Theoretical(1-D) Performance Prediction And Multi-Objective Optimization Of Pump-As-Turbine(PAT)

Pump-as-turbine(PAT)is a mechanical device that can simultaneously operate as a pump or turbine without any major geometrical change.Due to its advantages of costeffectiveness and availability of a wide range of heads and flows over conventional turbines,it has been widely applied in the micro-hydro power generation and other engineering fields.However,the main problem of using a PAT is the difficulty of predicting the turbine mode performance accurately.Pump manufacturers fail to provide the performance prediction data of their pump operating in reverse mode as PAT.Therefore,selecting an appropriate pump to be used as a PAT for an existing site presents a major problem.The performance prediction data of PAT is very essential because it determines whether the selected pump can achieve optimal performance.Thus,a wrong pump selection could result in a bad operating point which will deliver a non-desirable output.Hence,an accurate performance prediction method is vital to the PAT technology.Several methods have been proposed for PAT prediction in literature,but these methods generate an error margin of ±20% when compared to experimental data.Therefore,this study proposes a prediction method for PAT performance by using pump mode performance data to reduce the error margin.Also,other limitations that hinders the usage of PAT include low part load efficiency,narrow operating range and lack of flow control device.As centrifugal pumps are not designed to operate as PAT,the geometrical parameters are designed without taking into account its reverse operation.The fixed geometrical parameters(volute,impeller)of PAT have been one of the major challenges faced by this technology.Furthermore,in PAT mode operation,the change in flow direction increases the hydraulic and frictional losses sharply,rendering the PAT to be less efficient.Thus,optimizations based on the PAT flow zones,are required to improve the hydraulic performance and broaden its operational range to adapt to its multioperating conditions.Additionally,the visualization of internal hydraulic performance is also needed to explain the root cause of the poor off design performance of PAT.Following the study,the main ideas and innovations are as follows:1.An experimental study was carried out in an open test rig system to determine the hydraulic performance of a centrifugal PAT with a rotational speed of 1450 r/min,pressure head 8.5 m and flow rate of 25 m3/h.The head and efficiency characteristic curves were well predicted and agreed with the design specifications of the PAT.2.The computational domain was built and gridded with high-quality hexahedral mesh grids.The numerical model was well-validated with experimental results to demonstrate its suitability for hydraulic flow simulations.Numerical investigations were carried out to compare the pressure and velocity contour in all the optimized cases.3.A new theoretical prediction model is proposed in this study which is based on one dimensional(1D)velocity triangles.This prediction model reduces the error margin in predicting the performance characteristics of a PAT by using the pump mode performance data.The model introduces the use of detailed geometry and modelling of all the major hydraulic losses.The inputs of the PAT parameters made it possible to accurately model the head losses and obtain the actual velocity triangles.4.A one dimensional(1D)theoretical and multi-objective optimization of pump-asturbine(PAT)was performed on the original impeller to improve performance at the design point.A data set of 50 impellers were generated from Latin Hypercube Sampling method with its corresponding efficiencies obtained through the theoretical model.The multi-objective optimization problem was successfully solved and 2D Pareto solutions were achieved by using Pareto-based genetic logarithm(PBGA).5.The impact of surrogate models on the optimization of pump-as-turbine(PAT)was conducted to find a suitable surrogate model which is computational cost-effective and less complex to optimize the performance of PAT at design and off-design conditions.An optimization were performed on the impeller to improve the performance at design and off-design conditions of 0.6Qd,1.0Qd,and 1.4Qd using ANN,GRNN and ANFIS surrogate models.The three-objective optimization problem was successfully solved and 3D Pareto solutions were achieved.6.Multi-condition and multi-parameter optimization based on ANN and PBGA were performed on the original impeller to improve performance at design point,best efficiency point(BEP)and overload flow conditions of 1.0Qd,1.2Qd,and 1.4Qd,respectively.The three-objective optimization problem was successfully solved and 3D Pareto solutions were achieved.Numerical simulations were then carried out and the results were compared with the original model.7.To determine the unsteadiness in the PAT,unsteady flow simulations were carried out to study the flow behavior of the optimized PAT during velocity and pressure fluctuation.The internal flow analysis during different flow conditions revealed the intensity of pressure and velocity fluctuations within the optimized cases.The impeller design variables and the volute asymmetrical shape greatly affected the unsteady flow fields in PAT.The volute tongue region and the impeller lead edge region experienced the most irregular flow patterns in the PAT.The flow at the impeller trailing edge(impeller eye)also rendered weak pressure fluctuation intensities which can lead to cavitation.Furthermore,the main frequency harmonic excitations in the impeller,volute and the outlet pipe flow domain suggested that the exchanged flow times between the impeller and volute is mainly responsible for the pressure fluctuation which subsequently affects the noise and vibration generation in the PAT.8.The results from the entropy production analysis revealed that high entropy losses are generated in impeller flow passage compared with the volute and the outlet pipe passage of all three pump cases.The entropy produced in the impeller domain is mainly caused by the velocity due to vortices.Furthermore,the losses visualized at the blade leading edge and the suction side are higher than the blade trailing edge and the pressure side.The entropy losses decrease as the flow rate increases.In summary,the main objective of the dissertation is to carry out a 1D theoretical prediction model for PAT by using pump mode performance data.The proposed model was able to reduce the uncertainty in predicting turbine mode hydraulic performance.The study established a faster theoretical optimization which could be used to predict the effect of design variable on the objective function.This approach can also help pump designers to decide on which parameter to optimize.The numerical multi-objective and multi-condition optimization problems were successfully solved and the optimization target was achieved.Visualization of the PAT flow passage was present to reveal the root cause of the poor performance of PAT at design and off-design conditions.Thus,this study would provide a theoretical and numerical reference for the impeller optimization design of PAT.