Numerical Simulation Of The Gas-liquid Flow On The Surface Of Structured Packing Under Vertical And Inclined Conditions

Floating Liquefied-natural-gas Production Storageand Offloading(FLNG)is a new offshore natural gas production model and has become a research hotspot in ocean engineering.The natural gas pretreatment process in the upper module determines the final quality of LNG and plays a crucial role in ensuring the safety of downstream liquefaction process.As the key equipment in gas-pretreatment process,the packing cloumn has widely used in gas absorption field.The packing inside the tower provides solid wall conditions for gas-liquid two-phase contact,which is conducive to the completion of mass transfer and chemical reaction.When the packed tower is applied to FLNG,the gas-liquid two-phase flow will be disturbed by external tilt and movement,which will affect the absorption process of natural gas.Therefore,it is very important to study the gas-liquid two-phase flow of packed tower in non-vertical state.This paper focuses on the flow on the packing surface at the micro scale.The Mellapak 250 X is taken as the research object,which is the most commonly used orifice structured packing in industry.A CFD numerical model and algorithm are established to study the problem of falling film flow on two-dimensional "Z" model and three-dimensional inclined plate.On this basis,the sensitivity analysis of the numerical grid and the time step and the accuracy verification of the algorithm based on the three-dimensional plate model are carried out.In this study,the VOF model in computational fluid mechanics was used to capture the gas-liquid two-phase flow state and the interface changes,focusing on the concept definition and analysis method of the important indexes(liquid holdup,average liquid film thickness and effective contact area of gas-liquid two-phase)related to the gas-liquid two-phase flow state and reflecting the performance of the packing.On this basis,the CFD numerical simulation of gas-liquid two-phase flow in the vertical state of the above two models is carried out at first.And then the research on the inclined state condition(the ultimate angle of hull movement)was carried out.The effects of gas velocity,liquid velocity,contact angle,surface tension,viscosity and tilt angle on falling film flow in the packing were investigated.The results show that the liquid velocity is the decisive factor for the surface flow of the packing and the gas and liquid phases show different flow patterns at different Reynolds numbers.The influence of gas velocity on the two-phase flow state is relatively small.Only when the gas velocity is high will the gas phase affect the liquid phase flow state.Therefore,the liquid velocity is the operating parameter that needs to be paid attention to in the practical engineering.The contact angle and surface tension will significantly change the wetting area of the liquid phase on the packing surface,thus affecting the contact area of the gas-liquid phase.In addition,increasing the viscosity can increase the liquid holdup of the liquid phase.Therefore,the characteristics of contact angle,surface tension and viscosity can be used as important references for the choice of packing material and purification liquid in practical engineering.The influence of tilt angle on liquid holdup,liquid film thickness and effective area in the packing with or without gas velocity is given,which can be used to guide the design of packing structure under the condition of inclination and movement.