Experimental investigation on flow boiling heat transfer characteristics of R245fa/oil mixture in horizontal smooth tube

• Published in: International journal of multiphase flow Vol. 172; p. 104708
• Main Authors: Wang, Zhiqi, Zhang, Sifeng, Xia, Xiaoxia, Luo, Lan, Xu, Zehua, Peng, Deqi, Ren, Bowen
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.02.2024
• Subjects: Flow boiling; Lubricant; Organic rankine cycle; Pressure drop; Smooth tube; Organic rankine cycle; Flow boiling; Smooth tube; Pressure drop; Lubricant
• ISSN: 0301-9322
• DOI: 10.1016/j.ijmultiphaseflow.2023.104708

•Flow boiling HTC and FPD of R245fa-POE mixture in smooth tube are studied.•There is an optimal oil concentration of 2.3% to achieve the maximum HTC.•Correlations for HTC enhancement factor and FPD penalty factor are compared.•The POE can enhance the flow boiling heat transfer performance of R245fa. During the operation of the Organic Rankine Cycle (ORC) system, it's inevitable that the lubricating oil in the expander will mix with the working fluid, leading to a noticeable impact on its heat transfer performance. In this investigation, we conducted experimental research on the flow boiling heat transfer characteristics of a mixture of R245fa and POE (Polyolester) in a smooth horizontal tube. We explored the effects of various parameters, including oil concentration (ranging from 0% to 4%), heat flux (6 to 20 kW/m²), mass flux (ranging from 150 to 410 kg/(m²·s)), and vapor quality (ranging from 0 to 0.8). The results revealed that the heat transfer coefficient (HTC) for R245fa/POE increased with higher vapor quality and mass flux for each oil concentration. However, as the oil concentration increased, the rate of increase in HTC gradually diminished. Notably, an optimal oil concentration of 2.3% was identified to achieve a maximum HTC of 6 kW/(m²·K) in our experiments. Furthermore, our investigation into frictional pressure drop (FPD) indicated that higher vapor quality, mass flux, and heat flux all led to an increase in FPD for the R245fa/POE mixture. Additionally, we evaluated the impact of lubricants on the HTC and FPD of R245fa/POE mixtures using enhancement factor (EF) and penalty factor (PF), respectively. The EFs for HTC in the R245fa/POE mix ranged mainly between 0.95 and 1.6, while the PFs for FPD were concentrated between 0.75 and 1.4. The lubricating oil was found to enhance the flow boiling heat transfer performance of R245fa, with a comprehensive evaluation index range of 1.22 to 1.31. In terms of prediction accuracy, the Schlager-R22/150SUS correlation provided the most reliable estimates for the enhancement factor of HTC and the penalty factor of FPD, with mean absolute deviations (MAD) of 9.79% for EF and 14.03% for PF. [Display omitted]