior for fouling resistance is seen for CNT, while the rectilinear fouling behavior was seen for FCNT.