Design Of Axial Flow Turbines Research Articles

Multi-Objective Optimization of an Axial Flow Turbine Design Using Surrogate Modeling and Genetic Algorithm

Abstract A surrogate model-based multi-objective design optimization methodology using a nondominated sorting genetic algorithm is used to maximize the power output and efficiency of an axial flow turbine. A set of high-fidelity data obtained from ANSYS-CFD simulations on space-filling samples is used for developing a surrogate model. The methodology uses (i) a Latin hypercube experimental design for selecting space-filling samples, (ii) a genetic algorithm for determining parameters of a Kriging model, and (iii) axial gap, tip clearance, and rotation angle of nozzle profile of turbine as predictor variables. Flow in the turbine is characterized by the presence of jet and wake flow, shock in the rotor blade flow passage, and tip clearance vortex. Study shows that (i) the axial gap controls the mixing of the jet and the wake flows and provides proper blade incidence which reduces the shock strength in the rotor blade flow passages and (ii) an optimum sized tip clearance vortex controls tip clearance leakage. The optimization provided a set of Pareto-optimal solutions that are nondominated in terms of power and efficiency. Verification of a selected design configuration from the Pareto-optimal solution using computational fluid dynamics (CFD) analysis showed that the process of optimization has been able to fine-tune the axial gap and the tip clearance.

Preliminary Design of an Axial-Flow Turbine for Small-Scale Supercritical Organic Rankine Cycle

A small-scale organic Rankine cycle (ORC) with kW-class power output has a wide application prospect in industrial low-grade energy utilization. Increasing the expansion pressure ratio of small-scale ORC is an effective approach to improve the energy efficiency. However, there is a lack of suitable expander for small-scale ORC that can operate with a high efficiency under the condition of large expansion pressure ratio and small mass flow rate. Aiming at the design of high-efficiency axial-flow turbine in small ORC system, this paper investigates the performance of a kW-class axial-flow turbine and proposes a method for efficiency improvement. First, the preliminary design of an axial-flow turbine is conducted to optimize the geometric parameters and aerodynamic parameters. Then, the effects of tip clearance and trailing edge thickness on turbine performance are analyzed under design and off-design conditions. The results show that the efficiency of the two-stage or three-stage turbine is evidently better than that of the single-stage one. The output power and efficiency of the three-stage turbine are close to that of the two-stage turbine while the speed is lower. Meanwhile, the trailing edge loss and leakage loss can be significantly reduced via reducing the trailing edge thickness and tip clearance, and thus the turbine efficiency can be improved significantly. The estimated efficiency arrives at 0.82, which is 33% higher than that of the conventional turbine. Considering the limitation of turbine speed, three-stage axial-flow turbine is a feasible choice to improve turbine efficiency in a small-scale ORC.

On the design of propeller hydrokinetic turbines: the effect of the number of blades

A design study of propeller hydrokinetic turbines is explored in the present paper, where the optimized blade geometry is determined by the classical Glauert theory applicable to the design of axial flow turbines (hydrokinetic and wind turbines). The aim of the present study is to evaluate the optimized geometry for propeller hydrokinetic turbines, observing the effect of the number of blades in the runner design. The performance of runners with different number of blades is evaluated in a specific low-rotational-speed operating conditions, using blade element momentum theory (BEMT) simulations, confirmed by measurements in wind tunnel experiments for small-scale turbine models. The optimum design values of the power coefficient, in the operating tip speed ratio, for two-, three- and four-blade runners are pointed out, defining the best configuration for a propeller 10 kW hydrokinetic machine.

Automated axial flow turbine design with performance prediction

This paper describes the automation of preliminary 3D aerothermodynamic design and analysis of a single-stage axial flow turbine driving the compressor of the micro turbojet engine Jet Cat p200 using an analytical method. The geometrical parameters of the new design are based on the reverse engineering of the small gas turbine. Baseline stage dimensions and design parameters such as flow coefficient, stage loading factor are close to 0.5 and 1.14, respectively, with maximum expansion in the NGV (stator) row. In the thermal cycle and 1D analysis calculations, the turbine flow path and the meanline total conditions of all engine stations and all design controlling parameters are determined on the basis of mass, energy and momentum conservation. The results are compared with the published data in the engine manual and gave a good match. The axially symmetric assumption changes the 3D flow problem through the turbine stage into 2D problem, i.e. the flow through the profile cascade. Applying the simple radial equilibrium approach at the axial gapes with selecting the free vortex law of blading, the velocity triangles at different radii along the blade length are determined. The generated velocity triangles are used to design the profile cascades considering parabolic mean, convex and concave sides. The turbine stage performance is calculated at design point and off-design conditions based on the one-dimensional analytical approach with constant total pressure loss coefficient across the stator and constant relative velocity recovery factor across the rotor. A CFD model is constructed to evaluate the turbine stage performance numerically. The CFD results are compared with that of the analytical method which give a good match.

Combined Turbine and Cycle Optimization for Organic Rankine Cycle Power Systems—Part B: Application on a Case Study

Organic Rankine cycle (ORC) power systems have recently emerged as promising solutions for waste heat recovery in low- and medium-size power plants. Their performance and economic feasibility strongly depend on the expander. The design process and efficiency estimation are particularly challenging due to the peculiar physical properties of the working fluid and the gas-dynamic phenomena occurring in the machine. Unlike steam Rankine and Brayton engines, organic Rankine cycle expanders combine small enthalpy drops with large expansion ratios. These features yield turbine designs with few highly-loaded stages in supersonic flow regimes. Part A of this two-part paper has presented the implementation and validation of the simulation tool TURAX, which provides the optimal preliminary design of single-stage axial-flow turbines. The authors have also presented a sensitivity analysis on the decision variables affecting the turbine design. Part B of this two-part paper presents the first application of a design method where the thermodynamic cycle optimization is combined with calculations of the maximum expander performance using the mean-line design tool described in part A. The high computational cost of the turbine optimization is tackled by building a model which gives the optimal preliminary design of an axial-flow turbine as a function of the cycle conditions. This allows for estimating the optimal expander performance for each operating condition of interest. The test case is the preliminary design of an organic Rankine cycle turbogenerator to increase the overall energy efficiency of an offshore platform. For an increase in expander pressure ratio from 10 to 35, the results indicate up to 10% point reduction in expander performance. This corresponds to a relative reduction in net power output of 8.3% compared to the case when the turbine efficiency is assumed to be 80%. This work also demonstrates that this approach can support the plant designer in the selection of the optimal size of the organic Rankine cycle unit when multiple exhaust gas streams are available.

New efficiency charts for the optimum design of axial flow turbines for organic Rankine cycles

Turbine efficiency plays a key role in the design optimization of ORCs (organic Rankine cycles) and should be properly evaluated for an accurate estimate of the real power production. Its value is in general assumed as given in the design optimization procedure, without a check that it can be really achieved in the resulting working conditions. The peculiar properties of high molecular weight fluids markedly influence turbine design and ask for turbine design criteria specifically tailored to ORCs. In this work a meanline design procedure for single stage axial flow turbines is developed to find optimum turbine geometry and efficiency in a wide range of operating conditions. Unlike previous literature, real fluid properties and very recent loss models are implemented. The variation of the predicted turbine efficiency with loading coefficient, flow coefficient, specific speed and specific diameter is shown through new general maps that explicitly take into account the strong influence of compressibility and turbine size through the volumetric expansion ratio and size parameter, respectively. All these maps can be included in a general design optimization procedure of the ORC system to help select the optimum design point, overcoming any arbitrary assumptions on turbine efficiency.

On the aerodynamic design of axial flow turbines by an implicit upwind axisymmetric method

An inverse time-marching solution of Euler equations is performed in the meridional plane of a complete machine. The method predicts the flowfield and blade cambersurface geometry that correspond to a specified load distribution in the blade regions. The implicit approach is used to solve Euler equations with a specified mass flowrate at the outlet section. This boundary condition leads to typical turbine solutions of the inverse problem. The implicit upwind scheme is based on a time linearization of Osher's flux difference splitting operator. This operator employs a high-order Riemann solution that provides second-order space accuracy. For infinite-span turbine cascades working in the low-subsonic range, the new method has convergence times and residuals that are one order of magnitude lower than previous explicit methods. It keeps the same convergence properties up to the sonic outflow conditions, while explicit methods fail in the high-subsonic range. When endwalls are included in the computation to design finite-span cascades or complete turbine stages, the method still encounters some numerical instabilities that can be partly overcome using a sufficient mesh resolution in the spanwise direction.

Dimensions of the axial flow turbine design for wind power

The design, consisting of an axial flow turbine surrounded by an annular disc in the form of a rear edge flanged diffuser, is continued here showing: (i) The value of the theoretical and actual power coefficient, clarifying that the actual power coefficient is about twice that for normal horizontal axis wind turbine. (ii) The different critical dimensions specially that the overall horizontal length does not exceed the diameter of the rotor of the axial flow turbine, while the width of the annular disc is only 0.16 of the diameter of the same rotor.

Axial flow turbine design for wind power

Abstract This paper considers an axial flow turbine and a rear edge flanged diffuser to be used to harness wind energy. The drag of the rear edge of the diffuser can set up a pressure drop behind the axial flow turbine greater than the velocity head of the incoming wind. While the axial flow turbine has a higher efficiency than normal wind turbines at Betz Limit. The result is that this design can produce an overall energy at least three times the actual wind turbines working at Betz Limit.

Paper 2: Aerodynamic and Thermal Considerations in Designing Axial Flow Turbine Blades

The first part of this paper reviews the progress made in improving the aerodynamic design of axial flow turbines and describes experience at the National Gas Turbine Establishment (N.G.T.E.) in the use of some of these techniques. The second part of the paper discusses the present status of internal air cooling for turbine blades and shows how some of the design techniques established for aero-engines might be exploited in the field of power generation.