Heat transfer coefficients are reported for mixed convection in a 10 cm x 10 cm horizontal channel partially filled with a porous layer of 3 mm DIA glass beads and bottom heated over a 10 cm length. Measurements were made for Rayleigh-Darcy numbers of 35-655, Peclet numbers of 0-316, and several porous sublayer heights (0, 5, 7.5, 9 and 10 cm). Heat transfer coefficients are favorably compared to prior numerical results and thus validate predictions. The Nusselt number is sensitive to the fraction of the overall layer occupied by the porous sublayer at sublayer fractions less than 0.5, but is less so at larger sublayer fractions. The numerical prediction of a critical Peclet number at which the Nusselt number is minimum is confirmed experimentally, showing that a small cross flow can reduce heat transfer by disrupting natural convection currents. The transition to forced convection occurs at larger Peclet numbers than have been predicted numerically. (c) 2017 Elsevier Ltd. All rights reserved.