Simulations And Optimizations For Leakages In CO₂ Expanders Based On LBM

The research status of the structure and performance optimization of carbon dioxide expanders was investigated.On this basis,it was pointed out that the problem of leakage of working fluid is an important part of the performance loss of expander.Subsequently,the rolling rotor type two-cylinder expander was studied.After introducing its basic structure and working process,a feasible dimension design calculation scheme was proposed.Numerical calculation and analysis were carried out to obtain the changing physical properties of the working fluid in the expander.Subsequently,a calculation method to obtain the length of the leakage line,the leakage quantity and the change of the physical properties of the working fluid during the external leakage process was proposed.Meanwhile,the analytic analysis of the position change and structural motion of the leak position in the expander was carried out.A calculation method to obtain leakage quantity and the change of the physical properties of the working fluid during the internal leakage process was proposed based on the analytic analysis.Based on the design of the expander mentioned in the previous section,the leakage of each part and of each stage are calculated and counted.The external leakage of the suction stage of the expander was proposed as the main part of the leakage loss which accounts for 78.4% of the total leakage loss,far exceeding the sum of losses in other phases and other leakage.Therefore,a lattice Boltzmann model to study the blocking effect of lubricating oil on the external leakage in the suction process was introduced.The model automatically selects the adaptive time step according to the density change of the working fluid.The passive scalar model and Shan-Chen model were combined into the model to ensure the influence of the force between the phases in the two-phase leakage process while ensuring the pressure difference as the main driving force of the flow field.The concept of leakage ratio was also introduced.The model was tested using a low-pressure leakage,six phase distribution patterns generated in the simulations and the influence of them on the leak rate were listed.After the completion of the model,the model was applied to the external leakage process in the suction phase of the expander designed in a previous section,and under different lubricant inflows,leakage widths and back pressure of the equipment,multiple sets of simulation tests were performed.The results of simulation showed that the optimal lubricant input volume ratio has only a slight change under different leakage width and back pressure;Under optimized situations,the lubricating oil can reduce the mass leakage flow of the working medium by about half,and the loss of lubricating oil can be at a low level;The concentration of the liquid phase in the leakage field decreases as the leakage width increases.However,the decisive factor of the total leakage is still the leakage width of structural gaps of the expander.Therefore,only by further improving the machining accuracy of the equipment and reducing the value of the width of leakage gaps can the working fluid leakage be truly reduced by a large extent.