Hiteshkumar Mistry

Loss Audit of a Turbine Stage

In order to achieve high aerodynamic efficiency of a turbine stage, it is crucial to identify the source of aerodynamic losses and understand the associated loss generation mechanisms. This helps a turbine designer to maximize the performance of the turbine stage. It is well known that aerodynamic losses include profile, endwall, cooling/mixing loss, leakage, and trailing edge loss components. However, it is not a trivial task to separate one from the others because different loss sources occur concurrently and they interact with each other in a machine. Consequently, designers tend to rely on various empirical correlations to get an approximate estimate of each aerodynamic loss contribution. In this study, a systematic loss audit of an uncooled turbine stage has been undertaken by conducting a series of numerical experiments. By comparing entropy growth across the turbine stage, aerodynamic losses are broken down within the stator, rotor, and interblade row gap. Furthermore, losses across each blade row are broken down into profile, leakage, endwall, and trailing edge losses. The effect of unsteady interaction due to the relative motion of the stator and the rotor was also identified. For the examined turbine stage, trailing edge losses of the rotor were dominated, contributing to more than a third of the total aerodynamic loss. The profile loss across the stator and the rotor, unsteady loss between the stator and the rotor, and the stator endwall loss were also identified to be the significant loss sources for this turbine stage. The design implications of the findings are discussed.

A FLUX-CONSERVATION MIXING PLANE ALGORITHM FOR MULTIPHASE NON- EQUILIBRIUM STEAM MODELS

The accuracy of steady multistage turbomachinery calculations is dependent on the exchange of information at an interface, also known as mixing plane, between adjacent blade rows. This paper describes an extension of already available flux conservation mixing plane algorithm to the Eulerian-Eulerian non-equilibrium steam model. Since algorithms that conserve fluxes across the interface have been found to be more accurate and robust, greater emphasis is on the conservation of mixture and droplet fluxes across the interface. The key characteristics of the algorithm are provided, along with its application on simple two- and three-dimensional blade rows.

Performance Evaluation of Last Stage Bucket Tip Section Using Non-Equilibrium Wet Steam CFD Tool

In order to develop the next generation of low pressure steam turbines, it is imperative to understand the moisture effects and their impact on performance. The objective of the current study is to apply the recently developed in-house multiphase CFD code to the analysis of the tip section of the last stage bucket (LSB), and propose design improvements based on insights gained into wet steam flow physics. 2D simulations were first carried out with equilibrium and non-equilibrium condensation models, showing significant differences in losses which were attributed to non-equilibrium condensation. Details of the shock structures in the transonic blade tip passage under equilibrium and non-equilibrium condensation models were captured with best in class split blade grid topology. Sensitivity studies of major airfoil design parameters such as un-guided turning and trailing edge angle were carried out and their impact quantified on the basis of wetness losses. Based on the detailed investigation of the flow field, a new design feature on the suction side of the LSB was proposed. Numerical results show higher performance benefit of the new design at design point operation.Copyright

Numerical Sensitivity Studies on Nucleation of Droplets in Steam Turbine

This work presents the results of canonical test cases that highlight the importance of nucleation bulk surface tension factor (NBTF) on CFD predictions for condensing flows in steam turbines. Numerical simulations are carried out on nozzle and cascade geometries to explore modeling effects on the condensation of water vapor. The recent Euler-Euler approach [10] for modeling homogeneous condensation provides better results than the equilibrium assumption. Modeling of the nucleation rate plays a significant role in the non-equilibrium approach and it depends on the free surface energy of each droplet. NBTF is introduced in the classical homogeneous condensation nucleation rate expression to control the intensity of the homogeneous condensation event [3, 10]. It is observed that the NBTF controls the location of the condensation front, degree of super cooling, wetness fraction and droplet size. In addition, no unique value of NBTF is found in the range of simulations to match the experimental observations. Finally, by increasing the value of NBTF from 0.7 to 1.0 for a particular nozzle case, the location of condensation front is shown to be delayed by 60 mm and super cooling increased by 20%. This in turn will affect quantities such as the flow angle, pressure at the blade row exit and the thermodynamic loss which are relevant for the turbine designer.Copyright

Aerodynamic Performance Assessment of Part-Span Connector of Last Stage Bucket of Low Pressure Steam Turbine

Modern steam turbines often utilize very long last stage buckets (LSB’s) in their low-pressure sections to improve efficiency. Some of these LSB’s can range in the order of 5 feet long. These long buckets (aka “blades”) are typically supported at their tip by a tip-shroud and near the mid span by a part span shroud or part span connector (PSC). The PSC is a structural element that connects all the rotor blades, generally at the mid span. It is primarily designed to address various structural issues, often with little attention to its aerodynamic effects. The objective of the current work is to quantify the impact of PSC on aerodynamic performance of the last stage of a LP steam turbine by using detailed CFD analyses. A commercial CFD solver, ANSYS CFX™, is used to solve the last stage domain by setting steam as the working fluid with linear variation of specific heat ratio with temperature. A tetrahedral grid with prismatic layers near the solid walls is generated using ANSYS WORKBENCH™. The results show a cylindrical PSC reduces the efficiency of the last stage by 0.32 pts, of which 0.20 pts is due to the fillet attaching the PSC to the blade. The results also show insignificant interaction of the PSC with the bucket tip aerodynamics. The work presents a detailed flow field analysis and shows the impact of PSC geometry on the aerodynamic performance of last stage of steam turbine. Present work is useful to turbine designer for trade-off studies of performance and reliability of LSB design with or without PSC.Copyright