Multistage Axial Flow Turbines Research Articles

A Novel Hybrid Computational Method for Multi-Stage Axial Flow Turbine Performance Prediction

Axial flow gas turbines are used in many important applications including power production, gas transportation and jet propulsion. Computational tools are required to predict the performance of an existing turbine or to design a new one. Classical simple flow models, assisted with experimental correlations, provide useful and inexpensive tools for a limited number of configurations. Modern computational fluid dynamic (CFD) methods, on the other hand, are rather generally applicable but are not computationally affordable in many cases. This paper presents a hybrid approach in which a streamline curvature method employs both CFD results and correlated data to predict the performance of a multi-stage axial flow turbine. The proposed algorithm can be run on a personal computer and is not limited to standard blade profiles. Numerical test results are presented which show the applicability of the proposed method in both design and off-design working conditions.

Optimization of multistage turbines using a through-flow code

Fast and accurate flow calculation and performance prediction of multistage axial flow turbines at design and off-design conditions were performed using a compressible steady state inviscid through-flow code with high fidelity loss and mixing models. The code is based on a stream function model and a finite element solution procedure. A new design system has been developed which optimizes hub and shroud geometry and inlet and exit flow field parameters for each blade row of a multistage axial flow turbine. Optimization was performed using a hybrid constrained optimization code that switches among the modules automatically in order to avoid local minima and to accelerate design convergence rate. By automatically varying a relatively small number of geometric variables per turbine stage it is possible to find an optimal radial distribution of flow parameters at the inlet and outlet of every blade row. Thus, an optimized meridional flow path can be found that is defined by the optimized shape of the hub and shroud while keeping blade shapes intact. The multistage design optimization system has been demonstrated using an actual two-stage axial gas turbine as an example. The comparison of computed performance of an already very high efficiency initial design and its optimized design demonstrates more than 1 per cent improvement in the turbine efficiency at design and significant off-design conditions. The entire design optimization process is feasible on a typical single-processor computer workstation or a personal computer.

Off-Design Performance of Multistage Turbines

Nondimensional equations are developed for the performance of a multistage axial flow turbine operating at conditions which differ from those for which the turbine is designed. The effect of variations in inlet temperature, inlet pressure, exit pressure, and turbine speed is explored. Typical cases are evaluated. The method presented is thought to be useful particularly for preliminary design.

A Theoretical Research on the Stage Efficiency of Double-Rotation Axial-Flow Turbines

In the ideal middle stages of a multi-stage axial-flow turbine, we take, as one stage, any adjoining two rows of blades such as I, II which are moving to the opposite directions with each other. The adiabatic heat drops of the moving blades I and II are (1-ρ) H0 and ρH0 respectively, when we represent the total heat drop of the stage with H0 and adopt a factor ρ called "ratio of heat drops". For the blades I and II, we take c', c as circumferential velocities, ζ', ζ as coefficients of loss and β2', β2 as exit angles, respectively. We represent the relative velocity of fluid at the exit of the blades II with u2. Taking the velocity ratio ξ2= (c'+c)/u2 and the function about energy ratio Au22/(2gH0)=Φ, the circumferential efficiency ηc of an ideal middle stage may be given by the next equation [numerical formula] if we have the suitable value of the angle β2', calculated from the following equation [numerical formula] Using these equations, we have made clear numerically the performance of double-rotation axialflow turbines. And comparing these turbines with double-rotation radial-flow turbines, we have confirmed again our old deduction. In general we may expect to realize more efficient doublerotation turbines, if we are able to get the combinations of two blades having the sections suited to the given theoretical condition, not being restrained for any stage by such construction with two rows of blades having same sections as in Ljungstrom turbine.