Flow boiling was investigated on a hydrophobic surface by coating Teflon® onto a 1×1 cm2 copper surface, resulting in contact angle of 118°. The images depicted were taken using distilled water flowing at 299 kg/m2s with 3°C subcooling. In the first series, the number of active nucleation sites increased as heat flux increased. For lower values of heat flux (< 80 kW/m2), vapor bubbles remained almost stationary on the surface. The hydrophobic contact angle makes the horizontal component of surface tension force act radially outward, causing the bubble interface to grow. This leads to increased triple contact line and increased vertical component surface force. The buoyancy force due to the vapor bubble volume appears to be insufficient to overcome this vertical force for liftoff. This explains the stationary bubbles observed at the lower heat fluxes. The bubbles show an increase in size and number with heat flux. After this increasing trend, the bubble continues to grow larger when heat flux is higher than 80 kW/m2, eventually leading to the dryout at 117.5 kW/m2. The later bubble growth at high heat fluxes is caused primarily by the coalescences of neighboring bubbles. These larger bubbles are more affected by flow induced drag forces and move downstream. This can be seen in the lower sequential series at 100 kW/m2. The larger vapor masses slide across the surface, continue to absorb smaller bubbles as they move downstream, and are swept off the surface.

This content is only available via PDF.