Dynamic modeling and simulation of a pipeline used for transportation of a fluid near the saturation

A new district energy network has been developed by using CO2 as a heat transfer fluid. Only two pipes are required by the network, thus allowing the recovery of waste heat from the cooling users to be valorized by the heating users. Understanding and predicting the dynamics as well as identifying control methods and strategies is one of the key issues for the development of such a CO2 network that would operate in a safe, reliable and energy efficient manner. To assess the technical feasibility and address the safety problems, a physical-based model was developed to describe the fluid dynamics of CO2 pipelines used for transportation of a fluid near the saturation. Dynamic simulations were carried out to investigate pressure and flow fluctuation during transient flow and under changes of external perturbations. It shows that the system was able to response all these perturbations although the corresponding pressure wave propagates along the pipe.

Fundamental study on microlayer dynamics in nucleate boiling

Lubomír Bures

Dynamics of nucleate boiling are strongly affected by the formation and behaviour of the microlayer, a layer of liquid underneath growing bubbles. As a result of its minute thickness, very high heat fluxes occur within the microlayer and its evaporation contributes significantly to the overall heat transfer. Microlayer formation is, however, not guaranteed and the transition from the contact-line to the microlayer regime of nucleate boiling is not fully understood. The difficulty of experimental investigation of the microlayer and the uncertainties surrounding its formation and subsequent evolution motivate the use of Direct Numerical Simulation (DNS) to model its behaviour. In this work, a computational strategy for utilising DNS to model nucleate boiling by resolving explicitly the microlayer is developed. The numerical method is based on the resolution of continuum conservation equations for incompressible two-phase flows in Cartesian and axisymmetric cylindrical coordinates. The phasic interface is tracked by means of the geometric Volume-of-Fluid (VOF) method and the algorithm is applicable both to adiabatic and volatile flows. Online, implicit coupling of the fluid and solid domains for the solution of the conjugate heat-transfer problem is included and closure models for the treatment of the interfacial heat-transfer resistance and the dynamic contact angle are introduced. A rigorous verification and validation exercise is performed to evaluate the efficacy of the numerical algorithm. Subsequently, a theoretical criterion for modelling the transition between contact-line and microlayer regimes is derived, and tested. Very good agreement is found with the reference experimental and simulation data and the predictive power of the criterion is demonstrated with the aid of DNS results. The computational procedure is then validated for simulations of nucleate boiling with resolved microlayer using relevant experimental data recently measured at the Massachusetts Institute of Technology; it is shown that the main observed growth features and surface heat-transfer characteristics are well-reproduced using the overall method. A sensitivity study of the dependence of the initial microlayer thickness on the growth conditions is performed and a universal equation describing the thickness distribution is proposed with liquid properties and bubble expansion rate being the governing parameters. Finally, the computational method is extended to coarse-mesh problems by introducing several reduced-order models and the full bubble-growth cycle from nucleation to detachment is simulated. Good agreement with reference measurements is again achieved and the experimental findings regarding the force balance during nucleate boiling are confirmed.

EPFL2021

Modeling the fluid dynamics of the heart: from blood flows in idealized left ventricles to patient-specific aortic valve simulations

Simone Deparis, Elena Faggiano, Marco Fedele, Davide Forti, Alfio Quarteroni, Anna Tagliabue

The computational fluid dynamics of the heart represents a challenging task both in terms of mathematical and numerical modeling; this is mainly due to the pulsatile nature of the blood flow, to its complex interaction with the valves, and, more in general, to the reciprocal action of the components responsible for the heart functioning. Even if one focuses on the study of the left ventricle, the blood flow patterns result to be significantly dependent on the mechanical contraction and relaxation of the muscle, on the conformation of the chamber, and on the interaction with the valves, which define a complex fluid-structure interaction problem. In this respect, also the blood flow in the aorta, and hence in the downstream circulation, is strongly affected by the aortic valve, whose behavior should be suitably mathematically and numerically modeled. In this work, we firstly focus on the study of the fluid dynamics inside the left ventricle in idealized configurations, for which we propose a mathematical model based on the Navier-Stokes equations endowed with mixed, time-dependent boundary conditions, which allow a simplified treatment of the aortic and mitral valves’ behavior. In this idealized setting, we perform numerical simulations which highlight the role and influence of modeling the valves to study and characterize the blood flow patterns inside the ventricle, as well as other parameters of clinical relevance. In addition, we consider a reduced, patient-specific fluid-structure-interaction model for the simulation of the blood flow through the aortic valve. Specifically, we propose an efficient coupled model which represents the valve dynamics by means of a zero-dimensional (0D) equation with the opening angle as primitive variable, while the blood flow by means of the full 3D Navier-Stokes equations. In this coupled model, the valve’s leaflets, which are reconstructed from MRI data of the open and closed configurations for a specific patient, influence the Navier-Stokes equations by means of resistive immersed surfaces, whose position depends on the opening angle of the valve. Moreover, the dynamics of the valve described by the 0D model is dependent on the velocity and pressure variables, specifically on the pressure jump and the flow rate through the valve itself. We per form patient-specific numerical simulations of the aortic valve based on this reduced 3D-0D model, for which we highlight its ability to correctly capture the fluid dynamics indicators expected for the patient.

2015

Heterogeneous Catalytic Reactor for Hydrogen Production from Formic Acid and Its Use in Polymer Electrolyte Fuel Cells

Nordahl Autissier, Andrew Francis Dalebrook, Martin Grasemann, Gabor Laurenczy, Katerina Stefania Sordakis

A proof-of-concept prototype of a medium-scale heterogeneous catalytic reactor for continuous production of hydrogen through formic acid (FA) dehydrogenation has been developed. A commercial proton exchange membrane (PEM) fuel cell (FC) fed with the resulting gas outflow (H2 + CO2) was applied to convert chemical energy to electricity. To implement an efficient coupling of the reactor and FC, research efforts in three interrelated areas were undertaken: 1) catalyst development and testing; 2) computer modelling of heat and mass transfer to optimize the reactor design and 3) study of compatibility of the reactor gas outflow with a PEM FC. During the catalyst development, immobilization of a homogeneous Ru – metatrisulphonated triphenylphosphine (mTPPTS) catalyst on different supports was performed and Ru-mTPPTS supported on phosphinated polystyrene beads demonstrated the best results. A validated mathematical model of the catalytic reactor with coupled heat transfer, fluid flow and chemical reactions was proposed for catalyst bed and reactor design. Measured reactor operating characteristics were used to refine modelling parameters. In turn, catalyst bed and reactor geometry was optimised during an iterative adaptation of the reactor and model parameters. In the final phase, PEM FC operating conditions and (H2+CO2) gas treatment were optimized to provide the best FC efficiency and lifetime. Stable performance of a commercial 100W PEM FC coupled with the developed reactor prototype was successfully demonstrated. The low CO concentration (below 5 ppm) in the reformate was insured by preferential oxidation (PROX).