Experimental study of dual-impeller string vibration in a baffled mixing vessel

Vibration characteristics of 45∘ pitched blade impellers (PBIs) operating in a standard baffled mixing vessel were studied. Experiments were conducted for eight different impeller configurations with combinations of: numbers of impellers (single or double), diameter to tank diameter ratios D/DT  = 1/2 and 5/7, and impeller spacing of 0, 0.75D, and 1.5D. The impeller orbit frequency and shape, mean square deflection, and phase relationship were studied with a glycerine-water mixture having Reynolds number, Re≤ 5000, and N/Ncrit≤ 1.2. Two high speed cameras were used to measure two orthogonal components of lateral impeller deflection. It was observed that the impeller orbits are periodic at low speed and exhibit chaotic motion at high speed which can be modelled using a random normal distribution. The mean square deflection continues to grow beyond N*>1 exponentially, and the addition of a second impeller increases the mean square deflection. Frequencies associated with impeller orbits for the D/DT  = 1/2 showed that at low rotational speeds, the highest energy in power spectra of the impeller deflection is at twice the impeller rotational speed. With an increase in rotational speed, the energy in the power spectra shifts to synchronous and finally subsynchronous range with a broad band character. The phase relationship and whirl direction of top and bottom impeller were found to be dependent on the rotational speed and impeller spacing.