A level-set method for thermal motion of bubbles and droplets

Published under licence in Journal of Physics: Conference Series by IOP Publishing Ltd. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.A conservative level-set model for direct simulation of two-phase flows with thermocapillary effects at dynamically deformable interface is presented. The Navier-Stokes equations coupled with the energy conservation equation are solved by means of a finite-volume/level-set method. Some numerical examples including thermocapillary motion of single and multiple fluid particles are computed by means of the present method. The results are compared with analytical solutions and numerical results from the literature as validations of the proposed model.Peer ReviewedPostprint (published version

Simulation of fluid-structure interaction and impact force on a reed valve

The cyclic impact force between a reed valve and the seat plate is the main reason of the valve failure in many thermo-technical devices as compressors, engines, etc. According to experimental observations the latter is due to fatigue and usually occurs in the leading part of the valve ‘neck’. In this work, a complex numerical analysis is presented aimed to studying the external forces and internal stresses suffered by the valve. In particular, the impact force between the valve and the seat is studied. The numerical analysis relies on the coupled synergy of two different simulation concepts. In order to do so, two codes are used: (1) first, the in-house Computational Fluid Dynamics (CFD) code presented in [1] is employed to simulate the Fluid-Structure Interaction (FSI) between gas and valve, extracting reference data for valve displacement and external gas pressures; (2) second, the analysis of the internal structure stresses, together with the impact forces with the plate is implemented in a Computational Solid Dynamics (CSD) code developed in FreeFEM++ [2]. The impact force representation is based on the formulation presented in [3] where a conserving algorithm for frictionless dynamic contact/impact is developed. Due to the importance of obtaining an adequate impact force, an exhaustive study is carried out on its characterization in terms of numerical parameters, such as the penalty stiffness. Under this framework, the valve displacement and impact velocities are verified. Hence, impact forces are analysed in different scenarios, obtaining interesting observations about stresses distribution, with a particular focus on the points where failure is experienced.The authors acknowledge Voestalpine Precision Strip AB company for the previous research collaboration project that allowed to validate experimentally the presented numerical methods. P. Castrillo gratefully acknowledges the Universitat Politecnica de Catalunya and Banco Santander for the financial ` support of his predoctoral grant FPI-UPC (109 FPI-UPC 2018). E. Schillaci acknowledges the financial support of the Programa Torres Quevedo (PTQ2018-010060). This work has also been financially supported by a competitive R+D project (ENE2017-88697-R) by the Spanish Research Agency.Postprint (published version

Analysis of high-order interpolation schemes for solving linear problems in unstructured meshes using the finite volume method

Finite-volume strategies in fluid-structure interaction problems would be of crucialvimportance in many engineering applications such as in the analysis of reed valves in reciprocating compressors. The efficient implementation of this strategy passes from the formulation of reliable high-order schemes on 3D unstructured meshes. The development of high-order models is essential in bending-dominant problems, where the phenomenon of shear blocking appears. In order to solve this problem, it is possible to either increase the number of elements or increase the interpolation order of the main variable. Increasing the number of elements does not always yield good results and implies a very high computational cost that, in real problems, is inadmissible. Using unstructured meshes is also vital because they are necessary for real problems where the geometries are complex and depart from canonical rectangular or regular shapes. This work presents a series of tests to demonstrate the feasibility of a high-order model using finite volumes for linear elasticity on unstructured and structured meshes. The high-order interpolation will be performed using two different schemes such as the Moving Least Squares (MLS) and the Local Regression Estimators (LRE). The reliability of the method for solving 2D and 3D problems will be verified by solving some known test cases with an analytical solution such as a thin beam or problems where stress concentrations appear.P. Castrillo gratefully acknowledges the Universitat Politècnica de Catalunya and Banco Santander for the financial support of his predoctoral grant FPI-UPC (109 FPI-UPC 2018). The authors are supported by the Ministerio de Economía y Competitividad, Spain, RETOtwin project (PDC2021-120970-I00).Peer ReviewedPostprint (published version

A level-set model for thermocapillary motion of deformable fluid particles

A new level-set model is proposed for simulating immiscible thermocapillary flows with variable fluid-property ratios at dynamically deformable interfaces. The Navier–Stokes equations coupled with the energy conservation equation are solved by means of a finite-volume/level-set approach, adapted to a multiple marker methodology in order to avoid the numerical coalescence of the fluid particles. The temperature field is coupled to the surface tension through an equation of state. Some numerical examples including thermocapillary driven convection in two superimposed fluid layers, and thermocapillary motion of single and multiple fluid particles are computed using the present method. These results are compared against analytical solutions and numerical results from the literature as validations of the proposed model.Peer ReviewedPostprint (author's final draft

Analysis and design of a drain water heat recovery storage unit based on PCM plates

© 2016. This version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/This paper is focused on the detailed analysis of a PCM plate heat storage unit with a particular configuration, looking for the maximum contact area with the fluid (water) and the minimum volume to be used in a real household application. In that sense, a numerical study of the thermal and fluid dynamic behaviour of the water flow and the PCM melting-solidification processes, together with the thermal behaviour of the solid elements of the unit, has been carried out. On the other hand, an experimental set-up has been designed and built to validate the numerical model and characterise the performance of the heat storage unit. The purpose of the numerical and experimental study is to present a series of results to describe the heat storage unit performance in function of the time. Thus, after a preliminary design study three different cases have been simulated and tested. A 7.2% of discrepancy between numerical results and experimental data has been evaluated for the heat transfer. The PCM heat storage unit designed is capable to store approx. 75% of the thermal energy from the previous process wasted water heat, and recover part of it to supply around 50% of the thermal energy required to heat up the next process.Peer ReviewedPostprint (author's final draft

A level-set model for two-phase flow with variable surface tension: thermocapillary and surfactants

An unstructured conservative level-set method for two-phase flow with variable surface tension is introduced. Surface tension is a function of temperature or surfactant concentration on the interface. Consequently, the called Marangoni stresses induced by temperature gradients or surfactant concentration gradients on the interface lead to a coupling of momentum transport equation with thermal energy transport equation or interface surfactant transport equation. The finite-volume method discretizes transport equations on 3D collocated unstructured meshes. The unstructured conservative level-set method is employed for interface capturing, whereas the multiple marker approach avoids the numerical coalescence of fluid particles. The fractional-step projection method solves the pressure-velocity coupling. Unstructured flux-limiters are proposed to discretize the convective term of transport equations. A central difference scheme discretizes diffusive terms. Gradients are evaluated by the weighted least-squares method. Verifications and validations are reportedThe main author, N. Balcazar-Arciniega, as a Serra-Húnter Fellow (UPC-LE8027), acknowledges the Catalan Government for the financial support through this programme. Simulations were executed using computing time granted by the RES (IM-2021-1-0013, IM-2020-2-0002) and PRACE 14th Call (2016153612) on the supercomputer MareNostrum IV based in Barcelona, Spain. The authors acknowledge the financial support of the MINECO, Spain (PID2020-115837RB-100).Peer ReviewedPostprint (published version

Numerical dynamic analysis of reciprocating compressor mechanism. Parametric studies for optimization purposes

© 2016. This version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/A complete numerical dynamic analysis of reciprocating compressor mechanism is presented, coupling the instantaneous pressure in the compression chamber, the electric motor torque and the hydrodynamic reactions, which arise from the piston and crankshaft secondary movements. Additionally, non-constant crankshaft angular velocity and the piston and crankshaft misalignment torques have also been considered. Two sensitivity analyses have been carried out to prove that neither the inertial forces in the directions of the secondary movements, nor the oscillations of the angular velocity produce significant differences in the compressor behaviour. Finally, a set of parametric studies has been developed to evaluate the influence of geometrical parameters in the stability of the secondary movements, the friction power losses and the compressor consumptionPeer ReviewedPostprint (author's final draft

Unstructured Level-Set Method For Saturated Liquid-Vapor Phase Change

A novel conservative level-set method for saturated liquid-vapor phase change on unstruc-tured meshes is introduced. Transport equations are discretized by the finite-volume method on col-located unstructured grids. Mass transfer promoted by thermal phase change is computed using theenergy jump condition at the interface, as a function of the temperature gradient. The fractional-stepprojection method is used for solving the pressure-velocity coupling, convective terms are discretized byunstructured flux-limiter schemes, central difference scheme is used for discretization of diffusive terms.Verification and validation cases have been undertaken to prove the accuracy and robustness of the nu-merical methods, including simulation of the Stefan problem, and film boiling on a cylindrical surface.Excellent agreement between numerical solutions against analytical solution and empirical correlationsfrom the literature is reported.Peer ReviewedPostprint (published version

Minimization procedure of experimental tests for calibration purposes, within HVAC&R energy efficiency framework

Simulation models and predictive tools need to be fast, accurate and robust at the same time. The models that have to provide numerical solutions under transient conditions for a long period of time need to be simple with the aim of minimizing the time respond, without losing the accuracy. Thus, previous experimental data and a calibration methodology are necessary to assure this objective, both are strictly necessary to reproduce the behaviour with accuracy expected. Consequently, even accurate information (e.g. look-up tables) for HVAC&R components (e.g. heat exchangers, fan/compressor, auxiliary elements, etc.) is known and all coupling system is developed, the minimization of experimental tests for calibration purposed based scattered data interpolation is now an important aspect, which looks for reducing the quantity of experiments necessary to assure the accuracy expected from an optimization point of view. The present work shows an optimization procedure based on test number minimization according detailed error comparison against existing previous data. Illustrative results for a specific component are presented highlighting test number reduction without losing accuracy.This project has received funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation programme under grant agreement No 755517.Peer ReviewedPostprint (published version

Symmetry-preserving discretisation methods for magnetohydrodynamics

In this work, the symmetry-preserving method [1, 2, 3] is extended to include magnetohydrodynamic effects, using the collocated grid arrangement of Ni et al. [4, 5]. The electromagnetic part is solved explicitly using the induction-less approximation and an electric potential Poisson equation. The proposed solver is implemented in OpenFOAM and tested for accuracy and stability, and compared to the method of Ni et al. [4, 5]. A new benchmark case using a Taylor-Green vortex in a transverse magnetic field is used, for which kinetic energy budget terms are compared to the analytical solutions. Finally, Hunt’s case is used to compare flow profiles to the analytical solutions. Influence of the spatial discretisation on accuracy and stability is also examined by solving both cases on meshes with variable degrees of distortion. The symmetry-preserving method showed accuracy on Cartesian meshes and stability even on extremely distorted meshes, whereas the method of Ni et al. [4, 5] showed less accurate conservation of current density and was not able to produce stable solutions on the extremely distorted meshes.This project is part of the RIS3CAT-FEDER, FusionCAT project (001- P-001722) of Generalitat de Catalunya and the RETOtwin project (PDC2021-120970-I00) of Ministerio de Economía y Competitividad, Spain. J.A.H. is supported by the predoctoral grant FI 2022 (2022 FI B1 00204) of the Catalan Agency for Management of University and Research Grants (AGAUR).Peer ReviewedPostprint (published version