Experimental investigation and development of heat transfer correlation for flow boiling of nanorefrigerants through horizontal tubes

Nanorefrigerant attracts considerable academic attention for its significantly enhanced in the heat transfer performance. This study aims to experimentally investigate the comprehensive heat transfer characteristics of various working fluids, i.e., a pure working fluid (R123), nanorefrigerants (ZnO-R123, TiO₂-R123) and a hybrid nanorefrigerant (ZnO/TiO₂-R123 (4:6)), in a horizontal tube for an organic Rankine cycle system. The effects of mass flux (250, 300, 350, 420 and 500 kg/(m²·s)), dryness fraction and nanoparticle on the heat transfer coefficient are examined, with results indicating that the heat transfer coefficient keeps increasing with the mass flux and dryness fraction. When the mass flux is 500 kg/(m²·s) and the dryness fraction is 0.76, the maximum heat transfer coefficients for ZnO-R123, TiO₂-R123 and ZnO/TiO₂-R123(4:6) are 4.4 kW/(m²·K), 3.9 kW/m²·K, 3.5 kW/m²·K, which is 40 % higher, 25.4 % higher and 10 % higher than that for R123 of 3.1 kW/m²·K, respectively. Moreover, the experimental data are compared with the prediction results from four typical correlations and a new correlation is proposed to predict the heat transfer coefficient of nanorefrigerants. When the mass flux is 350 kg/(m²·s), Chen and Zhang correlations have good prediction accuracy with MRE of −10 % and −11 %, and MAE of 11.1 % and 13.6 %, respectively. The MRE of the new correlation varies within ± 12 %, which has a high prediction accuracy and meets the heat transfer coefficient prediction requirement for nanorefrigerant flowing in a horizontal tube. This study furnishes comprehensive experimental data of nanorefrigerants and offers theoretical support for their application in thermodynamic cycles.