2370 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Ind. Eng. Chem. Res. 1992, 31, zyxwvu 2310-2319 Comparison of Axial Flow Impellers Using a Laser Doppler Anemometer V. V. Ranade,? V. P. Mishra, V. S. Saraph, G. B. Deshpande, and J. B. Joshi* Department zyxwvutsrqp of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India zyxwvutsrqponmlkjihgfedcba Influence of shapes of eight axial flow impellers on flow in agitated vessels was studied using a laser Doppler anemometer. The tank diameter was zyxwv 500 mm with a flat bottom and provided with four standard (width zyxwvuts = T/10) baffles. In all cases the tank to impeller diameter ratio was 3 and the impellers were centrally located. The flow generated by different axial impellers zyx has been compared in terms of mean velocities, turbulent kinetic energy, pumping effectiveness, and hydraulic efficiency. The measured flow data near the impeller have been presented in the form suitable for specifying the boundary conditions to the numercial model. The two-equation (k-e) turbulence model has been shown to be adequate for predicting the bulk flow in the case of all impellers. 1. Introduction 1.1. Background. Agitated reactors are widely used in chemical and allied industries. There are many different ways to provide agitation and mixing in a vessel. The present paper is primarily concerned with impeller-induced agitation. Numerous parameters affect flow in agitated tanks. The tank geometry and the type and arrangement of impellers are the two most important parameters which determine the flow field in the vessel and therefore the performance of a stirred vessel. In most of the cases, the basic shape of a agitated vessel is cylindrical with either a flat or dished bottom and a number of baffles. However, a wide variety of impellers with different shapes and sizes are being used in practice (Tatterson, 1991;Rewatkar and Joshi, 1991). There are fundamental reasons for choosing one impeller over another, which depend on a complete definition of the mixing requirement. It therefore appears that a calculation method which predicts accurately the flow field around an impeller of arbitrary shape would be of enormous benefit to the reactor designer. Unfortu- nately, such a general method does not exist, even for the most commonly used radial flow impellers. The flow field is usually so complex and highly turbulent that it defies analytical description. Traditional designs have therefore had to rely on experience and a set of well-defined geo- metrical relationships between impeller dimensions, its positioning, and tank size. Recent advances in experi- mental and numerical modeling techniques can enhance our understanding about the influence of impeller design on flow in agitated zyxwvuts tanks. This paper describes an attempt to develop a coherent basis for impeller design by estab- lishing defiite relationships between impeller shape and the generated flow characteristics. 1.2. Types of Impellers. Impellers are classified as two general types: axial flow and radial flow. An axial flow impeller discharges fluid along the axis of the impeller (parallel to the impeller shaft) while radial flow impeller discharges flow along the impeller radius. General flow patterns of these impellers are well-known and well doc- umented (Joshi et al., 1982; Mann, 1983; Fort, 1986, Old- shue, 1984; Nagata, 1975). However, knowledge of tur- bulence characteristics of the flow generated by these im- pellers is not readily available. No adequate information is available to make proper selection of impellers. A widely cited guide for impeller selection uses arbitrary parameters * Author to whom correspondence should be addressed. + Present address: Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 0888-5885 I92 12631-2370%03.00 IO like scale of agitation and considers only standard im- pellers. Recently many new, modified impellers have been proposed for a variety of process results. These impellers need to be investigated and placed at their appropriate place in a general perspective. 1.3. Present Contribution. The influence of various impellers on flow in agitated vessels is investigated in detail. A combined approach of experimental as well as numerical simulation is adopted to generate a coherent picture of flow characteristics of various impellers. Ex- perimental data useful for specifying the impeller boundary conditions is reported for eight axial flow impellers. Hy- draulic efficiencies and pumping effectiveness factors of various impellers are reported to aid the designer for making the proper selection of an impeller. 2. Tools of Investigation Recent advances in measurement techniques such as the hot film anemometer/laser Doppler anemometer enable us to characterize complex, three-dimensional turbulent flows generated by impellers. However, detailed charac- terization of the flow field is very expensive and time consuming. Numercial simulations of the generated flow field can prove very economical if properly validated models are developed. Recently, Ranade and Jbshi (199Ob) and Ranade et al. (1989) have presented results of nu- merical simulations of flow generated by a disk turbine and pitched blade turbines. They have obtained good agree- ment between simulated and measured flow characteris- tics. Their models, however, require boundary conditions near the impeller. Flow near and inside the impeller can in principle be simulated using a suitable turbulence model. However, periodic movement of blades and co- herent structure behind the blade raise severe and often intractable problems for flow simulation. Alternatively, approximate models for the flow inside the impeller can be constructed using drag and lift coefficienta for the im- peller blades. Such models can give a reasonable estimate of mean flow characteristics near the impeller. This issue of flow near the impeller is discussed again in the sub- section of impeller boundary conditions. Since the ob- jective of the present study is to investigate the influence of impeller designs, uncertainty in the flow near the im- peller should be minimized. Therefore, we have adopted a hybrid approach in the present work (1) flow near the impeller will be characterized accurately using measure- ments with the laser Doppler anemometer; (2) using this information, flow in the bulk of the vessel will be simulated using a turbulence model. Details of these two stages are given in the following subsections. 1992 American Chemical Society