Insight into prediction of unsteady forced convection in a porous straight channel subjected to an inlet flow modulation: A REV latttice Boltzmann investigation

Hassane Naji1, Riheb Mabrouk2, Hacen Dhahri2
1 Univ. Artois, IMT Lille Douai, Junia, Univ. Lille, ULR 4515, Laboratoire de Génie Civil et géo-Environnement(LGCgE), F-62400 Béthune, France
2 Laboratoire d’Études des Systèmes Thermiques et Énergétiques (LESTE), École Nationale d’Ingénieurs de Monastir, Rue Ibn Jazza, 5019 Monastir
Mots clés : latent heat thermal energy storage system, Brinkman-Forchheimer extended Darcymodel, two-energy model, Pulsating flow, Energetic and exergetic efficiencies, thermal lattice Boltzmann method
Résumé :

The technologies involving unsteady forced convection with phase change in porous media with unsteady inlet flow are increasingly investigated. To gain further insight into this issue, the continuity, momentum and two-energy equations (LTNE model) describing the flow in the melted PCM and heat transfer between metal foam and PCM are solved within an open-ended straight porous channel as a latent heat thermal energy storage system (LHTES). The system is modulated by sin waves superimposed on a time-varying input flow. From a technical point of view, heat transfer under pulsed flow is often found in various industrial applications such as finned heat sinks for electronic chipsets, filtration devices, pulsed tube cryo-coolers, ducted air conditioners, aerosols transport in human respiratory tract, blood flow in the vessels, etc.

A thermal mesoscopic method at a representative elementary volume scale (REV) level is taken up to numerically address such a problem. Simply put, the thermal lattice Boltzmann method (TLBM) enthalpy-based is employed to deal the Brinkman-Forchheimer extended Darcy (BFD) model and the two-energy model . On the other hand, the multi-distribution functions (MDF) model was adopted to calculate the dynamic and thermal fields. Relevant values of the pulse amplitude A, porosity and Strouhal (St) number were deemed.

The comprehensive analysis of the results suggests that small amplitude speed up the melting rate and the heat spread where forced convection acts in the domain. In addition, the overall irreversibility of the system is reduced during the charging process with a low St number and a lower amplitude and during the discharging process with low St and larger amplitude. Furthermore, it turned out that an unsteady convective flow better improves the unit energy and exergy performance rather than a pulsed flow. Further detailed insights on intensity, phase field evolvement and streamlines, and overall characteristics in terms of average entropy generation rate (Nsav), energetic and exergetic efficiencies are also exhibited.

To sum up, the REV-scale enthalpy-based TLBM has been successfully applied to two-dimensional porous LHTES. In particular, solutions are shown to be achievable with today’s computing resources and to be extremely beneficial, shedding new light on phenomena otherwise inaccessible experimentally.

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