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  • 1.
    Fallahjoybari, Nima
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Performance improvement of a solar air heater by covering the absorber plate with a thin porous material2020In: Energy, ISSN 0360-5442, E-ISSN 1873-6785, Vol. 190, article id 116437Article in journal (Refereed)
    Abstract [en]

    The effect of covering the absorber plate of a solar air heater with a thin porous media is investigated in the present study. Simulations are carried out for turbulent flow and heat transfer in the solar heater and within the porous layer. The effects of different parameters such as Reynolds number, Darcy number and solid to fluid thermal conductivity ratio on the thermal and thermo-hydraulic performances of a solar air heater are studied. It is observed that the implementation of a thin porous layer over the absorber plate significantly increases the thermal and thermo-hydraulic performances of the solar air heater. The maximum increase in the thermal and thermo-hydraulic performances is more than 5 times of those obtained in a solar heater without porous medium. Meanwhile, the maximum increase in the frictions factor of the porous solar heater is 2 times of that in a solar heater without porous media at the maximum Reynolds number studied. The proposed configuration also reduces the risk of hot spots since no entrapped eddies are formed over the absorber plate. It is shown that the turbulence produced at the porous-fluid interface penetrates into the thin porous layer and enhances the heat transfer from the absorber plate. The results also reveal that the conduction heat transfer within the porous layer highly affects the thermal and thermo-hydraulic performances of the solar heater.

  • 2.
    Fallahjoybari, Nima
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Hellström, Gunnar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Maerefat, Mehdi
    Department of Mechanical Engineering, Tarbiat Modares University.
    Nimvari, Majid E.
    Department of Mechanical Engineering, Tarbiat Modares University, Faculty of Engineering Technologies, Amol University of Special Modern Technologies, Amol.
    Numerical Computation of Macroscopic Turbulent Quantities in a Porous Medium: an Extemsion to a macroscopic Turbulent model2016In: Journal of Porous Media, ISSN 1091-028X, E-ISSN 1934-0508, Vol. 19, no 6, p. 497-513Article in journal (Refereed)
    Abstract [en]

    A numerical study is conducted using a standard numerical model for a porous medium consisting of a staggered arrangement of square cylinders. Fully developed macroscopic turbulent kinetic energy and dissipation rate are derived and analyzed for different porosities of the medium at different Reynolds numbers. The results obtained are used to extend the applicability range of an existing macroscopic turbulence model in porous media to low-Reynolds-number turbulent flows. It is shown that the levels of normalized macroscopic turbulent kinetic energy and dissipation rate are not constant over the entire range of Reynolds number. These quantities increase from lower levels at low Reynolds numbers up to an asymptotic value being independent of Reynolds number. The constants in the closure expression of the macroscopic turbulence equations are modified using the present results. Finally, in order to highlight the importance of the present modifications, the results of the macroscopic turbulence model before and after the modifications are compared for two cases.

  • 3.
    Fallahjoybari, Nima
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Hellström, J. Gunnar I.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Investigation of Hydrodynamic Dispersion and Intra-pore Turbulence Effects in Porous Media2019In: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634Article in journal (Refereed)
    Abstract [en]

    The aim of the present paper is to evaluate and compare the pore level hydrodynamic dispersion and effects of turbulence during flow in porous media. In order to compute these quantities, large eddy simulations of turbulent flow in five unit cells comprised of spherical particles are performed and the results are averaged over the cells. Visualizations of vortical structures reveal that the size of the turbulence structures is of the size of the pores. Investigations furthermore yield that volume-averaged values of the hydrodynamic dispersion are of the same order as the Reynolds stress within the pores. It is also shown that the effect of intra-pore turbulence and hydrodynamic dispersion on the redistribution of macroscopic momentum within the porous medium is negligible compared to Forchheimer term. A discussion is provided on the accuracy of the eddy viscosity hypothesis in the modeling of the volume-averaged intra-pore Reynolds stresses. Finally, the effect of variation in the pore-scale geometry on the turbulence structures and averaged values of hydrodynamic dispersion and Reynolds stress is investigated.

  • 4.
    Fallahjoybari, Nima
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, Staffan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Hellström, J. Gunnar I.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Investigation of thermal dispersion and intra-pore turbulent heat flux in porous media2020In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 81, article id 108523Article in journal (Refereed)
    Abstract [en]

    In the present study, the importance of the thermal dispersion and the turbulent heat flux in porous media and their effects on the macroscopic distribution of thermal energy are investigated. To this end, turbulent flow and heat transfer within five unit-cells mimicking porous media are solved using large eddy simulation. It is shown that the thermal dispersion and the turbulent heat flux are negligible as compared to the convection term in the macroscopic energy equation. When further scrutinizing this equation, it is revealed that except for the longitudinal components of the thermal dispersion, the other components of thermal dispersion and turbulent heat flux may be neglected away from the boundaries as compared to the interfacial heat transfer. Visualizations of vortices show that the size of the turbulence structures within the cells is of the same order as the size of the pores; therefore, the turbulent heat flux is limited to the intra-pore level. Finally, a discussion is provided on the accuracy of the gradient type diffusion model commonly used for turbulent heat flux in porous media in the absence of macroscopic turbulence. It is shown that the intra-pore turbulence does not affect the macroscopic transport of thermal energy within the porous media studied.

  • 5.
    Fallahjoybari, Nima
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Lundström, Staffan T.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    A Subgrid-Scale Model for Turbulent Flow in Porous Media2019In: Transport in Porous Media, ISSN 0169-3913, E-ISSN 1573-1634, Vol. 129, no 3, p. 619-632Article in journal (Refereed)
    Abstract [en]

    Given the analogy between the filtered equations of large eddy simulation and volume-averaged Navier–Stokes equations in porous media, a subgrid-scale model is presented to account for the residual stresses within the porous medium. The proposed model is based on the kinetic energy balance of the filtered velocity field within a pore; hence, when using the model, numerical simulations of the turbulent flow in the pores are not required. The accuracy of the model is validated with available data in the literature on turbulent flow through packed beds and staggered arrangement of square cylinders. The validation yields that the model successfully captures the effect of the pore-scale turbulent motion. The model is then used to study turbulent flow in a wall-bounded porous media to assess its accuracy.

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