Computational fluid dynamics simulation of surge in a three stage axial compressor

A three dimensional unsteady Reynolds-Averaged Navier-Stokes solver was used to perform multistage unsteady simulations of a three and half stage compressor. Previously published research presented the simulation of the same compressor with mal-scheduling of the variable stator vanes (VSV) and predicted a rotating stall pattern in all stages. The nominal VSV schedule compressor was simulated to provide a reference case for comparison purposes. The compressor’s behaviour in the nominal case seemed to behave against conventional wisdom with mass flow and pressure fluctuations representing compressor surge. However further analysis showed that inherent design feature in the compressor which had a highly loaded third stator was the primary cause of surge initiation, a situation which was eased with mal-scheduling by unloading that stator. Full analysis of the simulation results of the nominal case and discussion are presented in this paper

Numerical investigation of VSVs mal-schedule effects in a three-stage axial compressor

Variable Stator Vanes (VSVs) are commonly used in multi-stage axial compressors for stage matching at part load operations and during start up. Improper VSVs settings or malfunction of the controlling actuator system can lead to compressor instabilities including rotating stall and surge. It is important to be able to predict the aerodynamic behaviour of compressors in such events to either produce tolerant designs or incorporate diagnosis and recovery systems. This paper presents a numerical study of a compressor operating near the stall boundary for a mal-scheduled VSVs case. A high-speed three-stage axial compressor with Inlet Guide Vanes (IGV) is used in the investigation because of its relative simplicity and availability of geometry and aerodynamic data. A 3D RANS viscous unsteady time-accurate flow solver was used to perform the full annulus simulation with a downstream variable nozzle to control outflow boundary conditions. The unstructured mesh contained about 25 million grid points and the simulation was performed on a high performance computing cluster for many engine rotations. Rotating stall with one single cell covering several passages in all three rotors was predicted which propagated at approximately half of the shaft speed. Full analysis of the flow features is presented in the paper

Effects of blade damage on the performance of a transonic axial compressor rotor

Gas turbine axial compressor blades may encounter damage during service for various reasons. Debris from casing or foreign objects may impact blades causing damage near the rotor’s tip. This may result in deterioration of performance and reduction in the surge margin. Ability to assess the effect of damaged blades on the compressor performance and stability is important at both the design stage and in service. The damage to compressor blades breaks the cyclic symmetry of the compressor assembly. Thus computations have to be performed using the whole annulus. Moreover, if rotating stall or surge occurs, the downstream boundary conditions are not known and simulations become difficult. This paper presents an unsteady CFD analysis of compressor performance with tip curl damage. Tip curl damage typically occurs when rotor blades hit a loose casing liner. The computations were performed up to the stall boundary, predicting rotating stall patterns. The aim is to assess the effect of blade damage on stall margin and provide better understanding of the flow behaviour during rotating stall. Computations for the undamaged rotor are also performed for comparison. A transonic axial compressor rotor is used for the time-accurate numerical unsteady flow simulations, with a variable choked nozzle downstream simulating an experimental throttle. One damaged blade was introduced in the rotor assembly and computations were performed at 60% of the design rotational speed. It was found that there is no significant effect on the compressor stall margin due to one damaged blade despite the differences in rotating stall patterns between the undamaged and damaged assemblies

Binary interaction uncertainty in the optimisation of a transcritical cycle: consequences on cycle and turbine design

Doping CO2 with an additional fluid to produce a CO2-based mixture is predicted to enhance the performance of the super critical CO2 power cycle and lower its cost when adapted to Concentrated Solar Power plants. A consistent fluid mixture modelling process is necessary to reliably design and predict the performance of turbines operating with CO2-based working fluids. This paper aims to quantify the significance of the choice of an Equation of State (EoS) and the uncertainty in the binary interaction parameter (Kij) on the cycle and turbine design. To evaluate the influence of the thermodynamic model, an optimisation study of a 100 MWe simple recuperated transcritical CO2 cycle is conducted for a combination of three mixtures, four equations of state, and three possible values of the binary interaction parameter. Corresponding multi-stage axial turbines are then designed and compared based on the optimal cycle conditions. Results show that the choice of the dopant fraction which yields maximum cycle thermal efficiency is independent from the fluid model used. However, the predicted thermal efficiency of the mixtures is reliant on the fluid model. Absolute thermal efficiency may vary by a maximum of 1% due to the choice of the EoS, and by up to 2% due to Kij uncertainty. The maximum difference in the turbine geometry due to EoS selection corresponded to a 6.3% (6.6 cm) difference in the mean diameter and a 18.8% (1.04 cm) difference in the blade height of the final stage. On the other hand, the maximum difference in turbine geometry because of Kij uncertainty amounted to 6.7% (5.6 cm) in mean diameter and 27.3% (2.73 cm) in blade height of the last stage.</p

Loss analysis in radial inflow turbines for supercritical CO2 mixtures

Recent studies have indicated the potential of CO2-mixtures to lower the cost of concentrated solar power plants. Based on aerodynamic and cost considerations, radial inflow turbines (RIT) can be a suitable choice for small to medium sized sCO2 power plants (about 100 kW to 10 MW). The aim of this paper is to quantify the effect of doping CO2 on the design of RITs. This is achieved by comparing the 1D mean-line designs and aerodynamic losses of pure sCO2 RITs with those of three sCO2 mixtures containing tetrachloride (TiCl4), sulphur dioxide (SO2), and hexaflourobenzene (C6F6). Results show that the optimal turbine designs for all working fluids will have similar rotor shapes and velocity diagrams. However, factors such as the clearance-to-blade-height ratio, turbine pressure ratio, and the difference in the viscosity of the fluids cause variations in the achievable turbine efficiency. Once the effects of these factors are eliminated, differences in the total-to-static efficiency amongst the fluids may become less than 0.1%. Moreover, if rotational speed limits are imposed, then greater differences in the designs and efficiencies of the turbines emerge amongst the fluids. It was found that limiting the rotational speed reduces the total-to-static efficiency in all fluids; the maximum reduction is about 15% in 0.1 MW CO2 compared to the 3% reduction in CO2/TiCl4 turbines of the same power. Among the mixtures studied, CO2/TiCl4 achieved the highest performance, followed by CO2/C6F6, and then CO2/SO2. For example, 100 kW turbines for CO2/TiCl4, CO2/C6F6, CO2/SO2, and CO2 achieve total-to-static efficiencies of 80.0%, 77.4%, 78.1%, and 75.5% respectively. Whereas, the efficiencies for 10 MW turbines are 87.8%, 87.3%, 87.5%, and 87.2%, in the same order

Loss analysis in radial inflow turbines for supercritical CO2 mixtures

Recent studies have indicated the potential of CO2-mixtures to lower the cost of concentrated solar power plants. Based on aerodynamic and cost considerations, radial inflow turbines (RIT) can be a suitable choice for small to medium sized sCO2 power plants (about 100 kW to 10 MW). The aim of this paper is to quantify the effect of doping CO2 on the design of RITs. This is achieved by comparing the 1D mean-line designs and aerodynamic losses of pure sCO2 RITs with those of three sCO2 mixtures containing tetrachloride (TiCl4), sulphur dioxide (SO2), and hexaflourobenzene (C6F6). Results show that the optimal turbine designs for all working fluids will have similar rotor shapes and velocity diagrams. However, factors such as the clearance-to-blade-height ratio, turbine pressure ratio, and the difference in the viscosity of the fluids cause variations in the achievable turbine efficiency. Once the effects of these factors are eliminated, differences in the total-to-static efficiency amongst the fluids may become less than 0.1%. Moreover, if rotational speed limits are imposed, then greater differences in the designs and efficiencies of the turbines emerge amongst the fluids. It was found that limiting the rotational speed reduces the total-to-static efficiency in all fluids; the maximum reduction is about 15% in 0.1 MW CO2 compared to the 3% reduction in CO2/TiCl4 turbines of the same power. Among the mixtures studied, CO2/TiCl4 achieved the highest performance, followed by CO2/C6F6, and then CO2/SO2. For example, 100 kW turbines for CO2/TiCl4, CO2/C6F6, CO2/SO2, and CO2 achieve total-to-static efficiencies of 80.0%, 77.4%, 78.1%, and 75.5% respectively. Whereas, the efficiencies for 10 MW turbines are 87.8%, 87.3%, 87.5%, and 87.2%, in the same order

Loss analysis in radial inflow turbines for supercritical CO2 mixtures

Recent studies suggest that CO2 mixtures can reduce the costs of concentrated solar power plants. Radial inflow turbines (RIT) are considered suitable for small to medium-sized CO2 power plants (100 kW to 10 MW) due to aerodynamic and cost factors. This paper quantifies the impact of CO2 doping on RIT design by comparing 1D mean-line designs and aerodynamic losses of pure CO2 RITs with three CO2 mixtures: titanium tetrachloride (TiCl4), sulfur dioxide (SO2), and hexafluorobenzene (C6F6). Results show that turbine designs share similar rotor shapes and velocity diagrams for all working fluids. However, factors like clearance-to-blade height ratio, turbine pressure ratio, and fluid viscosity cause differences in turbine efficiency. When normalized for these factors, differences in total-to-static efficiency become less than 0.1%. However, imposing rotational speed limits reveals greater differences in turbine designs and efficiencies. The imposition of rotational speed limits reduces total-to-static efficiency across all fluids, with a maximum 15% reduction in 0.1 MW CO2 compared to a 3% reduction in CO2/TiCl4 turbines of the same power. Among the studied mixtures, CO2/TiCl4 turbines achieve the highest efficiency, followed by CO2/C6F6 and CO2/SO2. For example, 100 kW turbines achieve total-to-static efficiencies of 80.0%, 77.4%, 78.1%, and 75.5% for CO2/TiCl4, CO2/C6F6, CO2/SO2, and pure CO2, respectively. In 10 MW turbines, efficiencies are 87.8%, 87.3%, 87.5%, and 87.2% in the same order.</p

Design of a 130 MW axial turbine operating with a supercritical carbon dioxide mixture for the SCARABEUS project

The application of supercritical carbon dioxide (sCO2) mixtures in power generation cycles could improve power block efficiency for concentrated solar power applications. Mixing CO2 with titanium tetrachloride (TiCl4), hexafluoro-benzene (C6F6), sulphur dioxide (SO2) or others increases the critical temperature of the working fluid, allowing it to condense at ambient temperatures in dry solar field locations. Therefore, transcritical power cycles, which have lower compression work and higher thermal efficiency compared to supercritical cycles, become feasible. This paper presents the flow path design of a utility-scale axial turbine operating with an 80-20% molar mix of CO2 and sulphur dioxide (SO2). A preliminary turbine design has been developed for the selected mixture using an in-house mean-line design code considering both mechanical and rotor dynamic criteria. Furthermore, 3D blades have been generated and blade shape optimisation has been carried out for the first and last turbine stages of the multi-stage design. It has been found that increasing the number of stages from 4 to 14 stages increase the total-to-total efficiency by 6.3%. The final turbine design has a total-to-total efficiency of 92.9% as predicted by the 3D numerical results with maximum stress less than 260 MPa and a mass flow rate within 1% of the intended cycle mass-flow rate. Optimum aerodynamic performance was achieved with a 14- stages design where the hub radius and the flow path length are 310 mm and 1800 mm respectively.</p

Integrated aerodynamic and mechanical design of a large-scale axial turbine operating with a supercritical carbon dioxide mixture

In this paper, the design of a large-scale axial turbine operating with supercritical carbon dioxide (sCO2) blended with sulfur dioxide (SO2) is presented considering aerodynamic and mechanical design aspects as well as the integration of the whole turbine assembly. The turbine shaft power is 130 MW, designed for a 100 MWe concentrated-solar power plant with turbine inlet conditions of 239.1 bar and 700 °C, total-to-static pressure ratio of 2.94, and mass-flow rate of 822 kg/s. The aerodynamic flow path, obtained in a previous study, is first summarized before the aerodynamic performance of the turbine is evaluated using both steady-state and unsteady three-dimensional numerical models. Whole-annulus unsteady simulations are performed for the last turbine stage and the exhaust section to assess the unsteady loads on the rotor due to downstream pressure field distortion and to assess the aerodynamic losses within the diffuser and exhaust section. The potential low engine order excitation at the last rotor stage natural frequency modes due to downstream pressure distortion is assessed. The design of the turbine assembly is constrained by current manufacturing capabilities and the properties of the proposed working fluid. High-level flow-path design parameters, such as pitch diameter and number of stages, are established considering a trade-off between weight and footprint, turbine efficiency, and rotordynamics. Rotordynamic stability is assessed considering the high fluid density and related cross coupling effects. Finally, shaft end sizing, cooling system design, and the integration of dry gas seals are discussed.</p

Integrated aerodynamic and mechanical design of a large-scale axial turbine operating with supercritical carbon dioxide mixtures

In this paper, the design of a large-scale axial turbine operating with supercritical carbon dioxide (sCO2) blended with sulfur dioxide (SO2) is presented considering aerodynamic and mechanical design aspects as well as the integration of the whole turbine assembly. The turbine is 130 MW, designed for a 100 MWe concentrated-solar power plant with turbine inlet conditions of 239.1 bar and 700 °C, total-to-static pressure ratio of 2.94 and mass-flow rate of 822 kg/s. The aerodynamic flow path, obtained in a previous study, is first summarised before the aerodynamic performance is evaluated using both steady-state and unsteady 3D numerical models to simulate the aerodynamic performance of the turbine. Whole-annulus unsteady simulations are performed for the last turbine stage and the exhaust section to assess the unsteady loads on the rotor due to downstream pressure field distortion and to assess aerodynamic losses of the diffuser and exhaust section. The potential low engine order excitation on the last rotor stage natural frequency modes due to downstream pressure distortion is assessed. The design of the turbine assembly is constrained by current manufacturing capabilities and the proposed working fluid properties. High-level flow-path design parameters, such as pitch diameter and number of stages, are established considering a trade-off between weight and footprint, turbine efficiency and rotordynamics. Rotordynamic stability is assessed considering the high fluid density related to cross coupling effects. Finally, shaft end sizing, cooling system design and the integration of dry gas seals are discussed