Thermodynamic Response Enhanced by Sloshing in Marine LNG Fuel Tanks: Experimental Work and Numerical Modelling

Abstract

This thesis presents studies of sloshing and how it influences thermodynamic conditions in liquefied natural gas (LNG) fuel tanks. Liquid in a moving tank mixes more efficiently with the gas, which condenses and is followed by a drop in pressure. This issue is relevant for the operation of LNG-fuelled vessels, where the system pressure may drop rapidly under severe motion. To compensate for the pressure loss, it may become necessary to derate the engines, or, in the worst case, perform a complete shutdown of the gas-fuel system. The long-term objective of the research is to improve the operational reliability of LNG-fuelled vessels by performing design improvements or operational measures. LNG has become increasingly popular as a marine fuel due to its low emissions compared to those from conventional fuels like heavy fuel oil (HFO) and marine diesel oil (MDO). Low-pressure LNG fuel systems are designed similarly to landbased storage facilities. The storage tanks are vacuum-insulated and pressurized such that heat ingress is minimal. But when the tank is pressurised, the liquid may be significantly colder relative to the saturated condition. The thermal equilibrium is controlled in the short term by the liquid due to its large mass. The sloshing enhances the internal energy transfer, and the final state corresponds to a state that is closer to the initial liquid temperature. This research is based on experimental work, analysis, modelling, and simulations. An experimental facility was designed and constructed, experimental tests were conducted separately, with a transparent tank for hydrodynamic studies and a pressurised steel tank for analysis of the thermodynamic response. Hydrodynamic sloshing tests were conducted with both rectangular and cylindrical tanks. Computational fluid dynamics (CFD) simulations of sloshing hydrodynamics were carried out with both tank geometries with the aim of replacing the hydrodynamic experiments to investigate any tank shape, inner structure, or motion. The resulting sloshing regimes were categorized and used systematically when preparing tests involving the thermodynamic response. Experiments with a pressurized steel tank were conducted using both liquid nitrogen (LIN) and water. A theoretical framework was developed and implemented into a lumped capacity model. The model provides a good starting point for development of correlations between motion parameters and the enhanced heat transfer. It can also be combined with other submodels into a system model. CFD simulations were found to represent sloshing with acceptable accuracy. New sloshing characteristics inside LNG fuel tanks are described. The thermodynamic response is influenced significantly by the severity of the sloshing. The final state depends on the initial liquid temperature and tank pressure, but the time to reach the final state depends on the sloshing intensity. The largest pressure drop rate occurs close to the primary resonance, f/f1,0 = 1. The PBU power needed to maintain the pressure was estimated from the measured pressure. It corresponds well with the PBU power used in the experiments. An existing CFD solver was modified to take into account the transport of thermal energy. The pressure drop was predicted with this model, and the result was found to correspond well with the experimental results for resonant sloshing. A case study is presented in which anticipated motion of a full-scale ship were used to simulate sloshing inside the LNG fuel tank. The L/D ratio is found to have a prominent effect on the sloshing, even at low frequencies.