Horizontal flow boiling of carbon dioxide inside a conventional tube and parallel minichannels of a square cross-section

Carbon dioxide is a good alternative for replacing hydrofluorocarbons (HFCs). As a result of this the present study deals with the experimental investigation of flow boiling of CO2 2 inside a 4.57 mm diameter tube and parallel 2 mm square cross-section minichannels operating with mass velocities ranging from 140 to 1000 kg m -2 s-1, -1 , pressures from 2 to 4 MPa and heat fluxes up to 85 kW m-2. -2 . A high-speed camera allowed recordings of flow patterns during flow boiling inside the parallel minichannel test section. For the conventional tube the tests were performed at high reduced temperatures, around 0.90, and mass velocities of 660 and 900 kg m -2 s -1 and heat fluxes up to 75 kW m-2. -2 . The heat transfer coefficient decreased for lower quality than the expected dryout incipience and the nucleate boiling was the dominant heat transfer regime. For the parallel minichannels, the heat transfer coefficient showed a considerable dependence on the heat flux, indicating nucleate boiling dominance over convective boiling. The images obtained in this study show active nucleation sites at the bottom of the channel for the parallel minichannels.