Natural convection heat transfer from vertical heated plates set in a vertical array

Abstract

Natural convective flow and heat transfer induced around a vertical row of heated plates were investigated experimentally. The experiments were carried out with the row of ten-plates of heights L=20, 30, 50mm. The plate row was placed in the air with vertical gaps ranging from G=5mm to 70mm. These enabled the experiments in the wide ranges of modified Rayleigh numbers, 2x10⁴<Ra_L^*<4x10⁶, and non-dimensional gaps (G/L)=0.1-3.5. The flow fields around the row were first visualized using a smoke. The results depicted that the plumes arising from the upstream plates remain laminar throughout the row when the gaps are smaller than 30mm. While when the gaps are larger than 50mm, the plumes sway randomly on the halfway of the row and the turbulent transition takes place. The average heat transfer coefficients from the plates in the row were subsequently measured. They showed a monotonous decrease toward downstream when the flow remains laminar. While when the turbulent transition occurs, they turn to increase. The coefficients were next arranged with various non-dimensional parameters to obtain heat transfer correlations. Among many parameters, the parameter Ra_G^*(G/L) was found to best correlate the present Nusselt numbers Nu_G, where Ra_G^* and Nu_G stand for the gap-based modified Rayleigh and the average Nusselt numbers. Moreover, from the plots in Nu_G - Ra_G^*(G/L) plane, the heat transfer correlations were proposed both for the laminar and turbulent flows. The proposed correlations are very simple, but can predict the Nusselt numbers quite satisfactory.