Modelling of a Two Phase Water Hammer

Jensen, Rune Kjerrumgaard ; Larsen, Jesper Kær ; Lassen, Kasper Lindgren

4. term

Energy Engineering, Master

2018

2018-06-07

149

Vandslag er pludselige trykstød i rør, som opstår, når strømningen ændres brat. I en tofaset situation indeholder strømningen både flydende vand og små lommer af damp eller gas, hvilket gør trykforløbet mere komplekst. Denne afhandling simulerer tofaset vandslag med to tilgange: en forenklet 1D-model, Karakteristikmetoden (MOC), med to varianter – den diskrete dampkavitetsmodel (DVCM) og den diskrete gaskavitetsmodel (DGCM) – samt en mere detaljeret 2D CFD-model i et aksialsymmetrisk (rundt) rør. Modellerne blev sammenlignet med fire publicerede forsøg, der dækker forskellige rørdimensioner og strømningsforhold. Rayleigh–Plesset-ligningen, som beskriver boblers vækst og kollaps, blev undersøgt med henblik på at indarbejde den i MOC, men implementeringen lykkedes ikke på grund af krav til meget små tidssteg og tidsbegrænsninger. Samlet set klarede DGCM sig bedst som tofaset vandslagsmodel, fordi den gav præcise trykberegninger, var robust selv ved høj gas-/dampandel (hulrumsfraktion), og ramte timingen af svingningerne godt.

Water hammer refers to sudden pressure surges in pipes when the flow changes abruptly. In a two-phase situation, the flow contains both liquid water and small pockets of vapor or gas, which makes the pressure response more complex. This thesis simulates two-phase water hammer using two approaches: a simplified 1D Method of Characteristics (MOC) with two variants—the Discrete Vapour Cavity Model (DVCM) and the Discrete Gas Cavity Model (DGCM)—and a more detailed 2D CFD model in an axisymmetric (round) pipe. The models were compared with four published experiments covering different pipe sizes and flow conditions. The Rayleigh–Plesset equation, which describes bubble growth and collapse, was investigated for inclusion in MOC, but could not be implemented due to very small time-step requirements and time limitations. Overall, DGCM performed best as a two-phase water hammer model, providing accurate pressure predictions, robustness at high void fractions (gas/vapor content), and good prediction of oscillation timing.

[This abstract was generated with the help of AI]

MOC ; CFD ; Water Hammer ; to-fase ; Two phase

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