The heat transfer performance of a heat exchanger which harnesses the effects of secondary flow in fluid streams is examined. The fluid streams within the heat exchanger are arranged to flow in curved passages of constant curvature and rectangular cross section. The aspect ratio of the cross section and the curvature of the flow channels are selected to obtain the secondary flow instability in the streams over a wide range of flow rates. The performance of the heat exchanger is tested using hot and cold water streams with adjustable flow rates and temperatures. The temperature of the fluids and heat transfer wall are measured at several cross sections along the length of the heat exchanger together with the inlet and outlet temperatures of the streams. The heat transfer between the two streams is evaluated and the heat exchanger effectiveness is estimated. An identical second heat exchanger, but without curvature in flow passages, is constructed and performance tested under similar operating conditions. Comparison of the performance characteristics of the two heat exchangers indicates about 30 percent improvement in heat exchanger effectiveness and overall heat transfer coefficient. The secondary flow patterns in the flow channels are observed and described. Details of a numerical simulation of the fluid flow through the heat exchanger are presented.