62 research outputs found
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Pore scale 3D modelling of heat and mass transfer in the gas diffusion layer and cathode channel of a PEM fuel cell
Flooding of the gas diffusion layer (GDL) of proton exchange membrane (PEM) fuel cells can be a bottleneck to the system’s efficiency and even durability under certain operating conditions. Due to the small scale and complex geometry of the materials involved, detailed insight into the pore scale phenomena that take place are difficult to measure or simulate. In the present effort, a direct 3D microscale model of a portion of the PEM cathode channel and carbon cloth GDL is used to parametrically investigate local heat and fluid flow at the GDL’s pore scale and their effects on condensation of water vapour that leads to flooding. The 3D simulation through the microscale geometry is among the first appearing in the international literature. The Navier–Stokes, energy and water vapour transport equations are solved at steady state and in three-dimensional space for a range of inlet velocities and cloth fibre material properties, using a conjugate heat transfer approach to calculate the temperature field within the solid fibres. Psychrometric calculations are applied to provide indications of the conditions and areas most prone to condensation based on the calculated local temperatures and water vapour concentration
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Off-centre binary collision of droplets: A numerical investigation
The paper presents results from a numerical investigation of the non-central binary collision of two equal size droplets in a gaseous phase. The flow field is two phase and three dimensional; the investigation is based on the finite volume numerical solution of the Navier–Stokes equations, coupled with the Volume of Fluid Method (VOF), expressing the unified flow field of the two phases, liquid and gas. A recently developed adaptive local grid refinement technique is used, in order to increase the accuracy of the solution particularly in the region of the liquid–gas interface. The reliability of the solution procedure is tested by comparing predictions with available experimental data. The numerical results predict the collision process of the two colliding droplets (permanent coalescence or separation) and in the case of separation the formation and the size of the satellite droplets. The time evolution of the geometrical characteristics of the ligament, for various Weber numbers and impact parameters, is calculated and details are shown of the velocity and pressure fields particularly at the ligament pinch off location not hitherto available. Gas bubbles due to collision are trapped within the liquid phase as it has also been observed in experiments and their volume is calculated
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Numerical investigation on the evaporation of droplets depositing on heated surfaces at low Weber numbers
The evaporation of water droplets, impinging with low Weber number and gently depositing on heated surfaces of stainless steel is studied numerically using a combination of fluid flow and heat transfer models. The coupled problem of heat transfer between the surrounding air, the droplet and the wall together with the liquid vaporisation from the droplet’s free surface is predicted using a modified VOF methodology accounting for phase-change and variable liquid properties. The surface cooling during droplet’s evaporation is predicted by solving simultaneously with the fluid flow and heat transfer equations, the heat conduction equation within the solid wall. The droplet’s evaporation rate is predicted using a model from the kinetic theory of gases coupled with the Spalding mass transfer model, for different initial contact angles and substrate’s temperatures, which have been varied between 20–90° and 60–100 °C, respectively. Additionally, results from a simplified and computationally less demanding simulation methodology, accounting only for the heat transfer and vaporisation processes using a time-dependent but pre-described droplet shape while neglecting fluid flow are compared with those from the full solution. The numerical results are compared against experiments for the droplet volume regression, life time and droplet shape change, showing a good agreement
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Numerical investigation of the evaporation of two-component droplets
A numerical model for the complete thermo-fluid-dynamic and phase-change transport processes of two-component hydrocarbon liquid droplets consisting of n-heptane, n-decane and mixture of the two in various compositions is presented and validated against experimental data. The Navier–Stokes equations are solved numerically together with the VOF methodology for tracking the droplet interface, using an adaptive local grid refinement technique. The energy and concentration equations inside the liquid and the gaseous phases for both liquid species and their vapor components are additionally solved, coupled together with a model predicting the local vaporization rate at the cells forming the interface between the liquid and the surrounding gas. The model is validated against experimental data available for droplets suspended on a small diameter pipe in a hot air environment under convective flow conditions; these refer to droplet’s surface temperature and size regression with time. An extended investigation of the flow field is presented along with the temperature and concentration fields. The equilibrium position of droplets is estimated together with the deformation process of the droplet. Finally, extensive parametric studies are presented revealing the nature of multi-component droplet evaporation on the details of the flow, the temperature and concentration fields
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Transient heating effects in high pressure Diesel injector nozzles
The tendency of today’s fuel injection systems to reach injection pressures up to 3000 bar in order to meet forthcoming emission regulations may significantly increase liquid temperatures due to friction heating; this paper identifies numerically the importance of fuel pressurization, phase-change due to cavitation, wall heat transfer and needle valve motion on the fluid heating induced in high pressure Diesel fuel injectors. These parameters affect the nozzle discharge coefficient (Cd), fuel exit temperature, cavitation volume fraction and temperature distribution within the nozzle. Variable fuel properties, being a function of the local pressure and temperature are found necessary in order to simulate accurately the effects of depressurization and heating induced by friction forces. Comparison of CFD predictions against a 0-D thermodynamic model, indicates that although the mean exit temperature increase relative to the initial fuel temperature is proportional to (1 − Cd2) at fixed needle positions, it can significantly deviate from this value when the motion of the needle valve, controlling the opening and closing of the injection process, is taken into consideration. Increasing the inlet pressure from 2000 bar, which is the pressure utilized in today’s fuel systems to 3000 bar, results to significantly increased fluid temperatures above the boiling point of the Diesel fuel components and therefore regions of potential heterogeneous fuel boiling are identified
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Friction-induced heating in nozzle hole micro-channels under extreme fuel pressurisation
Fuel pressurisation up to 3000 bar, as required by modern Diesel engines, can result in significant variation of the fuel physical properties relative to those at atmospheric pressure and room temperature conditions. The huge acceleration of the fuel as it is pushed through the nozzle hole orifices is known to induce cavitation, which is typically considered as an iso-thermal process. However, discharge of this pressurised liquid fuel through the micro-channel holes can result in severe wall velocity gradients which induce friction and thus heating of the liquid. Simulations assuming variable properties reveal two opposing processes strongly affecting the fuel injection quantity and its temperature. The first one is related to the de-pressurisation of the fuel; the strong pressure and density gradients at the central part of the injection hole induce fuel temperatures even lower than that of the inlet fuel temperature. On the other hand, the strong heating produced by wall friction increases significantly the fuel temperature; local values can exceed the liquid’s boiling point and even induce reverse heat transfer from the liquid to the nozzle’s metal body. Local values of the thermal conductivity and heat capacity affect the transfer of heat produced at the nozzle surface to the flowing liquid. That creates strong temperature gradients within the flowing liquid which cannot be ignored for accurate predictions of the flow through such nozzles
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Non-dimensionalisation parameters for predicting the cooling effectiveness of droplets impinging on moderate temperature solid surfaces
The conjugate problem of fluid flow and heat transfer during the impact of water droplets onto a heated surface is studied numerically using the Volume of Fluid (VOF) methodology; adaptive grid refinement is used for increased resolution at the droplet moving interface. The phenomenon is assumed to be 2D-axisymmetric and the wall temperature is moderated to prevent the onset of nucleate boiling. Parametric studies examine the effect of Weber number, droplet size, wall initial temperature and liquid thermal properties on the cooling process of the heated plate during the impaction period. The main variables describing the evolution of the phenomenon are non-dimensionalised with expressions arising from the transient conduction theory. It is proved that for all cases examined, these non-dimensional expressions can be grouped together for describing the hydrodynamic and thermal behavior in a similar manner. Additionally, semi-analytic expressions are derived, which, for a given range of variation, describe the spatial distribution and the temporal evolution of the temperature of the wall as well also the heat flux absorbed from the droplet, cooling effectiveness and mean droplet temperature
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Single droplet impacts onto deposited drops. Numerical analysis and comparison
The impact of a spherical water droplet onto a stationary sessile droplet lying on a solid wall is studied numerically using the volume-of-fluid methodology. The governing Navier-Stokes equations are solved both for the gas and liquid phase coupled with an additional equation for the transport of the liquid interface. An unstructured numerical grid is used along with an adaptive local grid refinement technique, which enhances the accuracy of the numerical results along the liquid-gas interface and decreases the computational cost. The stationary sessile droplet has been created from the prior impact of one or two water droplets falling onto the solid wall, while two solid walls have been studied−an aluminum substrate and a glass substrate. The material of the wall plays an important role because it has an impact on the droplet's wetting behavior. The numerical model is validated against corresponding experimental data presented in the first part of the present work (Nikolopoulos et al., 2010), showing good agreement. Furthermore, the numerical investigation sheds light on the governing physics of the phenomenon
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Vortex formation and recirculation zones in left anterior descending artery stenoses: computational fluid dynamics analysis
Flow patterns may affect the potential of thrombus formation following plaque rupture. Computational fluid dynamics (CFD) were employed to assess hemodynamic conditions, and particularly flow recirculation and vortex formation in reconstructed arterial models associated with ST-elevation myocardial infraction (STEMI) or stable coronary stenosis (SCS) in the left anterior descending coronary artery (LAD). Results indicate that in the arterial models associated with STEMI, a 50% diameter stenosis immediately before or after a bifurcation creates a recirculation zone and vortex formation at the orifice of the bifurcation branch, for most of the cardiac cycle, thus allowing the creation of stagnating flow. These flow patterns are not seen in the SCS model with an identical stenosis. Post-stenotic recirculation in the presence of a 90% stenosis was evident at both the STEMI and SCS models. The presence of 90% diameter stenosis resulted in flow reduction in the LAD of 51.5% and 35.9% in the STEMI models and 37.6% in the SCS model, for a 10 mmHg pressure drop. CFD simulations in a reconstructed model of stenotic LAD segments indicate that specific anatomic characteristics create zones of vortices and flow recirculation that promote thrombus formation and potentially myocardial infarction
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