Natural convection is measured in an enclosure that represents an integral collector storage solar system with an immersed heat exchanger. The enclosure has an aspect ratio of 9.3:1 and is inclined 30 deg to the horizontal. Heat transfer coefficients for bundles of 240 tubes are obtained for a range of transient operating modes and pitch-to-diameter ratios of 1.5, 2.4, and 3.3. Results for isothermal and stratified enclosures yield a correlation for the overall Nusselt number,
NuD=(2.45±0.03)RaD0.188,
230RaD9800.
Nusselt numbers are three times larger than those for a similarly configured single-tube and an eight-tube bundle. This increase is attributed to stronger fluid motion within the bundle and greater overall circulation rates in the enclosure.
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