Computational fluid dynamics simulations of blade damage effect on the performance of a transonic axial compressor near stall

Gas turbine axial compressor blades may encounter damage during service for various reasons such as damage by debris from casing or foreign objects impacting the blades, typically near the rotor’s tip. This may lead to deterioration of performance and reduction in the surge margin. The damage breaks the cyclic symmetry of the rotor assembly; thus, computational fluid dynamics simulations have to be performed using full annulus compressor assembly. Moreover, downstream boundary conditions are unknown during rotating stall or surge, and simulations become difficult. This paper presents unsteady computational fluid dynamics analyses of compressor performance with tip curl damage. Computations were performed near the stall boundary. The primary objectives are to understand the effect of the damage on the flow behaviour and compressor stability. Computations for the undamaged rotor assembly were also performed as a reference case. A transonic axial compressor rotor was used for the time-accurate numerical unsteady flow simulations, with a variable area nozzle downstream simulating an experimental throttle. Computations were performed at 60% of the rotor design speed. Two different degrees of damage for one blade and multiple damaged blades were investigated. Rotating stall characteristics differ including the number of stall cells, propagation speed and rotating stall cell characteristics. Contrary to expectations, damaged blades with typical degrees of damage do not show noticeable effects on the global compressor performance near stall

Using a Cubic Equation of State to Identify Optimal Working Fluids for an ORC Operating with Two-Phase Expansion Using a Twin-Screw Expander

For waste-heat recovery applications, operating an organic Rankine cycle (ORC) with two-phase expansion has been shown to increase the utilisation of the waste-heat stream, leading to a higher power output compared to a conventional ORC with single-phase expansion. However, unlike the conventional ORC, working-fluid selection for an ORC operating with two-phase expansion has not been explored in detail within the literature. Therefore, the aim of this paper is to explore which working-fluid parameters make a particular working fluid suitable for this type of cycle. This is conducted by coupling a thermodynamic model of the cycle with the Peng-Robinson cubic equation of state. Moreover, the effect of the expander volumetric ratio on the expander isentropic efficiency is accounted for using a performance model for a twin-screw expander. Ultimately, the adopted approach allows the effect of the working-fluid parameters, namely the critical temperature and ideal specific-heat capacity, on both the expander performance and the cycle to be evaluated in a generalised way. For the investigation, 15 theoretical working fluids are defined, covering five different critical temperatures, with a negatively-sloped, vertical and positively-sloped saturated vapour line respectively. The 15 working fluids are selected as they represent the feasible design space occupied by existing ORC working fluids. For each fluid, a cycle optimisation is completed for different heat-source temperatures ranging between 80 and 200 °C. The objective is to identify the optimal cycle operating conditions that result in maximum power output from the system. By analysing the results, the optimal characteristics of a working fluid are obtained, and this information can be used to identify physical working fluids which are good candidates for a particular heat-source temperature. In the final part of this paper, the cycle optimisation is repeated for the physical working fluids identified, thus validating the suitability of the approach developed. Ultimately, the results can help to narrow down the search space when considering working fluids for an ORC operating with two-phase expansion

GT2006-90434 THE EFFECT OF STAGGER VARIABILITY IN GAS TURBINE FAN ASSEMBLIES

ABSTRACT Fan blades of high bypass ratio gas turbine engines are subject to substantial aerodynamic and centrifugal loads, producing the well-known phenomenon of fan blade untwist. The accurate prediction of the running geometry, as opposed to the cold geometry at rest, is crucial in the assessment of aerodynamic performance, vibratory response and noise production of the fan. The situation is further complicated by the fact that some geometric variation is inevitable even for the state-ofthe-art manufacturing processes used. The aim of this paper is to investigate the effect of static stagger variability on the dynamic untwist behaviour of fan assemblies. An aeroelastic model was used to show that under certain conditions the stagger pattern changes significantly, both in form and amplitude, relative to the static configuration. At other conditions, a strong correlation between the running and static patterns is demonstrated. [Keywords: stagger variability, stagger pattern, untwist

Full Annulus Numerical Study of Hot Streaks Propagation in a Hydrogen-rich Syngas-Fired Heavy Duty Axial Turbine

This paper presents a study of the effect of fuel composition on hot streaks propagation in a high-pressure turbine using a full annulus unsteady computational fluid dynamics analysis of the first two stages. Hot streaks result from the inherent non-uniformities of temperature profiles at the exit of the combustion chamber. Variations in composition arise from current challenges requiring gas turbines to adapt to fuel variations driven by the need to reduce CO2 emissions through the use of synthetic hydrogen-rich fuels (syngas) typically generated from the gasification of coal or solid waste. Syngas containing 80% hydrogen has been used in this study in a heavy duty gas turbine modified to accommodate the low calorific value fuel. Calculations were conducted on the baseline gas turbine originally designed for natural gas for the comparative study. Applying combustor representative hot streak profiles, analyses were performed for different hot streak distributions and locations. Analysis of results focused on the segregation of cold and hot fluid patterns and the effects of hot streaks on secondary flows and temperature re-distributions up to the second turbine stage. The hot flow pattern is affected by the fuel composition, resulting in more concentrated thermal wake shapes for syngas when compared to the reference natural gas fuel. In effect, the interaction with the secondary flow leads to more intense flow turning of the pressure side leg of the horseshoe vortex in the first rotor passage. The higher temperature levels in the case of syngas, in combination with the effect of the enhanced secondary flow, result in higher radial spread of the hot fluid that tends to migrate towards the blade hub and tip with the effects being obvious further downstream the first turbine stage

A comparison of flow path designs for axial turbines operating with pure CO2 and CO2 mixtures

Supercritical CO2 (sCO2) mixtures have been found to be promising for enhancing the performance of power cycles for concentrated solar power (CSP) applications, with up to a 6% enhancement in cycle efficiency compared to a simple recuperated CO2 cycle depending upon the mixture and cycle configuration chosen. Given that turbine efficiency significantly affects the overall plant performance, it is important to confirm whether turbines operating with CO2 mixtures can achieve the same efficiencies compared to pure CO2, whilst exploring whether the use of mixtures introduces any differences in the turbine design. This study aims to investigate the differences in turbine flow path designs produced for pure CO2 compared to CO2 mixtures, whilst taking into account aerodynamic, rotordynamic and mechanical design aspects, as assessed during the mean-line design process. The aim of this study extends to evaluating the effect of key turbine design variables, such as the loading coefficient, flow coefficient and degree of reaction, on the flow path design and overall aerodynamic performance. Multiple flow path designs have been produced for axial turbines operating with pure CO2 and mixtures of CO2 with titanium tetrachloride (TiCL4), hexafluorobenzene (C6F6) and sulphur dioxide (SO2) for installation in a 100 MWe CSP plant. It is found that turbines operating with either pure CO2 or CO2 mixtures result in overall total-to-total efficiencies in excess of 92.5%; where the highest turbine efficiency is achieved for the turbine operating with pure CO2, whilst this reduces by a maximum of 1.1 percentage points for the CO2/TiCL4 mixture. This reduction in efficiency is because the CO2/TiCL4 turbine is limited to a maximum of six design stages in order to meet the imposed mechanical design criteria, whilst the pure CO2 turbine can accommodate thirteen stages leading to higher aerodynamic efficiency. The difference between the two cases is the result of a higher mass-flow rate for the CO2/TiCL4 mixture (66% greater than for pure CO2), which results in high rotor bending stresses and limits the number of stages to comply with the design criteria. It is also found that designing the turbine at loading and flow coefficients of 0.8 and 0.6 respectively, whilst fixing the degree of reaction and pitch-to-chord ratio to values of 0.5 and 0.85 respectively, resulted in an efficiency enhancement of 0.2% with respect to a baseline design produced at loading and flow coefficients of 1.0 and 0.5. This increase is due to being able to increase the number of stages from eleven to fifteen. This indicates that there is not much benefit in modifying key design parameters to improve the turbine efficiency as the 0.2% efficiency enhancement is considered within the margin of accuracy of mean-line flow path design

The Application of Similitude Theory for the Performance Prediction of Radial Turbines Within Small-Scale Low-Temperature Organic Rankine Cycles

For small-scale organic Rankine cycles (ORCs) to be a competitive technology, it is reasonable to assume that the same turbine design will be implemented into a range of different applications. It is therefore critical to be able to predict turbine off-design performance over a range of different operating conditions while utilizing different working fluids. Similitude theory can be used for this purpose, and it has been well validated for ideal gases. However, the same cannot be said for its applications to the organic fluids found within ORCs. This paper considers a candidate subsonic turbine design operating with R245fa and the corresponding turbine performance map. Similitude theory is used to predict the performance of the same turbine operating at different inlet conditions using R245fa, R123, and R1234yf. The similitude predictions are compared to computational fluid dynamics (CFD) results obtained using ansys CFX. The original similitude theory using turbine total inlet conditions was found to only apply within a small range of operating conditions, so a modified similitude theory has been suggested that uses the choked flow conditions instead. This modified similitude theory agrees with the CFD predictions to within 2%, right up until the choked mass flow rate. Further studies considering supersonic turbines are required to establish the applicability of similitude for applications beyond the choked pressure ratio