PhD Defence at DTU Mechanical Engineering

PhD Defence 23 March: "Nonlinear Analysis of Rotors Supported by Air Foil Journal Bearings – Theory & Experiment"

Monday 09 Mar 15

Jon S. Larsen from DTU Mechanical Engineering defends his PhD "Nonlinear Analysis of Rotors Supported by Air Foil Journal Bearings – Theory & Experiment" Monday 23th March at 14:00. The defence takes place in Building 101, room S09 at the Technical University of Denmark. Supervisors are Professor, Dr. Techn. Ilmar Santos from DTU Mechanical Engineering and PhD Frans Pløger from Essen, Germany.

Direct driven compressors supported by air foil bearings (AFB) are gaining increasing popularity, e.g. within the waste water treatment industry. At the same time, the demand for larger machines up to 250 kW is growing. In order to keep production costs low, the shaft and bearing design need to be simple and allow manufacturing using conventional materials and production facilities. As a consequence, the assembled rotor weight can be up to 50 kg.

The compressors are operated at variable speed and load and are subjected to several starts and stops per day. Therefore, the rotor bearing design must be robust with a good margin to rotor-dynamical instability. To ensure this, good mathematical models, capable of accurately predicting the dynamic behaviour the rotor-bearing system, are required at the design stage.

This thesis focuses on developing and improving existing mathematical models for predicting the non-linear dynamic behaviour of AFBs, both isolated and coupled with the rotor. Large emphasis is put on the experimental validation of the obtained theoretical results. Two experimental test rigs were designed and manufactured speci cally for this purpose.

With the first rig, the isolated non-linear mechanical behaviour of the bump foils was carefully examined. A mathematical model capable of predicting this non-linear behaviour was developed and compared to the experimental results with good agreement. With the second test rig, the overall non-linear behaviour of the rotor-bearing system was investigated experimentally, and the linearised stiffness and damping coefficients of the bearings were identified.

Results obtained theoretically were compared to these linearised coeficients with reasonably good agreement. Furthermore, the sensitivity to rotor unbalance and the non-linear response were analysed experimentally and a coupled rotorbearing mathematical model were compared to these results with very good agreement in terms of the non-linear rotor unbalance response and stability.