GAS-INDUCING IMPELLER DESIGN AND PERFORMANCE-CHARACTERISTICS

A theoretical and experimental study on the design and performance cha racteristics of gas inducing impellers is presented. In particular, th e model developed by Evans et al. (1991, A.I.Ch.E. Spring National Mee ting, Houston, TX, gaper 33e) is critically reviewed and, as a result, improvements to the kinetic energy pressure loss analysis and to the initial conditions are proposed. In addition, the model is successfull y extended to account for multiple gas outlet orifice on each blade. E xperimental measurements of the power consumption, rate of gas inducti on, mass transfer coefficient and detached bubble size for a partially optimised, 0.154 m diameter, six-bladed concave gas-inducing impeller are presented. A significant increase in the induced gas rate is obse rved by adding more outlet orifices to each blade. The principal advan tage of using multiple orifices is that similar size bubbles are produ ced, compared to a single orifice, but larger interfacial areas are ge nerated; the aerated power input is only slightly reduced from its ung assed value. Mass transfer coefficients, k(L)a, of the order of 0.02 s (-1) are attainable for a single outlet orifice on each blade; k(L)a i s significantly increased by using multiple orifices. The dimensionles s bubble size distributions, d/d(gm) are independent of the impeller s peed over the range 4-8 rps, and can be successfully represented by a log-normal distribution. (C) 1998 Elsevier Science Ltd. All rights res erved.