Combined convection and radiation in simultaneously developing laminar flow and heat transfer is numerically considered with a discrete-direction method. Coupled heat transfer in absorbing emitting but not scattering gases is presented in some cases of practical situations such as combustion of natural gas, propane, and heavy fuel. Numerical calculations are performed to evaluate the thermal radiation effects on heat transfer through combustion products flowing inside circular ducts. The radiative properties of the flowing gases are modeled by using the absorption distribution function model. The fluid is a mixture of carbon dioxide, water vapor, and nitrogen. The flow and energy balance equations are solved simultaneously with temperature dependent fluid properties. The bulk mean temperature variations and Nusselt numbers are shown for a uniform inlet temperature. Total, radiative and convective mean Nusselt numbers and their axial evolution for different gas mixtures produced by combustion with oxygen are explored.
Problem statement: Modeling thermal radiation with simultaneous buoyancy and forced convection for real gases flowing inside system with complex geometry is a difficult task encountered in engineering applications. Purpose of this study was to research numerically the interaction of mixed convection and thermal radiation in laminar air flow inside an inclined cylindrical duct with Uniform Wall Heat Flux (UWHF). This study highlighted the radiative double effects of water vapor in air flow on thermal and on dynamic fields. Approach: Flow equations and energy balance equation were solved simultaneously with temperature dependant thermophysical properties. An implicit finite difference technique was used to solve mass, momentum and energy equations. In order to take into account the non-gray radiative behavior of water vapor (H 2 O), a global absorption distribution function model was used to represent the infrared radiative properties. Results: Results were presented in term of temperature, velocity and radiative power fields and of evolution of bulk temperature and Nusselt numbers. Effects of thermal radiation on temperature and on velocity distributions were also examined. Conclusion/Recommendations: It was shown that inclination angle of duct had a significant effect on thermal and dynamic fields especially for thick medium. Radiation strongly affected the velocity profiles. Numerical results were discussed referring to available experimental data in order to improve estimations of engineering parameters.
This chapter provides a specific study of the performance of thermal systems, principally heat exchangers, which are applied in several industrial applications such as chemical industry, energetic industry, industrial lasers, and so on. These thermodynamics systems were critical in transferring heat from a higher to a lower temperature fluid. They have been used for several years and are available currently for various designs. Thermodynamic properties influence the heat transfer and the performance of heat exchangers. Therefore, it is important during the design of heat exchangers to select primary the accurate operating conditions in terms of thermodynamics to provide a minimum amount of entropy generation in the system. In this study, the concept of entropy is used to analyze heat transfer processes from the thermodynamic viewpoint through the second law of thermodynamics. To assess heat exchanger performance, investigations are given for entropy generation, entropy generation number, and efficiency. These studies offer a new way to obtain well-designed heat exchangers.
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