An Experimental Study of Subcooled Film Boiling on a Vertical Surface—Hydrodynamic Aspects

Interface and liquid velocities near the leading edge of a vertical wall 6.3 cm wide and 10.3 cm high were measured during subcooled film boiling of water at 1 atm pressure. The interface and liquid velocities in the boundary layer adjacent to the interface were measured using the hydrogen bubble flow visualization method. Photographs taken from the front and side showed the existence of a finite vapor layer at the leading edge and the existence of ripples and large-amplitude waves (bulges) on the interface. The bulges and ripples did not slide on the interface but moved in unison with the interface. The wave amplitude and wavelength were also measured. For a given subcooling and wall superheat, the amplitude, the interfacial velocity, and the wavelength were found to attain an equilibrium value several millimeters downstream of the leading edge. The waves were highly nonlinear and the interface velocities, which are found to be governed by the wave amplitude, were much larger than those predicted from the smooth interface, laminar flow theory. Streamlines in the liquid were found to expand into the wave valleys. At the wave peaks the streamlines appeared to be clustered together and the measured interface velocity gradients were high. The overall picture is one of expansion in the wave valleys and contraction (of flow) at the wave peaks. The flow field in turn is found to affect the liquid side heat transfer in subcooled film boiling significantly.