Heat transfer coefficients and pressure drop measurements have been obtained for internal flows in straight rectangular channels, and channels of converging and diverging cross-sectional area, with surface arrays of varying geometries of concavities. Four basic shapes of concavity features have been considered, including hemispherical sector dimples, inverted-truncated cones, shallow cylindrical pits, and a combination of cone and cylindrical pit. Average channel Reynolds numbers of 5000, 12000, and 20000 have been tested with each type of concavity array. Both single channel wall and opposing wall enhancements have been included, with wall-to-fluid temperature ratios ranging from 1.05 to 1.25. Concavity depth-to-diameter ratio of 0.23 was used, and concavity array densities varied around 0.4 in magnitude. Results show that hemispherical sector concavity arrays can achieve heat transfer enhancements of about 50% relative to smooth surfaces, with pressure loss increases of 25% or less. The approximation of the hemisphere shape by an inverted and truncated cone results in equal heat transfer enhancement, with similar or less pressure loss. Other shapes of simplified geometries show lower heat transfer enhancements with higher pressure losses. Effects of temperature ratio and double-wall concavities on heat transfer are observed to be negligible for the conditions tested. Pressure losses are however significantly affected by single or double wall concavity features.