Whenever a small amount of fluid has to be pumped against a high differential pressure, various types of piston pumps are typically used. However, the need for excellent suction behaviour, for the possibility of a dry run and for minimal manufacturing costs requires the development of alternative pump concepts. This paper presents the development of a novel miniature high-pressure fuel gear pump to replace the feed pump as well as the high-pressure pump in gasoline direct-injection fuel systems. It was found that the clearance between the housing and the rotating gears has to be reduced to a minimum in order to keep the internal leakage at a reasonable level. Based on theoretical predictions a radially and axially pressure-compensated pump concept was developed by incorporation of a floating sealing element able to minimize the tip and face leakages. The design of this sealing element was optimized in the course of experimental investigations. The experiments were carried out using a specifically developed test rig, and the obtained results prove that the optimized pump is able to generate the required differential pressure Dp = 40-50 bar at a specific speed n q = 0.005-0.400 r/min. The measured Dp-Q curves were almost vertical beyond a certain pressure level, yielding a nearly pressure-independent volumetric efficiency of approximately 75%. The developed pump concept finally offers outstanding performance characteristics in an operation range where conventional gear pump concepts usually lose importance owing to unacceptably high volumetric losses.
The present contribution addresses the analysis of the leakage behavior of a small hydro Francis turbine using an analytical approach, which was validated based on the results of computational fluid dynamics (CFD). For a custom-designed Francis turbine with a specific speed of nq = 41.9 rpm, the flow chambers resulting from the labyrinth geometry were added to a traditional full CFD model of the turbine and numerical simulations were performed for several operation points ranging from part load (Qmin = 0.5 × Qopt) to over load (Qmax = 1.3 × Qopt). Consequently, the single losses occurring in the runner seals on crown and band side as well as the pressure distribution within the runner side spaces could be evaluated and compared to the results gained with an analytical approach, which was originally developed to calculate the leakage flow of centrifugal pumps. The comparison of the pressure distribution achieved with the numerical simulation and the analytical calculation shows that both approaches match well if the angular velocity of the fluid ωFluid trapped in the runner side spaces is calculated in an appropriate way. Furthermore, the achieved results demonstrate that the use of the analytical model enables the calculation of the disk friction and leakage losses with sufficient accuracy. This paper contributes to the improvement of the performance prediction of Francis turbines based on combined numerical and analytical calculations.
Due to the low electricity prices in central Europe, cost optimisations related to all parts of a new hydropower plant have become increasingly important. In case of a run-of-river hydropower plant using a vertical axis Kaplan turbine, one of the cost drivers are the excavation works. Thus, a decisive factor for the reduction of construction costs is the minimisation of the construction depth of the elbow-type draft tube. In course of the design phase of a new hydropower plant in Austria, an analysis of the impact of draft tube modifications on the performance of the Kaplan turbine was carried out by applying computational fluid dynamics. The net head of the turbine with a diameter of D = 3.15 m accounts for Hnet = 9.00 m and the maximum discharge per unit is Qmax = 57.5 m3/s. After it was proven that there is a good agreement of the numerically calculated and experimentally measured turbine efficiency for the original turbine configuration, various draft tube designs were tested in order to find out their impact on the turbine efficiency and to analyse the sources of draft tube losses in detail. Finally, it was possible to find a new draft tube design representing a compromise of reduced construction costs and acceptable turbine efficiency.
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