<p>This study presents a comprehensive experimental investigation into the flow boiling heat transfer characteristics of R134a and its environmentally sustainable alternative, R513A, within horizontal micro-fin tubes. The primary objective is to develop a robust semi-empirical correlation for predicting the Nusselt number under a wide range of operating conditions. Experiments were conducted across mass fluxes ranging from 50 to 300 kg·m⁻²·s⁻¹, heat fluxes between 12 and 36 kW·m⁻², and saturation temperatures from 17°C to 27°C. The correlation was formulated based on critical dimensionless numbers, including the dimensionless vapor mass flux (<i>J</i>_g), Weber number (<i>We</i>_l), Jacob number (<i>Ja</i>), and Boiling number (<i>Bo</i>). The developed model was validated with experimental data, yielding a mean absolute error of less than 15%, demonstrating improved prediction accuracy compared to existing correlations. Comparative results revealed that R513A exhibits comparable thermal performance to R134a, supporting its viability as a low-GWP substitute in flow boiling applications. The findings offer valuable insights for the design and optimization of compact heat exchangers and contribute to the broader objective of enhancing energy efficiency while transitioning toward environmentally benign refrigerants. This work thus provides a significant step forward in the development of predictive tools for next-generation thermal systems operating with alternative refrigerants.</p>