EFFECT OF PAD FLEXIBILITY ON THE PERFORMANCE OF TILTING PAD JOURNAL BEARINGS - BENCHMARKING A PREDICTIVE MODEL
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Tilting pad journal bearings (TPJBs) supporting high performance turbomachinery rotors have undergone steady design improvements to satisfy ever stringent operating conditions that include large specific loads due to smaller footprints, and high surface speeds that promote flow turbulence and thus larger drag power losses. Simultaneously, predictive models continuously evolve to include minute details on bearing geometry, pads and pivots configurations, oil delivery systems, etc. In general, predicted TPJB rotordynamic force coefficients correlate well with experimental data for operation with small to moderately large unit loads (1.7 MPa). Experiments also demonstrate bearing dynamic stiffnesses are frequency dependent, best fitted with a stiffness-mass like model whereas damping coefficients are adequately represented as of viscous type. However, for operation with large specific loads (> 1.7 MPa), poor correlation of predictions to measured force coefficients is common. Recently, an experimental effort [1] produced test data for three TPJB sets, each having three pads of unequal thickness, to quantify the effect of pad flexibility on the bearings force coefficients, in particular damping, over a range of load and rotational speed conditions. This paper introduces a fluid film flow model accounting for both pivot and pad flexibility to predict the bearing journal eccentricity, drag power loss, lubricant temperature rise and force coefficients of typical TPJBs. A finite element pad structural model including the Babbitt layer is coupled to the thin film flow model to determine the mechanical deformation of the pad surface. Predictions correlate favorably with test data, also demonstrating that pad flexibility produces a reduction of up to 34% in damping for the bearing with the thinnest pads relative to that with the thickest pads. A parametric study follows to quantify the influence of pad thickness on the rotordynamic force coefficients of a sample TPJB with three pads of increasing preload, rp = 0, 0.25 (baseline) and 0.5. The bearing pads are either rigid or flexible by varying their thickness. For design considerations, dimensionless static and dynamic characteristics of the bearings are presented versus the Sommerfeld number (S). Pad flexibility shows a more pronounced effect on the journal eccentricity and the force coefficients of a TPJB with null pad preload than for the bearings with larger pad preloads (0.25 and 0.5), in particular for operation with a small load or at a high surface speed (S>0.8).
