Combined forced and free convective heat transfer characteristics in a narrow vertical rectangular channel with 2.5 mm in gap heated from both sides

Heat transfer characteristics of mainly combined forced and free convective flow in a vertical rectangular flow channel with a gap of 25 mm, which was quite narrow compared with those investigated in previous experiments, were studied experimentally for water. As a result, the following heat transfer characteristics were made clear, using a non-dimensional parameter Grx/Rex21/8Pr1/2 similarly to the case for the 18 mm gap which was already reported by the authors. (1) When the Grx/Rex21/8Pr1/2 is less than 10-4, both upward flow and downward flow show the nature of forced convective heat transfer. (2) When the Grx/Rex21/8Pr1/2 is between 10-4 and 10-2, heat transfer coefficients for both upward flow and downward flow are higher than any of those predicted by the previous correlations for turbulent forced convection along a flat plate and turbulent free convection along a vertical flat plate. This is, differently from the case of 18 mm gap, due to the effect of the acceleration of main flow induced by the development of the boundary layer along the channel. (3) When the Grx/Rex21/8Pr1/2 is larger than 10-2, the upward flow shows the nature of free convective flow even with the gap as narrow as 2.5 mm in the vertical rectangular flow channel. Heat transfer correlations which have been developed for the 18 mm gap channel, are also available for the described-above regions of 2.5 mm gap channel. (author)