Assessment of Rotor Stability for Steam Turbine Considering Labyrinth Seal Characteristics of Fluid Destabilization Force and Vibrational Frequency Effect of Bearing Coefficients

Accurate evaluation of the rotor stability is important for increasing the performance of the Steam Turbines. This paper discusses the important factors (such as destabilization force, bearing coefficient) for the evaluation of rotor stability. The destabilization force which varies with the type of seal suggests that seal shape plays an important role. In the past, several researchers have studied the fluid destabilization force both numerically and experimentally and prediction of the same can be done fairly accurately by applying CFD techniques. The characteristics of the fluid destabilization force can be accurately evaluated by investigating the sensitivity of parameters such as clearance and swirl velocity for each seal type using CFD. In the case of partial admission operation in which the bearing load changes (such as control stage of steam turbines), the frequency ratio effect on bearing coefficients is higher in the case of light-load (Sommerfeld number is large) than in the case of high-load (Sommerfeld number is small). In order to estimate the frequency ratio effects due to varying load accurately, an experimental study and analytical study were carried out. As a result of comparison of the test results to analytical results, the test results are in good agreement with thermo-elastic-hydrodynamic-lubrication (TEHL) analysis which considers deformation of pad obtained by 3D-FEM. The evaluation of rotor stability at each bearing load by partial admission (example: Governing Valve test) is in agreement with the field data of steam turbine. For new designs and modification designs, this assessment considering the characteristics of each parameter is effective for improving the quality of rotor design.

Twin-entry turbine losses: An analysis using CFD data

The current paper presents a computational fluid dynamics (CFD) flow behaviour and losses analysis of twin-entry radial turbines in terms of its Mass Flow Ratio ( MFR, the ratio between the flow passing through one of its intake ports and its total mass flow), focusing on the mixing phenomena in the unequal admission conditions cases. The CFD simulations are first validated with experimental data. Then, the losses mechanisms are analysed and quantified in the different parts of the twin-entry turbine in terms of the MFR value. A sudden expansion is found at the junction of both branches in the interspace between volutes and rotor for unequal and partial admission cases. Tracking the flow coming from each of the turbine intake ports, it has been observed that both flow branches do not fully mix with each other within the rotor. Another source of losses has been identified in the contact between both flow branches due to their momentum exchange that depends on the difference between both flow branches velocities. These losses have not been considered before, and they should be included in mean line loss-based models for twin-entry turbine since they are very significant for unequal admission conditions.

A Comparison of Partial Admission Axial and Radial Inflow Turbines for Underwater Vehicles

The metal fueled steam Rankine cycle has been successfully applied to Unmanned Underwater Vehicles. However, the suitable turbine configuration is yet to be determined for this particular application. In this paper, the mean-line design approach based on the existing empirical correlations is first described. The corresponding partial admission axial and radial inflow turbines are then preliminarily designed. To assess the performance of designed turbines, the three-dimensional Computational Fluid Dynamics (CFD) simulations and steady-state structural analysis are performed. The results show that axial turbines are more compact than radial inflow turbines at the same output power. In addition, since radial inflow turbines can reduce the exit energy loss, this benefit substantially offsets the increment of the rotor losses created by the low speed ratios and supersonic rotor inlet velocity. On the contrary, due to the large volume of dead gas and strong transient effects caused by the high rotor blade length of radial inflow turbines, the overall performance between axial and radial inflow turbines is comparable (within 4%). However, the strength of radial inflow turbines is slightly superior because of lower blade inlet height and outlet hub radius. This paper confirms that the axial turbine is the optimal configuration for underwater vehicles in terms of size, aerodynamics and structural performance.

Performance and Losses Analysis for Radial Turbine Featuring a Multi-Channel Casing Design

A novel control technique for radial turbines is under investigation for providing turbine performance controllability, especially in turbocharger applications. This technique is based on replacing the traditional spiral casing with a multi-channel casing (MC). The MC divides the turbine rotor inlet circumferentially into a certain number of channels. Opening and closing these channels controls the inlet area and, consequently, the turbine performance. The MC can be distinguished from other available control techniques in that it contains no movable parts or complicated control mechanisms. Within the casing, this difference makes it practical for a broader range of applications. In this investigation, a turbocharger featuring a turbine with MC has been tested on a hot gas test stand. The experimental test results show a reduction in the turbine operating efficiency when switching from full to partial admission. This reduction increases when reducing the admission percentage. To ensure the best performance of the turbine featuring MC while operating at different admission configurations, it becomes crucial to investigate its internal flow field at both full and partial admission to understand the reasons for this performance reduction. A full 3D computational fluid dynamics (CFD) model of the turbine was created for this investigation. It focuses on identifying the loss mechanisms associated with partial admission. Steady and unsteady simulations were performed and validated with available test data. The simulation results show that operating the turbine at partial admission results in highly disturbed flow. It also detects the places where aerodynamic losses occur and which are responsible for this performance reduction. This operation also shows flow unsteadiness even when operating at steady conditions. This unsteadiness depends mainly on the admission configuration and percentage.