Cavity for Flow Control and Performance Improvement of Airfoil

The fluid flow analysis over a cambered airfoil having three different cavity locations on the suction surface is reported in this paper. The Elliptical cavity is created at LE, MC, and TE along chordwise locations from the leading to trailing edge. In this regard, the steady simulation is carried out in the Fluent at Reynolds number of 105 based on their chord length. The lift and drag characteristics for clean and cavities airfoil are investigated at different angles of attack. For the clean airfoil, the stall point is observed at 18°. The presence of a cavity improves the stall and aerodynamic characteristics of airfoil. It has been seen that the lift and drag coefficients for pre-stalled or lower angles are nearly similar to clean and cavity at MC or TE positions. For the post-stall point, the improvement in the aerodynamic performance is seen for the cavity at MC or TE. The cavity placed at LE produces lower lift and higher drag characteristics against other configuration models. The overall cavity effect for the flow around the airfoil is that it creates vortices, thereby re-energizes the slower moving boundary layer and delays the flow separation in the downstream direction. The outcomes of this analysis are suggested that the cavity at a position before the mid chord from the leading edge does not improve the performance of the airfoil. Though vortex is formed in the confined spaces but it is unable to reattach the flow towards the downstream direction of an airfoil.

Contribution of Tip Shroud Cavity to Loss Generation in the Main Flow of a Low Pressure Turbine Using Steady and Unsteady Numerical Approach

A lot of studies on turbomachinery main flow optimisation have been performed in order to reach current efficiencies. To go further in the study of aerodynamic losses sources, a better understanding of technological effects is required. Tip shroud cavities in low pressure turbine is an example. Indeed, the by-pass flow causes additional pressure losses. In addition, interactions between main flow and cavity flows, as well as the re-entering flow, cause mixing losses and modifications of flow angle. This paper investigates the contribution of tip shroud cavities in a low pressure turbine stage on the overall performance and flow structures. The ability of a steady simulation to predict this kind of flow by comparison with time-resolved results is poorly documented in the literature, and is an objective of this paper. Computations are compared with experimental data from low speed turbine test rig. Entropy production shows that a large amount of additional losses comes from the cavities themselves whichever the steady or unsteady treatment of the simulation. Additional losses generated in the rotor are more dependent on the presence of the shroud or not than the unsteady feature of the simulations.

Influence of Probe Blockage Effect on Compressor Test Performance

In order to study the influence of probe blockage effect on the test performance of the compressor, a full-channel steady simulation method was used to establish a regression model of the influence of the probe on the compressor performance. Taking NASA Rotor67 compressor as research objects, different sizes and quantities of probes are arranged at different axial positions of the compressor inlet and outlet respectively, the flow-efficiency and flow-pressure ratio characteristic line are obtained under different probe blockage ratios, circumferential and axial layouts at the design speed. The calculation results show that due to the blockage effect of the probe, the flow rate of the blocking point is reduced, the flow rate of the stall point is increased, the efficiency and pressure ratio of the compressor are decreased. The separation of the rotor surface layer caused by the presence of the probe and the blending loss of the probe wake and the mainstream are the main reasons for the above changes. The response surface model was used to model the regression of the data, and the regression model of the compressor characteristics in the presence/absence of the probe was obtained, and the regression model was verified. The relative error was within 3%, indicating the established regression model of probe on the influence of compressor performance has good accuracy.

Improving Steady Computational Fluid Dynamics to Capture the Effects of Radial Mixing in Axial Compressors

The compressors of power-generation gas turbines (GTs) have a high stage count, blades with low aspect ratios, and large clearances. These features promote strong secondary flows. An important outcome deriving from the convection of intense secondary flows is the enhanced span-wise transport of fluid properties mainly involving the rear stages, generally referred to as “radial mixing.” An incorrect prediction of this key phenomenon may result in inaccurate performance evaluation and could mislead designers. In the rear compressor stages, the stream-wise vorticity associated with tip clearance flows is one of the main drivers of the span-wise transport phenomenon. Limiting it by averaging the flow at row interfaces is the reason why a steady analysis underpredicts radial mixing. To properly forecast the span-wise transport, an unsteady analysis should be adopted. However, this approach has a computational cost not yet suitable for industrial purposes. Currently, only the steady simulation can fit in a lean design chain and any model upgrade improving its radial mixing prediction would be highly beneficial. To attain some progresses in Reynolds-averaged Navier–Stokes (RANS) model, its lack of convection of stream-wise vorticity must be addressed. This can be done by acting on another mixing driver that is turbulent diffusion; by enhancing turbulent viscosity, one can promote span-wise diffusion, thus improving the radial mixing prediction. In this paper, this strategy to update the RANS model and its application on an existing compressor is presented, together with the model tuning that has been performed using unsteady results as the target.

A Hybrid Method for Fan Simulations in Full Vehicle Thermal Analyses

In full vehicle thermal flow analyses, the most often used procedure to simulate fluid motions driven by the cooling fan is the Moving Reference Frame (MRF) method. In the MRF approach, the fan is fixed in space and the fan rotation is modeled using grid fluxes. This method is widely used because it provides a fast and effective means of simulating fans. However, the MRF method does not always accurately predict the thermal wake and the mass flow rate through the fan, which causes errors in predicted temperatures on the parts downstream of the fan. Another method for fan simulation is the Rigid Body Motion (RBM) method in which the fan rotates in time. The RBM method models the fan motions directly, thus it can accurately predict the mass flow rate and thermal wake. However, an RBM simulation is transient and needs a time-average to obtain statistically steady-state results. The RBM method requires a significant amount of CPU resources and simulation time, which prevents it from being widely used in industry. In the current work, a Hybrid Rigid Body Motion (HRBM) method is developed and validated. The HRBM method splits the full vehicle thermal simulation into two simulations, and then couples them at the interface. The first simulation is transient, utilizes the RBM method for the fan, and only models the fan regions. The second simulation is steady, which models the full vehicle except the fan regions. The solution from the transient simulation is time-averaged on the exchange interface and used as boundary conditions for the steady simulation. Conversely, the solution for the steady simulation is used as boundary conditions for the transient simulation at the exchange interface. Due to the slight differences resulting from time-averaging, there is a mismatch in the physical quantities at the exchange interface. This causes stability issues which prevent the coupled simulations from converging. Special techniques have been used in this work to stabilize the solution at the interface, which ensured the convergence of the coupled simulations. The HRBM method greatly improves the accuracy of the full vehicle thermal flow simulation compared to using the MRF method. The thermal wake that results from using HRBM to model the fan is very similar to that produced by RBM, but HRBM utilizes ∼20–30% of the simulation resources required by RBM to achieve convergence.

Numerical Investigation of a Steam Nozzle with Focus on Non-Equilibrium Condensation and Unsteady Flow Behavior

Present work analyses the condensation of superheated water vapor in supersonic Barschdorff nozzle. The influence of pneumatic mounts in 3D laval nozzle is analyzed using steady and unsteady two phase non-equilibrium condensation steam flow model in Ansys CFX16. Mesh independency studies in 2D model showed that at a lower inlet total temperature and very fine mesh (e.g. 100.2°C and 77k mesh elements) there is problem with the convergence using steady simulation. This is possibly due to the ability of very fine mesh to capture the small flow unsteadiness. The variation in location of Wilson’s point, Wilson’s pressure and maximum sub-cooling rate at the centerline of the nozzle is below 1.5%. The 2D CFD nucleation rate is 50% stronger and droplet diameter is 18% higher compared to the 3D CFD results. The deviation in nucleation rate and droplet diameter at nozzle outlet is the result of dissipation due to wing structure in the 3D model. Nucleation zone predicted by Ansys CFX16 is far upstream the experimental one. Different correction factors in modified nucleation model were used to fit the computed pressure distribution with the experimental one. The correction factor is dependent on boundary conditions and nozzle profile. It is thus concluded that the significance of such correction factor is not unique.

CFD analysis of a twin scroll radial turbine

The contribution deals with the application of coupled implicit solver for compressible flows to CFD analysis of a twin scroll radial turbine. The solver is based on the finite volume method, convective terms are approximated using AUSM+up scheme, viscous terms use central approximation and the time evolution is achieved with lower-upper symmetric Gauss-Seidel (LU-SGS) method. The solver allows steady simulation with the so called frozen rotor approach as well as the fully unsteady solution. Both approaches are at first validated for the case of ERCOFTAC pump [1]. Then the CFD analysis of the flow through a twin scroll radial turbine and the predictions of the efficiency and turbine power is performed and the results are compared to experimental data obtained in the framework of Josef Božek – Competence Centre for Automotive Industry.

Numerical Investigation on Slot Casing Treatment in a Transonic Axial Compressor Stage: Part 2 — Unsteady Simulations and Analysis

Slot-type casing treatment generally has a great potential of enhancing the operating range for tip-critical compressor rotors, however, with remarkable efficiency drop. In the first part of this two-part words, several configurations of slot casing treatment were tested in a 1.5 transonic compressor stage by steady simulations. One kind of arc-curve skewed slot contributed to considerable stall margin improvement with minimum efficiency loss. However, interaction between main passage and casing treatment was inherently unsteady. Steady simulation was inadequate to provide accurate compressor performance prediction and precise flow field details. Thus, this part was aimed at clarifying the differences between steady and unsteady simulations. The unsteady interaction process between main passage flow and slots were also detailed interpreted. Unsteady simulation was conducted by applying sliding interface between rotor passage and arc-curved skewed slots. Firstly, differences of compressor performance were examined between steady and unsteady methods. Results showed that steady simulation underestimated stall margin improvement and efficiency drop by casing treatment. Then analysis on aerodynamic parameters and specific flow fields were carried out at smooth casing peak efficiency and casing treatment near stall conditions. Unsteady simulation provided more than 50% larger mass flow rate entering or exiting slots opening surfaces at both operating conditions. It revealed that in unsteady simulation, casing treatment contributed to stronger suction/injection process, which promoted tip flow fields more effectively than steady simulation. Axial velocity deficit at rotor outlet was refilled by slots more effectively in unsteady simulation. In steady result, a large low momentum blockage existed inside rotor passage near tip region and prevented flow from entering the passage at near stall condition. While in unsteady simulation at the same condition, incoming flow was still able to travel across rotor passage in a high velocity. Further, instantaneous flow fields near tip region and inside the slots were particularly examined during a rotor blade passing period to elaborate the unsteady flow interaction. The mid-pitch surface of a representative slot was selected to represent the re-circulation procedure inside slots. Unsteady flow fields and spectrum analysis manifested that tip flow field was dominated by slots passing, while re-circulation process inside slots was dominated by blade passing. Low pressure region inside the blade passage facilitated the injection process. Circulation inside slots lagged behind the pressure variations beneath slots. When the slot was striding over the blade tip, intense injection didn’t emerge immediately beneath slots’ front portion. Until the high pressure region moved away from the slot opening surface, fluids inside the slots started to inject into the main flow in high speed.

Effects of volute’s asymmetry on the performance of a turbocharger centrifugal compressor

The effects of the volute’s asymmetry on the performance of a turbocharger centrifugal compressor were studied using steady simulations and theoretical analysis. According to the steady simulation results, it is found that the volute’s asymmetry has significant influence on the performance of the centrifugal compressor. The variation of the stage efficiency due to volute’s asymmetry is up to 4%. Meanwhile, the volute’s asymmetry restricts the compressor stable flow range by imposing a distorted outlet pressure condition and forcing some certain impeller passages to suffer from a worse flow than the others. These certain passages are likely to stall first and trigger the surge, as the stage flow rate further decreases. In other words, the local stall triggers the surge. The relevant flow mechanisms were given to explain the effects based on the three-dimensional flow field, and a new model was developed to demonstrate how the local stall induced by the volute’s asymmetry triggers the system instability.