Entry region of louvered ®n heat exchangers Marlow E. Springer, Karen A. Thole * ,1 Mechanical Engineering Department, University of Wisconsin, Madison, Wisconsin 53706, USA Received 9 September 1998; received in revised form 17 June 1999; accepted 1 July 1999 Abstract The dominant thermal resistance for most compact heat exchangers occurs on the gas side and as such an understanding of the gas side ¯ow®eld is needed before improving current designs. Louvered ®ns are commonly used in many compact heat exchangers to increase the surface area and initiate new boundary layer growth. For this study, detailed ¯ow®eld measurements were made in the entry region of several louvered ®n geometries whereby the louver angle, ratio of ®n pitch to louver pitch, and Reynolds number were all varied. In addition to mean velocity measurements, time-resolved velocity measurements were made to quantify unsteady eects. The results indicated larger ®n pitches resulted in lower average ¯ow angles in the louver passages and longer development lengths. Larger louver angles with a constant ratio of ®n pitch to louver pitch resulted in higher average ¯ow angles and shorter development lengths. As the Reynolds number increased, longer development lengths were required and higher average ¯ow angles occurred as compared with a lower Reynolds number case. Time-resolved velocity measurements indicated some ¯ow periodicity behind the fully developed louver for a range of Reynolds numbers. The Strouhal number of these ¯uctuations was constant for a given louver geometry, but the value increased with increasing ®n pitch. Ó 1999 Elsevier Science Inc. All rights reserved. Keywords: Heat exchangers; Louvered ®ns; Interrupted surfaces 1. Introduction Louvered ®n compact heat exchangers are used ex- tensively in several automotive applications such as ra- diators, oil coolers, condensers, and charge air coolers. The purpose of placing louvers on the ®n is to provide additional heat transfer surface area and to interrupt the growth of the boundary layer forming along the ®n surface. This new boundary layer formation provides a high heat transfer region along the ®n. Under typical operating conditions of most ®n-and-tube air-and-water heat exchangers the dominating thermal resistance is on the air (external) side and can be as much as 95% of the total thermal resistance. By achieving a better under- standing of the ¯ows in the louvered ®ns, methods of reducing the thermal resistance can be developed which will ultimately lead to a reduction in space, weight, and cost of louvered ®n heat exchangers. One region, in particular, that is important for the understanding of the performance of a compact heat exchanger is the air-side entry region. As the ¯ow pro- gresses streamwise through several louver passages, the ¯ow becomes hydrodynamically fully developed. This paper describes the eects of geometry and Reynolds number on the air ¯ow in the entry region of a compact heat exchanger. In addition, there has been some evi- dence in the literature that periodic shedding from the louvers can occur. This paper also addresses when, if at all, this shedding occurs downstream of the ®rst louver in the fully developed ¯ow region. 2. Past studies Detailed studies of compact heat exchangers date back to the hot-wire anemometry and ¯ow visualization work performed by Beauvais [1]. Much later Davenport [2,3] performed ¯ow visualization studies and concluded that while hydraulic diameter is relevant to heat transfer in plain ®ns, the heat transfer for louvered ®ns is more appropriately described by a Reynolds number based on the louver pitch. Experimental Thermal and Fluid Science 19 (1999) 223±232 www.elsevier.nl/locate/etfs * Corresponding author. Tel.: +1-540-231-7192; fax: +1-540-231- 9100; e-mail: thole@vt.edu 1 Present address: Mechanical Engineering Department, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA. 0894-1777/99/$ - see front matter Ó 1999 Elsevier Science Inc. All rights reserved. PII: S 0 8 9 4 - 1 7 7 7 ( 9 9 ) 0 0 0 2 8 - X