PIV measurement of turbulent flow characteristics inside an open-cell metal foam replica

Abstract

Metal foam structures have been of interest because their geometrical characteristics are theoretically desirable to many applications such as heat exchanger, chemical reactor, filter, etc. Better understanding of flow characteristics inside metal foam is essential to utilize metal foam in thermo-fluid systems. A test section with a 3-D printed 8 times scaled-up transparent metal foam replica has been prepared. Time-resolved 2-D particle image velocimetry was used to measure velocity fields in the center planes at upstream, inside, and downstream of the replica model. The investigation was conducted for 3 different Reynolds numbers, 1000, 5,000 and 10,000 based on the channel hydraulic diameter. The turbulent flow characteristics inside an open-cell metal foam was compared between laminar and turbulent inlet flow conditions. Upstream turbulence intensity was 10% in case of turbulent inlet condition. For the ensemble averaged fields, clear similarities are found in mean velocity distributions and turbulent stresses inside the metal foam regardless of Reynolds number. The velocity magnitude inside metal foam increased up to 1.6 times of the channel bulk velocity for all Reynolds number. The temporal fluctuation of streamwise velocity component is increased up to 1.4 times to channel bulk velocity inside the metal foam replica. This increase was similar to that of laminar inlet condition even there is no fluctuations in the upstream. Normalized vorticity and turbulent kinetic energy also showed the similarity in three different Reynolds numbers, but the magnitude is higher at lower Reynolds number. Temporal analysis was also conducted at several points inside metal foam. The integral length scale is similar to 0.2 size of ligament in the case of Re = 10,000. Energy spectrum shows that the spectral slope is -5/3 in inertial subrange which confirm that flow inside metal foam is turbulent even the inlet flow is laminar.