Investigation of Surface Film Boiling under Free Convection : (1st Report, An Experiment on Small Wires)

Abstract

The experimental study on film boiling was undertaken to clarify how the behavior of bubble formation on the liquid-vapor interface depends upon the degree of subcooling and the size of heating surface, and to determine the influence of the stirring action of bubbles on film boiling heat transfer. The ranges of the experiment are as follows. Heating surface : horizontal platinum-wire of 0.5, 0.7 and 1.0 mm in diameter Test fluid : H₂O, CCl₄ Heat flux : (20∼80)×10⁴ kcal/m²hr for H₂O, (8∼50)×10⁴ kcal/m²hr for CCl₄ Degree of subcooling : 0∼15°C From the experiment performed, the following facts were found : (I) the assumption of smooth liquid-vapor interface, as often adopted in the theoretical analysis, can not be applied for low degree of subcooling ; (II) the behavior of bubble formation is affected by the size of heating surface ; (III) the distance between the bubble formation sites is larger than the critical and dangerous wave length determined by Taylor's instability ; and (IV) the effect of the size of heating surface on heat transfer coefficient can be correlated in a unified law by introducing the concept of equivalent Prandtl number.