Characterization of a radial turbocharger turbine in pulsating flow by means of CFD and its application to engine modeling (original) (raw)
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Energy Conversion and Management, 2014
The ever-increasing demand for low carbon applications in automotive industry has intensified the development of highly efficient engines and energy recovery devices. Even though there are significant developments in the alternative powertrains such as full electric, their full deployment is hindered by high costing and unattractive life-cycle energy and emission balance. Thus powertrain based on highly efficient internal combustion engines are still considered to be the mainstream for years to come. Traditionally, turbocharger has been an essential tool to boost the engine power, however in recent years it is seen as an enabling technology for engine downsizing. It is a well-known fact that a turbocharger turbine in an internal combustion engine operates in a highly pulsating exhaust flow. There are numerous studies looking into the complex interaction of the pulsating exhaust gas within the turbocharger turbine, however the phenomena is still not fully integrated into the design stage. Industry practice is still to design and match the turbine to an engine based on steady performance maps. The current work is undertaken with the mind to move one step closer towards fully integrating the pulsating flow performance into the turbocharger turbine design. This paper presents the development efforts and results from a full 3-D CFD model of a turbocharger turbine stage. The simulations were conducted at 30,000 rpm and 48,000 rpm (50% and 80% design speed respectively) for both 20 Hz and 80 Hz pulsating flow inlet conditions. Complete validation procedure using cold-flow experimental data is also described. The temporal and spatial resolutions of the incidence angle at the rotor leading edge suggest that the circumference variation is little (7%) as compared to its variation in time as the pulse progresses. The primary aim of this paper is to investigate the relationship of the turbine speed, as well as the pulsating flow frequency to its performance. It was found that there are no direct instantaneous relationship between the pulsating pressure at the turbine inlet and the turbine efficiency, except when one considers an additional parameter, namely the incidence angle. This paper also intends to investigate the potential loss of information if the performance parameters are simply averaged without considering the instantaneous effects.
Analyses of steady and unsteady flows in a turbocharger’s radial turbine
This study presents the aerodynamic behaviours of a twin-entry radial inflow turbine under steady and transient con-ditions. The influence of the volute tongue is depicted by a low momentum wake propagating toward the rotor entry, but its effect does not extend beyond a circumferential position of 60 , and more total pressure loss is revealed with respect to the hub side. The transient simulations carried out at different operating conditions and Fast Fourier Transform analysis of static pressure fluctuations induced by the components’ interactions have revealed a space-time periodic behaviour which has been described by a double Fourier decomposition. The flow simulations considering the two sides subject to both non-pulsatile and pulsatile flows conditions have revealed the existing rotor and tongue potential effects and interaction effects the rotor and volute, in addition to the circumferential and spanwise flow non-uniformities at the volute exit, which are more accentuated with a pulsatile flow at inlet. The results of Fast Fourier Transform analysis of temporal pressure fluctuations at the inter-space depict an unsteady behaviour related to the pulsatile frequencies which are characterised by high amplitudes. On the other hand, the spatial pressure fluctuations for the non-pulsatile and pulsatile flows conditions seem to have the same dominant modes since Fast Fourier Transform analysis was carried out at a fixed instant.
In the present paper, an unsteady approach to determine the performance of a small radial inflow turbine working under cold pulsating flow is presented. It has been concluded that a reasonably good characterization of turbine behavior working with pulsating flow can be obtained using, in a quasi-steady way, models of the turbine isentropic efficiency and turbo-charger mechanical efficiency. Both models have been fitted using data obtained from a steady flow characterization procedure. Turbocharger-measured parameters from the cold pulsating flow campaign have been compared with the ones obtained from one-dimensional gas dynamics computational modeling. The modeling approach is based on quasi-steady isentropic and mechanical efficiency models. Reasonably good accuracy in compressor and turbine variables prediction has been obtained for most of the operative conditions. Influence of amplitude and frequency of the pulsating flow over the instantaneous and average turbine efficiency has been studied to put some light on the analysis of the involved physical phenomena. The main conclusion is that the biggest effect of unsteady flow on turbine efficiency is through the influence on blade jet to speed ratio. It has been also concluded that, for the same average blade jet to speed ratio, pulses' amplitude does not influence turbine efficiency when it is closed, but does at other variable geometry turbine (VGT) positions. The effect of pulses' frequency is less evident and only influences VGT performance at the highest VGT openings.
Mixed-flow turbines for automotive turbochargers: steady and unsteady performance
International Journal of Engine Research, 2002
Turbochargers are finding increasing appliincreasingly more stringent emission standards. This cation to automotive diesel engines as cost effective means paper is part of an extensive experimental profor improving their power output and efficiency, and gramme focused on turbine design technology for reducing exhaust emissions; these requirements have led vehicle applications aiming to (a) explore available to the need for highly loaded turbocharger turbines. A means to modify the turbine operating characteristics mixed-flow turbine is capable of achieving its peak isen-in order to improve the engine/turbocharger matchtropic efficiency at reduced velocity ratios compared to a ing and (b) improve the transfer of energy from the typical radial inflow turbine; it is therefore possible to pulsating exhaust gases to the turbine. In order to improve the turbocharger/engine matching. These turbines achieve these aims, the project is centred on mixeddiffer from the commonly used radial turbines in that the flow turbine rotors as opposed to radial rotors comflow approaches the rotor in the non-radial direction; in monly used in automotive turbochargers. This choice the extreme a mixed-flow turbine would become an axial overcomes one of the principal limitations of the machine. The steady and unsteady performances of a radial inflow turbine, where stress and material conmixed-flow turbocharger turbine with a constant blade siderations dictate that the blade angle must be zero inlet angle have been investigated. The steady flow results to maintain radial blade sections and to keep the indicated that the mixed-flow turbine obtains a peak centrifugal load in the blades purely tensile. A efficiency (total-to-static) of 75 per cent at a velocity ratio mixed-flow geometry permits the use of a non-zero of 0.61, compared with that of a typical radial-inflow turblade angle without departure from the above bine which peaks at a velocity ratio of 0.7. The performance requirement and introduces an extra degree of freeand flow characteristics were found to deviate significantly dom into the rotor design, which has particular from the equivalent steady state values commonly used in implications for the range of operation of the turbine, turbocharger turbine design.
Comparison of Experimental, 3D and 1D Model for a Mixed-Flow Turbine Under Pulsating Flow Conditions
Jurnal Teknologi
The pulse flow performance of a turbocharger turbine is known to be different than its corresponding steady flow performance. This often leads to less-than-satisfactory 1D engine model prediction. In this study, the effectiveness of a 1D pulse flow turbine model is assessed against experimental data with the aid of 3D CFD model. The turbine under study is a single-entry variable geometry mixed-flow turbine. The result shows highly comparable pulse flow swallowing capacity and actual power characteristics between 1D and 3D models. The over-prediction in 1D actual power magnitude is found to be due to the simplification of combining nozzle and rotor stage pressure loss together.
Conceptual Design of a Variable Geometry, Axial Flow Turbocharger Turbine
SAE Technical Paper Series
The modern automotive industry is under strict regulations to reduce emissions to comply with the Kyoto Protocol, a universally acknowledged treaty aiming at reducing exhaust gas emissions. In order to achieve the required future emission reduction targets, further developments on gasoline engines are required. One of the main methods to achieve this goal is the application of engine downsizing. Turbocharging is a cost-effective method of downsizing an engine whilst reducing exhaust gas emissions, reducing fuel consumption and maintaining prior performance outputs. For these reasons, the turbocharging is becoming the most widely adopted technology in the automotive markets. In 2012, 32% of passenger and commercial vehicles sold had a turbocharger installed, and is predicted to be 40% of 2017 [1]. Even if the engine turbocharging is a widespread technology, there are still drawbacks present in current turbocharging systems. The main problem is overcoming the issue of turbo-lag, which is the poor initial response of the turbocharger to the driver commands due to its inertia. Indeed, the system turbine plus compressor is characterized by an own rotational inertia, therefore, the turbocharger will take a certain time to accelerate and produce the desired boost when a higher amount of exhaust gas is sent to the system. In this work, an innovative solution to the turbo-lag phenomenon will be analyzed: a vaneless stator-axial flow turbine. The proposed turbine configuration would improve the transient response of the system since the axial turbine has intrinsically a lower inertia than the radial turbine as stated by the research works of Ford first [2] and Honeywell after [3]. The whole design process is presented in this paper and particular relevance has been given to the thermo-fluid-dynamic aspect of the machine. Several CFD investigations have been carried out in order to deeply understand the new turbine behavior and, thanks to a 1D model of the target engine, it has been possible to validate the new design simulating the performance of the system engine + turbocharger.
Performance of a Mixed Flow Turbocharger Turbine Under Pulsating Flow Conditions
Volume 2: Aircraft Engine; Marine; Microturbines and Small Turbomachinery, 1995
The performance of a high pressure ratio (P.R.=2.9) mixed flow turbine for an automotive turbocharger has been investigated and the results revealed its better performance relative to a radial-inflow geometry under both steady and pulsating flow conditions. The advantages offered by the constant blade angle rotor allow better turbocharger-engine matching and maximization of the energy extracted from the pulsating engine exhaust gases. In particular, the mixed inlet blade geometry resulted in high efficiency at high expansion ratios where the engine-exhaust pulse energy is maximum. The efficiency characteristics of the mixed flow turbine under steady conditions were found to be fairly uniform when plotted against the velocity ratio, with a peak efficiency at the design speed of 0.75. The unsteady performance as indicated by the mass-averaged total-to-static efficiency and the swallowing capacity exhibited a departure from the quasi-steady assumption which is analysed and discussed.
Computer Simulations of the Static State of the Turbocharger Turbine
Acta Mechanica Slovaca, 2011
In the present work, a numerical study of a turbulent flow through a turbocharger turbine in a static state is presented. Solutions of the time averaged Navier-Stokes in conjunction with the standard k-f turbulence model were developed using a control volume discretization method. The resolution was found by the commercial code Fluent 5.6. Geometry and the meshing were built by the software Gambit 6.1. This study is expected to provide a finer knowledge of structures of the flow such as velocity, pressure, turbulent kinetic energy, dissipation rate of the turbulent kinetic energy and viscosity. The proposed model has been tested on a turbocharger turbine of type Garrett TA03 automotive engines. The numerical results have been compared with ones found by other experimental results.
CFD Investigation of Flow Structures in Rotor-Stator Disc Cavity Systems
2014
Ingestion of hot gas from the main annulus to the rotor-stator disc cavity is a major concern related to industrial turbines. Fluctuating pressure structures which are previously shown to form inside the disc cavity can influence the gas ingestion from the main flow stream. These rotating low pressure regions can reduce the lifetime of the turbine by overheating the components, induced by ingestion of hot gas, and influencing the mechanical integrity of the device. The hot gas ingestion is usually reduced by supplying a cooling air stream to the cavity and a proper rim seal design. The amount of the cooling air however needs to be minimized so as not to adversely affect the turbine performance and efficiency. A preliminary numerical study was conducted using computational fluid dynamics (CFD) on a one-stage test turbine in order to investigate the flow structure and pressure distribution in the turbine disc cavity. The test turbine consists of a newly designed blisk which is soon to...
Journal of Turbomachinery
Radial turbines are frequently submitted to unsteady inlet flows, for example, in turbocharging applications. Complex flows dominated by waves propagation take place, and advanced methodologies are required. Such complexity is hardly compatible with industrial constraints and design time scales. Also, the validity of the usual performance indicators, such as efficiency, is questionable in unsteady flows. However, the need for simplification led the community to develop modeling strategies for unsteady effects, based on hypotheses. One of those is that the rotor flow is assumed quasi-steady. This assumption is assessed by different criteria of the literature. It also enables an adaptation of performance indicators such as efficiency and pressure ratio. But the validity of such an assumption is still under discussion. The present paper is a contribution to this discussion. It focuses on a physical analysis of the physics involved in unsteady flows and the consequences that it produces...