Experiments have been performed to investigate the effects of longitudinal fins on laminar mixed convection in airflow between parallel plates, with the bottom plate heated isothermally and the top plate cooled isothermally. In addition to the un-finned condition, nondimensional fin spacings in the range 0.25 < A < 2.0 were considered for relative fin heights of F = 0.5 and 0.9 and for the Rayleigh number range 760 < Ra < 29700. For fully developed flow over the unfinned surface, square longitudinal vortices (A = 1.0) were observed for Ra > Rac and persisted until Ra ≈ 20,000. While data for closely spaced fins showed considerable enhancement at lower Rayleigh numbers due to the additional surface area, the development of vigorous secondary flow was delayed. Hence, for fully developed conditions and F = 0.5, Nusselt numbers for the finned surface were, in some instances, lower than those for mixed convection without fins. Results are compared to previous numerical predictions, and it is shown that agreement is good until the flow violates modeling assumptions of steady, symmetric flow between fins.

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