Pressure drop and heat transfer of arrays of in-line circular blocks on the wall of a parallel channel are measured. Diameter and height of the blocks are 40 and 18 mm, respectively, while pitches of the blocks are varied. The effects of the number of lines and rows and other factors on pressure drop and heat transfer are investigated. The pressure loss coefficient zeta is the sum of the pressure drop across three regions, the inlet, intermediate and outlet regions, and is formulated as an empirical equation that agrees with experimental data to within +/-10%. Average heat transfer coefficient of the first row of blocks is 10% lower than that of the second row. Coefficients of the first 5 rows Nu(mi), are approximated to within 10% by Nu(mi) = 0.118(Re/beta)(0.75), where beta is the opening ratio. The average Nusselt number of the second to fifth rows is also correlated to fan power, P-w, to within 5% by Nu(ave) = 190P(w)(0.25), where P-w = DeltaPU(m)A(0), DeltaP is the pressure difference, U-m is the mean velocity and A(0) is the cross-sectional area of the duct. Finally, the Nusselt number is represented by a non-dimensional expression as Nu(ave) = 0.134(zeta(1/3)Re)(0.75). (C) 2004 Elsevier Ltd. All rights reserved.