EXPERIMENTAL-STUDY OF THE 2-PHASE FLOW DYNAMICS IN NUCLEATE AND FILM POOL BOILING

An experimental study is conducted to measure the transient characteri stics of the local boiling process close to the heated wall. Nucleate and film boiling two-phase parameters were measured for power oscillat ions in a small horizontal heater immersed in stagnant liquid. A metho d for measuring fast temporal variations of void fraction and the inte rfacial impact rate is presented. Ensemble averages of the data have b een carried out to obtain time evolution of the void fraction and the interfacial impact rate at the probe tip. R-113, R-11 and water have b een used as working fluids, finding important differences between the behavior of the freons and water at the boiling crises. The critical h eat flux in freons is associated with a sudden transition in the impac t rate and the void fraction, which supports the theory of hydrodynami c instability. On the other hand, the experiments in water suggest the existence of two transitions: a hydrodynamic transition from bubbling regime to ''mushroom'' regime followed by a liquid film dryout type o f CHF, in agreement with the theory of Haramura & Katto.