Stabilization and heat transfer enhancement of nano-organic fluids in ORC systems: role of surfactant type and concentration in evaporation and condensation processes

The agglomeration of nanoparticles in nanofluids remains a critical challenge in practical applications. This study experimentally investigates the stabilization mechanisms and heat transfer characteristics of nano-organic working fluids (TiO2/R123) modified with three surfactants (anionic SDBS, cationic CTAB, and non-ionic Span80) during evaporation and condensation processes in organic Rankine cycle (ORC) systems. Particle size analysis, absorbance measurements, and viscosity assessments are systematically conducted to evaluate stability, while heat transfer coefficients and pressure drops are analyzed under varying mass fluxes (200–500 kg/(m2·s)). Results indicate that the optimal surfactant concentrations for dispersion stability are 0.3 % CTAB, 0.4 % SDBS, and 0.3 % Span80. Surfactants improve evaporation heat transfer coefficients by 2.1–39.6 %, with 0.3 % Span80-TiO2/R123 achieving the maximum enhancement (39.6 % at G = 450 kg/(m2·s)), while evaporation pressure drops decrease by 9.5–48.6 %, with 0.3 % CTAB-TiO2/R123 showing the largest reduction (48.6 % at G = 350 kg/(m2·s)). Surfactant-modified nano-organic working fluids increase condensation heat transfer coefficients by 14.9–72.5 %, with 0.4 % SDBS-TiO2/R123 delivering the highest enhancement (72.5 % at G = 450 kg/(m2·s)). The condensation pressure drops of surfactant-modified nano-organic working fluids are in range of 0.04–0.10 MPa, which is 6–50 % lower than that of unmodified nano-organic working fluids of 0.08–0.12 MPa. This work provides a framework for designing surfactant-modified nanofluids in ORC systems.