Experimental Development of a Novel Aeration Device

Abstract

Theoretical and experimental studies were carried out with the aim of developing an improved aeration device. Many of the aeration devices currently in use contain fine pores, through which air is pumped to produce small bubbles in the diameter range of 2 to 3 mm in water, the objective being optimum performance in terms of mass transfer. Because of the very small pore size, these devices are often subject to clogging, consequent deterioration of efficiency and escalation of power requirements. The development of the new aeration device, named the SAF diffuser, is an attempt to generate bubbles of the optimum size range using relatively larger orifices so that the diffuser clogging and the energy requirements can be reduced. The SAF Diffuser comprises a vertical draft tube immersed in the water body to be aerated. A buoyancy: induced flow is created within the tube by injecting air through 0.5 mm diameter peripheral orifices located near the inlet end. The induced flow exerts a drag force on the bubbles that are being formed at the peripheral orifices. This drag force causes the bubble to detach, from the orifice before it grows to the normal size attained in stagnant water. Utilizing this concept, hole sbes that are larger than (about 15 times) those used in conventional fine pore diffusers are possible. For a given air flowrate, the induced velocity and hence the bubble size depends upon the tube dimensions. This dependence provides a novel technique to control the bubble size. Theoretical and experimental study of the air-water flow as well as the mass transfer characteristics of the device were undertaken. Simple methods of measuring the two phase flow parameters were developed. Through theoretical analysis and experimentation, non-dimensional correlations to predict the phase velocities within the device as well as the mass transfer characteristics of the device were developed. The studies showed that for medium depth applications, the device can operate with 10 to 20 percent higher aeration efficiencies than conventional designs. Even higher efficiencies are possible in the low depth applications typical in aquaculture.