The Influence of Tube Shape on Heat-Transfer Coefficients in Air to Air Heat Exchangers

Heat-transfer coefficients are reported for air passing through round tubes, through partially flattened tubes, and through partially flattened tubes which have also been dimpled. It was found that flattened tubes maintained fully turbulent flow and, consequently, a higher heat-transfer coefficient at lower Reynolds number values than did round tubes. The addition of dimples to the flattened tubes maintained fully turbulent flow from the transition point and increased the heat-transfer coefficient substantially. The addition of the dimples also improved the relation between heat transfer and pressure drop in the flattened tubes.