Investigation of passive bearing for biomedical applications

• Main Author: Ho, Ivan Jun Hui
• Other Authors: Chan Weng Kong
• Format: Final Year Project
• Language: English
• Published: 2015
• Subjects: DRNTU::Engineering::Mechanical engineering::Fluid mechanics; DRNTU::Engineering::Bioengineering
• Online Access: http://hdl.handle.net/10356/64033
• Institution: Nanyang Technological University

The area of interest is to maintain an axial clearance gap of 0.2mm between the centrifugal rotating impeller and stationary housing so minimum power is required to drive the impeller and hydrodynamic lift, hence it was proposed to use a Generalized Reduced Gradient (GRG) algorithm to optimize the parameters obtained from Muijderman’s equation [1]. Due to abnormal phenomena observed in previous experimental results, investigation revealed that erroneous eddy current sensor measurement technique and lack of temperature control within the fluid mixture contributed to the abnormalities. Improved procedures were proposed to rectify the errors. Verification using CFD simulations showed similar trend, phenomena and closely related experimental values. However, studies revealed that theoretical predictions differ greatly. In addition, different trends were observed when compared. Newly optimized SGBs performed poorly as compared to previous designs. Investigation of the issue concluded that the difference was due to the fact that Muijderman neglected inertia effects when deriving the equation. Additionally, Muijderman assumed that pressure varies linearly at the entrance of the groove and ridge whereas findings in CFD simulations show that the pressure actually fluctuates instead. Due to the findings, attempt to include the inertia term within Muijderman’s equation was carried out. The modified Muijderman’s equation was able to predict both positive and negative pressure generation trend while closely relating to experimental values. Optimization was reattempted and values obtained were closely related to Low’s k15 SGB parameters when run at 2100rpm with 0.1mm film thickness. However attempts to achieve positive pressure generation at 0.2mm film thickness was not successful, even after removing manufacturing constraints. Hence, it shows that if one wishes to achieve positive pressure generation, new hydrodynamic bearing designs need to be created. Various approaches were used after analysing results and findings obtained from CFD simulations. Seven different hydrodynamic designs were created using such approaches. One approach is by using a different spiral angle for the outer spiral curve while the spiral angle for the inner spiral curve remains (I14O16 and I14O18 bearing). Another approach is by varying the inner spiral angle from 14 degrees to 20 degrees linearly, by emulating a positive quadratic curve or a negative quadratic curve (I14O1420L, I14O1420QP, I14O1420QN, I16O1420L, I18O1420L). Results obtained from the various new designs showed improved pressure generation within the grooves as compared to previous SGB designs. Ultimately, I18O1420L bearing possesses the best performance among the new hydrodynamic bearing designs, as no sudden dip in pressure was experienced while offering potential of achieving positive pressure generation at 0.2mm film thickness. Additionally, improved fluid flow within the grooves and its inlet were also observed for I18O1420L bearing.