Experimental study and correlation fit on boiling heat transfer coefficients of R134a/ethane mixtures

Non-azeotropic mixtures, characterized by temperature glide, can effectively enhance the heat transfer efficiency of the Organic Rankine Cycle. A thorough understanding of their heat transfer characteristics is essential for the rational design of evaporator structures and the overall enhancement of system efficiency. This study investigates the boiling heat transfer behavior of R134a/ethane mixtures in a horizontal tube. The deviations of the experimental and numerical results are compared, and the effects of various operating parameters on the heat transfer coefficient are assessed. The experimental conditions include a heat flux range of 1–4.7 kW/m², mass flow rate of 28–37 kg/(m²·s), inlet temperature of 235–248 K, inlet pressure of 0.4–0.75 MPa, and vapor quality between 0.06 and 0.95. Experimental results are compared with predictions from existing correlations, and new modified correlations are proposed. The results indicate that increasing both heat flux and mass flux leads to a consistent rise in heat transfer coefficients. Meanwhile, high mass flow rate can accelerate the wetting rate of the mixture on the pipe wall, thereby increasing its critical vapor quality. Higher inlet pressures increase the vapor-to-liquid density ratio, which negatively impacts heat transfer. Conversely, higher inlet temperatures enhance nucleate boiling and convective heat transfer. The ethane mole fraction positively impacts heat transfer coefficient owing to its superior thermal conductivity, reduced vapor density, lower liquid viscosity, and decreased latent heat of vaporization compared to R134a. A modified correlation, based on experimental data, is presented for predicting the heat transfer coefficient of mixtures. This new correlation predicts 84.2 % of the experimental data within ±20 % error, with an average error rate of 10.6 %.