Influence of oil on heat transfer and pressure drop during flow boiling of CO2 at low temperatures

First experimental results for flow boiling heat transfer and pressure drop of CO2-oil mixtures at low temperatures inside a horizontal smooth tube of 14 mm inner diameter are presented. For the experiments, the nominal concentration of the fully miscible lubrication oil (0-3 wt%), saturation pressure (14.3-26.4 bar), mass flux (75-300 kg m−2 s−1), inlet vapor quality (0.1-0.9) and heat flux (0-100 kW m−2) were varied. Measurements were carried out inside a 14 mm smooth tube under near isothermal wall condition. This was realized using a test section that consists of 6 independent electrically heated segments around the perimeter. Pressure drops and local heat transfer coefficients are presented. Results for CO2-oil mixtures reveal a significant increase in pressure drop with increasing local oil concentration (i.e. nominal oil concentration and vapor quality) and heat flux compared to measurements with pure CO2. Heat transfer coefficients tend to decrease with increasing oil concentration. As local measurements depict, the addition of oil can lead to a significant decrease in liquid heat transfer coefficient (i.e. bottom tube segments). However, local heat transfer coefficients at the top of the tube can also be enhanced due to an increase in tube wetting induced by foam formation.