Numerical modelling of entropy production in mixed convection heat transfer

Ogban, Peter Unor (2020) Numerical modelling of entropy production in mixed convection heat transfer. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Energy losses in fluids engineering systems occur due to thermal and viscous irreversibilities. These irreversibilities can be tracked to identify regions of design modification for efficiency improvement in thermofluid systems. The rate of entropy production in numerical heat transfer is an important parameter that characterizes the degree of these irreversibilities. This can lead to improved designs with higher system efficiency levels for energy savings in various engineering applications. Previous conventional techniques have generally detected energy losses on a global scale or end-to-end basis. This thesis focuses on two-dimensional numerical modeling of entropy generation and the Second Law of Thermodynamics in mixed convection heat transfer. A Control-Volume Based Finite Element Method (CVFEM) is used to discretize and solve the governing conservation equations. An entropy-based algorithm is developed by post-processing of the velocity and temperature fields to obtain numerical predictions of the rate of entropy production. The new model is used to analyze heat transfer and entropy production for both natural and mixed convection in enclosures filled with different fluids, including nanofluids. The optimal conditions for which viscous and thermal irreversibilities are minimized is analyzed. The results from Computational Fluid Dynamics (CFD) are validated using available benchmark data. A new approach for minimizing the rate of entropy production in different flow configurations with nanofluids is also obtained. In addition, the local entropy production rates are obtained from two forms of the discretized Second Law – namely, transport and positive-definite forms of the entropy transport equation. The computed local entropy generation rates from both methods are compared and related to expected numerical errors from available benchmark solutions. An entropy-based error indicator is determined to assess the solution accuracy of fluid flow simulations with heat transfer using the Second Law of Thermodynamics. The formulation presents a new approach for the characterization of numerical error using a parameter called the “apparent entropy production difference.” Furthermore, a corrective mechanism on the numerical algorithm is developed. The transport entropy is used to calculate an artificial viscosity (named as an entropy-based artificial viscosity) to reduce the numerical error and ensure closer compliance with the Second Law.

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