Aljaž Škerlavaj - Academia.edu (original) (raw)
Papers by Aljaž Škerlavaj
10th International Conference on Nuclear Engineering, Volume 3, 2002
One of the most well-known experiments on atmosphere stratification in a nuclear power plant cont... more One of the most well-known experiments on atmosphere stratification in a nuclear power plant containment at severe accident conditions is the test E11.2 'Hydrogen distribution in loop flow geometry', which was performed in the Heissdampf Reaktor containment test facility in Germany. In the present work, the simulation of the test E11.2 with the CONTAIN computer code is presented. An input
An orifice plate is installed as the pressure-reducing device in the feedwater (FW) long recircul... more An orifice plate is installed as the pressure-reducing device in the feedwater (FW) long recirculation line of NPP Krško. During operations very intensive cavitation was developed, generating vibrations and noise of high level. Besides, fractures of certain welds in the piping system were noticed during a plant outage. The aim of the research is an analysis of the existing orifice FWRO-004 and hydraulic analysis and design of a new multi-stage orifice. The main goal in the design process is minimization of cavitation and reduction of pressure pulsations and vibrations. In the literature the information about the pressure losses in dependence on pipe diameter and opening of the orifice are given. In the single-stage orifice, the pressure drop is known. In the multi-stage case, due to avoiding the occurrence of cavitation it is important that the pressure drop on the last two stages is lower than on the first stages. According to the recommendations in the literature the pressure drop...
IOP Conference Series: Earth and Environmental Science, 2014
A double-suction pump operating at relatively low suction head and with poorly designed suction c... more A double-suction pump operating at relatively low suction head and with poorly designed suction chambers was analysed by the computational fluid dynamics (CFD). Two impeller geometries were considered -one with thicker and one with thin layer of predicted vapour cavity on blades. Steady-state simulations (SSS) were performed with shear-stresstransport (SST) turbulence model with curvature correction (CC). Transient simulations were performed with scale-adaptive-simulation SST (SAS-SST) model with CC. For both analysed geometries, transient simulations predicted higher maximal thickness of cavities than SSS. In transient simulations it was observed that, because of poor design of suction chambers, near the rib of the suction chambers two stronger (non-cavitating) vortices appeared. Near the main vortical structures, vortices with smaller intensity appeared, with direction of rotation opposite to the main vortices. Depending on their position and direction of rotation, the vortices either decreased or increased the extent of cavitation. The most important adverse effect was to increase the size of the sheet cavity by local elongation and thickening. The local effect seemed to be more pronounced for impeller with smaller thickness of sheet cavity.
ASME/JSME 2011 8th Thermal Engineering Joint Conference, 2011
ABSTRACT
Journal of Physics: Conference Series, 2015
ABSTRACT The paper presents numerical simulations of flow in a model of a high head Francis turbi... more ABSTRACT 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 k-ɛ and SST model, while for transient simulations the SAS SST ZLES model was used. With proper grid refinement in distributor and runner and with taking into account losses in labyrinth seals very accurate prediction of torque on the shaft, head and efficiency was obtained. Calculated axial and circumferential velocity components on two planes in the draft tube matched well with experimental results.
In this article, single-phase, computational-fluid-dynamics simulations of free-surface vortices ... more In this article, single-phase, computational-fluid-dynamics simulations of free-surface vortices are presented. The purpose of the simulations is to determine the appropriate turbulence model for free-surface vortices, which could later be applied to simulations of flow in various engineering systems. The water flow in the laboratory model of a free-surface vortex was numerically simulated by unsteady single-phase computations. The vortex circumferential velocity, the downward velocity inside the vortex core and the predicted length of the free-surface vortex gas core were compared with available measurements. For the two-equation turbulence models, the results indicated the importance of the curvature correction (CC). The effect of the time-step size and the choice of the advection scheme were analyzed. For the tested case, it was determined that the unsteadiness of the flow was insufficient for the correct behavior of the scale-adaptive simulation (SAS) turbulence model. With the CC option, the shear-stress-transport (SST-CC) turbulence model and the SAS-CC turbulence model can both be used for such predictions; however, the SAS-CC model was found to be more reliable. Single-phase simulations successfully predicted the gas-core length for vortices with a short gas core. However, for long cores, the length was under-predicted.
A comparison between numerical simulations and measurements of a six-blade Kaplan turbine is pres... more A comparison between numerical simulations and measurements of a six-blade Kaplan turbine is presented in order to determine an appropriate numerical setup for accurate and reliable simulations of Kaplan turbines. Values of discharge, torque and losses obtained by different turbulence models are compared to each other and to the measurements. Steady state simulations with various turbulence models tend to predict large errors at full discharge rate, which are the result of underestimated torque on the shaft and overestimated flow energy losses in the draft tube. The results were slightly improved with the curvature correction (CC) and Kato-Launder (KL) limiter of turbulence production. Transient simulations were performed with shear-stress-transport (SST) turbulence model, the scale-adaptive-simulation (SAS) SST model, and with zonal large-eddy-simulation (ZLES). Details about turbulent structures in the draft tube are illustrated in order to explain the reasons for differences in flow energy losses obtained by different turbulence models. The effects of advection schemes and mesh refinement were tested. It was shown that all of the transient simulations considerably improved results at full discharge rate. The largest improvement was achieved with the SAS SST and the ZLES models in combination with the bounded central differential scheme. In addition, it was shown that the ZLES model produced accurate results at all operating points, with discrepancy lower than 1%.
There are three vertical mixed flow cooling water pumps installed in a nuclear power plant. At th... more There are three vertical mixed flow cooling water pumps installed in a nuclear power plant. At the maximum plant output all three of them are in operation. If one is switched off, the plant output must be reduced. All three cooling water pumps needed to be overhauled after each cycle (18 months), what was too often comparing with similar plants. The main reasons were cracks which were often noticed at the impeller blades inlet near the hub. Sometimes a cooling water pump was switched off also between two planed overhauls due to the broken blade. Besides, air bubbles were noticed at the cooling water outflow. The plant staff put forward the goal to increase pumps reliability and to increase the time between two pump overhauls on three cycles.
The supposition was, that the pumps sucked the air through the air entraining vortices. Pump sump is not “classical” according to recommendations (1), but pumps have turning vanes in front of suction bell inlet. Detailed analysis of existing situation was carried out on hydraulic model and also by computational flow dynamics (CFD). There were no not-allowed surface or subsurface (bottom or wall) vortices noticed, and also flow conditions in front of the impeller were acceptable. The basic advantage of this solution should be, that it requires lower submergence and shorter pumps – cheaper solution! But the research work indicated submerged vortices generated by turning vanes, which entered the impeller. Modifications were introduced and studied, but without success. After accepting the conclusion, that no practical solution could be expected on the side of pump sump, the problem was solved another way - on the impeller side. The level of stresses in the point of the highest concentration was reduced for ca 75 % with better impeller design and the position was shifted away from the impeller inlet - the sensitivity of impeller on dynamic loading was highly reduced!
The basic contribution of the paper is the increase of knowledge of the solution with turning vanes in front of the vertical pump impeller.
Numerical prediction of an efficiency of a 6-blade Kaplan turbine is presented. At first, the res... more Numerical prediction of an efficiency of a 6-blade Kaplan turbine is presented. At first, the results of steady state analysis performed by different turbulence models for different operating regimes are compared to the measurements. For small and optimal angles of runner blades the efficiency was quite accurately predicted, but for maximal blade angle the discrepancy between calculated and measured values was quite large. By transient analysis, especially when the Scale Adaptive Simulation Shear Stress Transport (SAS SST) model with zonal Large Eddy Simulation (ZLES) in the draft tube was used, the efficiency was significantly improved. The improvement was at all operating points, but it was the largest for maximal discharge. The reason was better flow simulation in the draft tube. Details about turbulent structure in the draft tube obtained by SST, SAS SST and SAS SST with ZLES are illustrated in order to explain the reasons for differences in flow energy losses obtained by different turbulence models.
A double suction storage pump has been refurbished because of the strong cavitation which resulte... more A double suction storage pump has been refurbished because of the strong cavitation which resulted in cavitation damage on blade and consequently in frequent repairs of the impeller. The analyses of the old and the new impeller were done by the computational fluid dynamics (CFD), performing transient simulations with the commercial solver Ansys CFX. In the simulations, the scale-adaptive-simulation with the curvature correction (SAS-CC) turbulence model was used. No model tests were carried out. Additionally, observations with the digital camera were made through the specially designed plexi-glass window, mounted at the lid at the suction side. The predicted pump head at the operating point agrees well with the pump characteristics measurements, performed with the direct thermodynamic method. The extent of the cavitation predicted by CFD is smaller than the observed one because the cloud cavitation was not predicted. The observations of the cavitation extent show that the impeller design is better than the old one, which was also possible to anticipate based on the CFD results.
A pump intake can have an important impact on a pump operation due to production of strong unstea... more A pump intake can have an important impact on a pump operation due to production of strong unsteady vortices which may cause air intake problems. Constructing a pump sump model and experimental testing is expensive, therefore numerical simulations are expected to help or even replace the experimental testing in the future. In order to understand and eventually be able to predict such surface vortices numerically, a vortex in a small chamber was simulated. A benefit of such isolated vortex test case is small number of elements in computational mesh, compared to the whole pump intake, and a controlled testing environment. For a small chamber vortex simulation, various turbulence model simulations as well as laminar and Euler simulations were evaluated. The results indicate that the SAS-CC turbulence model might be a good choice for a simulation of a pump intake. Time step increase had a moderate influence on SAS-CC results.
This article is focused on the choice of a suitable turbulence model for simulations of an indust... more This article is focused on the choice of a suitable turbulence model for simulations of an industrial pump's intake, from the perspective of accuracy and, partially, also the CPU time. Twelve steady-state and transient simulations were made on a fine computational mesh, using turbulence models such as: the shear stress transport (SST), the scale-adaptive simulation (SAS), the Reynolds stress model, the explicit algebraic Reynolds-stress model, the detached eddy simulation and the large eddy simulation (LES). The curvature-correction (CC) option was assessed for the SST and SAS turbulence models. The results were compared with the LES and with published experimental results. Although all the models could predict the main floor vortex, there were still some substantial differences. It can be able to conclude that it is better to use either the SST-CC turbulence model, due to its low-computational resources and far better results than the SST model, or the SAS-CC turbulence model, since its predictions are quite similar to the LES results. In the final step, good agreement with experimental results was shown for a longer simulation with the SAS-CC turbulence model.
Computational fluid dynamics (CFD) calculations of pump sumps are troublesome due to the nature o... more Computational fluid dynamics (CFD) calculations of pump sumps are troublesome due to the nature of the flow. Pump sump flow is turbulent and unsteady, and pump sump dimensions are large compared to diameter of vortices occurring near the sump walls or in the pump column. Therefore, to capture the general and important phenomena of the flow, the computational grid should be fine enough at certain areas of the sump. Combined with unsteady calculations, this usually results in computationally expensive cases. The decision for a suitable turbulent model plays an important role in adding or reducing the computational costs. The present study first compares some different turbulent models on a fine computational grid to the published experimental model. The intention of the present work is to get an answer whether the Unsteady Reynolds Averaged Navier-Stokes (URANS) model really fails in predictions of vortex modeling, since the usage of Large Eddy Simulation (LES) model for industrial cases would represent huge computational power demands. In the second part of the paper a real case pump sump is analyzed.
10th International Conference on Nuclear Engineering, Volume 3, 2002
One of the most well-known experiments on atmosphere stratification in a nuclear power plant cont... more One of the most well-known experiments on atmosphere stratification in a nuclear power plant containment at severe accident conditions is the test E11.2 'Hydrogen distribution in loop flow geometry', which was performed in the Heissdampf Reaktor containment test facility in Germany. In the present work, the simulation of the test E11.2 with the CONTAIN computer code is presented. An input
An orifice plate is installed as the pressure-reducing device in the feedwater (FW) long recircul... more An orifice plate is installed as the pressure-reducing device in the feedwater (FW) long recirculation line of NPP Krško. During operations very intensive cavitation was developed, generating vibrations and noise of high level. Besides, fractures of certain welds in the piping system were noticed during a plant outage. The aim of the research is an analysis of the existing orifice FWRO-004 and hydraulic analysis and design of a new multi-stage orifice. The main goal in the design process is minimization of cavitation and reduction of pressure pulsations and vibrations. In the literature the information about the pressure losses in dependence on pipe diameter and opening of the orifice are given. In the single-stage orifice, the pressure drop is known. In the multi-stage case, due to avoiding the occurrence of cavitation it is important that the pressure drop on the last two stages is lower than on the first stages. According to the recommendations in the literature the pressure drop...
IOP Conference Series: Earth and Environmental Science, 2014
A double-suction pump operating at relatively low suction head and with poorly designed suction c... more A double-suction pump operating at relatively low suction head and with poorly designed suction chambers was analysed by the computational fluid dynamics (CFD). Two impeller geometries were considered -one with thicker and one with thin layer of predicted vapour cavity on blades. Steady-state simulations (SSS) were performed with shear-stresstransport (SST) turbulence model with curvature correction (CC). Transient simulations were performed with scale-adaptive-simulation SST (SAS-SST) model with CC. For both analysed geometries, transient simulations predicted higher maximal thickness of cavities than SSS. In transient simulations it was observed that, because of poor design of suction chambers, near the rib of the suction chambers two stronger (non-cavitating) vortices appeared. Near the main vortical structures, vortices with smaller intensity appeared, with direction of rotation opposite to the main vortices. Depending on their position and direction of rotation, the vortices either decreased or increased the extent of cavitation. The most important adverse effect was to increase the size of the sheet cavity by local elongation and thickening. The local effect seemed to be more pronounced for impeller with smaller thickness of sheet cavity.
ASME/JSME 2011 8th Thermal Engineering Joint Conference, 2011
ABSTRACT
Journal of Physics: Conference Series, 2015
ABSTRACT The paper presents numerical simulations of flow in a model of a high head Francis turbi... more ABSTRACT 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 k-ɛ and SST model, while for transient simulations the SAS SST ZLES model was used. With proper grid refinement in distributor and runner and with taking into account losses in labyrinth seals very accurate prediction of torque on the shaft, head and efficiency was obtained. Calculated axial and circumferential velocity components on two planes in the draft tube matched well with experimental results.
In this article, single-phase, computational-fluid-dynamics simulations of free-surface vortices ... more In this article, single-phase, computational-fluid-dynamics simulations of free-surface vortices are presented. The purpose of the simulations is to determine the appropriate turbulence model for free-surface vortices, which could later be applied to simulations of flow in various engineering systems. The water flow in the laboratory model of a free-surface vortex was numerically simulated by unsteady single-phase computations. The vortex circumferential velocity, the downward velocity inside the vortex core and the predicted length of the free-surface vortex gas core were compared with available measurements. For the two-equation turbulence models, the results indicated the importance of the curvature correction (CC). The effect of the time-step size and the choice of the advection scheme were analyzed. For the tested case, it was determined that the unsteadiness of the flow was insufficient for the correct behavior of the scale-adaptive simulation (SAS) turbulence model. With the CC option, the shear-stress-transport (SST-CC) turbulence model and the SAS-CC turbulence model can both be used for such predictions; however, the SAS-CC model was found to be more reliable. Single-phase simulations successfully predicted the gas-core length for vortices with a short gas core. However, for long cores, the length was under-predicted.
A comparison between numerical simulations and measurements of a six-blade Kaplan turbine is pres... more A comparison between numerical simulations and measurements of a six-blade Kaplan turbine is presented in order to determine an appropriate numerical setup for accurate and reliable simulations of Kaplan turbines. Values of discharge, torque and losses obtained by different turbulence models are compared to each other and to the measurements. Steady state simulations with various turbulence models tend to predict large errors at full discharge rate, which are the result of underestimated torque on the shaft and overestimated flow energy losses in the draft tube. The results were slightly improved with the curvature correction (CC) and Kato-Launder (KL) limiter of turbulence production. Transient simulations were performed with shear-stress-transport (SST) turbulence model, the scale-adaptive-simulation (SAS) SST model, and with zonal large-eddy-simulation (ZLES). Details about turbulent structures in the draft tube are illustrated in order to explain the reasons for differences in flow energy losses obtained by different turbulence models. The effects of advection schemes and mesh refinement were tested. It was shown that all of the transient simulations considerably improved results at full discharge rate. The largest improvement was achieved with the SAS SST and the ZLES models in combination with the bounded central differential scheme. In addition, it was shown that the ZLES model produced accurate results at all operating points, with discrepancy lower than 1%.
There are three vertical mixed flow cooling water pumps installed in a nuclear power plant. At th... more There are three vertical mixed flow cooling water pumps installed in a nuclear power plant. At the maximum plant output all three of them are in operation. If one is switched off, the plant output must be reduced. All three cooling water pumps needed to be overhauled after each cycle (18 months), what was too often comparing with similar plants. The main reasons were cracks which were often noticed at the impeller blades inlet near the hub. Sometimes a cooling water pump was switched off also between two planed overhauls due to the broken blade. Besides, air bubbles were noticed at the cooling water outflow. The plant staff put forward the goal to increase pumps reliability and to increase the time between two pump overhauls on three cycles.
The supposition was, that the pumps sucked the air through the air entraining vortices. Pump sump is not “classical” according to recommendations (1), but pumps have turning vanes in front of suction bell inlet. Detailed analysis of existing situation was carried out on hydraulic model and also by computational flow dynamics (CFD). There were no not-allowed surface or subsurface (bottom or wall) vortices noticed, and also flow conditions in front of the impeller were acceptable. The basic advantage of this solution should be, that it requires lower submergence and shorter pumps – cheaper solution! But the research work indicated submerged vortices generated by turning vanes, which entered the impeller. Modifications were introduced and studied, but without success. After accepting the conclusion, that no practical solution could be expected on the side of pump sump, the problem was solved another way - on the impeller side. The level of stresses in the point of the highest concentration was reduced for ca 75 % with better impeller design and the position was shifted away from the impeller inlet - the sensitivity of impeller on dynamic loading was highly reduced!
The basic contribution of the paper is the increase of knowledge of the solution with turning vanes in front of the vertical pump impeller.
Numerical prediction of an efficiency of a 6-blade Kaplan turbine is presented. At first, the res... more Numerical prediction of an efficiency of a 6-blade Kaplan turbine is presented. At first, the results of steady state analysis performed by different turbulence models for different operating regimes are compared to the measurements. For small and optimal angles of runner blades the efficiency was quite accurately predicted, but for maximal blade angle the discrepancy between calculated and measured values was quite large. By transient analysis, especially when the Scale Adaptive Simulation Shear Stress Transport (SAS SST) model with zonal Large Eddy Simulation (ZLES) in the draft tube was used, the efficiency was significantly improved. The improvement was at all operating points, but it was the largest for maximal discharge. The reason was better flow simulation in the draft tube. Details about turbulent structure in the draft tube obtained by SST, SAS SST and SAS SST with ZLES are illustrated in order to explain the reasons for differences in flow energy losses obtained by different turbulence models.
A double suction storage pump has been refurbished because of the strong cavitation which resulte... more A double suction storage pump has been refurbished because of the strong cavitation which resulted in cavitation damage on blade and consequently in frequent repairs of the impeller. The analyses of the old and the new impeller were done by the computational fluid dynamics (CFD), performing transient simulations with the commercial solver Ansys CFX. In the simulations, the scale-adaptive-simulation with the curvature correction (SAS-CC) turbulence model was used. No model tests were carried out. Additionally, observations with the digital camera were made through the specially designed plexi-glass window, mounted at the lid at the suction side. The predicted pump head at the operating point agrees well with the pump characteristics measurements, performed with the direct thermodynamic method. The extent of the cavitation predicted by CFD is smaller than the observed one because the cloud cavitation was not predicted. The observations of the cavitation extent show that the impeller design is better than the old one, which was also possible to anticipate based on the CFD results.
A pump intake can have an important impact on a pump operation due to production of strong unstea... more A pump intake can have an important impact on a pump operation due to production of strong unsteady vortices which may cause air intake problems. Constructing a pump sump model and experimental testing is expensive, therefore numerical simulations are expected to help or even replace the experimental testing in the future. In order to understand and eventually be able to predict such surface vortices numerically, a vortex in a small chamber was simulated. A benefit of such isolated vortex test case is small number of elements in computational mesh, compared to the whole pump intake, and a controlled testing environment. For a small chamber vortex simulation, various turbulence model simulations as well as laminar and Euler simulations were evaluated. The results indicate that the SAS-CC turbulence model might be a good choice for a simulation of a pump intake. Time step increase had a moderate influence on SAS-CC results.
This article is focused on the choice of a suitable turbulence model for simulations of an indust... more This article is focused on the choice of a suitable turbulence model for simulations of an industrial pump's intake, from the perspective of accuracy and, partially, also the CPU time. Twelve steady-state and transient simulations were made on a fine computational mesh, using turbulence models such as: the shear stress transport (SST), the scale-adaptive simulation (SAS), the Reynolds stress model, the explicit algebraic Reynolds-stress model, the detached eddy simulation and the large eddy simulation (LES). The curvature-correction (CC) option was assessed for the SST and SAS turbulence models. The results were compared with the LES and with published experimental results. Although all the models could predict the main floor vortex, there were still some substantial differences. It can be able to conclude that it is better to use either the SST-CC turbulence model, due to its low-computational resources and far better results than the SST model, or the SAS-CC turbulence model, since its predictions are quite similar to the LES results. In the final step, good agreement with experimental results was shown for a longer simulation with the SAS-CC turbulence model.
Computational fluid dynamics (CFD) calculations of pump sumps are troublesome due to the nature o... more Computational fluid dynamics (CFD) calculations of pump sumps are troublesome due to the nature of the flow. Pump sump flow is turbulent and unsteady, and pump sump dimensions are large compared to diameter of vortices occurring near the sump walls or in the pump column. Therefore, to capture the general and important phenomena of the flow, the computational grid should be fine enough at certain areas of the sump. Combined with unsteady calculations, this usually results in computationally expensive cases. The decision for a suitable turbulent model plays an important role in adding or reducing the computational costs. The present study first compares some different turbulent models on a fine computational grid to the published experimental model. The intention of the present work is to get an answer whether the Unsteady Reynolds Averaged Navier-Stokes (URANS) model really fails in predictions of vortex modeling, since the usage of Large Eddy Simulation (LES) model for industrial cases would represent huge computational power demands. In the second part of the paper a real case pump sump is analyzed.