Fluctuating lift forces and pressure distributions due to vortex shedding in tube banks

Abstract

Experimental measurements of fluctuating lift forces and surface pressure distributions are presented for staggered and in-line tube banks consisting of four rows, with transverse pitch-to-diameter ratio ( P T/ d) of 2.67 and longitudinal pitch-to-diameter ratio ( P L/ d) of 2.31. A strain gauge system was used to measure the fluctuating force on one instrumented tube located at various positions in the tube banks. A second instrumented tube was equipped with a static pressure tapping and a pressure transducer to provide time average and rms fluctuating surface pressure distributions, respectively. Investigations of the staggered tube bank revealed that the highest levels of fluctuating lift coefficients occurred in the second row. For the in-line geometry, on the other hand, similar levels of fluctuating lift coefficient were found in the second, third, and fourth tube rows. The normal configuration for staggered tube banks as used here is symmetrical. Further tests were undertaken for an asymmetric geometry, revealing a major reduction in the fluctuating lift forces as compared with symmetrical geometry. It is concluded that the use of irregular geometries in staggered tube banks should be further investigated as a practical solution to the reduction of vortex-induced vibrations in tubular heat exchangers.