Study on the flow condensation flow patterns and heat transfer characteristics of low-GWP zeotropic mixture R1234ze(E)/R1336mzz(Z) in macro- and mini-channels

This study systematically investigates the flow condensation characteristics of the zeotropic mixture R1234ze(E)/R1336mzz(Z) in horizontal smooth tubes through experimental methods. A sapphire-quartz coaxial visualization heat exchanger was developed to enable simultaneous measurement of heat transfer coefficients (HTCs) and flow patterns. Experimental parameters encompassed tube diameters (8 mm and 2 mm), mass fluxes (100–600 kg/(m²·s)), and bubble-point temperatures (75 °C and 85 °C). Results demonstrated that stratified and annular flows dominated in the macro-channel (8 mm), while intermittent and annular flows prevailed in the mini-channel (2 mm). The modified Breber flow pattern map is suitable for zeotropic mixtures. Heat transfer analysis revealed a positive relationship between condensation HTCs and both mass flux and vapor quality, with limited sensitivity to bubble-point temperature variations. In the macro-channel, all models (Shah, Marinheiro, and Cavallini et al. with the Bell and Ghaly and Silver correction) overpredicted HTCs by 120–300 % under non-annular flow conditions, which is attributable to non-negligible thermal resistance induced by concentration gradients. By incorporating an attenuation factor related to vapor-liquid composition differentials ( y ₁ − x ₁ ) and Bond number (Bo), a modified heat transfer correlation accounting for non-equilibrium effects was proposed, reducing the total mean absolute relative deviation from over 60 % (in non-annular flow) to 12.2 % for macro- and mini-channels. This work provides valuable insights and a reliable tool for the design of compact condensers in high-temperature heat pumps and organic Rankine cycles using zeotropic mixtures.