Investigation of Natural Convection Heat Transfer in Converging Channel Flows Using a Specklegram Technique

Natural convection heat transfer characteristics in converging vertical channel flows were studied by nonintrusively measuring the wall temperature gradients using a laser specklegram technique. Local and average heat transfer coefficients were obtained for forty different configurations, including five different inclination angles from the vertical, γ = 0, 15, 30, 45 and 60 deg, with eight different channel exit openings for each inclination angle. Correlations for both local and average Nusselt numbers, based on the channel length L, were determined as functions of Grashof number, where the local Grashof number, based on the channel length L, ranged up to 7.16×106 and the overall Grashof number varied from 3.58×106 (γ = 60 deg) to 7.16×106 (γ = 0), depending upon the inclination angle. As the top opening was decreased, both local and average Nusselt numbers deviated from the single inclined plate theory and significant reductions in heat transfer resulted. The minimum opening ratio, at which the average Nusselt number started decreasing from that for the single plate, was determined as (b/L)min = 0.07, 0.1, 0.3, 0.35, and 0.4 for inclination angles of 0, 15, 30, 45 and 60 deg, respectively. For Ra* larger than 105, average Nusselt numbers, based on the channel opening b, approached the single-plate limit of the vertical channel flow theory, which was modified to incorporate the reduced gravity due to the inclination. When Ra* was smaller than 105, however, neither the single-plate limit nor the fully developed limit properly described the heat transfer characteristics in the converging channel.