Session: 25-02 Annular Seals 2

Paper Number: 83356

83356 - Rotordynamic Performance of Smooth Liquid Annular Seals in the Laminar, Transition, and Turbulent Regimes Over a Range of Imposed Pre-Swirl Ratios 

Rotordynamic results are presented for smooth annular seals with a length-to-diameter (L/D) ratio of 0.50, the nominal radius R = 51.00 mm, at the nominal radial clearance C_r = 0.2032 mm.  Tests were conducted for angular shaft speeds, ω = 2, 4, 6, 8 krpm; axial pressure drops,  = 2.1, 4.13, 6.21, 8.27 bars; eccentricity ratios;   = 0.0, 0.27, 0.53, 0.8 where  is the static eccentricity.  Three preswirl inserts were used to target zero, medium and high target preswirl ratios (0.0, 0.4, and 0.8) for a set of pre-determined operating conditions with ISO VG 2 oil at 46.1°C.                

Pitot tubes measured the circumferential velocity at separate upstream and downstream seal locations and were used to calculate preswirl ratio, PSR = v_inlet/Rω, and outlet-swirl ratio, OSR = v_outlet/Rω. PSR tended to converge to a value of ~0.4-0.5 at  = 6, 8 krpm for all tested preswirl inserts.

As expected (and predicted), the cross-coupled stiffness coefficients K_xy and K_yx increased steadily with increasing ω and PSR. The remaining results are generally consistent with expectations and predictions using Zirkelback and San Andrés’ 1996 model except concerning the direct stiffness coefficients K_xx and K_yy at 2 krpm.  Here,  K_xx and K_yy increased as  advanced from 2 to 4 bars, then dropped in moving from 4 to 6 bars before then again increasing (at a lower rate) in moving from 6 to 8 bars.  The remaining rotordynamic coefficients displayed no corresponding changes.  The authors believe that the observed changes in K_xx and K_yy   at 2 krpm as  increased arose because the flow shifted from laminar to transition to turbulence.  Based on the observed behavior of K_xx and K_yy with increasing , the shift from laminar to transition occurred at a vector Reynolds number Re on the order of 1200 versus the 1000 value suggested by Zirkelback and San Andrés.  The corresponding observed shift from transition to turbulence was at approximately 1500 versus 3000. 

A much less pronounced drop in K_xx and K_yy with increasing  arose at 4 krpm.  However, at 6 and 8 krpm, K_xx and K_yy increased in a roughly linear fashion with increasing .  These results indicate that the accompanying flow for these higher speeds had shifted to turbulence versus Zirkelback and San Andrés’ assumption that the flow would still be in the transition regime.

The remaining rotordynamic coefficients were generally in reasonable agreement with predictions.  The direct virtual mass coefficients were an exception to this outcome.  They generally followed the predicted trends; however, they were on the order of 2 to 3 times larger than predicted.  The agreement between results and predictions for the stiffness and damping coefficients generally improved with increasing ω and .

Presenting Author: Dara Childs Texas A&M University

Presenting Author Biography: Dara Childs is the retired Leland T. Jordan Professor of Mechanical Engineering at Texas A&M University (TAMU). He worked at TAMU from June 2080 to December 2018, having previously taught at the University of Louisville and Colorado State University. He worked from 1962 to 1965 at Rocketdyne division of North American Aviation. He was the director of the Turbomachinery Laboratory at TAMU from September 1984 until December 2018. He is the author of over 100 paper on rotordynamics of turbomachinery and the book, Turbomachinery of Rotordynamics with Case Studies.

Authors:

Dara Childs Texas A&M University
Joshua Bullock Valero Energy Corp.