Experimental investigation and CFD analysis of pressure drop in an ORC boiler for a WHRS implementation

Waste heat dissipated in the exhaust system of a combustion engine represents a major source of energy to be recovered and converted into useful work. The Waste Heat Recovery System (WHRS) based in an Organic Rankine Cycle (ORC) is an approach for recovering energy from heat sources, achieving a significant reduction in fuel consumption and, as a result, exhaust emissions. This paper studies pressure drop in an ORC shell-and-tubes boiler for a WHRS implementation experimentally and with computational simulations based on a 1-dimensional heat transfer model coupled with 3D calculations. An experimental database is developed, using ethanol in a pressure range of 10–15 absolute bar as working fluid, with mass fluxes inside the tubes in the range of 349.31 kg/s-m2 and 523.97 kg/s-m2, and inlet temperatures in the range of 60 °C and 80 °C. Thus, the friction factor of different regions of the boiler were estimated using both CFD simulations, experimental data, and bibliographic correlations. Simulations of operating points and the results of the experimental test bench showed good agreement in pressure drop results, with a mean absolute error of 15.47%, without a significant increment in the computational cost

Development of a pattern recognition methodology with thermography and implementation in an experimental study of a boiler for a WHRS-ORC

Waste heat dissipated in the exhaust system in a combustion engine represents a major source of energy to be recovered and converted into useful work. A waste heat recovery system (WHRS) based on an Organic Rankine Cycle (ORC) is a promising approach, and it gained interest in the last few years in an automotive industry interested in reducing fuel consumption and exhaust emissions. Understanding the thermodynamic response of the boiler employed in an ORC plays an important role in steam cycle performance prediction and control system design. The aim of this study is, therefore, to present a methodology to study these devices by means of pattern recognition with infrared thermography. In addition, the experimental test bench and its operating conditions are described. The methodology proposed identifies the wall coordinates, traces the paths, and tracks the wall temperature along them in a way that can be exported for subsequent post-processing and analysis. As for the results, through the wall temperature paths on both sides (exhaust gas and working fluid), it was possible to quantitatively estimate the temperature evolution along the boiler and, in particular, the beginning and end of evaporation

Numerical study of a thrombus migration risk in aneurysm after coil embolization in patient cases: FSI modelling

Purpose There are still many challenges for modelling a thrombus migration process in aneurysms. The main novelty of the present research lies in the modelling of aneurysm clot migration process in a realistic cerebral aneurysm, and the analysis of forces sufered by clots inside an aneurysm, through transient FSI simulations. Methods The blood fow has been modelled using a Womersley velocity profle, and following the Carreau viscosity model. Hyperelastic Ogden model has been used for clot and isotropic linear elastic model for the artery walls. The FSI coupled model was implemented in ANSYS® software. The hemodynamic forces sufered by the clot have been quantifed using eight diferent clot sizes and positions inside a real aneurysm. Results The obtained results have shown that it is almost impossible for clots adjacent to aneurysm walls, to leave the aneurysm. Nevertheless, in clots positioned in the centre of the aneurysm, there is a real risk of clot migration. The risk of migration of a typical post-coiling intervention clot in an aneurysm, in contact with the wall and occupying a signifcant percentage of its volume is very low in the case studied, even in the presence of abnormally intense events, associated with sneezes or impacts. Conclusions The proposed methodology allows evaluating the clot migration risk, vital for evaluating the progress after endovascular interventions, it is a step forward in the personalized medicine, patient follow-up, and helping the medical team deciding the optimal treatment.Universidade de Vigo/CISU

Analysis of the local growth and density evolution of soot deposits generated under hydrocarbon condensation: 3D simulation and detailed experimental validation

The utilization of the Exhaust Gas Recirculation (EGR) system during atypical engine operating conditions in order to meet future type-approval criteria exposes the internal surfaces of the devices to exhaust gas with elevated concentrations of particulate matter and greater amounts of hydrocarbon species, leading to the formation of dense and wet sludge deposits. To broaden the understanding of this phenomenon and contribute to the development of advanced EGR devices, this study presents an extended Computational Fluid Dynamics (CFD) model that, in addition to simulating the growth of fouling deposits caused by the accumulation of soot particles, also takes into account the condensation of hydrocarbons. Two scenarios with varying hydrocarbon concentrations in the exhaust flow are analysed, and the evolution of the deposit's thickness and density is determined. A sequential validation process is carried out by comparing the numerical results to actual deposit profiles at different stages of the fouling process. Additionally, hyperspectral images of the fouling layer have been acquired and analysed to validate the regions where hydrocarbon condensation is predicted to play a crucial role, enabling the verification of the hydrocarbon condensation phenomenon predicted by the numerical model. The results obtained under the studied conditions indicate that, on average, 77.4% of the analysed area exhibits a low level of relative error, demonstrating that the proposed model and the methodology used serve as a valuable tool for examining the propensity for deposit formation in devices subjected to fouling exacerbated by hydrocarbon condensation.Agencia Estatal de Investigación | Ref. PDC2021-121778-10

Sistemas fluidomecánicos no transporte: prácticas de simulacións numéricas

A tecnoloxía CFD, sigla de Computational Fluid Dynamics, en galego Dinámica de Fluídos Computacional, é unha técnica de análise numérica que permite determinar o comportamento dos fluídos mediante a realización de experimentos virtuais no ordenador. Esta técnica conta con varias décadas de desenvolvemento científico de respaldo, converténdose a día de hoxe nun dos estándares de análise para numerosos sectores industriais. É por iso que as simulacións numéricas son empregadas actualmente como eficaces ferramentas de diagnóstico nas etapas de deseño, mellora e optimización de produtos e procesos de fabricación en industrias como a naval, a automobilística, a aeroespacial, a hidráulica, as enerxías renovables, a enxeñería biomédica ou a construción. Co fin de achegar o emprego desta técnica ao alumnado do grado de Enxeñería Mecánica, este manual foi creado para servir de apoio á docencia da materia Sistemas fluidomecánicos e materiais avanzados para o transporte, enfocándose na aplicación práctica de simulacións numéricas en casos reais da enxeñería nos que se atopan involucrados fluxos de fluídos. Esta colección de exercicios prácticos pretende servir de axuda na etapa formativa do alumnado, facilitándolle a información necesaria e a metodoloxía que deberá empregar para levar a cabo o estudo de diferentes casos, e favorecendo así unha comprensión intuitiva dos problemas propostos mediante a análise dos resultados obtidos nas simulacións numéricas. Por tratarse dun dos programas de CFD máis empregados na industria, o software utilizado nas actividades deste manual é ANSYS Fluent 2020. Así tamén, as actividades propostas neste manual foron deseñadas para poder ser levadas a cabo contando cunhas esixencias computacionais accesibles para o alumnado, o cal poderá realizar nun tempo axeitado os exercicios aquí recollidos

CFD transient simulation of the cough clearance process using an Eulerian wall film model

In this study, a cough cycle is reproduced using a computational methodology. The Eulerian wall film approach is proposed to simulate airway mucus flow during a cough. The reproduced airway domain is based on realistic geometry from the literature and captures the deformation of flexible tissue. To quantify the overall performance of this complex phenomenon, cough efficiency (CE) was calculated, which provided an easily reproducible measurement parameter for the cough clearance process. Moreover, the effect of mucus layer thickness was examined. The relationship between the CE and the mucus viscosity was quantified using reductions from 20 to 80%. Finally, predictions of CE values based on healthy person inputs were compared with values obtained from patients with different respiratory diseases, including chronic obstructive pulmonary disease (COPD) and respiratory muscle weakness (RMW). It was observed that CE was reduced by 50% in patients with COPD compared with that of a healthy person. On average, CE was reduced in patients with RMW to 10% of the average value of a healthy person

On the width and mean value of bubble size distributions under subcooled flow boiling

In this work, a new comprehensive dataset about bubble sizes in flow boiling of water is presented. The experimental setup basically consists of a lower copper heated plate embedded in a horizontal rectangular channel flow. Several experimental conditions, ranging bulk velocities: 0.1–0.9 [m·s−1]; mass fluxes: 96.9–871.8 [kg·m−2·s−1]; subcooling degrees: 16–36 [°C]; heat fluxes: 200–650 [kW·m−2] and pressures: 110–190 [kPa] were set for three cooper plates of different roughness, with Sa: 0.45, 1.23 and 7.43 [µm], respectively. Based on the current samples, the probability function for the normalized bubble diameter has been described by a lognormal pdf for any plate and experimental condition and a dimensionless correlation for the standard deviation of the bubble size distribution is presented. In order to completely describe the diameter distribution for bubbles in subcooled boiling systems, an improved correlation for the mean bubble diameter value is also presented here, after a deep review of previously published datasets. Further work is required to accommodate different channel configurations and surface morphologies.Agencia Estatal de Investigación | Ref. RTC2019-006955-

An image-processing algorithm for morphological characterisation of soot agglomerates from TEM micrographs: Development and functional description

The inspection of soot agglomerates from microscopy images usually relies on manual human measurements, whereas image processing tools for faster analysis are highly demanded. In this study, an automated algorithm for the extraction of morphological soot descriptors from transmission electron micrographs is presented. The proposed algorithm involves the detection of the image scale (the conversion from pixels to nanometres) using a Hough transform and an optical character recognition process. Primary particles are identified through a two-step circle Hough transform combining phase-coding and edge-based approaches, whereas size descriptors are obtained through spatial and frequency filtering. Finally, the fractal dimension is obtained for each agglomerate as a projected-area derived measurement due to an iterative process. Results were validated by comparison of a set of micrographs taken at three different magnifications with manual image processing, obtaining p-values greater than 0.05 and around 91.5% time saving

Experimental evaluation of the critical local wall shear stress around cylindrical probes fouled by diesel exhaust gases

The problem of fouling in the heat exchangers of exhaust systems has yet to be resolved. This results in enormous costs for engine manufacturers due to the required over-sizing during design and due to unscheduled maintenance needs. This article presents an experimental layout developed for measuring fouling in diesel engine exhaust gas systems. This facility was based on a circular cylindrical cross-flow device, with one straight and smooth stainless steel probe positioned transverse to the flow of exhaust gases. The probe can be cooled from the inside with water and fouled on the outside as a result of particle deposition from exhaust gases. The tests were conducted under constant engine operating conditions. Therefore, the asymptotic depth of the fouling layer could be measured at different angular positions at the end of each test. The critical wall shear stress rate is proposed as the controlling mechanism of the local removal process that leads to different fouling depths around each probe. This is in contrast to the critical velocity concept, which cannot be applied at a local scale due to its formulation. The experimental results, although subject to the usual uncertainties of fouling processes, seem to support this idea

Experimental study of soot particle fouling on ribbed plates : applicability of the critical local wall shear stress criterion

This article examines the notion of critical wall shear stress as the key control parameter of the local fouling removal process. In this study, an experimental setup was developed for measuring fouling on enhanced surfaces. Specifically, the experimental configuration consists of a forced convection plate heat exchanger containing a one-pass rectangular channel with two ribbed plates arranged in a symmetrically staggered manner. The exhaust gases flow by the rib-roughened sides of the plates, and the flat sides can be cooled with water from the independent external coolant circuit. As a result of soot particle deposition from exhaust gases, a layer of fouling is deposited over the ribs. After asymptotic conditions were reached during the tests, detailed fouling thickness measurements were conducted. The dimensionless particle relaxation time during these tests was determined to be in the range of 0.3–10. The measurements were then complemented with a numerical analysis. In particular, the local wall shear stress was calculated using a commercial computational fluid dynamics (CFDs) software package. The fouling thickness profiles deposited over the ribs and the local critical shear stress values were compared and discussed for two different geometries. The results obtained clearly support the idea that critical wall shear stress is an appropriate criterion for facilitating the understanding of the local behaviour of fouling deposits