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Axial flow impellers with hydrofoil blades, APV-B2, i.e., B2-30 and B2-45 (30' and 45' angle of attack of blades) have recently been designed, mainly for use in the up-pumping mode, and some new results are reported here for single, dual and triple impeller configurations in the turbulent flow regime. The results are compared with dual Lightnin' A-315's pumping downwards as they have traditionally been used and with dual radial flow Rushton turbines. The ungassed power numbers of the APV-B2's are low, allowing these impellers to be used at large impeller-to-tank diameter ratios. The drop in P P is lower with all combinations of APV-B2 impellers, i.e., for single, dual and triple impellers ing axial hydrofoils. Mixing times measured using a decolourisation technique for dual and triple impeller configurations of APV-B2 under unaerated and aerated conditions showed that the mixing times are much shorter than those for the dual Rushton turbine and similar to those found for dual down-pumping A-31 5's. Detailed mixing characteristics and flow patterns for the dual and triple APV-B2's are also discussed. when compared to ot ni er configurations tested. Torque fluctuations are less than those with single or dual down pump-Des turbines a ecoulement axial munies de pales profilees, APV-B2, p.ex. B2-30 et B2-45 (angle d'attaque des lames de 30" et 45O), ont ete recemment concues principalement pour utilisation en mode de pompage ascendant, et de nouveaux resultats sont presentees ici pour 'des configurations de turbines simples, doubles ou triples en regime d'ecoulement turbulent. Les resultats sont compares a ceux de doubles turbines Lightnin A-3 15 en mode de pompage descendant tel qu'on les utilise traditionnellement et de doubles turbines Rushton a ecoulement radial. Les nombres de puissance en I'absence de gaz des APV-B2 sont faibles, ce qui permet d'utiliser ces turbines a de larges rapports de diametre turbinereservoir. La diminution du rapport P$P est plus faible dans toutes les combinaisons de turbines APV-B2, comme les turbines simples, doubles et triples par exemple, comparativement aux autres configurations testees. Les fluctuations de couple sont inferieures a celles des turbines a pales profilees simples ou doubles a ecoulement axial et en mode de pompage descendant. Les temps de melange mesures au moyen d'une technique de decoloration pour des configurations de turbines APV-B2 doubles ou triples dans des conditions non aerees ou aerees, montrent que les temps de melange sont plus courts que ceux de la turbine Rushton double et similaires a ceux trouves pour les turbines A-3 15 doubles en mode de pompage descendant. Les caracteristiques de melange et les modes d'ecoulement detailles sont egalement analyses pour les turbines APV-B2 doubles et triples.

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A wide blade, high solidity ratio, axial flow hydrofoil impeller has been developed particularly for gas dispersion when used in the upward-pumping mode. It has a low power number so that a relatively low torque is required when using it with a large impeller to tank diameter ratio, a commonly desired aim of retrofitting. As part of the work characterising these impellers, laser Doppler anemometry has been used to study the single phase flow both down-and uppumping, the latter for the first time. Mean and root-mean-square velocities were measured throughout a fully baffled, flat bottomed cylindrical tank. When down-pumping, the discharge flow was almost axial but when up-pumping, it had a strong radial component. Turbulence energy dissipation rates in the impeller region and the discharge stream when uppumping were found to be higher than those when down-pumping. In both modes, the impeller was found to have a high circulation efficiency. In Part 11, the data presented here are used for obtaining computational fluid dynamic solutions of the flow field. Keywords: up-and down-pumping hydrofoils, LDA data, power, flow patterns, turbulence characteristics.or gas-liquid mixing, the main requirements of agitation F are breakage and dispersion of gas bubbles and bulk mixing of all the phases. Rushton turbines, which have traditionally been used have a number of disadvantages, e.g., a significant decrease in aerated power number as compared to a high unaerated power number, which reduces mass transfer potential due to a decrease in the volumetric mass transfer coefficient (Nienow, 1990). Also, radial flow turbines give poor top to bottom mixing in multiple impeller sy\tems, leading to a large increase in mixing time (Otomo et al., 1995). Liquid pumping and gas handling can be improved by the use of large diameter impellers but, the replacement of standard disc turbines with larger ones is not a simple retrofitting operation (Nienow, 1990). As a result, down-pumping axial hydrofoil impellers of high solidity ratio were developed for gas-liquid systems. McFarlane and Nienow ( 1995) have reviewed their performance and concluded that such impellers can be best used at high impeller to tank diameter ratios, especially to give improved bulk blending in multiple impeller configurations (Otomo et al., 1995). However, they produce rather large gassed power fluctuations and in a later review, Nienow (1996) suggested that further improvements could be made by using such hydrofoils in the up-pumping mode.*Author to whom correspondence should be addressed. E-mail address: A.W.Nienow@bham.ac.uk A new impeller design, APV-B2 (Figure 1 a), with smooth and elongated wide blades to give a solidity ratio of I has therefore been developed at The University of Birmingham where this work was conducted. It is especially for use in the up-pumping mode in gassed reactors. Ozcan-Taskin et al. (1995,1996 reported its gassed power characteristics alongside those of two down-pumping impellers, namely the high solidity ratio Lightnin' A-3 15 and a p...

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