Energy Transfer Enhancement Inside an Annulus Using Gradient Porous Ribs and Nanofluids

Porous media and nanofluid utilization are two passive heat transfer improvement tools, which have been employed extensively in recent years. Porous media with gradient properties result in both a higher effective thermal conductivity and better local convective heat transfer because of conducting the flow to the desired regions. In this study, distinct porous ribs are located on the internal border of an annulus. Four different conditions are considered for permeability change of ribs, including the minimum and maximum Darcy numbers and linearly increasing or decreasing variation in the radial direction, called LIV and LVD, respectively. In the first step, effects of porous rib relative height, porous rib porosity, and flow Reynolds number on the thermal efficiency and pressure drop are investigated. The results show that the configuration with Da = LVD and W/R_h = 0.25 has the maximum performance number PN = 2, that is the Nusselt improvement over pressure drop increment. Porous ribs arrangement with W/R_h = 0.25 and the minimum porosity (ɛ = 0.9) give the best PN. In the next step, the effects of nanoparticle addition with different volume fractions to the base fluid in different Reynolds numbers are investigated. In this step, porous rib relative height is set to W/R_h = 0.25. The results show that the maximum volume fraction has the highest heat transfer enhancement (about 2–2.5 times) but the lower volume fractions have higher PNs (PN ≈ 2.5 at ϕ = 1% and Re = 500).