Thermohydrodynamic Analysis of Gas Foil Bearings for High-Speed Turbomachinery

Abstract

High-speed turbomachinery such as turboexpander, turbocompressors, turbogenerators, and many more whose applications lie in aircraft, etc. needs to have better and high-quality performance with no compromises. If for example in the case of a turboexpander a high-speed is a must-have criterion in order to achieve desired performance output and thus imposes a constraint on a turbo-pair design. To avoid the issue of contamination and implement a cost-effective design the obvious choice is to switch from ‘liquid film lubrication’ to ‘gas film lubrication’. Thus, the use of gas bearings is popular in the past few decades due to its merits such as low frictional losses, low power consumption, ability to operate at high temperatures, and low noise operations, etc. However, the demerits associated with simple gas bearings are low stiffness and damping characteristics.
The current dissertation proposes the use of gas foil bearings as they possess the ability to tailor the stiffness and damping characteristics by modifying the compliant structure along with all the merits of simple gas bearings which further increases its scope of applications to a wider range. The mathematical model is formulated to study the thermohydrodynamic behaviour of both gas foil thrust and journal bearing accounting gas rarefaction effects. The non-linear Reynold’s and energy equation using first-order velocity slip terms are coupled together to evaluate the pressure and temperature distribution of the gas foil bearings. Additionally, the dynamic coefficients are predicted for gas foil journal bearing. Further, the use of temperature-dependent viscosity is used in non-isothermal Reynold’s equation for analysing the thermohydrodynamic behaviour of gas foil bearings under the influence of slip flow. The various performance parameters like load carrying capacity, power loss, frictional torque, and temperature is investigated and compared for both no-slip and slip flow phenomenon.
Additionally, the thermal analysis of rotor supported on gas foil journal and thrust bearing is also attempted in the current dissertation. The rotor is divided into different regions on the basis of a component surrounding it. The rotor of any turbomachine operating on gas bearings tends to have gas frictional losses. The thesis attempts to evaluate frictional losses for different regions of a rotor analytically along with the heat flow and convective heat transfer coefficient which acts as boundary conditions. The ‘Steady State Thermal’ module of the ANSYS WORKBENCH is used to predict the temperature distribution of the rotor. The effect of a rise in temperature on the structural behaviour i.e. thermal deformation and induced thermal strain of the rotor is also investigated. Also, the transient analysis of the rotor to predict the time-varying temperature is attempted.
Further, the fabrication methodology of various components is presented in the current dissertation to test the performance of gas foil bearings. The fabrication methodology of components like rotor dies to form bump foils, top foils, bearing assembly, and bearing housing is presented. Additionally, the test setup is also developed to test the thermal performance of gas foil bearings. The K-type thermocouples are attached to the gas foil bearings via NI temperature card. The various sensors and equipment’s like pressure transducer, NI vibration card, oscilloscope, accelerometers, high-pressure compressor air facility, and test bench to mount turboexpander’s bearing housing are used to fully develop the experimental test facility. The preliminary vibrational and thermal analysis is conducted on an experimental setup and found to be in close agreement with the numerical results.
Overall, the thesis highlights the importance of using a full thermohydrodynamic model instead of using just an isothermal Reynold’s and energy equation to predict the hydrodynamic and thermal characteristics of gas foil bearings. The results also conclude that the no-slip assumptions underpredicts the performance of gas foil bearings and thus slip flow assumption should be involved while modelling. The temperature rises of the rotor due to low clearance bearings are also highlighted along with the methodology to predict it. Finally, the experimental investigation is carried out to validate the numerical models formulated in the thesis.