FLOODING AND CHURN FLOW IN VERTICAL PIPES

Experimental studies are reported on the flooding phenomenon and on the closely associated churn flow regime. Flooding experiments were carried out with air-water flow in a 32 mm dia vertical pipe with various forms of liquid outlet, namely a porous wall, a tapered outlet and a square-edged outlet. For the first time, downflow (penetration) measurements were made with the porous wall outlet and showed significant differences between penetration rates beyond flooding and the flooding rate. This contrasts with other types of injectors, where the mechanisms of flooding are somewhat different. Measurement of pressure drop and holdup in the churn flow regime were made both with and without a co-existing falling film below the liquid injector. These showed that the falling-film and churn flow regions were essentially decoupled. Analysis of the data for churn flow showed that the minimum pressure gradient does not, for this data, correspond to the condition of zero wall shear stress as had been suggested by some earlier analytical studies. Interfacial shear stresses in churn flow were compared with those used in current reactor safety codes (TRAC and RELAP) and it was found that, for the churn flow region, the relationships used in the RELAP code were more appropriate.