Optimization Of Ejectors Based On Entropy Production Theory

In the process of gas field exploitation,the decay of wellhead pressure is widespread.And there are both high-pressure and low-pressure wells in a filed at the same stage,especially in the middle and the late,due to the variety position of stratum and the different period of exploitation.The common gathering and transportation processes in gas fields exist the problems of severe throttling loss and high operating cost of compressors.To pressurize the low-pressure gas directly by high-pressure gas through using ejector can reduce energy consumption and investment.In addition,the use of ejector can reduce the wellhead back pressure making the production of low-pressure well boosted.However,there are two outstanding problems in the application of ejectors in gas fields: low entrainment ratio and inability to adapt to the change of working conditions.The main reason for the low entrainment ratio is that the structure of ejector designed by semi-empirical theory is unreasonable and the irreversible loss is heavy.Therefore,adjusting the structure of the ejector,reducing its internal irreversible loss,and studying its operating law under variable working conditions are of great significance for its application in gas field gathering and transportation.Based on the theoretical research,a two-dimensional numerical simulation model of the ejector was established by using FLUENT.The model is validated by relevant experiments and the influence of natural gas viscosity on the simulation results is considered.Based on the numerical model,the operation law of the ejector under different working conditions is analyzed,and the working characteristic layout of the ejector is studied.In order to ensure that its entrainment ratio is not less than 20%,it is recommended that the applicable range is: the expansion ratio is 4～10,and the compression ratio is not higher than 2.1.Through the analysis of the simulation result trace map,it is found that the main cause of the shock of the ejector diffusion chamber is the formation of the vortex at the entrance.The irreversible loss inside the ejector is analyzed by the entropy production analysis method.Turbulence entropy production accounts for 97% of total entropy production,and the total proportion of viscous entropy production,mean temperature gradients entropy production,fluctuating temperature gradients entropy production and wall entropy production is less than 3%.Entropy production is mainly caused by shock waves,vortices and recirculation inside the ejector.Under normal working conditions,the entropy production mainly concentrates at mixing chamber and the entrance of diffusion chamber;under abnormal working conditions,the entropy production mainly concentrates in the suction chamber.The influence of nozzle area ratio,nozzle distance,the length and obliquity of diffusion chamber,the length and diameter of cylindrical section and obliquity of conical section on entropy generation is analyzed by using control variable method.The most important structural parameters affecting the value of entropy production are nozzle area ratio,nozzle distance and diameter of cylinder section.For the above three parameters,factor ranking and level optimization are performed by orthogonal simulation method.The order of the three factors is: cylinder diameter,nozzle area ratio and nozzle distance,and the optimum values of the three parameters are 25.4mm,1.51 and 54 mm respectively.Under the same conditions,the optimized structure turbulence entropy production decreases by 50.82%,and the ejector rate increases by 189.98%.Entropy production mainly distributes at the entrance of mixing chamber,and the distribution of entropy production in diffusion chamber decreases obviously.However,whether the optimized structure can truly achieve this theoretical value still needs to be verified by relevant experiments.