On the Effect of Phase Transition On Impact Pressures Due to Sloshing

Sloshing assessment for a new membrane LNG vessel relies on Sloshing Model Tests (SMT) at small scale (scale 1:40 most of the time) with 6 degree-of-freedom excitations reproducing the full-scaleship motions by Froude-scaling. Numerous local physical phenomena involved during each impact do not follow a Froude similarity. GTT, in a patient R&D effort, aims to study each of them one after the other, in order to limit the biases induced by the experimental modeling. Recent studies on the Density Ratio (DR) between the gas and the liquid (Maillard et al., 2009) and on the compressibility of the gas phase (Braeunig, Brosset, Dias, Ghidaglia, 2009) have led GTT to use systematically a heavy mixture of gases instead of air during its SMT in order to match the DR with the real one on board LNG carriers and reduce the compressibility bias. In this communication, the objective is to address, in the context of sloshing impacts, the complex situation of phase transitions between a liquid and its vapor in thermo-dynamical equilibrium along the phase boundary, as it is the case within tanks of LNG carriers. Through a simple 1D semi-analytical model of a gas pocket compression, the influence of phase transition is analysed in the context of general fluids with application to LNG/NG, different thermal boundary conditions and different scales. The model is first requested to explain qualitatively the trends observed experimentally during SMT with water and steam in a pressure vessel: the significant reduction of the statistical impact pressures and the disappearance of the pressure oscillations when gas-pocket impacts occurred.