Thermo-hydraulic Instability Due to a Blockage of Flow by Vapor in a Boiling Two-phase Flow System (Experimental Results Concerning the Mechanism) : Heat Transfer, Combustion, Power, Thermophysical Properties

Abstract

Thermo-hydraulic instability has been investigated experimentally in a boiling two-phase flow system composed of a vertical N-shaped boiling channel and an adiabatic bypass between an inlet plenum and an outlet plenum, using Freon 113. Instability caused by the blockage of flow by vapor, which cannot be found in Boure's classification of two-phase flow instabilities, has been observed. The flow maps in each section, static characteristics, stable flow limits and characteristics of this instability have been studied under various conditions to clarify the mechanism. The inlet velocity decreases when a vapor slug combining in the inverted U-shpaed bend obstructs the flow. The vapor slug flowing into the downcomer increases the void fraction in the downcomer, resulting in increased gravitational loss. Consequently, flow instability is brought about.