Achanta Ramakrishna Rao’s research while affiliated with Indian Institute of Science Bangalore and other places

Incipient motion of spheres, of different sizes and densities, over riding on a bed of closely packed spheres is experimentally investigated, based on the critical velocity criterion. The significance of inter-particle geometry of over-riding spheres and bed spheres on the incipient motion of over-riding spheres is explicitly brought out in this study. The results are generalized by showing their validity with the data of others. The second part of the paper deals with critical shear approach.

Critical shear criterion is followed to summarize the hydrodynamic conditions leading to incipient motion of spheres, of different sizes and densities, over-riding on a bed of closely packed spheres. A relationship governing the incipient motion conditions, based on critical shear stress criterion is developed, correlating the various physical properties of water and over-riding spheres, relative sizes as well as the inter-particle geometry of over-riding spheres and bed spheres. The results are generalized by showing their validity with others data of over-riding spheres on sphere-beds as well as on sand roughened beds. Results have indicated that conventional Shields' curve is not applicable because of protrusion height of the over-riding spheres over and above or below the general bed level, and an equation for Shields' parameter valid for over-riding spheres is presented.

Literature suggests that apart from a significant percentage of water loss from seepage, downward seepage from the channel causes an increase in the mobility of channel bed materials and thus changes the channel stability. Consequently, regime conditions (which provide the relationship among hydraulic parameters) should also be revised by incorporating downward seepage as an additional parameter. In the present work, regime conditions for the prediction of channel geometry in alluvial channels affected by downward seepage have been formulated on the basis of experimental observations.

The oxygen transfer rate is a parameter that characterizes the gas-liquid mass transfer in surface aerators. Gas-liquid transfer mechanisms in surface aeration tanks depend on two different extreme lengths of time; namely, macromixing and micromixing. Small scale mixing close to the molecular level is referred to as micromixing; whereas, macromixing refers to mixing on a large scale. Using experimental data and numerical simulations, macro- and micro-scale parameters describing the two extreme time scales were investigated. A scale up equation to simulate the oxygen transfer rate with micromixing times was developed in geometrically similar baffled surface aerators.

Downward seepage (suction) increases the mobility of the channel. In this study, experimental investigations were carried out to analyse the suction effect on stream power along the downstream side of the flume. It was observed that stream power has a major influence on the stability and mobility of the bed particles, due to suction. Stream power is found to be greater at the upstream side and lower at the downstream side. This reduces the increment in the mobility of the sand particles due to suction at the downstream side. Thus, there is more erosion at the upstream side than the downstream side. It was also found that the amount of deposition of sand particles at the downstream side, because of the high stream power at the upstream side, is greater than the amount of erosion of sand particles from the downstream side.

The scale up or scale down of the process variables in a surface aerator requires information about the shear rate prevailing in the system. In fact, the performance of surface aerator depends upon the shear rate. Shear rate affects the mass transfer operation needed by the surface aerator. Theoretical analysis of shear rate suggests a nonlinear behavior with rotational speed of the impeller, which has been shown in the present work. Present work also shows that in a geometrically similar system of baffled surface aerator, shear rate can be used as a governing parameter for scaling up or down the mass transfer phenomena.

Oxygen transfer and power requirement with rotor speed has always been a central topic for design and scale-up of surface aerators. Present study develops design curves correlating the oxygen transfer coefficient, power per unit volume and rotor speed. Based on the design curves, energy conservation by using right sized circular tank surface aerators has been discussed and demonstrated that it is economical to use a big sized circular tank surface aerator rather than using multiple numbers of smaller sized circular tank to aerate the same volume of water. KeywordsEnergy consumption-Surface aerators

The efficiency of the surface aeration systems is generally governed by the geometric and dynamic parameters. The geometry is important because successful translation of the laboratory finding can be scaled up to field installations. Experimental optimization of the geometrical parameters (classical approach of one parameter variations at a time) has certain limitations, because it assumes a linear relationship among the various geometric parameters. In the real experimental process, it is not possible to vary all the parameters simultaneously. In such a case, the model of the system is built through computer simulation, assuming that the model will result in adequate determination of the optimum conditions for the real system. In this paper, two approaches have been used to model the phenomena in unbaffled circular surface aerators: i) Multiple regression and ii) Neural network. It has been found that neural network approach is showing better predictability compared to the multiple regression approach. In process of optimization, the pertinent dynamic parameter is divided into a finite number of segments over the entire range of observations. For each segment of the dynamic parameter, the neural network model is optimized for the geometrical parameters spanning over the entire range of observations. Thus each segment of the dynamic parameter has its set of optimal geometrical conditions. Results obtained are having less variation among them and they are very nearer to the experimental optimal conditions. Input parameter significance test of neural network model reveals that blade width of the rotor is the most significant geometric parameter for the aeration process.

The oxygen transfer rate and the corresponding power requirement to operate the rotor are vital for design and scale-up of surface aerators. The aeration process can be analyzed in two ways such as batch and continuous systems. The process behaviors of batch and continuous flow systems are different from each other. The experimental and numerical results obtained through the batch systems cannot be relied on and applied for the designing of the continuous aeration tank. Based on the experimentation on batch and continuous type systems, the present work compares the performance of both the batch and continuous surface aeration systems in terms of their oxygen transfer capacity and power consumption. A simulation equation developed through experimentation has shown that continuous flow surface aeration systems are taking more energy than the batch systems. It has been found that batch systems are economical and better for the field application but not feasible where large quantity of wastewater is produced. Key wordsActivated Sludge Process-Continuous Flow-Geometric Similarity-Oxygen Transfer-Power Number-Surface Aerator-Two Film Theory

Seepage through sand bed channels in a downward direction (suction) reduces the stability of particles and initiates the sand movement. Incipient motion of sand bed channel with seepage cannot be designed by using the conventional approach. Metamodeling techniques, which employ a non-linear pattern analysis between input and output parameters and solely based on the experimental observations, can be used to model such phenomena. Traditional approach to find non-dimensional parameters has not been used in the present work. Parameters, which can influence the incipient motion with seepage, have been identified and non-dimensionalized in the present work. Non-dimensional stream power concept has been used to describe the process. By using these non-dimensional parameters; present work describes a radial basis function (RBF) metamodel for prediction of incipient motion condition affected by seepage. The coefficient of determination, R-2 of the model is 0.99. Thus, it can be said that model predicts the phenomena very well. With the help of the metamodel, design curves have been presented for designing the sand bed channel when it is affected by seepage. (C) 2010 Elsevier B.V. All rights reserved.