斯特林机活塞杆帽式密封泄漏率预测及其优化设计

Prediction and Optimal Design of Leakage Rate of Piston Rod Cap Seal for Stirling Engine

斯特林机活塞杆帽式密封处总有微量气体泄漏,但目前大多数研究是通过分析密封面接触载荷而开展的。为探究干摩擦状态下帽式密封泄漏问题,基于分形理论从微观角度对密封面形貌进行表征,结合环形轴向缝隙流体流动模型建立帽式密封泄漏分形模型。通过试验验证泄漏模型的准确性。根据已验证模型对泄漏率影响因素进行交互分析,获取工况条件对不同粗糙度密封面泄漏率的影响规律,并提出使用遗传算法对特定工况下的C型环密封表面形貌与O型圈的预压缩率进行最优匹配。结果表明:泄漏率随着O型圈的预压缩率增大而下降,随着工质压力及活塞杆速度的增大而增大,且密封面越粗糙时泄漏率变化幅度越大;当密封表面粗糙度增大时泄漏率增大,但密封面形貌越粗糙时,增大接触载荷泄漏率更易减小;最终确定当斯特林机的工质压力P1=5 MPa,活塞杆运动速度V=1 m/s时,应匹配密封面粗糙度Ra=0.27μm的改性聚四氟乙烯C型环,并设定O型圈的预压缩率W=23.13%,帽式密封泄漏率将会达到最低。所建泄漏模型可提前为密封件的选择做出决策,而优化设计方法可为后续密封件表面粗糙度的确定和加工方法的选择提供指导。

Stirling engine is an external combustion engine that reciprocates a piston by heating a gas working medium with an external heat source.Its internal working medium is high-pressure hydrogen or helium;therefore,there is always trace gas leakage at the piston rod cap seal,which significantly affects the reliability and operational efficiency of the Stirling machine.However,most current studies have analyzed sealing-surface-contact loads to obtain the best sealing performance by optimizing the contact loads;therefore,there is an urgent need to study the leakage problem directly.The contact between the cap seal and piston rod is not an ideal surface but two rough surfaces.The contact between the micro-convex body and the rough surface forms a leakage gap,resulting in a small amount of leakage at the seal.Therefore,the surface morphology of the cap seal is a key factor affecting the leakage rate.Based on fractal theory,the contact between the cap seal and piston rod is simplified as the contact between the rough surface and ideal rigid plane.The rough contact surface topography was characterized microscopically.The response surface center complex method was used to fit the functional relationship between the contact load of the sealing surface and the effective sealing length and working condition factors.The real contact area of the cap-sealing surface was calculated using the sliding friction surface contact mechanics model,and the leakage fractal model of the cap sealing was constructed by combining it with the annular axial gap fluid flow model.Based on the verified model,an interactive analysis of the influencing factors of the leakage rate was conducted,and the influence law of the working conditions on the leakage of sealing surfaces with different roughness values was obtained.A genetic algorithm was proposed to optimize the surface topography of the cap seal and the working parameters to minimize the leakage rate at the cap seal of the piston rod and ensure long-term effective operation of the Stirling engine.The results showed that the leakage rate of the cap seal decreased with an increase in the precompression ratio of the O-ring and increased with an increase in the working medium pressure and speed of the piston rod,and the rougher the sealing surface the greater the magnitude of the change in the amount of leakage.When the sealing surface is rough,the leakage rate decreases rapidly with an increase in the fractal dimension of the sealing surface or a decrease in the feature-scale coefficient.However,the leakage rate changed only slightly when the sealing surface was smooth.When the sealing surface roughness increases when the leakage increases,but the sealing surface morphology of the rougher,increase the contact load leakage is easier to reduce,this is because the rougher surface of the micro-convex body less,resulting in the sealing surface leakage channel increases,but the sealing surface bearing capacity will be reduced.Using the genetic algorithm with the lowest cap-seal leakage as the goal,the current in-use engineering seal surface morphology and Stirling machine working condition factors were used for global optimization in the genetic algorithm.Additionally,qualifying the genetic algorithm when generating the independent variables to prevent the generation of rougher or smoother sealing surface morphologies.Finally,it was determined that when the working medium pressure of Stirling machine is 5 MPa and the piston rod movement speed is 1 m/s,the modified polyteflon C-ring with a sealing-surface roughness of 0.27μm should be matched.Additionally,when the precompression ratio of the O-ring was set to 23.13%,the leakage rate of the cap seal was minimized.The leakage model can decide on the selection or continued use of seals in advance and realize safe,reliable,and more efficient operation of the Stirling machine.The optimized design method can provide guidance for the determination of the surface roughness of subsequent seals and the selection of processing methods to produce seals with the best sealing performance.