This paper discusses the importance of the unsteady interaction in transonic turboma chinery stages. Although the flow in a turbomachine is inherently unsteady, most cur rent calculations for routine design work exploit the steady state assumption. In fact, unsteady flow effects are often taken into account for mechanical integrity checks, such as blade flutter or forced response, or heat transfer issues associated with circumferen tial nonuniformities, whereas steady state calculations are usually selected for the aer odynamic design. In this work, some cases are discussed in which significant departures are found between steady and time-averaged results, and the basic fluid mechanisms responsible for them are examined. Finally, a current perspective of unsteady computational fluid dynamics (CFD) calculations for the aerodynamic design is given.
This work deals with the influence of roughness on high-pressure steam turbine stages. It is divided in three parts. In the first one, an experimental campaign on a linear cascade is described, in which blade losses are measured for different values of surface roughness and in a range of Reynolds numbers of practical interest. The second part is devoted to the basic aspects of the numerical approach and consists of a detailed discussion of the roughness models used for computations. The fidelity of such models is then tested against measurements, thus allowing their fine-tuning and proving their reliability. Finally, comprehensive computational fluid dynamics (CFD) analysis is carried out on a high-pressure stage, in order to investigate the influence of roughness on the losses over the entire stage operating envelope. Unsteady effects that may affect the influence of the roughness, such as the upcoming wakes on the rotor blade, are taken into account, and the impact of transition-related aspects on the losses is discussed.
The usual ways to measure the aerodynamic forcing function are complex and expensive. The aim of this work is to evaluate the forces acting on the blades using a relatively simpler experimental methodology based on a time-resolved pressure measurement at the rotor discharge. Upstream of the rotor, a steady three holes probe (3HP) has been used. The postprocessing procedures are described in detail, including the application of a phase-locked average and of an extension algorithm with phase-lag. The algorithm for the computation of the force components is presented, along with the underlying assumptions. In order to interpret the results, a preliminary description of the flowfield, both upstream and downstream of the rotor, is provided. This gives an insight of the most relevant features that affect the computation of the forces. Finally, the analysis of the results is presented. These are first described and then compared with overall section-average results (torque-sensor), and with the results from 3D unsteady simulations (integral of pressure over the blade surface) in order to assess the accuracy of the method. Both the experimental and the numerical results are also compared for two different operating conditions with increasing stage load
This paper describes some experiences about impact of unsteadiness in turbine flows, with a special focus on the effects of potential interaction on aerodynamic performance. The main motivation consists in trying to identify some design areas in which some further margins of improvement could be found, provided the designer chooses the proper computational framework. The underlying idea is that the approximations associated with the steady-state picture of a turbine stage might prevent the designer from unlocking the full potential of the stage, especially when the design requirements imply a challenging aerodynamics. To this end, three common design topics are presented in which the step from the classical steady-state approach to the time-accurate one unveils relevant issues, which in turn have an impact on aerodynamic performance: stator/rotor interaction in transonic stages, the choice of the axial gap between stator and rotor, and the choice of the blade count ratio. In all reported cases, significant departures are found between steady and time-averaged results, and the basic fluid mechanisms responsible for them are examined. In particular, an attempt is made to emphasize limitations deriving from of the steady-state picture of the turbine flow field, in order to warn the designer about the possible traps of the steady-state assumption.
This work aims at investigating the impact of partial admission on a steam turbine stage, focusing on the aerodynamic performance and the mechanical behavior. The partialized stage of a small steam turbine was chosen as test case. A block of nozzles was glued in a single “thick nozzle” in order to mimic the effect of a partial admission arc. Numerical analyses in full and in partial admission cases were carried out by means of three-dimensional, viscous, unsteady simulations. Several cases were tested by varying the admission rate, that is the length of the partial arc, and the number of active sectors of the wheel. The goal was to study the effect of partial admission conditions on the stage operation, and, in particular on the shape of stage performance curves as well as on the forces acting on bucket row. First of all, a comparison between the flow field of the full and the partial admission case is presented, in order to point out the main aspects related to the presence of a partial arc. Then, from an aerodynamic point of view, a detailed discussion of the modifications of unsteady rows interaction (potential, shock/wake), and how these ones propagate downstream, is provided. The attention is focused on the phenomena experienced in the filling/emptying region, which represent an important source of aerodynamic losses. The results try to deepen the understanding in the loss mechanisms involved in this type of stage. Finally, some mechanical aspects are addressed, and the effects on bucket loading and on aeromechanical forcing are investigated.
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