Contents lists available at ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs Impact of pore density on oscillating liquid flow in metal foam Özer Bağcı a , Nihad Dukhan b, ⁎ a Department of Flow, Heat and Combustion Mechanics, Ghent University, Sint-Pietersnieuwstraat 41, 9000 Ghent, Belgium b Department of Mechanical Engineering, University of Detroit Mercy, Detroit, MI 48221, USA ARTICLE INFO Keywords: Oscillating flow Water Metal foam Pores per inch Regenerator Kinetic friction factor ABSTRACT Oscillating water flow in commercial, highly-porous, open-cell aluminium foam with two common pore densities of 10 and 40 pore per inch (ppi) was investigated experimentally. Flow displacements of 74.35, 97.20 and 111.53 mm were produced in each foam; a sizable range of flow frequency from 0.116 to 0.696 Hz was imposed. The effect of pore density was investigated by comparing the two pore densities of the current study with a third from the literature having 20 ppi. The comparison was at the same porosity of approximately 88%. Results have shown that the pore density has a considerable effect on important oscillating water flow parameters including inlet pressure, pressure drop and friction factor. For high frequencies, the 20-ppi foam produced the highest inlet pressure followed by the 40-ppi and then the 10-ppi foam. For low frequencies, the 10-ppi foam produced the highest inlet pressure followed by the 40-ppi foam and then the 20-ppi foam. These trends were explained by considering viscous and form drag pressure losses according to permeabilities and Forchheimer drag coefficients of the three foams. For low frequencies, the pressure drop was periodic for the three pore densities, while for high frequencies, the periodic behavior of the pressure drop was not clearly discernable, indicating the presence of nonlinear processes and multiple transitions among flow regimes. The friction factor for the oscillating flow was seen to lie above that for steady flow for 10- and 20-ppi foam for all frequencies, as expected. However, there was no difference between the steady-state and oscillating flow friction factors for the 40-ppi foam at low frequencies. The behaviour of friction factors indicated that there was two different regimes based on frequency with the boundary between them occurring at Reynolds number around 4000 for 10- and 20-ppi foams, and at Reynolds number 5500 for the 40-ppi foam. The friction factor correlated with the Reynolds number, with the correlation being generally better for low frequencies. Friction factors for oscillating water flow in metal foam were compared to those obtained for oscillating flow of air and water in some other porous media from the literature. Various effects at play, as well as the interactions among them, were highlighted and discussed. Major conclusions and practical implications were summarized. 1. Introduction Many commercial open-cell metal foams are known to have good thermal conductivity, and large surface area per unit volume. Their internal morphology are composed of thin ligaments forming windows (pores) that surround cells. The foams also have high porosity (around 90% or higher). The last two attributes endows the foams with high permeability (ease of fluid flow). Their morphology hinders the growth of any boundary layers and at the same time fosters admixing of fluid travelling through the foams. The combination of these features are sought after in heat transfer cores. Aside from having a good core, heat transfer can be further augmented by having the fluid oscillate within a given core. The effective thermal diffusivity in oscillating flow can be several orders of magnitudes higher than the fluid molecular diffusivity [1]. The heat transfer due to oscillating flow can be comparable to that of heat pipes [2]. Metal foams belong to the general porous-media class of materials. Applications of oscillating flow (and heat transfer) in porous media include normal and oscillating heat pipes [3], various types of re- generators, [4,5], Stirling engine regenerators, [6–8] and heat ex- changers of pulse tubes [9,10]. Certain coolers of nuclear power plants, thermo-acoustic engines, magnetic refrigerators and reverse-flow re- actors also experience oscillating flow and heat transfer. Grasping various characteristics of oscillating flow is indispensable for eluci- dating heat transfer resulting from such flow. Oscillating flow in certain porous media (e.g. spherical particles, granular beds, wire netting and mesh screens) have received sizable consideration. Khodadadi [11] solved oscillatory flow through a porous channel bounded by two impermeable walls for two limiting cases: highly-inertial and highly-viscous flow. For highly-inertial flow, there https://doi.org/10.1016/j.expthermflusci.2018.04.020 Received 7 March 2018; Received in revised form 1 April 2018; Accepted 23 April 2018 ⁎ Corresponding author. E-mail address: nihad.dukhan@udmercy.edu (N. Dukhan). Experimental Thermal and Fluid Science 97 (2018) 246–253 Available online 24 April 2018 0894-1777/ © 2018 Elsevier Inc. All rights reserved. T