A study of an up‐ and a down‐pumping wide blade hydrofoil impeller: Part I. LDA measurements

Abstract

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...

Using ultrasonic Doppler velocimetry and CFD modeling to investigate the mixing of non-Newtonian fluids possessing yield stress

“…5 and 6 validate the CFD model developed in this study. Pumping flowrate (Q P ) that encompasses impeller swept volume can be estimated as follows (Mishra et al, 1998):…”

Study of mixing behavior of cstr using CFD

“…(11), mixing is ideal when * 0 → V Q . In general, the expression of Q for impeller is (Mishra et al, 1998) …”

Investigation of Mixing in Shear Thinning Fluids Using Computational Fluid Dynamics

“…Saeed et al (2009) used CFD to calculate the flow and circulation numbers for three axial-flow impellers in the mixing of a shear-thinning fluid with yield stress. The pumping flowrate encompassing the impeller swept volume is given by (Mishra et al, 1998) ( )…”

“…Table 3 shows the circulation numbers calculated using CFD for three axial flow impellers in the mixing of xanthan gum solution (Saeed, 2007;. The impeller pumping efficiency (η p ) and circulation efficiency (η c ) can be used to compare the performances of the different impellers in the mixing of shear-thinning fluids possessing yield stress (Mishra et al, 1998;Aubin et al, 2001):…”