| In the natural gas liquefaction process,multiple heat exchange units in a large cold box are assembled in parallel.The non-uniform flow deviation of the two-phase medium in the piping flow is often encountered in the parallel process of multiple production equipment.The maldistribution of medium flow between parallel heat exchange units can lead to serious consequences.A mild maldistribution reduces the heat transfer coefficient and economic performance of the system.A serious one causes the local fluid channel of the heat exchanger to be clogged or evaporated,and the equipment temperature changes rapidly,increasing the thermal stress of the cold box structure and causing damage to the internal components.This paper studies the physical mechanism,influencing factors and optimization schemes of the medium maldistribution caused by the piping system in the LNG cold box.Four parts are covered in this study,including flow maldistribution of single-phase fluid in diversion and converging pipeline systems,flow maldistribution of homogenous two-phase fluid in diversion pipeline systems,component maldistributions of two-phase flows in T-tube,and flow maldistributions of two-phase flows in T-tube and diversion pipelines for annular and stratified flow regimes.According to the structural characteristics of the plate-fin natural gas cold box and the phases and flow patterns of the fluids in different stages of the liquefaction process,the flow process of the natural gas in the liquefaction cold box is decomposed into fivetypical processes,including the parallel shunting and confluence of the single-phase fluid,split parallel flow of mist flow,fractionation and distribution of mist flow in T-tubes,fraction and distribution of annular flow in T-tubes,and split parallel flow of annular flow.The physical mechanisms and distribution characteristics that caused the maldistributions in these five typical processes are studied respectively,and the main influencing factors and optimization strategies of medium flow distribution among the parallel heat exchange units in the natural gas liquefaction cold box are obtained.The flow distribution characteristics of single-phase and mist flow in parallel shunt branches are studied numerically.The results show that along the flow direction of the mother tube,the static pressure gradually increases,the dynamic pressure gradually decreases,and the total pressure slightly decreases for right-angle T-type parallel shunt piping system.A large reverse flow vortex is generated near the right angle T-joint inlet of the branch pipe in the upstream direction of the main pipe,the vortex region and strength in the downstream branch pipe are smaller,and the flow resistance in these branch pipes decreases.Since the flow rate of the pipe is determined by the pressure difference at both ends of the pipe section and the flow resistance in the pipe section,the flow ratio at the outlet of the downstream branch pipe gradually increases.Therefore,the inconsistency of pressure drop of each branch pipe and the internal resistance coefficient are the direct reasons for the maldistributions of the branch pipes in the right-angled T-joint shunt piping system.This is also the hydrodynamic mechanisms that lead to flow maldistributions between the parallel units in the cold box.The flow characteristics and distribution uniformity of the liquefied natural gas flowing through the right-angled T-junction and elbow transition and in the manifold channel are studied numerically.With increasing mass flux in the main pipe,the dynamic pressure on the axis of the converging system increases along the flow direction,and the static pressure gradually decreases.The pressure drop in the branch pipe is low,and the flow resistance coefficient of each branch is nearly the same.The inhomogeneity of flow between each branch is mainly caused by the change of pressure drop,and the resistance loss caused by vortexes mainly occurs in the main pipe.The connection type of the branching and converging pipes is optimized to be an elbow transition structure.For this novel design,the pressure drop and resistance characteristic coefficient of each branch pipe are nearly the same.The uniformity of flow in branch pipes is enhanced,while the pressure drops of the whole system are reduced.The vector of the flow velocity in the elbow transition system changes greatly,resulting in a greater resistance loss in the branch pipe than in a right-angled T-junction system.But the resistance loss in the mother tube and entire piping system is reduced,and the flow distribution is more uniform.The maldistribution of phase components separation and liquid-phase flow rate for the mist flow regime of the flow splitting process in T-tubes are studied.The inlet recirculation zone occurred in right-angle T-junction branch pipe causes a low-concentration annular zone,resulting in parallel distribution of liquid phase content in the lower part of the branch pipe.The standard deviation of non-uniformities for fraction and flow ranges from 0.1 to 0.4.The gas flow velocity vector in the left side of the junction between the main pipe and the straight pipe has changed too much,causing a significant difference in gas-liquid two-phase velocity.As a result,the liquid phase buffers to the right side of the lower branch and is distributed to the right,leading to separation of the components of the gas-liquid two phases.The general rule of separation is that both the liquid concentration and the flow uniformity decrease with the increase of the velocity and the liquid phase particle size.The use of full-bend and half-bend transition joints instead of the right-angled T-shaped structure between the main pipe and the branch pipe eliminates the left recirculation zone of the lower branch pipe,increases the flow area between the main pipe and the branch pipe,and reduces the gas-liquid two-phase flow speed.It facilitates flow and components fraction uniform distribution.The flow and liquid-gas mass distributions of the two-phase mist flow in the piping system of multiple outlet branches are studied when the ratio of liquid phase content changes.The flow distribution of two-phase medium is the same as the single-phase fluid medium.Increased kinetic energy relate to increased flow rate and two-phase flow density,resulting in lower flow rates in the outlets at upstream and higher flow rates in the outlets at downstream for the right angle T-junction system.This also reduces the uniform distribution of the system.The total flow inhomogeneity index of the bend transition was significantly lower than that of the right angle T-junctionThe study shows that the liquid phase content at the outlet of each branch of the piping remained basically unchanged,indicating that the correlation between the total flow distribution and the mass components in individual branch is not strong.Thus,the two-phase flow problem for the piping flow can be decomposed into multiple outlets mass flow problems and the phase composition inside a single outlet problem.The main flow characteristics and distribution rules of the annular two-phase flow medium in the T-tube and multi-branch diversion pipe are studied experimentally,and the flow characteristics of the two-phase mixture in the liquefaction process of natural gas are analyzed.The experimental results show that the flow rate of the parent and branch pipes varies linearly with the inlet liquid-phase flow rate when the gas-phase flow ratio is distributed equally in a single T-shaped pipe.While the gas-phase flow rate is fixed,the liquid-phase flow rate of the branch pipe increases with the increase of the gas-phase component fraction.The increase of the inlet gas phase flow rate leads to thegradual:increase of the liquid-phase distribution flow rate at the outlet of the main tube in the T-tube,while the liquid-phase flow of the branch tube increases first and then decreases.When the transition between the main pipe and the branch pipe adopts an elbow or a half bending structure,the flow resistance is reduced,and the two-phase and single-phase fluid media are distributed more uniformly.It also enhances the two-phase composition and flow uniformity within a single branch.These two connection structure can be used as a solution to the maldistribution of the flow between parallel heat exchange units in a natural gas liquefaction cold box. |
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