Research On The Optimal Design Of Swirl Brakes And Its Influence On The Performance Of Annual Seal

The annular seal is a key component in pumps as well as other rotating fluid machineries and has been widely used in mechanic engineering since its function is to prevent flow leakage and improve the whole system’s efficiency.As the demand on the properties of working medium and operating parameters increases,the operating instabilities in systems induced by seals become more and more significant.Sealing technology with swirl brakes is one of the important methods to suppress the rotor instability caused by the vibration force of sealing fluid.The optimization design of swirl brakes is related to many factors such as multi-parameters,multiconditions,multi-objectives and so on.However,most of the current optimization design was based on the deterministic single-point method with few on the multi-point way.In present work,in order to improve the sealing performance and the running stability of the rotor,the annular seal with swirl brakes was selected and more further optimization design has been carried out for the swirl brakes based on the multi-parameter,multi-condition and multiobjective optimization methods.Moreover,the whole optimization design was carried out based on the numerical simulation combined with orthogonal and uniform experimental design,prediction model as well as multi-objective genetic algorithm.The main work and conclusions are as follows:1.Firstly,the research background on the annular seal with swirl brakes from home and abroad were reviewed,and the future research on the optimization design of swirl brakes based on multi-condition and multi-objective optimization methods shows important.The relevant theory and method of numerical calculation were briefly introduced.Moreover,the theoretical model of the dynamic characteristics of the annual seal with swirl brakes was described in detail,which provides a significant theoretical basis for the future research.2.Secondly,based on the SST k-ω turbulence model,the numerical simulation was carried out for the annular seal with swirl brakes,and compared with the corresponding experimental results.Meanwhile,the effect of the geometrical parameters of square groove(such as the length l,height h,thickness t and number p)of the swirl brakes on the static and dynamic characteristics of the annual seal was analyzed.The main conclusions are as follows:(1)The static characteristics of the annular seal with swirl brakes were analyzed: by adding the swirl brakes in the annular seal inlet,the pressure difference between the inlet and the outlet of the annular seal as well as the fluid excitation force in the seal decrease,and the flow leakage is therefore reduced;meanwhile,the circumferential velocity of the leakage flow will change together with the flow resistance in the seal clearance increasing,so the circumferential velocity and the pre-swirl ratio at the seal inlet decrease.(2)The dynamic characteristics of the annual seal with swirl brakes was analyzed: by adding the swirl brakes in the annual seal inlet,the cross-coupled stiffness coefficient of the annual seal decreases while the direct damping coefficient increases,and the eddy coefficient decreases.(3)The effects of each geometrical parameters as selected on sealing performances were analyzed and compared,respectively,and the results are as follows: the length and number of square grooves have the largest influence on the static and dynamic characteristics of annual seal.The flow leakage of the annular seal increases as the length of the square grooves increase,while decreases with the number of the square grooves increasing;the inlet pre-swirl ratio of the annular seal decreases when the length and number of square grooves increase where the cross-coupled stiffness coefficient decreases while the direct damping coefficient increases;the eddy coefficient decreases showing the operating stability of the rotor improved.3.Thirdly,the parameters of the length l,height h,thickness t and number p of the square grooves have been chosen as the optimization design variables for the annual seal with swirl brakes under different working conditions with the rotor speed ω equal to ω=2000rpm,4000 rpm,6000rpm as well as 8000 rpm,respectively.Moreover,the minimize weighted averaged flow leakage and inlet pre-swirl ratio were selected as the optimization objectives.Based on the orthogonal experimental design and range analysis of each parameters,the combinations of the optimal parameters were finally set as two different design schemes named Opt 1 and Opt 2,respectively.The static and dynamic characteristics of the annual seal with swirl brakes before and after the optimization design were compared.The main conclusions are as follows:(1)Based on the scheme Opt 1 with the flow leakage as the optimization objective,the weighted averaged flow leakage of the optimized annular seal decreases by 11.01%,while the weighted averaged inlet pre-swirl ratio increases by 12.2%.It can be obtained that the optimized swirl brakes have slight effect on the dynamic characteristics of the annular seal.(2)Based on the scheme Opt 2 with the inlet pre-swirl ratio as the optimization objective,the weighted averaged leakage of the optimized annular seal decreases by 1.38% and the weighted averaged inlet pre-swirl ratio decreases by 20.4%.Under the working conditions where the pressure difference between the seal inlet and outlet is equal to(35)P =2.07 bar with the eccentric distance e equal to102.87μm,and the rotor speeds ω are equal to 2000 rpm,4000rpm,6000 rpm and 8000 rpm,respectively,the cross-coupled stiffness coefficient of the annular seal after optimization decreases by 5.72%～10.15%,and the direct damping coefficient increases by 7.67%～22.67%,which shows that the eddy coefficient has been reduced and the performance of the annual seal has been improved in the various ranges with the operating parameters changing.4.Furthermore,in order to establish the prediction model between the optimization objective and the optimization parameters in the optimization design of the annual seal with swirl brakes,the parameters of the length l,height h,thickness t and number p of the square grooves the annual seal with swirl brakes were determined as the optimization variables,under the working conditions when the rotor speeds ω are equal to 2000 rpm,4000rpm,6000 rpm and8000rpm,respectively.By changing the sizes of each parameters an appropriate mixing uniformity test table was designed together with the combination of the application of numerical simulation and uncertainty analysis,a prediction model was built between the optimization objective and parameters.The predictive ability of the three models is compared and analyzed.The results show that the prediction of BP neural network model is more accurate among others and can be used as an appropriate evaluation function in the future optimization algorithm in present work.5.Finally,the optimization combination of the optimal parameters was obtained as the scheme Opt 3 where the corresponding parameters of the square grooves for the swirl brakes are: the length l=6.985 mm,the height h=7.458 mm,the thickness t=3.954 mm,the number p=40based on the BP neural network prediction model together with the application of the multiobjective genetic algorithm method and the optimal parameter combination as well as the Matlab.The static and dynamic characteristics of the annular seal were analyzed before and after optimization.The main conclusions are as follows:(1)The prediction model using BP neural network is much more accurate.(2)After optimization,the weighted averaged flow leakage of the annular seal has been reduced by 8.69% with the weighted average inlet pre-swirl ratio reduced by 16.33%.(3)Under the working condition as selected,the cross-coupled stiffness coefficient of the annular seal decreased by 8.11%～17.97% while the direct damping coefficient increased by20.80%-35.94% after optimization where the eddy coefficient has been further reduced with the sealing performance of the annular seal improved in the various ranges with the changing operating parameters.The optimization effect on the annual seal performances has been significant.(4)The comparison of the seal performances between different optimization schemes(such as Opt 1,Opt 2 and Opt 3)were analyzed.It shows that the optimization scheme of Opt3 has the best optimization effect on the overall static and dynamic characteristics of the annual seal with swirl brakes.