M. H. Padzillah

Numerical Assessment of Unsteady Flow Effects on a Nozzled Turbocharger Turbine

In order to extract maximum amount of energy possible from the automotive reciprocating engine exhaust gas, the turbocharger usually installed closely downstream the exhaust valve thus exposing it to highly pulsating flow conditions. This condition induces highly complex flow field within the turbocharger stage and significantly impact its performance characteristics which is not fully understood. The main objective of this paper is to provide understanding of unsteady flow feature using a Computational Fluid Dynamics (CFD) approach validated with experimental data. Despite focusing on unsteady feature of the flow, this research also emphasizes the importance of accurately modelled geometry in the early section of the paper. A steady state validation against experimental data is performed prior to unsteady calculations. The effect of different phase shifting methods is described and the relationship of instantaneous efficiency with incidence angle is established. In the final section of this paper, the turbocharger stage is sectioned where its instantaneous performance is evaluated individually in each section. The unsteady simulation is performed at fixed 30 000 RPM with 20Hz pulsing flow.Copyright

A high performance low pressure ratio turbine for engine electric turbocompounding

In order to enhance energy extraction from the exhaust gases of a highly boosted downsized engine, an electric turbo-compounding unit can be fitted downstream of the main turbocharger. The extra energy made available to the vehicle can be used to feed batteries which can supply energy to electric units like superchargers, start and stop systems or other electric units. The current research focuses on the design of a turbine for a 1.0 litre gasoline engine which aims to reduce the CO2 emissions of a “cost-effective, ultra-efficient gasoline engine in small and large family car segment”. A 1-D engine simulation showed that a 3% improvement in brake specific fuel consumption (BSFC) can be expected with the use of an electric turbocompounding. However, the low pressure available to the exhaust gases expanded in the main turbocharger and the constant rotational speed required by the electric motor, motivated to design a new turbine which gives a high performance at lower pressures. Accordingly, a new turbine design was developed to recover energy of discharged exhaust gases at low pressure ratios (1.05–1.3) and to drive a small electric generator with a maximum power output of 1.0 kW. The design operating conditions were fixed at 50,000 rpm with a pressure ratio of 1.1. Commercially available turbines are not suitable for this purpose due to the very low efficiencies experienced when operating in these pressure ranges. The low pressure turbine design was carried out through a conventional non-dimensional mixed-flow turbine design method. The design procedure started with the establishment of 2-D configurations and was followed by the 3-D radial fibre blade design. A vane-less turbine volute was designed based on the knowledge of the rotor inlet flow direction and the magnitude of the absolute speed. The overall dimensions of the volute design were defined by the area-to-radius ratios at each respective volute circumferential azimuth angle. Subsequently, a comprehensive steady-state turbine performance analysis was performed by mean of Computational Fluid Dynamics (CFD) and it was found that a maximum of 76% of total-static efficiency ηt-s can be achieved at design speed.Copyright

Numerical and Experimental Investigation of Pulsating Flow Effect on a Nozzled and Nozzleless Mixed Flow Turbine for an Automotive Turbocharger

To achieve better flow guidance into the turbine blades, nozzle vanes were added as an integral part of the stator design. However, the full investigation that directly addresses the comparison between the two turbine arrangements under pulsating flow conditions is still not available in literature. This work represents the first attempt to observe differences, particularly in the circumferential flow angle distribution between both volute arrangements under steady and pulsating flow operating conditions. Experimentally validated Computational Fluid Dynamics (CFD) simulations have been conducted in order to achieve this aim. As the experimental data within the Turbocharger Group at Imperial College are extensive, the simulation procedures are optimized to achieve the best compromise between the computational cost and prediction accuracy. A single operating pressure ratio is selected for the steady and pulsating environment in order to provide consistent comparison for both volute arrangements. The simulation results presented in this work are conducted at the turbine speed of 48000rpm and 60Hz flow frequency for the pulsating flow simulations. The results indicated that there are significant differences in the flow angle behavior for both volutes regardless of the flow conditions (steady or unsteady). It is also found that the differences in flow angle distribution between increasing and decreasing pressure instances during pulsating flow operation is more prominent in the nozzleless volute than its nozzled counterpart.Copyright

Comparison of the influence of unsteadiness between nozzled and nozzleless mixed flow turbocharger turbine

Nozzled and nozzleless volutes are the most commonly used stator configurations for the turbocharger turbine. In this paper detailed experimental investigation was carried out to compare the performance of a mixed flow turbine with nozzled and nozzleless stator under steady and pulsating conditions which replicates the pulsating exhaust flow from the internal combustion engine. The results show that the turbine with nozzled volute has higher efficiency at low load conditions in both steady and unsteady conditions. The steady peak efficiency is nearly the same as the nozzleless one but it shifts to higher velocity ratio. The improvement of the cycle average efficiency for the nozzled volute under pulsating conditions is influenced on the pulse frequency. The hysteresis loops of the swallowing capacity, reflecting the filling and emptying as well as the wave action effect in the turbine, are similar for two configurations. The comparison shows that the volute configuration has no direct influence on the ‘unsteady’ behaviour in the turbine. Instead, the results of the 1-D model show that the swallowing capacity curve has an evident influence on the unsteadiness in the turbine. Furthermore, the unsteadiness of a turbine is enhanced by reducing the swallowing capacity or increasing the turbine loading.

Modification of Aortic Cannula With an Inlet Chamber to Induce Spiral Flow and Improve Outlet Flow: Modification of Aortic Cannula to Induce Spiral Flow

Physiologically, blood ejected from the left ventricle in systole exhibited spiral flow characteristics. This spiral flow has been proven to have several advantages such as lateral reduction of directed forces and thrombus formation, while it also appears to be clinically beneficial in suppressing neurological complications. In order to deliver spiral flow characteristics during cardiopulmonary bypass operation, several modifications have been made on an aortic cannula either at the internal or at the outflow tip; these modifications have proven to yield better hemodynamic performances compared to standard cannula. However, there is no modification done at the inlet part of the aortic cannula for inducing spiral flow so far. This study was carried out by attaching a spiral inducer at the inlet of an aortic cannula. Then, the hemodynamic performances of the new cannula were compared with the standard straight tip end-hole cannula. This is achieved by modeling the cannula and attaching the cannula at a patient-specific aorta model. Numerical approach was utilized to evaluate the hemodynamic performance, and a water jet impact experiment was used to demonstrate the jet force generated by the cannula. The new spiral flow aortic cannula has shown some improvements by reducing approximately 21% of impinging velocity near to the aortic wall, and more than 58% reduction on total force generated as compared to standard cannula.

A Detailed Comparison on the Influence of Flow Unsteadiness Between the Vaned and Vaneless Mixed-Flow Turbocharger Turbine

A turbocharger is a key enabler for lowering CO2 emission of an internal combustion engine (ICE) through the reutilization of the exhaust gas energy that would otherwise have been released to the ambient. In its actual operating conditions, a turbocharger turbine operates under highly pulsating flow due to the reciprocating nature of the ICE. Despite this, the turbocharger turbines are still designed using the standard steady-state approach due to the lack of understanding of the complex unsteady pressure and mass propagation within the stage. The application of guide vanes in a turbocharger turbine stage has increased the complexity of flow interactions regardless of whether the vanes are fixed or variable. Although it is enticing to assume that the performance of the vaned turbine is better than the one without (vaneless), there are currently no tangible evidences to support this claim, particularly during the actual pulsating flow operations. Therefore, this research looks into comparing the differences between the two turbine arrangements in terms of their performance at flow field level. For this purpose, a threedimensional (3D) full-stage unsteady turbine computational fluid dynamics (CFD) models for both volutes are constructed and validated against the experimental data. These models are subject to identical instantaneous inlet pressure profile of 60 Hz, which is equivalent to an actual three-cylinder four-stroke engine rotating at 2400 rpm. A similar 95.14mm diameter mixed-flow turbine rotor rotating at 48,000 rpm is used for both models to enable direct comparison. The complete validation exercises for both steady and unsteady flow conditions are also presented. Results have indicated that neither vaned nor vaneless turbine is capable of maintaining constant efficiency throughout the pulse cycle. Despite that, the vaneless turbine indicated better performance during peak power instances. This work also showed that the pulsating pressure at the turbine inlet affected the vaned and vaneless turbines differently at the flow field level.

Experimental Work on the Characterization of the Hysteresis Behavior of the Vaned and Vaneless Mixed-Flow Turbocharger Turbine

AbstractIt has been proven that the turbocharger turbine operates differently under unsteady flow conditions as compared to steady flow. Despite this, the turbine wheel is still designed under the ...

Risk of rupture analysis for advanced level of AAA under combined physiological and physical conditions

The risk of rupture for advanced level of aneurysm is very much investigated by medical practitioners and researchers. Numerical modelers have also been contributing to the prediction of this rupture. In this study, the effect of various physiological and physical conditions to the increased of the risk rupture is investigated. This study uses simplified model of aneurysm based on actual shape for the advanced level of aneurysm. Three-dimensional model was used and various flow situations representing various physical conditions were evaluated. Both normal blood pressure (NBP) and high blood pressure (HBP) effect were studied in this case. This study shows that a person with advanced level of aneurysm is observed to have vortex formation as the flow reaches late systole, in the distal region under both resting and exercise conditions. This phenomenon will increase the risk of rupture due to the high fluctuation of wall shear stress in this area. Lower risk of rupture is also observed in the proximal area with the presence of weak vortex. The results also show that high blood pressure under exercise conditions exhibits approximately double the risk for rupture as compared to normal blood pressure under resting condition.

Accuracy and Efficiency Improvements of the Navier Stokes Equation Using Modified Splitting Method

Solution to Navier-Stokes equation using Splitting Method is tailored to promote accuracy and efficiency of the original flow algorithm. Two-dimensional unsteady lid-driven cavity flow based on basic Splitting algorithm on Cartesian Coordinates is compared with two advanced codes, Splitting on Stretched Coordinates and Splitting with Spectral Approach. In both Cartesian and stretched Splitting methods, the linear terms in Navier-Stokes equations are solved by Crank-Nicholson method while the non-linear terms are solved by second order Adams-Bashforth method while the spectral method approximates the fluid as linear combination of non-zero continuous Chebyshev Polynomials function over solution domain. Spectral method immensely improves the accuracy of solver with a drawback of large computational cost while Splitting in Stretched coordinates is superior in efficiency but less accuracy progression. The graphical correlations between accuracy, efficiency and grid resolutions suggest the use of Splitting-Stretched method for comparable accuracy and intensive flow profile with least time consumption.