Simulation Of Flow Distribution Characteristics Of Natural Gas Liquefaction Heat Exchanger Networks

Parallel heat exchanger system is a common form of heat exchanger application in industrial production.The uneven fluid distribution is the natural characteristic of the parallel system.And the change of fluid properties and phase in heat exchangers intensifies the complexity of fluid flow between heat exchanger units.In this paper,three plate-fin heat exchangers parallel operation is taken as the research object,and the simulation system of hydraulic and heat calculation of parallel heat exchanger system is established.Using the simulation system,the influences of structural factors,operating conditions and phase change on heat exchanger network flow distribution with Z type and U type parallel pipe are analyzed.Conclusion plays great guiding significance for the design,optimization and operation of parallel heat exchanger system.The main research contents are as follows:Based on the analysis of the structural characteristics of the parallel heat exchanger system,the principle of the parallel loop node balance is selected as the theoretical foundation.The physical model of parallel heat exchangers is divided into several nodes and extents,the fluid flow and heat transfer process in the heat exchanger unit is described by the hydraulic calculation equations and thermodynamic calculation equation.The relation of media parameters between nodes is expressed in matrix form.The simulation of coupling hydraulic and heating conditions of parallel heat exchangers with variable properties is realized.The model not only takes full account of the influence of pressure and temperature changes on:fluid properties,but also considers the changes of fluid temperature and mass gas content rate after fluid mixed through T-junction.The method and simulation system in this paper have good generality,which can be used to boiler,solar collector and other occasions of parallel heat transfer channels by replacing fluid physical properties and criterion correlation.Taking typical structure and working condition of natural gas liquefaction cold box as the object of research,the flow distribution characteristics of parallel heat exchanger system are simulated.The main conclusions are as follows:The characteristics of single-phase fluid flow distribution in Z-pipe parallel heat exchanger system:Increasing the diameter of main pipe and decreasing the length of main pipe both make flow tend to be uniform in parallel heat exchanger system.The smaller the heat exchanger resistance is,the more obvious the influence which structure of main pipe has on the flow distribution will be.The main pipe geometric parameters have small influence on the flow of last branch pipe,which mainly affects the flow distribution in front branch pipes.In the case of small Reynolds number,the unevenness of flow distribution increases greatly with the increase of Reynolds number as the flow resistance coefficient increases.When the Reynolds number is in the drag square area,the flow distribution unevenness is less affected by Reynolds number.The characteristics of gas-liquid two-phase fluid flow distribution in Z-pipe parallel heat exchanger system:When the structural parameters is same,the unevenness of gas-liquid two-phase fluid in parallel heat exchangers is larger than that of single-phase fluid,and uneven distribution is more serious.Increasing the diameter of main pipe still can reduce the flow distribution unevenness,but the decrease amplitude is less than that of single-phase fluid.The unevenness of flow distribution tends to decrease linearly with the increase of Reynolds number.When the heat flux density in the parallel heat exchangers grows synchronously to fluid phase transition,the unevenness will increase significantly.Compared with single-phase fluid,two-phase fluid flow distribution unevenness changes greater with the heat flux density.The characteristics of single-phase fluid flow distribution in U-pipe parallel heat exchanger system are different from Z-pipe’s:The main pipe geometric parameters have large influence on the flow of last branch pipe,which has small effect on the flow distribution in front branch pipes.The characteristics of gas-liquid two-phase fluid flow distribution in U-pipe parallel heat exchanger system:When the structural parameters is the same,the unevenness of gas-liquid two-phase fluid in parallel heat exchangers is less than that of single-phase fluid,and the decrease amplitude of two-phase fluid flow distribution unevenness is larger than that of single-phase fluid by increasing the diameter of main pipe.The increase of unevenness with Reynolds number is slower than that of single-phase fluid.When the heat flux density in the parallel heat exchangers grows synchronously to fluid phase transition,the unevenness will decrease,and the change rate of two-phase fluid unevenness with the heat flux is the same as that of the single-phase fluid.Relations of flow distribution unevenness and impedance ratio in Z-type and U-type piping are got.When condition parameters are certain,I compare the system pressure drop and flow distribution unevenness in Z-type piping with those of U-type piping,which can provide references for piping design.