The effect of liquid-solid contacts on the low limit of film boiling.

An experimental study was conducted to investigate the effect of liquid-solid contacts on the low limit of film boiling in water under atmospheric pressure. The test specimens selected for the present experiments were a silver sphere (30 mm in diameter) and two kinds of silver cylinders (20 mm in diameter and 60 mm long). The heat transfer surface was coated with a thin, electrically insulating layer, and the impedance between the metal substrate and the boiling liquid was measured. This impedance was related to the average fraction of the heat transfer area which was wetted by the liquid. Direct liquid-solid contacts were observed on local portions of the heated surface in saturated film boiling, and the frequency of contact ranged between 1.0 and 3.1 Hz. In subcooled boiling, however, the contact frequency became extremely low with higher subcooling.