Bubble behavior and its contribution to heat transfer of subcooled flow boiling in a vertical rectangular channel

In the present work, a visual investigation was carried out to study the bubble behavior including growth, sliding and coalescence on subcooled flow boiling in a narrow channel with a gap of 2.0 mm. At low heat flux, it was found that the bubble grew up rapidly on its nucleation site and then condensed subsequently at the pressures of 0.1 MPa and 0.3 MPa. Under the system pressure of 0.6 MPa and 1.0 MPa, the bubble generated and slid continually along the heating surface with very slow growing. At low pressure of 0.1 MPa and 0.3 MPa, the bubbles coalesced occasionally during growing on different nucleate sites and then condensed by subcooled bulk flow. At high pressure of 0.6 MPa and 1.0 MPa, the coalescence of bubbles mainly occurred during sliding on the heating wall with high heat flux. According to the energy balance, it was found that the sliding process of bubble had significant contribution to the enhancement of heat transfer than that that of stationary bubble. The bubble coalescence was analyzed, and found that the bubble growth rate and the bubble sliding velocity played an important role on bubble coalescence with different pressures.