Pergamon Int. Comm. Heat Mass Transfer, Vol. 27, No. 2, pp. 159-168, 2000 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0735-1933/00IS-see front matter PII S0735-1933(00)00097-X HEAT TRANSFER BEHAVIOR OF TEMPERATURE -DEPENDENT VISCOELASTIC NON-NEWTONIAN FLUID WITH BUOYANCY EFFECT IN 2:1 RECTANGULAR DUCT Chang-Hyun Sohn, Seong-Tae Ahn, and Sehyun Shin School of Mechanical Engineering Kyungpook National University Taegu, Korea 702-701 (Communicated by J.P. Hartnett and W.J. Minkowycz) ABSTRACT This numerical study investigates the flow characteristics and heat transfer mechanism of a viscoelastic non-Newtonian fluid in a 2:1 rectangular duct. The combined effect of temperature-dependent viscosity, buoyancy, and secondary flow caused by a second normal-stress difference is considered. The Reiner-Rivlin constitutive equation was adopted to model the viscoelastic fluid characteristics. An axially constant heat flux on the bottom wall and peripherally adiabatic boundary condition (H2) were both used. The numerical results for a polyacrylamide (Separan AP-273) solution showed a significant heat transfer enhancement compared to those of a constant property fluid, and exhibited a good consistency with experimental results for both thermal developing and thermally developed regions. In a bottom-wall-heated 2:1 rectangular duct, the main cause of the heat transfer enhancement of the viscoelastic fluid was viscoelastic-driven secondary flow, with temperature-dependent viscosity and buoyancy-induced secondary flow playing supporting roles. © 2000 Elsevier Science Ltd Introduction The fluid flows and heat-transfer behavior of non-Newtonian viscoelastic fluids has attracted special interest in recent years due to the wide application of these fluids in the chemical, pharmaceutical, petrochemical, and food industries, along with their dramatic ability to enhance heat transfer in a laminar flow through a non-circular duct. Hartnett et al. [1-3] reported on significant laminar heat transfer enhancements (up to 300%) with viscoelastic fluids in rectangular ducts, which had never been observed in a circular pipe flow. Hartnett[1] suggested that the increased heat transfer was probably due to a secondary flow resulting from the viscoelastic behavior of normal stress differences. Gao and Hartnett[4-5] 159