Transient CFD Analysis of Multi-Lobe Bearings at 60000RPM for A Gas Turbine

Abstract

In this paper the problem of a 3 lobe was selected which had the lobes placed at a distance of 120 degrees for transient CFD analysis . The rotation speed of the shaft was considered to be 60000 rpm. The results show that the presence of lobes highly affect the performance of the multi lobe as the critical quantities developed here are comparatively lesser to the other zones in the bearing. The unsteady analysis was carried with help of Ansys Fluent software. In this study , six time steps of 10, 30, 50, 70, 90 and 100 seconds are taken respectively. After 100 seconds the unsteady condition becomes steady. After 100 seconds the maximum values of static pressure, total pressure, static temperature and total temperature are 7.68e+06 Pascal, 440 K are respectively and becomes steady. The study concludes that at high rotational speeds, lobes must be given in bearings to have a good life span of the bearing. Nearly all heavy industrial turbo machines use fluid film bearings of some type to support the shaft weight and control motions caused by unbalance forces, aerodynamic forces, and external excitations from seals and couplings. The two primary advantages of fluid film bearings over rolling element bearings are their superior ability to absorb energy to dampen vibrations, and their longevity due to the absence of rolling contact stresses. The damping is very important in many types of rotating machines where the fluid film bearings are often the primary source of the energy absorption needed to control vibrations. Fluid film journal bearings also play a major role in determining rotadynamic stability, making their careful selection and application a crucial step in the development of superior rotor-bearing systems. Fixed-geometry bearings differ from tilting pad bearings in that the fixed-geometry has no moving parts, making the lobes or arcs stationary around the shaft. As the shaft is forced from its centered position under the downward load, the clearance becomes a converging-diverging wedge. Oil is supplied through two axial grooves located diametrally opposite each other at the horizontal split line. After entering the arc leading edge, the oil is drawn by shaft friction into the converging radial clearance where it is compressed to a much higher pressure, giving the its load carrying capability. Notice that the shaft does not move vertically downward under the vertical load but, rather, also moves in the horizontal (positive X) direction as well. This is because of the cross-coupling effects that are inherent to fixed-geometry journal bearings. These effects can contribute to rotor dynamic instability in some applications StanislawStrzelecki(1) worked on Effect of lobe profile on the load capacity of 2-lobe journal bearing His main findings were:-The results of calculations of load capacity of 2-lobe journal characterized by different profiles of upper and bottom lobe. The load capacity of combined 2-lobe journal type 2-LCOF is smaller than the load capacity of 2-lobe and Offset-Halves one. At assumed type and aspect ratio an increase in lobe relative clearance causes the decrease in load capacity of combined and another considered 2-lobe bearings. Except of 2-lobe with offset upper half and cylindrical bottom one, the largest load capacity shows the 2-lobe journal bearing, particularly in the range of larger relative eccentricities. All considered 2-lobe bearings show small differences in the values of load capacity for the lower range of relative eccentricities of bearings. Strzelecki and Ghonheam(2) worked on Dynamically loaded cylindrical journal with Their main findings were:-The profile of the journal centre trajectory changes with the presence of recess in it. They considered two types of load one characterised by internal combustion engine and other by needle punching machine. They also calculate the journal centre trajectory with and hence found out various parameters oil film pressure distribution and oil film resultant force. They eventually found out that the trajectory is affected by the presence of recess. The presence of recess on the peripheral position of the affects the trajectory too, hence this method could be subsequently applied to the study of multi- lobe bearing. Ghoneam and Strzelecki(3) worked on Thermal problems of multilobe journal tribosystem Their main findings were:-They found an approximate method for finding the condition of the lubricating oil film temperature. Oil film temperature was obtained from the basis of the known quantities like Reynolds's number and viscosity equation based on empirical calculations and theoretical data.It could help in solving the problems related to 4-lobe with known parameters. The oil film temperature distribution and maximum oil film temperature have been obtained from the numerical solution of geometry, Reynolds, energy and viscosity equations. Memmott and Santiago(4) worked on A classical sleeve instability in an overhung Their main findings were:-They enumerated the use of 2- lobe lemon bore bearings to solve the problem. As sleeve bearings are incapable to solve the various conditions required to extensively increasing speed and vibration the introduction of the lemon bore with suitable lubricating oil have been proposed. The that was installed on thehigh-pressure side of the compressor actually worked as a seal. They also