A quasi 3D approach for the modelling of an automotive turbocharger's compressor

In this work the 3DCell method has been extended to the thermo-fluid dynamic simulation of an automotive turbocharger's compressor. The 3DCell, an approach continuously developed by the authors at Politecnico di Milano, is based on a pseudo-staggered leapfrog method that allows to decompose a generic 3D problem in a set of 1D scalar equation arbitrarily oriented in space. The system of equations has been solved referring to a relative rotating framework for the moving components, whereas to an absolute reference elsewhere. The domain has been discretized on a basis of a polar coordinate system, identifying five macro sub-domains, namely the inlet pipe, impeller, vaneless diffuser, volute, outlet pipe, each treated numerically in a specific way. The diffuser's momentum in the tangential direction has been modelled resorting to the conservation of the angular momentum, while the rotor channels are modelled as rotating pipes that exchange work and momentum with the blades as they experience a relative source term due to the centrifugal force field and its potential. The model has been validated against measurements carried out on a steady state flow test bench at University of Genoa

Blood flow rate estimation in optic disc capillaries and vessels using Doppler optical coherence tomography with 3D fast phase unwrapping

The retinal volumetric flow rate contains useful information not only for ophthalmology but also for the diagnosis of common civilization diseases such as diabetes, Alzheimer's disease, or cerebrovascular diseases. Non-invasive optical methods for quantitative flow assessment, such as Doppler optical coherence tomography (OCT), have certain limitations. One is the phase wrapping that makes simultaneous calculations of the flow in all human retinal vessels impossible due to a very large span of flow velocities. We demonstrate that three-dimensional Doppler OCT combined with three-dimensional four Fourier transform fast phase unwrapping (3D 4FT FPU) allows for the calculation of the volumetric blood flow rate in real-time by the implementation of the algorithms in a graphics processing unit (GPU). The additive character of the flow at the furcations is proven using a microfluidic device with controlled flow rates as well as in the retinal veins bifurcations imaged in the optic disc area of five healthy volunteers. We show values of blood flow rates calculated for retinal capillaries and vessels with diameters in the range of 12-150 µm. The potential of quantitative measurement of retinal blood flow volume includes noninvasive detection of carotid artery stenosis or occlusion, measuring vascular reactivity and evaluation of vessel wall stiffness

Do coastal wetlands drive or buffer coastal acidification?

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Validation of a Theoretical Model for the Correction of Heat Transfer Effects in Turbocharger Testing through a Quasi-3D Model

In the last few years, the effect of diabatic test conditions on compressor performance maps has been widely investigated, leading some Authors to propose different correction models. The accuracy of turbocharger performance map constitute the basis for the tuning and validation of a numerical method, usually adopted for the prediction of engine-turbocharger matching. Actually, it is common practice in automotive applications to use simulation codes, which can either require measured compression ratio and efficiency maps as input values or calculate them "on the fly" throughout specific sub-models integrated in the numerical procedures. Therefore, the ability to correct the measured performance maps taking into account internal heat transfer would allow the implementation of commercial simulation codes used for engine-turbocharger matching calculations. In the paper the main results of a wide experimental activity are reported to provide a general understanding of heat transfer mechanism occurring in turbochargers and relationships for heat transfer rate useful to derive the adiabatic efficiency. The compressor steady flow performance maps were measured at different operating temperatures for compressor and turbine, with and without water-cooling and under quasi-adiabatic condition achieved by maintaining the lubricating oil average temperature equal to compressor outlet temperature and turbine inlet temperature to minimize internal heat fluxes. Furthermore, a mathematical model for the correction of compressor steady flow maps, developed by the University of Genoa, is adopted and compared to the quasi-adiabatic condition. In the context of this work a quasi-3D CFD code developed at Politecnico di Milano has been extended and applied to simulate the flow inside the compressor. To this purpose the adiabatic assumption has been removed and the heat transfer between the gas and the stationary and rotating components has been taken into account. Correlations for the heat transfer coefficient have been taken from the literature and implemented in the code. The quasi-3D model is then used to simulate the compressor both in adiabatic and diabatic condition. The quasi-3D CFD code was validated against the experimental results, confirming also the validity of the mathematical model used to correct the maps

A quasi 3D approach for the modelling of an automotive turbocharger's compressor

In this work the 3DCell method has been extended to the thermo-fluid dynamic simulation of an automotive turbocharger's compressor. The 3DCell, an approach continuously developed by the authors at Politecnico di Milano, is based on a pseudo-staggered leapfrog method that allows to decompose a generic 3D problem in a set of 1D scalar equation arbitrarily oriented in space. The system of equations has been solved referring to a relative rotating framework for the moving components, whereas to an absolute reference elsewhere. The domain has been discretized on a basis of a polar coordinate system, identifying five macro sub-domains, namely the inlet pipe, impeller, vaneless diffuser, volute, outlet pipe, each treated numerically in a specific way. The diffuser's momentum in the tangential direction has been modelled resorting to the conservation of the angular momentum, while the rotor channels are modelled as rotating pipes that exchange work and momentum with the blades as they experience a relative source term due to the centrifugal force field and its potential. The model has been validated against measurements carried out on a steady state flow test bench at University of Genoa

0d/1d thermo‐fluid dynamic modeling tools for the simulation of driving cycles and the optimization of ic engine performances and emissions

The prediction of internal combustion engine performance and emissions in real driving conditions is getting more and more important due to the upcoming stricter regulations. This work aims at introducing and validating a new transient simulation methodology of an ICE coupled to a hybrid architecture vehicle, getting closer to real‐time calculations. A one‐dimensional computational fluid dynamic software has been used and suitably coupled to a vehicle dynamics model in a user function framework integrated within a Simulink® environment. A six‐cylinder diesel engine has been modeled by means of the 1D tool and cylinder‐out emissions have been compared to experimental data. The measurements available have been used also to calibrate the combustion model. The developed 1D engine model has been then used to perform driving cycle simulations considering the vehicle dynamics and the coupling with the energy storage unit in the hybrid mode. The map‐based approach along with the vehicle simulation tool has also been used to perform the same simulation and the two results are compared to evaluate the accuracy of each approach. In this framework, to achieve the best simulation performance in terms of computational time over simulated time ratio, the 1D engine model has been used in a configuration with a very coarse mesh. Results have shown that despite the high mesh spacing used the accuracy of the wave dynamics prediction was not affected in a significant way, whereas a remarkable speed‐up factor was achieved. This means that a crank angle resolution approach to the vehicle simulation is a viable and accurate strategy to predict the engine emission during any driving cycle with a computation effort compatible with the tight schedule of a design process

Detailed prediction of HRR and NOx emissions in CI engines via a novel thermodynamic model with constant equivalence ratio zones

This paper presents a novel, quasi dimensional-model for the simulation of the combustion process in compression-ignition engines. The model discretizes with a multiple number of zones the in-cylinder air-fuel mass on fixed values of local equivalence ratio, with the charge stratification determined from a 2D reconstruction via a one-dimensional control-volume-based spray approach. Reacting multi-zones are further split into three conceptual sub-zones: liquid, unburnt vapor and burnt vapor, in which chemical reactions proceed according to a tabulated kinetics of ignition (TKI) model. This approach provides a simple methodological framework for the combustion of direct injection of virtually every kind of liquid fuel and relies on a detailed phenomenological chemical/physical link of jet’s reacting phenomena. To account for engine geometry, a spray-wall interaction sub-model has been added to the axial spray. The model has been validated against experimental data and detailed CFD simulation results. First, the direct injection model (as a free jet) has been assessed with respect to ECN sprays A, C, and D experiments. For both the long injection and split injection cases, all the combustion phases are well predicted: premixed peak, mixing-controlled combustion, burn-out are seamlessly described and in good agreement with experimental and detailed CFD data. The wall interaction sub-model was validated with a suitable experiment where a reacting jet impinges on a mock-up wall inside a combustion vessel. Finally, the model has been validated against real heavy-duty engine experimental observations of 151 points of a complete engine map. The tuning of the model consists in two parameters, that are engine-specific, hence constant for the whole map. With these assumptions, the AHRR traces are well described in all simulated conditions. Mean predicted BSFC is only slightly underestimated (−2.3%), as it is the NOx production (−1.6%)

Validation of a Theoretical Model for the Correction of Heat Transfer Effects in Turbocharger Testing through a Quasi-3D Model

In the last few years, the effect of diabatic test conditions on compressor performance maps has been widely investigated, leading some Authors to propose different correction models. The accuracy of turbocharger performance map constitute the basis for the tuning and validation of a numerical method, usually adopted for the prediction of engine-turbocharger matching. Actually, it is common practice in automotive applications to use simulation codes, which can either require measured compression ratio and efficiency maps as input values or calculate them "on the fly" throughout specific sub-models integrated in the numerical procedures. Therefore, the ability to correct the measured performance maps taking into account internal heat transfer would allow the implementation of commercial simulation codes used for engine-turbocharger matching calculations. In the paper the main results of a wide experimental activity are reported to provide a general understanding of heat transfer mechanism occurring in turbochargers and relationships for heat transfer rate useful to derive the adiabatic efficiency. The compressor steady flow performance maps were measured at different operating temperatures for compressor and turbine, with and without water-cooling and under quasi-adiabatic condition achieved by maintaining the lubricating oil average temperature equal to compressor outlet temperature and turbine inlet temperature to minimize internal heat fluxes. Furthermore, a mathematical model for the correction of compressor steady flow maps, developed by the University of Genoa, is adopted and compared to the quasi-adiabatic condition. In the context of this work a quasi-3D CFD code developed at Politecnico di Milano has been extended and applied to simulate the flow inside the compressor. To this purpose the adiabatic assumption has been removed and the heat transfer between the gas and the stationary and rotating components has been taken into account. Correlations for the heat transfer coefficient have been taken from the literature and implemented in the code. The quasi-3D model is then used to simulate the compressor both in adiabatic and diabatic condition. The quasi-3D CFD code was validated against the experimental results, confirming also the validity of the mathematical model used to correct the maps

A Constant Equivalence Ratio Multi-Zone Approach for a Detailed and Fast Prediction of Performances and Emission in CI Engines

The paper illustrates and validates a novel predictive combustion model for the estimation of performances and pollutant production in CI engines. The numerical methodology was developed by the authors for near real-time applications, while aiming at an accurate description of the air mixing process by means of a multi-zone approach of the air-fuel mass. Charge stratification is estimated via a 2D representation of the fuel spray distribution that is numerically derived by an axial one-dimensional control-volume description of the direct injection. The radial coordinate of each control volume is reconstructed a posteriori by means of a local distribution function. Fuel mass clustered in each zone is further split in 'liquid', 'unburnt' and 'burnt' sub-zones, given the local properties of the fuel spray control volumes with respect to space-time location of modelled ignition delay, liquid length, and flame lift-off. Multiple injections are described on the same numerical grid to account for jet-to-jet axial interactions, whose effects reflect on improved ignition characteristics. The multi-zones are open systems which are discretized on the equivalence ratio; mass is allowed to travel from one to another, causing a 2D charge stratification. For each zone, local thermodynamic properties and NOx production are determined to estimate cylinder-average performances and emissions. The apparent heat release rate, in-cylinder pressure, BSFC and NOx emissions are validated against experimental data of full map of a light-duty engine. The computational effort of the model is relatively low, which makes the approach suitable for static optimization, to be used in 1-D simulation codes for transient's optimization and can be run in parallel with a real time 1-D gas model for the simulation of long driving cycles

Unsteady modeling of turbochargers for automotive applications by means of a quasi-3D approach

This work describes the development and the application of a quasi-3D method for the simulation of turbochargers for automotive applications under unsteady flow conditions. The quasi-3D approach is based on the solution of conservation equations for mass, momentum and energy for unsteady flows and applied to 0D and 1D elements arbitrarily oriented in the space. The compressor is divided into different regions, each one treated numerically in a different way. For the impeller region a relative reference system has been used and the presence of a centrifugal force field has been introduced both in the momentum and energy conservation equation. The direction of the ports at the inlet and outlet of the impeller are used to determine the design flow angles and therefore the deviation during off-design conditions. Conversely in the vaneless diffuser the conservation of the angular momentum of the flow stream has been imposed in the tangential direction and then combined with the solution of the momentum equation in the radial direction. The model has been validated against measurements carried out on the test bench of the University of Genoa both in diabatic and adiabatic conditions