Pool Boiling Heat Transfer of Horizontally Oriented Double-Sided Enhanced Tubes

This study experimentally investigates the pool boiling heat transfer characteristics of R134a on the outer surfaces of three double-sided enhanced tubes with different surface structures at a saturation temperature of 6 °C (corresponding to a saturation pressure of 361.98 kPa) and heat fluxes ranging from 25 to 60 kW/m2. The internal helical grooves enhance boundary-layer disturbance and turbulent mixing, while the external fin arrays increase the available surface for bubble nucleation and the density of active nucleation sites, thereby promoting bubble generation and departure. The experimental results show that the refrigerant-side heat transfer coefficients of the E1, E2, and E3 tubes are 5.78–6.79, 5.33–6.71, and 4.31–6.02 times higher than those of the smooth tube, respectively. When the heat flux exceeds 35 kW/m2, the E1 tube facilitates bubble growth and timely detachment, reduces thermal resistance, and exhibits superior heat transfer performance. Four existing correlations were evaluated, and a new correlation was developed based on the present experimental data. The proposed correlation predicts 93% of the measured data within ±15% error.