Experimental investigation of the influence of lubricating oil on the flow boiling heat transfer and pressure drop of CO2 inside an enhanced tube.

Experimental results on flow boiling heat transfer coefficients and pressure drop of CO2-oil mixtures inside an electrically heated 9.52 mm enhanced tube are presented. The nominal mass fraction of fully miscible POE lubrication oil (0 %, 1.3 %, 3 %), saturation pressure (10-39 bar), mass flux (75-500 kg m-2s-1), inlet vapor quality (0.1-0.9), and heat flux (0-120 kW m-2) were varied. An isothermal peripheral boundary condition was realized by dividing the test section into six independently heated segments around the perimeter. Flow patterns, pressure drop, as well as average heat transfer coefficients are presented, discussed with respect to observed flow patterns, and compared to experimental results for pure CO2 obtained in the same test facility. As it was previously observed for smooth tube experiments, the oil mass fraction significantly influences the flow boiling heat transfer coefficient and pressure drop.