Effect of lubricating oil on flow boiling heat transfer of carbon dioxide

The flow boiling heat transfer of carbon dioxide with PAG-type lubricating oil entrained from 0% to 5% in horizontal smooth tubes was examined. Experiments were conducted using test tubes with inner diameter of 2–6 mm at mass fluxes of 360–1440 kg m−2 s−1 and heat fluxes of 4.5–36 kW m−2. The saturation temperature was 15 °C.At low oil concentrations of 0.5–1%, the heat transfer coefficient decreased to less than half that under oil-free conditions. The heat transfer coefficient did not decrease further with increasing oil concentration up to 5%. The heat flux positively influenced the heat transfer coefficient in low vapor quality regions, not the high vapor quality regions. The presence of oil caused the mass flux to significantly influence the heat transfer coefficient at a low heat flux till dryout, while no significant influence of the mass flux at a high heat flux was observed. The dryout quality decreased at a large mass flux. The measured pressure drops increased monotonously because of the lubricating oil.

► The heat transfer coefficient decreases sharply at a critical oil concentration.
► Critical oil concentration is 0.5% for a 2-mm tube and 1% for 4- and 6-mm tubes.
► Both the heat flux and mass flux have positive influence on heat transfer coefficient.
► The oil concentration/heat flux does not influence the measured dryout parameters.
► Decreasing the tube diameter significantly increases the heat transfer coefficient.