The volute of a radial inflow turbine has to be designed to ensure that the desired rotor inlet conditions like absolute Mach number, flow angle etc. are attained. For the reasonable performance of vaneless volute turbine care has to be taken for reduction in losses at an appropriate flow angle at the rotor inlet, in the direction of volute, whose function is to convert gas energy into kinetic energy and direct the flow towards the rotor inlet at an appropriate flow angle with reduced losses. In literature it was found that the incompressible approaches failed to provide free vortex and uniform flow at rotor inlet for compressible flow regimes. So, this paper describes a non-dimensional design procedure for a vaneless turbine volute for compressible flow regime and investigates design parameters, such as the distribution of area ratio and radius ratio as a function of azimuth angle. The nondimensional design is converted in dimensional form for three different volute cross sections. A commercial computational fluid dynamics code is used to develop numerical models of three different volute cross sections. From the numerical models, losses generation in the different volutes are identified and compared. The maximum pressure loss coefficient for Trapezoidal cross section is 0.1075, for Bezier-trapezoidal cross section is 0.0677 and for circular cross section is 0.0438 near tongue region, which suggested that the circular cross section will give a better efficiency than other types of volute cross sections.
This paper aims to study the flow pattern in and around a bucket of a Traditional and a Hooped Pelton runner at single injector operation and illustrates different stages of jet interaction. High speed photography is used to study the flow pattern, keeping the camera in different positions relative to the jet and to the bucket. It is concluded from the results that the flow visualization study, provides exceptional observations with an absolute frame of reference to mark the bucket duty period of a single-jet Pelton runner. The small scale models display erosion damages at the bucket lips, this indicated that the high pressure occur in the early stage of interaction. This fact is substantiated by the present flow visualization studies for the first time. The uncertainty of the free surface outflow within the Pelton turbine bucket establishes good documentation. The results are helpful to know the interaction between the jet and bucket of Pelton turbine.
The work arose initially from an interest in design of radial turbine for small scale gas turbine applications typically suitable for distributed power generation system which demands compact installations. The paper describes an investigation in to the design and performance of radial inflow turbines having a capacity of 25kW at 1,50,000 rpm. First a non-dimensional design philosophy is deduced to design a turbine rotor. The design approach is largely one dimensional along with empirical correlations for estimating losses used to obtain the main geometric parameters of turbine. From the proposed design approach, turbine total-to-static efficiency is calculated as 84.91% which is reasonably good. After that a modified vortex design procedure is developed to derive the non-dimensional volute geometry as a function of azimuth angle for actual flow condition. Once a specific turbine is designed, the flow is analyzed in detail using a three-dimensional Computational Fluid Dynamics (CFD) code in order to assess how accurately the performance is predicted by simple meanline analysis. Finally, a fully instrumented experimental setup is developed. The experimental investigations have been carried out to study the temperature and pressure distribution across turbine and total-to-static efficiency is calculated. The limitations of surging and choking in compressor as well as in the bearings to take up load at such high speed has allowed the tests to be conducted upto 70000 rpm only, with turbine inlet temperatures ranging from 900 K to 1000 K and a pressure ratio upto 1.79, which developed power in the range of 1.69 kW to 10.22 kW. The uncertainty bands are in order of ±13.76% to ±3.12%. It is observed that the CFD results are in good agreement with test results at off design condition. CFD models over predicted total to static efficiency by order of 7-8% at lower speed. These deviations are reduced as turbine runs close to design point.
Paper details the numerical investigation of flow patterns in a conventional radial turbine compared with a back swept design for same application. The blade geometry of a designed turbine from a 25kW micro gas turbine was used as a baseline. A back swept blade was subsequently designed for the rotor, which departed from the conventional radial inlet blade angle to incorporate up to 25° inlet blade angle. A comparative numerical analysis between the two geometries is presented. While operating at lower than optimum velocity ratios (U/C), the 25° back swept blade offers significant increases in efficiency. In turbocharger since the turbine typically experiences lower than optimum velocity ratios, this improvement in the efficiency at offdesign condition could significantly improve turbocharger performance. The numerical predictions show offdesign performance gains of the order of 4.61% can be achieved, while maintaining design point efficiency.
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