Heat transfer behavior of a temperature-dependent non-Newtonian fluid withReiner-Rivlin model in a 2 : 1 rectangular duct

The present study investigates the heat transfer enhancement mechanism obse rved with a non-Newtonian fluid having temperature-dependent viscosity in a 2:1 rectangular duct. The Reiner-Rivlin constitutive equation was used to model the non-Newtonian fluid characteristics. The H2 thermal boundary cond ition, corresponding to an axially-constant heat flux with a uniform heat f lux at the top wall, was used. The local Nusselt numbers calculated for a p olyacrylamide (Separan AP-273) solution showed significant heat transfer en hancements over those of a constant property fluid and gave excellent agree ment with experimental results in both the regions of thermally developing and fully developed. The heat transfer enhancement results from an increase d fluid mixing near the heated top wall, which is attributed to both the ef fects of the temperature-dependent viscosity and secondary flow induced by second normal stress difference. The present study concludes that the heat transfer enhancement of the viscoelastic fluid in a 2:1 rectangular duct is caused by the favorably combined effect of temperature-dependent viscosity and normal stress-induced secondary flow. (C) 1999 Elsevier Science Ltd. A ll rights reserved.