Consider window blinds that are installed in the air space between the two panes of a vertical double-pane window. The window is H = 0.5 m high and w = 0.5 m wide, and includes N = 19 individual blinds that are each L = 25 mm wide. When the blinds are open, 20 smaller, square enclosures are formed along the height of the window. In the closed position, the blinds form a nearly continuous sheet with two t = 12.5 mm open gaps at the top and bottom of the enclosure. Determine the convection heat transfer rate between the inner pane, which is held at Ts,i = 20°C, and the outer pane, which is at Ts,o = - 20°C, when the blinds are in the open and closed positions, respectively. Explain why the closed blinds have little effect on the convection heat transfer rate across the cavity.

1 = 12.5 mm L = 25 mm - Ts0 H = 0.5 m Txi Baffle Baffle Closed Open 1 = 12.5 mm