In-tube flow condensation of R-1233zd(E) and R-1336mzz(Z): Thermal-hydraulic performance for medium temperature heat pumps

The transition toward low-GWP refrigerants has brought renewed interest in R-1233zd(E) and R-1336mzz(Z) for industrial heat pumps operating at medium to high temperatures. However, experimental condensation data for these fluids at saturation temperatures above 60 °C and in small-diameter tubes remain scarce, limiting reliable design guidance. This study presents an extensive experimental evaluation of their in-tube condensation heat-transfer and frictional pressure-drop behavior in a 2 mm horizontal smooth copper tube over saturation temperatures of 60–85 °C and mass fluxes from 250 to 450 kg m −2 s −1 . Both refrigerants exhibit annular-dominant flow across all conditions, confirmed by Soliman's criterion and the El Hajal–Thome–Cavallini flow map. R-1336mzz(Z) achieved heat transfer coefficients up to ∼10% higher than R-1233zd(E), driven mainly by its higher liquid-to-vapor density ratio, while R-1233zd(E) produced lower frictional pressure drops. HTC reductions of 18–24% for R-1233zd(E) and 16–23% for R-1336mzz(Z) were observed as saturation temperature increased from 60 to 85 °C. Among evaluated correlations, Cavallini and Zecchin achieved the best HTC predictions ( η 30 % ≈ 97.1%, MARD ≈ 14%), while the Friedel correlation best captured frictional pressure drop ( η 30 % ≈ 97.9%, MARD ≈ 11.2%). These findings provide validated design-level data and emphasize the need for refined models tailored to high-temperature HFO condensation in small-diameter tubes.