Numerical investigation of the flow in axial water turbines and marine propellers with scale-resolving simulations

The accurate prediction of the performances of axial water turbines and naval propellers is a challenging task, of great practical relevance. In this paper a numerical prediction strategy, based on the combination of a trusted CFD solver and a calibrated mass transfer model, is applied to the turbulent flow in axial turbines and around a model scale naval propeller, under non-cavitating and cavitating conditions. Some selected results for axial water turbines and a marine propeller, and in particular the advantages, in terms of accuracy and fidelity, of ScaleResolving Simulations (SRS), like SAS (Scale Adaptive Simulation) and Zonal-LES (ZLES) compared to standard RANS approaches, are presented. Efficiency prediction for a Kaplan and a bulb turbine was significantly improved by use of the SAS SST model in combination with the ZLES in the draft tube. Size of cavitation cavity and sigma break curve for Kaplan turbine were successfully predicted with SAS model in combination with robust high resolution scheme, while for mass transfer the Zwart model with calibrated constants were used. The results obtained for a marine propeller in non-uniform inflow, under cavitating conditions, compare well with available experimental measurements, and proved that a mass transfer model, previously calibrated for RANS (Reynolds Averaged Navier Stokes), can be successfully applied also within the SRS approaches

Numerical Predictions of Cavitating Flow Around a Marine Propeller and Kaplan Turbine Runner with Calibrated Cavitation Models

Cavitating phenomena, which may occur in many industrial systems, can be modelled using several approaches. In this study a homogeneous multiphase model, used in combination with three previously calibrated mass transfer models, is evaluated for the numerical prediction of cavitating flow around a marine propeller and a Kaplan turbine runner. The simulations are performed using a commercial computational fluid dynamics (CFD) solver and the turbulence effects are modelled using, alternatively, the Reynolds averaged Navier Stokes (RANS) and scale adaptive simulation (SAS) approaches. The numerical results are compared with available experimental data. In the case of the propeller the thrust coefficient and the sketches of cavitation patterns are considered. In the case of the turbine the efficiency and draft tube losses, along with the cavitation pattern sketches, are compared. From the overall results it seems that, for the considered systems, the three different mass transfer models can guarantee similar levels of accuracy for the performance prediction. For a very detailed investigation of the fluid field, slight differences in the predicted shapes of the cavitation patterns can be observed. In addition, in the case of the propeller, the SAS simulation seems to guarantee a more accurate resolution of the cavitating tip vortex flow, while for the turbine, SAS simulations can significantly improve the predictions of the draft tube turbulent flow

Numerical predictions of the PPTC model propeller in oblique flow

The numerical predictions of the non-cavitating and cavitating flow around the PPTC model scale propeller, working in oblique flow, are presented. The simulations are performed using both commercial and open source CFD codes. The homogeneous multiphase model is used and three widespread mass transfer models, previously calibrated, are employed. The turbulence is modelled using the RANS (Reynolds Averaged Navier Stokes) approach with a two-equation turbulence model

Cavitation prediction in a Kaplan turbine using standard and optimized model parameters

The paper presents efficiency and cavitation prediction in a 6-blade Kaplan turbine. The study is a result of a collaboration between the University of Trieste (Italy) and Kolektor Turboin\u161titut (Slovenia), which recently joined in the ACCUSIM EU project with the aim to develop reliable, high fidelity methods for accurate predictions and optimization of the performances of hydro-machinery and marine propellers. Numerical simulations were done at one operating point for maximal runner blade angle and nominal head. Steady state results obtained with the SST (Shear Stress Transport) turbulence model were improved by transient simulations, where the SAS (Scale Adaptive Simulation) SST model was used. Cavitating flow was simulated using the homogeneous model. Mass transfer rate due to cavitation was regulated by the Zwart et al. model with default model constants used in ANSYS CFX commercial code and also with the evaporation and condensation parameters previously calibrated considering the sheet cavity flow around a hydrofoil. For a Kaplan turbine the numerical results were compared with the observation of cavity size on the test rig and with the measured sigma break curve. Steady state simulations predicted a significant too small efficiency level and too small extent of cavitation on the runner blades. With transient simulations, the shape and size of the predicted sheet cavitation agreed well with the cavitation observed on the test rig. In addition, also the predicted efficiency was more accurate, although the value of \u3c3 (cavitation or Thoma number) where the efficiency dropped for 1% was a bit too large. The difference between the results obtained with standard and calibrated model parameters of the Zwart mass transfer model was small

Vpliv izgub v labirintnih tesnilih na izkoristek visokotla\u10dne Francisove turbine (Effect of Losses in Labyrinth Seals on Efficiency of a High Head Francis Turbine)

The paper presents numerical simulations of flow in a model of a high head Francis turbine and comparison of results to the measurements. Numerical simulations were done by two CFD (Computational Fluid Dynamics) codes, ANSYS CFX and OpenFOAM. Steady-state simulations were performed by standard k-\u25b and SST models, while for transient simulations the SAS SST model in combination with ZLES in the draft tube was used. With proper grid refinement in distributor and runner and taking into account losses in labyrinth seals quite accurate prediction of torque on the shaft, head and efficiency was obtained

Numerical investigations of a cavitating propeller in non-uniform inflow

The numerical predictions of the non-cavitating and cavitating flow around a marine model scale propeller, working in non-uniform inflow, are presented. The simulations are performed using commercial and open source CFD codes. The homogeneous model is used and three widespread mass transfer models, previously calibrated on a two-dimensional hydrofoil, are employed. The turbulence effect is modelled using the RANS (Reynolds Averaged Navier Stokes) approach. In addition, the more advanced SAS (Scale Adaptive Simulation) method is briefly evaluated in the case of marine propeller. The overall numerical results compare well with the experimental data. The three different calibrated mass transfer models guarantee similar accuracy

Kuhljevi dnevi 2019

The paper presents a numerical analysis of flow in a 6-jet Pelton turbine for 1-nozzle operation. Flow in the manifold with injectors was analysed in order to get inlet conditions for runner analysis, which was at first performed without cavitation. Numerically obtained efficiency value was compared to the measured value from the test rig. The simulation was repeated with cavitation included. Small vapour cavity at the inner side and a larger one at the back side of the buckets were observed. Detailed analysis of results showed that the conditions for cavitation pitting were not fulfilled. Additionally, with cavitation modelling the accuracy of efficiency prediction improved

Napoved izkoristka nizkotla\u10dne cevne turbine z numeri\u10dno analizo toka (Efficiency Prediction for a Low Head Bulb Turbine with Numerical Flow Analysis )

This paper presents numerical analysis of the flow in a 3-blade bulb turbine. Numerically predicted efficiency values for maximal runner blade angle are compared to the measurements. A bulb turbine, for which deviation of steady state results from the measured values was extremely large, was chosen for analysis. Transient simulation with SST turbulence model did not give satisfactory results, but with SAS SST and ZLES models the agreement between numerical and experimental results was significantly improved. The interdependence between turbulence models, flow in the draft tube and calculated efficiency values can be seen. Also the effect of grid density on results is presented

Detailed Analysis of Flow in Two Pelton Turbines with Efficiency and Cavitation Prediction

This paper presents results of the numerical analysis of two Pelton turbines: a 6-jet turbine for middle head and a 2- jet turbine for high head. For the 6-jet turbine losses in manifold, quality of the jets and turbine efficiency were predicted and validated with the experimental data. Additional improvement of accuracy of efficiency prediction was obtained with cavitation modelling. It was also checked that there was no interaction between the evacuating water sheets and the incoming jets. For a 2-jet turbine cavitation prediction was done. Small vapour cavity at the inner side and a larger one at the back side of the bucket were observed. Detailed analysis of cavitation and condensation processes showed that the conditions for cavitation pitting were not fulfilled

Numerical Predictions of a Model Scale Propeller in Uniform and Oblique Flow

The numerical predictions of a model scale propeller working in uniform and oblique flow are presented. Both non-cavitating and cavitating flow conditions are numerically investigated using homogeneous (mixture) model. Two previously calibrated mass transfer models are alternatively used to model the mass transfer rate due to cavitation. The turbulence effect is modelled using the Reynolds Averaged Navier Stokes (RANS) approach. The simulations are performed using an open source solver. The numerical results are compared with the available experimental data. For a quantitative comparison the propeller thrust is considered, while for a qualitative comparison, snapshots of cavitation patterns are shown. From the overall results it seems that, with the current simulation approach it is possible to predict with a reasonable accuracy the propeller performances. Nevertheless, for detailed reproduction of complex cavitation phenomena, such as bubble cavitation for instance, a more sophisticated modelling approach is probably required