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A new approach for the design of diffuser-augmented hydro turbines using the blade element momentum

Silva, Paulo A.S.F., Rio Vaz, Deborah A.T.D., Britto, Vinicius, de Oliveira, Taygoara F., Vaz, Jerson R.P., Brasil Junior, Antonio C.P.
Energy conversion and management 2018 v.165 pp. 801-814
diffusers, mass flow, mathematical models, momentum, shear stress, turbines, turbulent flow
It is known surrounding a turbine with a diffuser may significantly increase its power. This effect has attained considerable attention as it shows theoretically the possibility of achieving a power coefficient about 2 times greater than an ordinary turbine. However, the effect of the diffuser efficiency has not been implemented into blade element momentum yet. Hence, this paper presents a novel approach to design diffuser-augmented hydro turbines considering the diffuser efficiency. Based on the blade element momentum, new expressions for the axial induction factor and thrust are obtained. To assess the proposed model, a comparative evaluation of two different diffusers (flanged conical diffuser and flanged lens diffuser) is performed. A numerical modeling investigation using computational fluid dynamics is carried out based on the Reynolds Averaged Navier-Stokes formulation, using the κ-ω shear-stress transport turbulence model. Evaluations for both turbine and diffuser are performed using experimental data available in the literature. Numerical and theoretical results are compared for a shrouded turbine equipped with a 83% efficiency diffuser. The relative difference observed for the maximum power coefficient between the proposed model and an actuator disk model with diffuser is about 5.3%. For the hydro turbine with flanged conical diffuser, the mass flow rate is about 20% higher than for a bare turbine, while for the turbine with flanged lens diffuser the increase is only 2.4% . Also, for the flanged conical diffuser the power is increased by 53%. Furthermore, it is observed that the proposed blade element momentum with diffuser achieved good agreement with the numerical model, providing improved results compared to other models available in the literature.