| Summary: | 博士 === 國立清華大學 === 動力機械工程學系 === 101 === This work numerically analyzes the dynamic behavior of natural convection from horizontal rectangular fin arrays. A parametric study is made using a 3-D unsteady model for fin lengths of L = 56500 mm, fin heights of H = 6.438 mm, and fin spacing of S = 6.420 mm. With an increasing L, the flow pattern evolves from a steady single-chimney to an oscillating sliding-chimney flow in which cold air is partly drawn downward from the upper ambience. The average convection heat transfer coefficient decreases with increasing fin length. For an intense sliding-chimney flow pattern from long and low fin arrays, an unsteady simulation yields higher average convection heat transfer coefficients than those using a steady-state simulation. The h–S relation exhibits a steep drop when S is narrowed below a threshold, which is larger for lower and longer fins. The optimum fin spacing Sopt occurs near the threshold S, below which the benefit of increasing heat transfer area surrenders to the decrease of h caused by excessive viscous drag. The predicted dependence of Sopt on H and L agrees well with experimental results and is explained based on numerical results of flow and heat transfer characteristics. The present predictions of Nu agree well with the correlations in the literature which use L/2 or S as the characteristic lengths.
For horizontal rectangular fin arrays with length L 200 mm, the overall convection heat transfer coefficients are quite low because the inner surfaces of the long fin arrays are poorly ventilated with cold surrounding air. In this study, we introduce perforations through the fin base to draw cold air directly from below the fin base. The perforations, especially locating in the inner region, improve ventilation and heat transfer performance significantly. The conditions with more but shorter perforations exhibit the most significant improvement in heat transfer. The overall heat transfer coefficients with short and distributed perforations, whose total perforated length equals L/2, can be as large as 2.7 times that without perforations.
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