Development of Research on Unsteady Flow Performance for Vehicle Turbocharger Turbine
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Vehicle turbocharger turbines usually operate under pulsating inlet conditions,and the unsteady flow performance,which departs substantially from that at steady flow conditions,can make benefits for turbine design and engine match.The development of some aspects in this fleld,which include turbine dynamometry,experiments and prediction of turbine unsteady flow performance,are summarized in this paper.Keywords:
Unsteady flow
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We have simulated the performance of a simple engine model with a gas turbine engine simulation program based on CFD. 2-dimensional Navier-Stokes code for the viscous flow was applied to simulate a compressor and a turbine, and the chemical equilibrium code with the lumped method was applied to simulate the combustor. Unsteady-flow phenomenon between rotor and stator of the compressor and the turbine was analyzed by steady mixing-plane method. In this way, the influence of the turbine blade pitch on the engine was investigated. It was shown that the compressor is operated at more higher pressure conditions as narrower the pitch distance of the turbine.
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Current trends in the automotive industry towards engine downsizing means turbocharging now plays a vital role in engine performance. A turbocharger increases charge air density using a turbine to extract waste energy from the exhaust gas to drive a compressor. Most turbocharger applications employ a radial inflow turbine. However, to ensure radial stacking of the blade fibers and avoid excessive blade stresses, the inlet blade angle must remain at zero degrees, creating large incidence angles. Alternately, mixed flow turbines can offer non-zero blade angles while maintaining radial stacking of the blade fibers and reducing leading edge separation at low velocity ratios. Furthermore, the physical blade cone angle introduced reduces the blade mass at the rotor outer diameter reducing rotor inertia and improving turbine transient response. The current paper investigates the performance of a mixed flow turbine under a range of pulsating inlet flow conditions. A significant variation in incidence across the LE span was observed within the pulse, where the distribution of incidence over the LE span was also found to change over the duration of the pulse. Analysis of the secondary flow structures developing within the volute shows the non-uniform flow distribution at the volute outlet is the result of the Dean effect in the housing passage. In-depth analysis of the mixed flow effect is also included, showing that poor axial flow turning ahead of the rotor was evident, particularly at the hub, resulting in modest blade angles. This work shows that the complex secondary flow structures that develop in the turbine volute are heavily influenced by the inlet pulsating flow. In turn, this significantly impacts the rotor inlet conditions and rotor losses.
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For a turbocharger working under internal combustion engine operating conditions, the flow will be highly pulsatile and the efficiency of the radial turbine will vary during the engine cycle. In addition to effects of the inflow unsteadiness, there is also always a substantial unsteady secondary flow component at the inlet to the turbine depending on the geometry upstream. These secondary motions may consist of swirl, Dean vortices and other cross-sectional velocity components formed in the exhaust manifold. The strength and the direction of the vortices vary in time depending on the unsteady flow in the engine exhaust manifold, the engine speed and the geometry of the manifold itself. The turbulence intensity may also vary during the engine cycle leading to a partially developed turbulent flow field. The effect of the different perturbations on the performance of a radial nozzle-less turbine is assessed and quantified by using Large Eddy Simulations. The turbine wheel is handled using a sliding mesh technique, whereby the turbine wheel, with its grid is rotating, while the turbine house and its grid are kept stationary. The turbine performance has been compared for several inflow conditions. The results show that an inflow-condition without any perturbations gives the highest shaft power output, while a turbulent flow with a strongly swirling motion at the inlet results in the lowest power output. An unexpected result is that a turbulent inflow yields a lower shaft power than a turbulent inflow with a secondary flow formed by a pair of Dean vortices. The flow field for the different cases is investigated to give a better insight into the unsteady flow field and the effects from the different inlet conditions.
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The aerodynamic design and analysis of a turbine exhaust volute manifold is described. This turbine exhaust system will be used with an advanced gas generator oxidizer turbine designed for very high specific work. The elevated turbine stage loading results in increased discharge Mach number and swirl velocity which, along with the need for minimal circumferential variation of fluid properties at the turbine exit, represent challenging volute design requirements. The design approach, candidate geometries analyzed, and steady state/unsteady CFD analysis results are presented.
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Nonsynchronous excitation under low volume operation is a major risk to the mechanical integrity of last stage moving blades (LSMBs) in low-pressure (LP) steam turbines. These vibrations are often induced by a rotating aerodynamic instability similar to rotating stall in compressors. Currently extensive validation of new blade designs is required to clarify whether they are subjected to the risk of not admissible blade vibration. Such tests are usually performed at the end of a blade development project. If resonance occurs a costly redesign is required, which may also lead to a reduction of performance. It is therefore of great interest to be able to predict correctly the unsteady flow phenomena and their effects. Detailed unsteady pressure measurements have been performed in a single stage model steam turbine operated with air under ventilation conditions. 3D computational fluid dynamics (CFD) has been applied to simulate the unsteady flow in the air model turbine. It has been shown that the simulation reproduces well the characteristics of the phenomena observed in the tests. This methodology has been transferred to more realistic steam turbine multistage environment. The numerical results have been validated with measurement data from a multistage model LP steam turbine operated with steam. Measurement and numerical simulation show agreement with respect to the global flow field, the number of stall cells and the intensity of the rotating excitation mechanism. Furthermore, the air model turbine and model steam turbine numerical and measurement results are compared. It is demonstrated that the air model turbine is a suitable vehicle to investigate the unsteady effects found in a steam turbine.
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The flow through radial turbines in automotive turbocharging applications is inherently highly unsteady, exhibiting high amplitude pulsations of varying frequencies. This can have a significant effect on the performance of the turbine, and therefore accurate modelling of these effects is key in good engine design, particularly in the realm of 1-D modelling. Assuming that the turbine stage acts in a quasi-steady manner can result in modelling errors. To better capture the physical effects of unsteady flow, a novel 1-D model of a radial turbine volute was developed, within the engine modelling software GT-Power. A meanline model was developed and integrated into the volute model as a means of modelling the rotor, which was assumed to be quasi steady. Experimental work was done to characterise the unsteady performance of the turbine, and the results were used to validate the model. The model showed good agreement with experimental data as well as some small improvements compared to industry standard methods of modelling.
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Unsteady three-dimensional computations have been implemented on a turbocharger twin-scroll turbine system (volute-turbine wheel-diffuser). The flow unsteadiness in a turbocharger turbine system is essentially driven by a highly pulsating flow from the upstream combustor which causes a pulsating stagnation pressure boundary condition at the inlet to the turbine system. Computed results have been post-processed and interrogated in depth in order to infer the significance of the induced flow unsteadiness on performance. The induced flow unsteadiness could be deemed important since the reduced frequency of the turbine system (based on the time scale of the inlet flow fluctuation and the flow through time) is higher than unity. Thus, the computed time-accurate pressure field and the loss generation process have been assessed to establish the causal link to the induced flow unsteadiness in the turbine system. To do this consistently both for the individual subcomponents and the system, a framework of characterizing the operation of the turbine system linked to the fluctuating inlet stagnation pressure is proposed. The framework effectively categorizes the operation of the unsteady turbine system in both spatial and temporal dimension; such a framework would facilitate determining whether the loss generation process in a subcomponent can be approximated as unsteady (e.g. volute) or as locally quasi-steady (e.g. turbine wheel) in response to the unsteady inlet pulsation in the inlet-to-outlet stagnation pressure ratios of the two inlets. The notion that a specific subcomponent can be approximated as locally quasi-steady while the entire turbine system in itself is unsteady is of interest as it suggests a strategy for an appropriate flow modeling and scaling as well as for the turbine system performance improvement. Also, computed results are used to determine situations where the flow effects in a specific subcomponent can be approximated as quasi-one-dimensional; thus for instance the flow mechanisms in the volute can reasonably be approximated on an unsteady one-dimensional basis. For a turbine stage with sudden-expansion type diffuser, the framework for integrating sub-component models into a turbine system is formulated. The effectiveness and generality of the proposed framework is demonstrated by applying it to three distinctly different turbocharger operating conditions. The estimated power from the integrated turbine system model is in good agreement with the full unsteady CFD results for all three situations. The formulated framework will be generally applicable for assessing the new design configurations as long as the corresponding high fidelity steady CFD results are utilized to determine the quasi-steady (or acoustically compact) behavior of each new sub-component.
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