Study on the behavior of a wetted area right after liquid-wall contact in pool film boiling

摘要

The behaviors of the rewetting on pool film boiling, focusing on observations of a collapse of a vapor film, a wetted area and a contact angle on a superheated surface were investigated experimentally. Using a U-shaped platinum wire with 2 mm diameter and 140 mm long, pool film-boiling experiments were performed for saturated water at atmospheric pressure. After a stable film-boiling state at a prescribed initial wall temperature was established, saturated water from a nozzle, set above the test wire, was injected on the superheated surface in the stable film-boiling, then artificial rewettting was forced on the superheated surface. During forming the artificial rewetting, the temperature fluctuation right under the liquid injection and the behaivor of the rewetting were obtained for the initial wall temperatures of 600, 420, 400 and 300 degree-C, respectively. The present experimental results showed that the propagative collapse of the vapor film occurred when the initial wall superheat was below 300 K. On the other hand, above the initial wall superheat of 320 K, the rewetting declined after liquid injection was finished then film boiling recovered. the temperature was related to the thermodynamic limit of superheat, according to the measured temperature fluctuation during supplying the liquid. The wetted area right after liquid-wall contact and the advancing velocity of the rewetting front increased as the initial wall superheat decreased. The measured angles between the liquid-vapor interfacial line and the heated wall corresponded to dynamic contact angles were close to the dynamic advancing contact-angles in room temperature.