Mixing Performance Experiments in an Agitated Vessel Equipped with a Pitched Paddle Subjected to Unsteady Agitation

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Recently, several investigations for unsteady agitation in an agitated vessel has been carried out because it is useful for laminar mixing but rarely been for pitched paddle. Laminar mixing is often inefficient because of the formation of segregated regions. Two unsteady stirring approaches, co-reverse periodic rotation and time-periodic fluctuation of rotational speed, are adopted to eliminate the unmixed regions, which are two doughnut rings formed above and below the impeller in an agitated vessel equipped with a pitched paddle or a flat paddle. It is well known that the pitched paddle creates upward or downward flow according to rotational directions, which may result in an unique effect on mixing performance especially when co-reverse periodic rotation was conducted. The relationship between mixing time, tm, measured by the decolorization method with Reynolds number, Re, is used as a quantitative method to compare the mixing performance under an unsteady stirring with that obtained under a steady one for pitched paddle in addition to for flat paddle. The experimental results show that the mixing time can be drastically reduced in an agitated vessel equipped with pitched paddle when both unsteady stirring approaches adopted in this work are used. However, it is also found experimentally that for the method of time-periodic fluctuation of rotational speed, only when Re is smaller, can significant enhancements in mixing be obtained. In contrast to this, for the case of co-reverse periodic rotation, denoting improvements are observed in the range of all Re covered in this work. These results were explained by the precise measurements of positions of doughnut rings at different impeller speeds and different rotational directions.

Mixing Performance Experiments in an Agitated Vessel Equipped with a Pitched Paddle Subjected to Unsteady Agitation

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