This paper presents experimental pressure drop and heat transfer results of compact heat exchangers made with plain rectangular fins of short flowlengths tested with air at very low Reynolds numbers. Experiments were performed at sea level and at simulated elevated altitudes up to 25,298 m (83,000 ft). From the experimental results, the additional pressure drop of the air caused by the developing boundary layers and the hydrodynamic entrance length were determined. An equation was produced that predicted the average Nusselt number of the air, which significantly decreased with nondimensional length. The experimental results varied with respect to the aspect ratio of the rectangular duct and nondimensional length, which is inversely related to the Reynolds number.
