Experimental and numerical study on natural convection around a plate with rectangular grooves

Abstract

Laminar natural convection around a two-dimensional plate with rectangular grooves is studied experimentally and numerically. The heat transfer around a vertical plate with horizontal rectangular grooves, horizontal plate with upward and downward rectangular groove is investigated. A Mach-Zehnder interferometer is used in the experiment and the local Nusselt numbers at each groove are measured quantitatively from the interferograms. A modified SIMPLER algorithm for the pressure boundary is used in the numerical simulation. At low Rayleigh number $(Ra_L ≡ 10^6)$, increase of the total heat transfer rate from grooved surface is not notable in spite of the increased surface area. The total heat transfer rate from a grooved fin increases very rapidly with the increase of the Rayleigh number, while the growth rate for a flat plate is not that high. As revealed by the numerical analysis, secondary recirculation flows are observed in the groove. The protrusions prevent the main flow from flowing into the groove and cause recirculations. Consequently, the heat transfer rate at the inner surfaces of the groove is significantly smaller than that at the outer surface. The effect of Rayleigh number for each aspect ratio is also studied. The effects of the number of protrusions and the location of the groove are investigated. A general correlation for the average Nusselt number is finally made to obtain the heat transfer rate for rectangular grooves for the three alignments (vertical, upward-facing and downward-facing horizontal alignments), for any number of protrusions (5~13) and Rayleigh number $(10^6 ≤ Ra_L ≤ 10^9)$.