Multistage centrifugal pumps with variable speed drives are currently widely used in a variety of industrial and commercialapplications. However, there are limitations to defining the efficiency of variable speed drive pumps. As an alternative method,energy saving rates can be evaluated with flow patterns and mean duty cycles. Computational fluid dynamics (CFD) is beingused as a good tool to understand this and is less time consuming in terms of analyzing performances the experimentalmethod. Research attention was focused on the energy saving rates of a multistage centrifugal pump for variable flow withvariable speed drive through numerical and experiment methods. For this investigation Reynolds-averaged Navier-Stokes(RANS) equations were discretized by the finite volume method and a two equations SST model was used to account forthree dimensional steady state flows. In the experimental system, an experimental set-up of a variable flow system was madeto obtain energy saving rates and computational results were validated. The energy saving rates of the pumps depend on theflow patterns and specific mean duty cycles on which the machine or system operates. Mean duty cycles were divided intodifferent flow operating conditions and a weighting for the mean value was given for each segment according to interval time.The pump system was operated at 5070% of maximum flow rates. The energy saving rates were obtained from input powerthrough CFD simulation and experimentally, and the mean duty cycle was obtained from flow patterns in the field of the pump.Energy saving rates were evaluated as a function of mean duty cycle and input power of the system operation. The total energyconsumed for the constant speed drive was 25,922 kWh and for the variable speed drive pump was 17,687 kWh through CFD.The total annual energy saving rates were annually 33.81% through computational and 31.77% through experimental methodwith the variable speed drive system when compared to the constant speed drive system.
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