Frost formation: optimizing solutions under a finite volume approach

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 three-dimensional transient formulation of the frost formation process is developed by means of a finite volume approach. Emphasis is put on the frost surface boundary condition as well as the wide range of empirical correlations related to the thermophysical and transport properties of frost. A study of the numerical solution is made, establishing the parameters that ensure grid independence. Attention is given to the algorithm, the discretised equations and the code optimization through dynamic relaxation techniques. A critical analysis of four cases is carried out by comparing solutions of several empirical models against tested experiments. As a result, a discussion on the performance of such parameters is started and a proposal of the most suitable models is presented.Peer ReviewedPostprint (published version

Heat and moisture insulation by means of air curtains: application to refrigerated chambers

The present study is devoted to the determination of the efficiency of air curtain units (ACUs) applied to heat and moisture insulation of refrigerated chambers. A detailed study of the fluid dynamics and heat and mass transfer of the ACU in the refrigerated space and the external ambient is carried out by means of large eddy simulations (LES). The heat and moisture entrainment through the doorway and their transport inside the inner space are fully described. Three different configurations are studied: non-recirculating, recirculating and twin-jet air curtains. The condensation produced in the cool walls of the refrigerated space is evaluated considering the warm humid air from the ambient which penetrates inside the chamber through the doorway. The influence of both the discharge velocities and the discharge angles on the sealing capabilities of the three different tested ACU configurations is determined.Peer ReviewedPostprint (author's final draft

Vapor Compression System Modelica Library for Aircraft ECS

The aeronautical industry is developing new environmental control system architectures, based on the more electrical aircraft approach, with the aim of improving the overall performance of aircrafts. The analysis of theses architectures is very complex as they include different thermal systems interacting between each other. Their design is commonly carried out with the support of numerical simulations. The aim of this work is to present a Modelica library to simulate vapor compression cycles at both steady and transient conditions. The study of these specific cycles is particularly challenging from both the phenomena and the numerical resolution point of views. The main goal of the library is to provide, not only accuracy and robustness, but also very low coputational time. Most of the components developed for the library are simulated by means of look-up tables or efficiencies for the sake of accuracy and quickness. Therefore the efforts were focused on two critical aspects of the system. On the one hand, the heat exchangers were developed based on a switching moving boundary approach in order to have a relatively high ratio between accuracy and resolution time. This method takes into account the distribution of phases along heat exchangers (this is a crucial thermal aspect for the accurate simulation of evaporators and condensers). The time consumption is very low compared to distributed models as only one control volumen per phase is used. On the other hand, several features of the whole vapor compression cycle resolution were tackled with meticulosity, namely, the refrigerant amount management, the initilization procedure, the thermostatic control loop, and both the steady and transient simulation aspects. The results shown in this work are devoted to highlight three main characteristics of the simulations carried out from the developed library. 1) The numerical robustness of the system and its components. In particular, the heat exchanger switching moving boundary model was tested at all posible transitions, while the vapor compression cycle was also subjected to a wide range of boundary conditions. 2) The low computational time required for simulations. The system resolution at steady state conditions needs very few seconds to converge. 3) The accuracy of predictions. The simulations at both component and system levels showed good agreement with experimental data

Experimental Analysis of a Draft Beer Ice Bank Machine

In a long-term established application as draft beer cooling and servicing machines, seems difficult to see any research application at first sight. However, this perspective change when one considers that the costs associated to maintenance/operating problems are very high for breweries with a very extensive network of machines. Moreover, the strong competitive market among different companies means that any cooling failure could be an opportunity for the competence. An additional aspect to consider, is that the climate and installation variability among different machines asks for a complete a-priori understanding of the unit behavior under integrated operating conditions, in order to select for each location and ambient the optimum machine (lowest cost for the brewery while keeping the cooling needs for the customer). All these statements justify a complete research analysis, based on experimental testing of the machines under different situations, in terms of beer storage temperature, room temperature, consumption profile, and integration effects (ventilation, connecting ducting lengths, etc.). Under this framework, this paper presents in detail an experimental set-up developed ex-professo for this application, showing how the different aspects have been instrumented in order to provide the needed performance parameters. Special attention has been taken to monitor the ice bank status along a particular operating and consumption scenario. To close the paper, illustrative results will be given for some representative cases, showing temperature evolution for the ambient, the inlet/outlet beer, the refrigerating system, machine consumption, ice presence at different locations, etc. From the data obtained were determined the effect of some operating conditions on the machine performance and gathered for comparison purposes. From the presented information, the level of understanding achievable with the obtained data will be clearly demonstrated, and thus the corresponding economic benefits for the breweries covering a similar research work

Numerical Model of Capillary Tubes: Enhanced Performance and Study of Non-Adiabatic Effects

In this work a numerical model to simulate the thermal and fluid-dynamic phenomena inside non-adiabatic capillary tubes is presented. The model presented herein is an improved version of the distributed model detailed in [1]. It is based on a pseudo-homogeneous two-phase flow model where the governing equations (continuity, momentum, energy and entropy) are integrated over the discretized fluid domain and solved by means of a step-by-step scheme. The main novelty of the improved algorithm is its enhanced capability to address many of the convergence issues typically found in distributed models as stated by [2] and [3]. In addition, the newest version of the model allows the simulation of both concentric and lateral configurations whereas only the concentric configuration was available in the previous version. The initial section of this work is focused on explaining the new resolution procedures and features included in the enhanced algorithm that allow a better convergence performance. The predictions of the new model are compared against simulations carried out with other distributed models proposed in the open literature. The comparison shows that the new model succeeds (convergence is attained) in cases where high convergence difficulties have been reported. The subsequent section is devoted to the implementation and validation of the lateral configuration. On one hand, both the resolution approach and the hypotheses considered are briefly described, and on the other, the model predictions are compared against experimental data found in the open literature where good agreement is observed. And finally, the last section presents parametric studies on capillary tubes used in household refrigerators working with isobutane. The influence of the heat exchanger length and its relative position over the whole capillary tube are analysed for both configurations: concentric and parallel. In addition, the influence of the capillary tube inlet condition at a constant pressure (subcooling degree for single-phase flow and vapour quality for two-phase flow) is analysed. REFERENCES [1] N. Ablanque, J. Rigola, C.D. Pérez-Segarra, A. Oliva, “Numerical simulation of capillary tubes. Application to domestic refrigeration with isobutane”, International Refrigeration and Air Conditioning Conference at Purdue, 2377, Purdue, IN, USA, 2010. [2] C.J.L. Hermes, C. Melo, J.M. Gonçalves, “Modeling of non-adiabatic capillary tube flows: A simplified approach and comprehensive experimental validation”, International Journal of Refrigeration, vol. 37, pp. 1358-1367, 2008. [3] P.K. Bansal, C. Yang, “Reverse heat transfer and re-condensation phenomena in non-adiabatic capillary tubes”, applied Thermal Engineering, vol. 25, pp. 3187-3702, 2005

Virtual Household Refrigerators at Steady-state and Transient Conditions. Numerical Model and Experimental Validation

The aim of the present work is to present a comprehensive validation of a numerical model to simulate vapor compression refrigeration systems at steady state and transient conditions. The model simulates both the refrigeration cycle itself and the refrigerated compartments network, based on a modular approach where each element of the system is solved independently by means of appropriate models, while the whole system is solved iteratively based on the links established between the elements. The methodology implemented to achieve the transient simulation of the whole system combines a steady-state approach for the refrigerating cycle loop with a transient approach for the refrigerated compartments loop. The model has already been tested numerically [1], and therefore, the focus of this work is to present a comprehensive validation of the model at both steady and transient conditions using experimental measurements found in the technical literature [2,3]. The refrigerating cycle includes appropriate numerical models for the main components, namely, hermetic compressor, wire-and-tube condenser, non-adiabatic capillary tube, plate evaporator, and low-pressure-side accumulator, while only one refrigerated chamber is considered. On the one hand, for steady state conditions, the relevant temperatures of the main elements are compared at different ambient temperatures and heat loads. On the other, for transient conditions, the evolution of the relevant temperatures during one on/off cycle are compared against the experimental data. In both cases similar trends, good qualitative results, are observed between the numerical predictions and the experimental data.  Finally, parametric studies to analyse the influence of different parameters and elements of the cycle are carried out in order to show the model potential. REFERENCES [1] N. Ablanque, C. Oliet, J. Rigola, O. Lehmkuhl, C.D. Pérez-Segarra,“Simulation of Household Refrigerators with a Flexible Numerical Tool, International Refrigeration and Air Conditioning Conference at Purdue, Purdue, IN, USA, 2014. [2] Erik Björk, Björn Palm,“Refrigerant Mass Charge Distribution in a Domestic Refrigerator. Part I: Transient Conditions, Applied Thermal Engineering 26, pp 829-837, 2006. [3] Erik Björk, Björn Palm,“Refrigerant Mass Charge Distribution in a Domestic Refrigerator. Part II: Steady State Conditions, Applied Thermal Engineering 26, pp 866-871, 2006

Fluid-Structure Interaction of a Reed Type Valve Subjected to Piston Displacement

In the field of reciprocating compressors, the developing of reed type valves is a challenging task. The understanding of the fluid flow behaviour through the valve reed is essential to improve the valve design. Hence, this work attempts the dynamic simulation of this fluid-structure interaction (FSI) problem, taking into account valve movement due to piston displacement. In this work attends the in-house implemented CFD&HT and moving mesh coupled code TermoFluids [1]. The CFD&HT solver consists of a three-dimensional explicit finite volume fractional-step algorithm formulated in a second-order, conservative and collocated unstructured grid arrangement. The turbulence is modelled by means of large-eddy simulation. The moving mesh technique is based on a radial basis function interpolation method, which allows the dynamic deformation of the mesh according to the displacement of the valve [2,3]. The CFD&HT and moving mesh coupling is built by the space conservation law. The newly implemented FSI global solver is based on a partitioned coupled algorithm in which the dynamic action of the valve is modelled through a specific law based on modal analysis of valve reed theory [4]. In contrast to conventional CSD solvers, this alternative performs a fast prediction of the structure displacement. As a preliminary approach, a simplified geometry of an axial hole plus a rectangular diffuser with a piston based inlet condition is considered. An immersed body procedure is used to simulate solid parts inside the domain and, particularly, to reproduce the bottom and inlet boundaries. This strategy has been enhanced with respect to previous studies [5]; thus, the flow phenomena is now better captured specially when the opening of the valve is narrow. As an addition, a parametric study will be carried out in order to analyse the influence of the valve thickness. REFERENCES [1] O. Lehmkuhl, R. Borrell, C.D. Pérez-Segarra, M. Soria, A. Oliva. TermoFluids: A new parallel unstructured CFD code for the simulation of turbulent industrial problems on low cost PC Cluster, Parallel Computational Fluid Dynamics 2007, Vol. 67, pp. 275-282, 2009. [2] O. Estruch, O. Lehmkuhl, R. Borrell, C.D. Pérez-Segarra, A. Oliva. A parallel radial basis function interpolation method for unstructured dynamics meshes, Computer and Fluids, 80:44-54, 2013. [3] O. Estruch, O. Lehmkuhl, R. Borrell, C.D. Pérez-Segarra. Large-eddy simulation of turbulent dynamics fluid-structure interaction, 7th International Synopsium on Turbulence, Heat and Mass Transfer, Palermo, 2012. [4] W. Soedel. Mechanics, simulation and design of compressor valves, gas passages and pulsation mufflers, Purdue University Short Courses, IN, USA, 1992. [5] O. Estruch, O. Lehmkuhl, J. Rigola, A. Oliva and C.D. Pérez-Segarra. Transient and dynamic numerical simulation of the fluid flow through valves based on large eddy simulation models, 8th International Conference on Compressors and their Systems, London, 2013

Dynamic simulation of indirect air conditioning systems with optimized computational time

Peer ReviewedPostprint (published version

Thermal System Oriented Simulation of Aircraft Electrical Environmental Control Systems Including its Electric Coupling

A flexible numerical platform based on libraries has been developed within the Dymola/Modelica framework to simulate Environmental Control Systems (ECS). The goal was to build up a flexible tool to analyse complex systems including their thermal and electrical perimeters at both steady and transient conditions focusing on three key characteristics: numerical robustness, optimal time consumption, and high accuracy. This document aims to underline both the most relevant features of the numerical tool and the main challenges addressed during its development. Some illustrative simulations are shown in order to highlight the tool capabilities.Peer ReviewedPostprint (published version

3D Compressible Simulation Of a Muffler With Pseudosound Prediction Levels

The main objective of this paper is to present a numerical resolution of a suction muffler configuration by using an in-house object oriented CFD & HT code TermoFluids (Lehmkuhl et al. 2007), able to handle tridimensional geometries, unstructured meshes and parallelization availability. This code has been adapted to be able to resolute 3D Navier-Stokes equations in their compressible form and coupled with the numerical resolution of the whole compressor domain by means of a parallel and object-oriented called NEST tool (Lopez, 2016). The numerical results aim to study the influence of the suction muffler inner geometry in the fluidynamic behavior inside the muffler while considering how this internal geometry affects the global performance of the compressor. Hence, the inlet and outlet boundary conditions at the muffler are obtained from the numerical simulation of the whole compressor using NEST, while the fluid behavior inside the muffler is numerically simulated by means of detailed analysis. In addition, the paper presents a methodology that handles with Large Eddy Simulation (LES) models for the turbulent motion of fluid inside the muffler, the formulation of Navier-Stokes in their compressible form, dealing with numerical problems derived from the compressible part, and the coupling of the whole compressor simulation to set boundary condition. Finally, the obtained results will be compared with the empirical data obtained in the CTTC facilities from the study of a real muffler placed in a reciprocating compressor