On the Effect of Phase Transition on Impact Pressures due to Sloshing

摘要

Sloshing assessment for a new membrane LNG vessel relies onrnSloshing Model Tests (SMT) at small scale (scale 1:40 most of therntime) with 6 degree-of-freedom excitations reproducing the full-scaleshiprnmotions by Froude-scaling. Numerous local physical phenomenarninvolved during each impact do not follow a Froude similarity.rnGTT, in a patient R&D effort, aims to study each of them one after thernother, in order to limit the biases induced by the experimentalrnmodeling.rnRecent studies on the Density Ratio (DR) between the gas and thernliquid (Maillard et al., 2009) and on the compressibility of the gasrnphase (Braeunig, Brosset, Dias, Ghidaglia, 2009) have led GTT to usernsystematically a heavy mixture of gases instead of air during its SMT inrnorder to match the DR with the real one on board LNG carriers andrnreduce the compressibility bias.rnIn this communication, the objective is to address, in the context ofrnsloshing impacts, the complex situation of phase transitions between arnliquid and its vapor in thermo-dynamical equilibrium along the phasernboundary, as it is the case within tanks of LNG carriers.rnThrough a simple 1D semi-analytical model of a gas pocketrncompression, the influence of phase transition is analysed in the contextrnof general fluids with application to LNG/NG, different thermalrnboundary conditions and different scales.rnThe model is first requested to explain qualitatively the trends observedrnexperimentally during SMT with water and steam in a pressure vessel:rnthe significant reduction of the statistical impact pressures and therndisappearance of the pressure oscillations when gas-pocket impactsrnoccurred.