Experimental Study: An Effect of Rhamnolipid Bio‐Surfactant on Heat Transfer Performance of <scp> Al ₂ O ₃ </scp> Nanofluids in Horizontal Tube

ABSTRACT The utilization of bio‐surfactants in heat transfer enhancement studies remains underexplored compared to synthetic surfactants. This study investigates the effects of Rhamnolipid, a bio‐surfactant, on the thermal performance of distilled water (DW) and Al 2 O 3 water‐based nanofluids (NFs). Rhamnolipid concentrations ranging from 200 to 600 ppm and nanoparticle (NP) concentrations from 0.02 to 0.5 wt.% were studied. The addition of Rhamnolipid to distilled water resulted in the maximization of Thermal Conductivity (TC) at 300 ppm, which aligns with the critical micelle concentration (CMC), leading to a 19.46% improvement in the heat transfer coefficient (HTC). Al 2 O 3 NPs alone showed a 32.07% HTC enhancement at 0.05 wt.% concentration, although the performance declined after a few hours due to particle sedimentation. When Rhamnolipid and NPs were combined, HTC increased with Rhamnolipid concentrations up to 550 ppm, after which the performance declined. The combined addition resulted in maximum enhancement of 92.88% HTC at 0.1 wt.% concentrated nanofluid with 550 ppm RHL. These findings demonstrate that eco‐friendly bio‐surfactants, such as Rhamnolipid, can serve as effective surfactants for enhancing heat conduction performance in NFs.