Large Eddy Simulations of a Turbocharger Radial Turbine Under Pulsating Flow Conditions

The pulsating flow conditions which a turbocharger turbine is exposed cause important deviations of the turbine aerodynamic performance when compared to steady flow conditions. Indeed, the secondary flows developing in the turbine are determined by the inflow aerodynamic conditions, which largely vary during the pulse cycle. In this paper, a high-resolved Large Eddy Simulation is performed to investigate and characterize the flow field evolution in a turbocharger radial turbine over the pulse cycle. At first, the model is validated against experimental results obtained in gas-stand flow conditions. Then, the instantaneous flow field at the rotor mid-span section is compared to the one given by the equivalent cycle-averaged steady flow conditions. The results highlight five distinct flow features. At low mass flow rates, when the relative inflow angle assumes large negative values, the flow separates at the blade pressure side, causing a secondary flow consisting in two counter-rotating vortices characterized by a diameter comparable to the blade passage. As the mass flow rate increases, the first vortex persists at the blade tip while the second one moves closer to the blade trailing edge. This corresponds to the second characteristic flow field. With increasing relative inflow angle, for the third characteristic flow feature, only the recirculation at the blade leading edge is displayed and its size gradually reduces. For the fourth characteristic flow feature, at moderate negative values of the relative inflow angle, the flow field is well aligned with the blade profile and free of secondary flows. Then, as the relative inflow angle gradually grows towards large positive values, the flow separates on the blade suction side causing the mixing of the flow with the stream flowing on the pressure side of the previous blade.

Optimising Computational Fluid Dynamic Conditions for Simulating Copper Vertical Casting

A 2-D finite volume Computational Fluid Dynamic (CFD) model, using Ansys Fluent vR.1 of a vertically oriented upwards continuous casting (VUCC), was investigated for 8 mm, oxygen free copper (OFCu). The simulations enabled the mapping of the cast OFCu solidification front (SF) interface from liquid to solid. Optimisation of the simulation parameters were investigated which included mesh size and the Ansys specific ‘mushy zone’ constant (Amush), which is used to account for fluid flow dampening at SF within the model. Observations of the SF, the change in fluid volume in the die, the simulation convergence and the total simulation time, revealed that the optimised casting parameters were for mesh size 1×10-4 m and Amush 106 kg/m3s. These parameters were compared with the cast rod and highlighted qualitatively the relationship between grain growth direction and SF position during a casting pulse cycle.

Analysis of Retinal Vessel Pulsations with Electrocardiographic Gating

Purpose The origin of retinal venous pulsations has been a matter of debate for some time. One classical explanation to the origin of these pulsations has been that the cardiac cycle induces systolic peaks in the intraocular pressure (IOP) which leads to decreases in retinal vein diameters. Recently, theoretical concepts have been published which postulate that IOP changes during the pulse cycle is not the primary driving force for venous pulsation, and hence, predict that the retinal vein diameter is indeed reduced during IOP diastole. The aim of the study was to test this hypothesis in a clinical trial. Subjects and Methods Continuous IOP and retinal vessel analyser (RVA) measurements were taken from 21 subjects, ages 20 to 30 years, with no known ophthalmologic diseases, while connected to a standard electrocardiograph (ECG). With this methodology, average and synchronised curves for the pulse cycle of IOP and retinal vessel pulsations were calculated for each subject. Each pulse cycle was standardised to 50 timepoints, which enabled direct phase shift comparisons. Results All subjects showed comparable results. Close to the optic disc (within 0 to 1.5 optic disc diameters away from the disc), retinal arteries led with the first peak at the 16/50 pulse cycle position, followed by IOP peak at the 23/50 cycle position, and then by veins at the 26/50 cycle position. Conclusion The present method indeed shows that retinal veins do not collapse when the IOP is highest, on the contrary, IOP and retinal vein diameters seem to be in phase, which lends support to the hypothesis that IOP is not the major driving force of the retinal vein pulsations.

Parameter control and concentration analysis of graphene colloids prepared by electric spark discharge method

In this study, graphene colloids were prepared using the electric spark discharge method (ESDM) with graphite rods (99.9% purity) in deionized water (DW) at a normal temperature and pressure. Five different types of graphene colloids were prepared using an electrical discharge machine (EDM) with five different pulse cycle switching times (Ton:Toff ) = 10:10, 30:30, 50:50, 70:70, and 90:90 μs. According to the Ultraviolet-visible spectra (UV-Vis) and Zetasizer analysis, the results showed that the 10:10 μs parameter was the most suitable for the preparation of graphene colloids. UV-Vis was also used to detect the concentration of the graphene colloids; a comparison with a graphene oxide (GO) confirmed that this method could be used to calculate the discharge time needed to produce graphene colloids with different concentrations in the ESDM process.

Micro-Structured Polydopamine Films via Pulsed Electrochemical Deposition

Polydopamine (PDA) films are interesting as smart functional materials, and their controlled structured formation plays a significant role in a wide range of applications ranging from cell adhesion to sensing and catalysis. A pulsed deposition technique is reported for micro-structuring polydopamine films using scanning electrochemical microscopy (SECM) in direct mode. Thereby, precise and reproducible film thicknesses of the deposited spots could be achieved ranging from 5.9 +/− 0.48 nm (1 pulse cycle) to 75.4 nm +/− 2.5 nm for 90 pulse cycles. The obtained morphology is different in comparison to films deposited via cyclic voltammetry or films formed by autooxidation showing a cracked blister-like structure for high pulse cycle numbers. The obtained polydopamine spots were investigated in respect to their electrochemical properties using SECM approach curves. Quantitative kinetic data in dependence of the film thickness, the substrate potential, and the used redox species were obtained.

Comparison of NARX and Dual Polarization Models for Estimation of the VRLA Battery Charging/Discharging Dynamics in Pulse Cycle

The following work presents the model-assisted research on Valve-Regulated Lead-Acid (VRLA) Absorbent Glass Mat (AGM) battery in pulse operation cycle. The experimental research was conducted for a constant value of State of Charge (SOC) of the battery, for values ranging from 0.2 to 0.8. Based on the conducted test stand research, the parameters of the battery were identified, which were later used to model the battery using the equivalent circuit based on dual polarization (DP) model with double Resistive-Capacitive (RC) loop. Simulations were performed for the identified parameters of the battery which are described by the general form of the polynomial. The second part contains the research on utilization of Nonlinear AutoRegressive eXogenous (NARX) recurrent neural network to predict SOC and a terminal voltage of the battery. Obtained validation results with the use of the identified parameters of the double RC loop and NARX model were discussed in the following work. The article also features the advantages and disadvantages of NARX model and DP model utilization for the use of in Battery Managements Systems (BMS) and micro-installations based on renewable energy sources. Furthermore, the advantages of the addition of more RC loops to describe the dynamic states of batteries in pulse states were discussed in the article.

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 three-dimensional (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 95.14 mm 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. Furthermore, results also indicated that both the turbines matched its optimum incidence angle for only a fraction of pulse cycle, which is unfavorable.

The development of a novel unsteady flow control method: Controlling the rotating nozzle ring

The turbocharger is continuously fed with highly unsteady exhaust flow from a reciprocating engine. Despite that, the pulsating exhaust flow can provide more kinetic energy to the turbine compared with the moderated flow in a constant pressure turbocharging, it still significantly deteriorates the efficiency of the turbocharger, as the turbocharger turbine works at off-design point at most instances in an exhaust cycle. In order to address the issue, a novel mechanism named ‘rotating nozzle ring’ has been developed. It was shown that by rotating a nozzle ring around the turbine, the deviation of the flow angle from the design point can be reduced and, therefore, the performance of the turbocharger can be improved. This novel idea is further presented in this paper, by introducing a passive control method to control the speed of the rotating nozzle ring. It will be demonstrated that the rotating nozzle ring can be controlled by the exhaust flow by means of pre-setting a particular nozzle angle, and the rotation will stabilise at an approximately constant speed. An optimised nozzle profile will also be presented, with the intention to reduce the incidence loss on the rotating nozzle ring. A detailed full-stage computational fluid dynamics model will be built to investigate this passive control method. Results of both quasi-steady and transient calculation will demonstrate that, the passively controlled rotating nozzle ring can effectively suppress the unsteadiness level of turbine’s unsteady operation. As a result, the performance of the turbocharger turbine is improved, with the variation of the velocity ratio through a pulse cycle reduced by 8.5% and the isentropic energy weighted cycle averaged efficiency increased by 4.7%, compared to a traditional stationary nozzle ring.