A multi-faceted experimental investigation has been carried out to study heat transfer and pressure drop for airflow in arrays of heat generating rectangular modules deployed along one wall of a flat rectangular duct. Experiments were performed with fully populated arrays, arrays in which there are missing modules, arrays where barriers are implanted to obtain heat transfer enhancement, and arrays in which there is both a missing module and a barrier. For the fully populated array without barriers, row-independent (fully developed) heat transfer coefficients were encountered for the 5th and all subsequent rows. When there is a missing module in the array, the heat transfer coefficients at neighboring modules are increased, with the greatest enhancement (about 40%) occurring when the missing module is just upstream of the module of interest. The enhancement due to side-by-side pairs of missing modules differs very little from that induced by a single missing module. The implantation of a barrier in the array is shown to be an effective enhancement device, with the greatest effect (about a factor of two) being felt in the 2nd row downstream of the barrier but with residual enhancement persisting considerably farther downstream. Under some conditions, the enhancing effects of a missing module and a barrier were found to be mutually reinforcing. Pressure distributions were measured in arrays with and without barriers, and the barrier-induced pressure losses identified.