An experimental study of free liquid surface motion in stirred tanks - paper I2.1

We consider the liquid flow in a stirred tank with a rotary mechanical impeller as a highly complex, multicomponent dynamical system exhibiting unstable (pseudo-stationary) dynamic behaviour. Within the variety of the flow components in a stirred tank, large-scale low-frequency pseudo-periodic fluctuations that substantially influence mixing characteristics of the stirred vessel are frequently encountered. These flow fluctuations – instabilities – are associated to recurrent fast liquid jets originating at the vessel bottom and ascending along the baffles and the tank wall. In this paper we develop a statistical and non-linear dynamic analysis of the free liquid surface motion in the stirred vessels and investigate on the occurrence of local macro-swells of the liquid surface near a baffle. The experiments were carried out in three standard flat-bottomed cylindrical mixing vessels with four radial wall baffles (inner diameter, T, of 0.29 m, 0.50 m, and 1 m, respectively). Water was used as a working liquid. The filling height was equal to the tank diameter. Pitched blade turbines (PBT) of diameter equal to T/3 with six blades (pitch angle 45°) pumping towards the vessel bottom were used for stirring. The impeller off-bottom clearance was T/3 in all tanks. The impeller speed was varied from 300 rpm to 450 rpm in the smallest tank, from 200 rpm to 400 rpm in the medium-size tank and from 126 rpm to 220 rpm in the large one. The data analysis of experimental time series initiated with statistical and spectral analysis of the measured time series in order to reveal basic features of the surface motion (mean values, variance, probability density function, autocorrelation functions, etc.) and to detect the presence of the low-frequency components of the measured signals related to the macro vortex.