Flow phenomena leading to surge in a centrifugal compressor

© 2016 Elsevier Ltd.Surge is a global flow instability occurring in centrifugal compressors at low mass-flow rate operation. Due to its violent nature, it is the limiting factor for operability. To enhance the operating range, understanding of the flow instability inception when approaching surge is essential. Therefore, the flow evolution along a speed line is analysed by performing unsteady, three-dimensional flow simulations using a centrifugal compressor geometry with ported shroud. A stable operating condition, at high mass-flow rates, is compared to lower mass-flow rate operating conditions close to and at surge. The particularities of the flow-fields are analysed and described. A smooth flow-field is observed for the stable operating condition, whereas flow reversal manifesting as tip leakage at the outer periphery of the impeller occurs for all off-design operating conditions. The reversed flow exhibits swirling motion in the impeller rotation direction. This induces a globally swirling flow upstream of the impeller, which influences the flow incidence angles at the blades and hence, their efficiency. Proper orthogonal decomposition and dynamic mode decomposition have been performed to analyse the flow structures appearing with surge more thoroughly. For the lowest mass-flow rate operating condition, low frequency modes describing the filling and emptying processes during surge have been found

Fluidic injection scenarios for shock pattern manipulation in exhausts

Screening numerically internal fluidic injection scenarios for the manipulation of the double diamond shock pattern in convergent-divergent nozzle exhausts, we demonstrate the individual importance of design parameters. We find that the evolving shock pattern is sensitive to the injection location, while the persistence of the induced counterrotating vortex pairs is primarily governed by the injection pressure. Injection close to the nozzle exit generates secondary vortical structures amplifying the fluctuations in the nozzle vicinity

Acoustic signature of flow instabilities in radial compressors

Rotating stall and surge are flow instabilities contributing to the acoustic noise generated in centrifugal compressors at low mass flow rates. Their acoustic generation mechanisms are exposed employing compressible Large Eddy Sim- ulations (LES). The LES data are used for calculating the dominant acoustic sources emerging at low mass flow rates. They give the inhomogeneous char- acter of the Ffowcs Williams and Hawkings (FW-H) wave equation. The blade loading term associated with the unsteady pressure loads developed on solid surfaces (dipole in character) is found to be the major contributor to the aerodynamically generated noise at low mass flow rates. The acoustic source due to the velocity variations and compressibility effects (quadrupole in character) as well as the acoustic source caused by the displacement of the fluid due to the accelerations of the solid surfaces (monopole in character) were found to be not as dominant. We show that the acoustic source associated with surge is generated by the pressure oscillation, which is governed by the tip leakage flow. The vortical structures of rotating stall are interacting with the impeller. These manipulate the flow incidence angles and cause thereby unsteady blade loading towards the discharge. A low-pressure sink between 4 and 6 o’clock causes a halving of the perturbation frequencies at low mass flow rates operat- ing conditions. From two point space-time cross correlation analysis based on circumferential velocity in the diffuser it was found that the rotating stall cell propagation speed increases locally in the low pressure zone under the volute tongue. It was also found that rotating stall can coexist with surge operat- ing condition, but the feature is then seen to operate over a broader frequency interval

Large eddy simulation of fluidic Injection into a supersonic convergent-divergent duct

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Transforming the shock pattern of supersonic jets using fluidic injection

Double shock diamonds establish in the exhaust of modular convergent-divergent nozzles. These consist of two shock structures; one originating from the nozzle throat and another from its exit. Analyzing the shock pattern developing for different fluidic injection operating conditions, it is shown that fluidic injection allows the rearrangement of the shock structures relative to each other. Overlapping the two structures caused large pressure oscillations in the exhaust and high amplitudes of shock associated noise, whereas staggering the shock structures mitigated these effects. The screech tone frequency did not change for all injection operating configurations, although the shock diamonds had been shifted drastically with respect to each other. Hence, the screech phenomenon is dominated by the primary shock spacing originating from the nozzle throat

Flow induced energy losses in the exhaust port of an internal combustion engine

A numerical study of the flow in the exhaust port geometry of a Scania heavy-duty diesel engine is performed using the large eddy simulation (LES) and an unsteady Reynolds-Averaged Navier–Stokes (URANS) simulation approach. The calculations are performed at fixed valve positions and stationary boundary conditions to mimic the setup of an air flow bench experiment, which is commonly used to acquire input data for one-dimensional engine simulations. The numerical results are validated against available experimental data. The complex three-dimensional (3D) flow structures generated in the flow field are qualitatively assessed through visualization and analyzed by statistical means. For low valve lifts, the major source of kinetic energy losses occurs in the proximity of the valve. Flow separation occurs immediately downstream of the valve seat. Strong helical flow structures are observed in the exhaust manifold, which are caused due an interaction of the exhaust port streams in the port geometry.</jats:p

Effects of Hypoxia on Egg Capsule Conductance in Ambystoma (Class Amphibia, Order Caudata)

Aquatic amphibian eggs frequently encounter hypoxic conditions that have the potential to limit oxygen uptake and thereby slow embryonic development and hatching. Oxygen limitation might be avoided if egg capsule surface area and oxygen conductance increased in response to hypoxia. We investigated this possibility in two salamander species, Ambystoma annulatum and Ambystoma talpoideum. The effective surface area of egg capsules increased in response to hypoxia, which increased the conductance for oxygen and enhanced oxygen transport. The ability of amphibian eggs to adjust their conductance in response to oxygen availability may increase survival in hypoxic environments

Numerical flow analysis of a centrifugal compressor with ported and without ported shroud

Turbochargers are commonly used in automotive engines to increase the internal combustion engine performance during off design operation conditions. When used, a most wide operation range for the turbocharger is desired, which is limited on the compressor side by the choke condition and the surge phenomenon. The ported shroud technology is used to extend the operable working range of the compressor, which permits flow disturbances that block the blade passage to escape and stream back through the shroud cavity to the compressor inlet. The impact of this technology on a speed-line at near optimal operation condition and near surge operation condition is investigated. A numerical study investigating the flow-field in a centrifugal compressor of an automotive turbocharger has been performed using Large Eddy Simulation. The wheel rotation is handled by the numerically expensive sliding mesh technique. In this analysis, the full compressor geometry (360 deg) is considered. Numerical solutions with and without ported shroud for a near optimal operation condition and near-surge operation condition. The flow-field of the different cases is analyzed to elucidate the functionality of the ported shroud. In agreement with previous observations, it was found that the ported shroud reduces the flow disturbances in the blade passage for all operating conditions. However, the compressor efficiency for the off-design operation condition was found to be higher without the ported shroud, supporting the findings reported recently by an experimental investigation. The computational results are validated with experimental measurements in terms of the performance parameters and available Particle Image Velocimetry data. Copyright © 2014 SAE International