Numerical Simulation of Cascade Flow: Vortex Element Method for Inviscid Flow Analysis and Axial Turbine Blade Design

This study aims to design an axial turbine rotor blade and predict the turbine performance at preliminary design stage. Quasi three dimensional method was applied to design including blade to blade flow analysis. The blade profile uses a NACA 0015 airfoil by varying the profile thickness from hub to tip. The profile is divided into eleven segments which has different parameters. The profile was analysed using blade to blade flow/cascade flow analysis called vortex panel method to obtain lift coefficient. The analysis of cascade flow was performed in potential flow and prediction of turbine perfomance is carried out involving common best practice to give drag effect on the blade. The design of the turbine was applied on three different rotors, which also have a different discharge, head, and design rotation. The outer diameter of turbine 1 is 0.65 m, while turbine 2 and turbine 3 have an outer diameter of 0,60 m. The calculation result show that the efficiency of turbines 1, 2, and 3 were 88,32%, 89,67%, and 89,04%, respectively.

Physical Mechanisms Investigation of Sharkskin-Inspired Compressor Cascade Based on Large Eddy Simulations

In this paper, the effectiveness of sharkskin-inspired riblets in reducing the aerodynamic loss of compressor cascade flow was investigated by using high-fidelity numerical simulation method. Two key normalized parameters were selected to parameterize various riblet designs, and the corresponding relative change in cascade performance was first investigated based on the uRANS simulations with/without transition model. Then, the large eddy simulations were conducted to investigate the cascade flow with the selected riblet design cases. By comparing the flow resistance, transition positions, vortex formations and turbulence fluctuations of the boundary flow, the flow control mechanisms of the riblets were finally studied. Simulation results show that compared with the prototype case, the total pressure loss can be reduced by up to 20.5% in the fully turbulent environment. This is because the spanwise fluctuation of the turbulent vortices is impeded, and the turbulent vortices are lifted above the riblet tip. However, when considering transition from laminar to turbulent boundary flow, the aerodynamic performance of compressor cascade strongly depends on the riblet position relative to the transition on cascade SS. The total pressure loss can only be reduced by up to 8.1%, and even most riblet designs will degrade the cascade performance. The major reason is that the riblets are located upstream of the transition zone, especially at the small incidence angles. Due to the installation of riblets, the contact area between the laminar flow and the wall surface is increased, and the downstream laminar-to-turbulent transition is promoted.

Influence of Pressure Surface Winglets on the Tip Leakage Flow in a Compressor Cascade With High Subsonic Mach Numbers

The improvement of compressor performance is facing a new technological challenge, as the compressor is considered as one of the core components in a gas turbine. Tip leakage flow affects the aerodynamic performance of the compressor rotor directly, then the compressor performance can be improved by reasonably controlling it. In recent years, the blade tip winglet has been certainly concerned as an effective flow control method for reducing the leakage loss. The mechanism of using tip winglets to control tip leakage flow in compressor cascade has been investigated in the condition of low Mach number, whereas the research in high subsonic incoming conditions also needs to be considered. To investigate the effect of the pressure surface winglet on the aerodynamic performance of a compressor cascade at high subsonic inlet Mach numbers, an experiment compared cascades with no winglet and different width pressure surface tip winglets at different inlet Mach numbers (Ma = 0.5, 0.6 and 0.7). Results show that the pressure surface winglet weakened the pressure gradient on both sides of the blade and reduced flow loss in the condition of high subsonic Mach numbers, which in turn tip clearance flow. When pressure surface tip winglet width increased, the improving degree is increased. At the same time, a change in Mach number had a proportional the effect on tip leakage flow control. The most effective pressure surface winglet was PW2.0 at the inlet Mach number of 0.7, which produced the most significant cascade loss reduction of 6.53% when compared to the original cascade at the same inlet Mach number. To investigate the characteristics of the compressor cascade at different incidences, the Mach number was set at 0.7 and the characteristics of cascade flow at −6°, −3°, 0°, +3° and +6°were studied. Pressure surface winglets with different widths reduced both the influence range of the leakage flow and the strength of the leakage vortex. As the tip winglet width increased, the influence of the tip winglet on the cascade flow increased. When incidences moved from negative to positive, the improvement effect of the cascade flow field with the pressure surface winglet was enhanced. When the incidence was+6°, for example the improvement effects the PW2.0 on cascade loss was 12.4%. The flow characteristics in the compressor cascade with the pressure surface winglets behave better at different Mach numbers and incidences. Through the research in this paper, the improvement effect and mechanism of the aerodynamic performance of the pressure surface winglet in high subsonic Mach number are clearer, and the application range of the winglet is widened, which provides a rich reference for the optimization design of compressor with high subsonic Mach number.

Physical Mechanisms Investigation of Sharkskin-Inspired Compressor Cascade Based on Large Eddy Simulations

In order to survive in a complex environment, nature has produced efficient and versatile resource-rich structures. One of the novel drag reduction designs comes from the efficient movement of sharks through microscope riblets aligned along the flow direction. In this paper, the effectiveness of sharkskin-inspired riblets in reducing the aerodynamic loss of compressor cascade flow was investigated by using high-fidelity numerical simulation method. Two key normalized parameters were selected to parameterize various riblet designs, and the corresponding relative change in cascade performance was first investigated based on the uRANS simulations with/without transition model. Then, the large eddy simulations in conjunction with the wall-adapted local eddy viscosity model were conducted to investigate the cascade flow with the selected riblet design cases. By comparing the flow resistance, transition positions, vortex formations and turbulence fluctuations of the boundary flow, the flow control mechanisms of the riblets were finally studied. Simulation results show that compared with the prototype case, the total pressure loss can be reduced by up to 20.5% in the fully turbulent environment. This is because the spanwise fluctuation of the turbulent vortices is impeded, and the turbulent vortices are lifted above the riblet tip. Low-speed streaks inside the riblet valleys generate relatively low shear stresses, while the high-shear stresses occur only at the riblet tips. However, when considering transition from laminar to turbulent boundary flow, the aerodynamic performance of compressor cascade strongly depends on the riblet position relative to the transition on cascade SS. The total pressure loss can only be reduced by up to 8.1%, and even most riblet designs will degrade the cascade performance. The major reason is that the riblets are located upstream of the transition zone, especially at the small incidence angles. Due to the installation of riblets, the contact area between the laminar flow and the wall surface is increased, and the downstream laminar-to-turbulent transition is promoted.

Adaptive Simulation Based on URANS and Ensemble Kalman Filter for Resolving Turbulent Flow on LES

Turbulence is one of the most important phenomena in fluid dynamics. In general, turbulent phenomena can be resolved more clearly with Large Eddy Simulation (LES) compared with Unsteady Reynolds Averaged Navier-Stokes (URANS), and the numerical solution shows good agreements with that based on Direct Numerical Simulation (DNS). However, more time and computational power are needed on LES than those on URANS. If possible, the ideal simulation method is that the method is able to resolve the turbulent phenomena same quality as the results based on DNS and LES with less time and less computational power same as that on URANS. This paper shows an adaptive simulation method based on URANS and Ensemble Kalman Filter (Enkf) to reproduce the flow fields based on LES. In this study, a two-dimensional turbine cascade flow has been solved with URANS and LES. The adaptive simulation method has been also applied to the cascade flow. As the results, in the flow field of URANS with the assimilated turbulence model’s parameters, the separation phenomenon and the boundary layer thickness was close to that of the time averaged LES.

Comparative Study on Modal Decomposition Methods of Unsteady Separated Flow in Compressor Cascade

The present work investigated the vortex structure and fluctuation frequency characteristics generated by boundary layer separation of a high-load compressor cascade using modal decomposition methods. The dominant modes and dynamic behaviors of unsteady flow in the cascade were obtained, and the differences of three modal decomposition methods (Proper Orthogonal Decomposition, Dynamic Mode Decomposition and Spectral Proper Orthogonal Decomposition) in feature recognition of cascade flow were discussed. The results show that:(1) The POD method can accurately extract the dominant spatial structure of the flow field, but the modal coefficients are multi-frequency coupled, which makes the dominant modal characteristics of cascade flow unclear. (2) The standard DMD method can obtain the spatial-temporal single frequency mode of cascade flow, as well as their growth rates and frequencies. However, this method is likely to capture the suboptimal mode of large amplitude with large attenuation rate, and fails to obtain the high-frequency coherent structure, which makes it impossible to obtain the dominant feature with limited mode number. (3) The SPOD method, based on spectral characteristics, can obtain spatial and temporal single frequency modes, and there is no modal screening problem. The use of spectral estimation method (SPOD) reduces the sensitivity to numerical noise. This method can obtain the low-rank behavior of cascade flow, which is helpful to understand cascade flow mechanisms. Therefore, SPOD method is more advantageous for the modal analysis of unsteady separated flow in high-load compressor cascade.

Direct Numerical Simulation of Turbine Cascade Flow with Heat Transfer

Two- and three-dimensional direct numerical simulation (DNS) of turbine cascade flow at low Reynolds number with heat transfer are performed using high-order finite difference method. Two-dimensional laminar computation which is used to construct the initial flow of three-dimensional DNS fails to predict Stanton number on the second half of suction side where the flow is turbulent in experiment. In three-dimensional DNS, transition is triggered by periodic blow-and-suction disturbances. Numerical experiments show that phase randomness of the disturbance is not necessary to trigger the transition which can be induced by disturbances with fixed phases. In a range of time fundamental frequency of disturbance, when increasing the frequency, transition moves downstream. When time fundamental frequency is big enough, transition disappears. With fixed space phases, time phases and selected time fundamental frequency, time averaged pressure and Stanton number distributions of three-dimensional DNS coincide with the experimental datum. Averaged velocity and temperature, Root-Mean-squares (RMS) of velocity pulse，temperature pulse, Reynolds shear stress and heat flux are extracted from the DNS database. All statistics agree well with experimental and theoretical results which verify the accuracy of present database.