Parametric Studies on Dynamic Performance of Hybrid Airfoil Bearing

Abstract

Airfoil bearings offer many advantages over oil-lubricated bearings, but they have reliability issues during start∕stops (wear) and limited heat dissipation capability. To address these issues, a hybrid airfoil bearing (HAFB) combining hydrodynamic airfoil bearing with hydrostatic lift was introduced previously by one of the authors of this paper. Their studies show that HAFB has superior performance compared to its hydrodynamic counterpart in load capacity and cooling performance. In this article, the bearing stiffness and damping coefficients of HAFB are calculated using a linear perturbation method developed for HAFB. Simulations showed that feed parameter and supply pressure affect the dynamic characteristics of HAFB. With an increase in either the supply pressure or the feed parameter, the rotor centers itself and hence one sees a decrease in direct stiffness. Simulations showed that the cross-coupled stiffness could be reduced by increasing either the supply pressure or the feed parameter. Direct damping showed increasing trend with the supply pressure and the feed parameter. Frequency-domain analysis of the bearing coefficients was also performed. The direct damping showed marginal changes with supply pressure but showed rapid increase with increasing excitation frequencies. The damping converged to null values for all the pressures for supersynchronous excitations. The loss in damping with high stiffness values for high frequency excitation is a typical hardening effect of gas bearings. In almost all the cases, there are rapid decreases in cross-coupled stiffness and damping and the values show converging trends in supersynchronous regime.

Experimental Identification of Force Coefficients of Large Hybrid Air Foil Bearings

“…8 and Fig. 9 overlapped with Kxx calculated using the linear perturbation method [8,10] at zero excitation frequency. In Fig.…”

“…Kumar and Kim [10] present the simulated force coefficients of single pad HAFB with four orifice feeds using the linear perturbation method. The investigation reports the direct and cross coupled stiffness decrease with either supply pressure or feed parameter while direct and cross coupled damping increase with either supply pressure or feed parameter.…”

Evaluation of Foil Bearing Performance and Nonlinear Rotordynamics of 120 kW Oil-Free Gas Turbine Generator

“…To extend AFB to larger systems than current applications, hybrid foil bearing combining traditional AFB with hydrostatic injection was introduced [17][18][19][20][21], and their enhanced performances in load capacity at low speeds and superior cooling capacity were demonstrated [19,21]. A component level thermohydrodynamic behavior of AFBs was also investigated by many researchers [22][23][24][25][26][27][28][29].…”

Gas foil bearings with radial injection: Multi-domain stability analysis and unbalance response

“…Theoretically, the predictions indicated a significant improvement of the Onset Speed of Instability (OSI). In [28] the authors investigated the bearing force coefficients as a function of pressurized injection, and in [29], a three-pad preloaded HGFB with three orifices is designed which is theoretically as well as experimentally analysed in a number of papers. Experimental wear testing during 1000 start-stop cycles was carried out by Kim and Zimbru [30] with loads up to 445 N at 10 kRPM without visible wear for a 356 N load.…”

“…The isentropic inflow assumption for injection in gas bearings can be traced to as early as 1964 where Lund [32] calculated the orifice mass flow in a hydrostatic gas journal bearing using these formulas. More recently the same assumption has been used by several authors, among these [26,28,29,[33][34][35][36][37][38][39][40][41]. It is important to recall that the flow is assumed to be restricted at a certain choking area and that the flow through this cross section is considered uniform.…”