Influence of fully miscible lubricating oil on the pressure drop during flow boiling of CO2 inside an enhanced tube

The total static pressure drop in both adiabatic and diabatic two-phase flow conditions during flow boiling of CO2-oil mixtures inside a horizontal enhanced tube of 8.62 mm inner diameter were investigated, yielding interesting insights and supporting the design process of evaporators. The nominal mass fraction of fully miscible POE lubricating oil (1.3-3%), saturation pressure (10-39 bar), mass flux (75-500 kg m(-2) s(-1)), inlet vapor quality (0.1-0.9), and heat flux (0-120 kW m(-2)) were varied over a wide range of parameters. An isothermal peripheral boundary condition was realized for the electrically heated tube. Experimentally determined pressure drops are discussed with respect to operating conditions as well as observed flow patterns. The findings are compared to experimental results for pore CO2 obtained in the same test facility under similar conditions. The pressure drop of both pure fluid and a mixture with an oil mass fraction of 1.3% can be predicted satisfactorily by using established correlations including the hydraulic diameter of the enhanced tube. A significant influence of oil on the pressure drop is observed especially for high oil mass fractions, high vapor qualities, and high heat fluxes. (C) 2016 Elsevier Inc. All rights reserved.