Fluid flow and heat transfer of natural convection induced around a vertical row of heated horizontal cylinders

摘要

Natural convective flow and heat transfer induced around a vertical row of heated horizontal cylinders were investigated experimentally. The experiments were carried out with the 10-cylinder rows of diameters d=8.4, 14.4, 20.4 mm. The cylinder rows were placed in air with vertical gaps ranging from C = 3.6 mm to 150.6 mm. These enabled the experiments in the wide ranges of the modified Rayleigh numbers 5 × 10~2 ＜ Ra_G~* ＜ 10~5, and the non-dimensional gaps (G/d) = 0.176-17.9. The flow fields around the row were first visualized using a smoke. The results depicted that the plumes arising from the upstream cylinders remain laminar throughout the rows when the gaps between cylinders are smaller than 20.6 mm. While when the gaps are larger than 30.6 mm, the plumes begin to sway and undergo the turbulent transition on the halfway of the rows. The average heat transfer coefficients from the individual cylinders 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 parameters to obtain non-dimensional heat transfer correlations. Among those parameters, the parameter Ra_G~* (G/d) was found to best correlate the present Nusselt numbers Nu_G where, Ra_G~* and Nu_G are the gap-based modified Rayleigh and the average Nusselt numbers. Moreover, the non-dimensional heat transfer correlations were proposed both for the laminar and turbulent flows based on the data plots in Nu_G-Ra_G~* (G/d) plane. The proposed correlations are very simple but can predict the Nusselt numbers quite satisfactory.