Abstract
The main objective of this paper is to explore the existence of thermodynamic irreversibility due to laminar fluid flow with heat transfer under fully-developed conditions through a duct of circular cross-section. The evaluation of thermodynamic trade-offs caused by simultaneous heat transfer under a finite temperature-difference and fluid friction has been examined in terms of dimensionless entropy generation as a performance criterion. The temperature dependence of viscosity is taken into consideration in the analysis. Expressions involving relevant variables for entropy generation and pumping power for constant viscosity and temperature-dependent viscosity have been derived. The dimensionless entropy-generation defined on the basis of total heat-transfer rate attains a minimum along the duct length and the ratio of pumping power to total heat-transfer rate increases considerably along the duct length when the fluid is heated. The dimensionless entropy-generation increases as the dimensionless ratio of inlet wall to fluid temperatures (
α) increases, but the pumping power ratio decreases as
α increases. The results correspond to the constant viscosity assumption and temperature-dependent viscosity cases are compared and it was found that the constant viscosity assumption may yield a significant amount of deviation in entropy-generation and pumping power from those for the temperature-dependent viscosity case, especially for more viscous fluids.