Rollover and interfacial studies in LNG mixtures

Abstract

An experimental investigation into LNG rollover has been performed, using cryogenic liquids to simulate a two-layered LNG system. A vacuum insulated glass vessel was designed and constructed for rollover simulation experiments. Thin metal oxide coatings on the inner jacket of the vessel enabled the simultaneous heating and visualisation of the liquid in the vessel. Mixtures of liquid nitrogen and liquid oxygen were successfully used to form two differing density layers. An oxygen analysing system with an accuracy of 0.01% by volume oxygen, and a fifteen junction copper-constantan thermocouple array were used for primary measurements of mass concentration and temperature. For a number of initial density differences between layers, various liquid layer heating configurations were used to obtain the variations in evaporation flowrate, and detailed temperature and concentration profiles during experiments. Convective flow patterns in single and two-component liquid mixtures were obtained, using the Schlieren method. Results show that the mixing of layers is primarily due to entrainment of fluid from the intermediate layer separating the two convective layers. The intermediate layer can be described by Double-Diffusive convection theory, and controls the transport of heat and mass between layers. The measured peak flowrate is a function of the initial density difference between layers. The peak to equilibrium flowrate values are much lower than those reported in rollover incidents, due to the enhanced mixing processes occurring in the simulation mixture. A correlation between the evaporative mass flux and bulk fluid superheat fails during transient heating conditions, and cannot predict very high flowrates. Schlieren flow - visualisation studies clearly show various surface patterns for increasing surface evaporation mass fluxes.

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