Modélisation du transfert thermique par convection naturelle dans les géométries pratiques.

Abstract

The objective of this doctoral thesis is to study the thermal and dynamic structures of the flow resulting from natural convection heat transfer within different practical geometries, applying the most recent methods used to improve the heat transfer. For this reason, two studies on this subject have been carried out. In the first study, a numerical study was carried out on heat transfer by natural convection using two nanofluides inside a horizontal cylindrical enclosure with partial heating of its lower part at constant temperature or constant heat flux, the length of the heat source is changed from 5% to 25% of the total perimeter of the enclosure, the rest of the unheated parts of the bottom wall are considered as adiabatic, the two side parts of the enclosure are considered at a low constant temperature, each one of them has a length of 25% of the total perimeter of the enclosure, the top part of the enclosure is considered as adiabatic, it has a length of 25% of the total perimeter. To analyze the effect of using nanofluids with different particles volume fractions on heat transfer inside cylindrical horizontal enclosures, two nanofluids (Cu-water, TiO2-water) were used with volume fraction of nanoparticles being varied in the range of 0 corresponding to pure water) to 0.05. The steady state forms of Navier-Stokes equations and the equations of conservation of mass and energy in twodimensional cylindrical coordinates have been solved by the finite volume method and the SIMPLE algorithm was used for the pressure-velocity coupling. The Rayleigh number was varied in the interval 103 to 106. This research is characterized by a detailed analysis of the effect of changing both the Rayleigh number, the heated length and the value of nanoparticles volume fraction on the dynamic and thermal fields, on the percentage enhancement of the average Nusselt numbers, and also on the variation of the temperature and the vertical velocity component at the vertical and horizontal central lines of the enclosure. The results obtained were summarized in the form of correlation equations of the average Nusselt number as a function of the heated length, the Rayleigh number and volume fraction for both types of nanofluids. In the second study, a numerical study was carried out on the effect of using two techniques for the optimization of heat transfer by natural convection in a three-dimensional spherical enclosure of solar water heater. The outer wall (solar collector) is considered as a heated wall at a constant temperature, the wall which is located behind the solar collector is considered as adiabatic wall. In the enclosure there is a spiral tube, its wall is under a low temperature which increases linearly in terms of enclosure height. The first technique is to exploit nanotechnology using two types of nanofluids Cu-Water and CNT-Water, to improve the heat transfer in the enclosures of spherical solar water heaters, the volume fraction of nanoparticles was varied in the range of 0 (corresponding to pure water) to 0.05. The second technique involves making geometric modifications to the enclosure of spherical solar water heater to improve the natural convection heat transfer, these modifications consist in creating concentric annular orifices in the lower part of the internal adiabatic wall of the solar water heater, the number of annular orifices varies from 1 to 6. The Rayleigh number was varied in the interval 104 to 106. The steady state forms of Navier-Stokes equations and the equations of conservation of mass and energy in three-dimensional spherical coordinates have been solved by the finite volume method. The SIMPLE algorithm was used for the pressure-velocity coupling. This research is characterized by a detailed analysis of the effect of changing both the Rayleigh number, the number of annular orifices and the value of nanoparticles volume fraction on the dynamic and thermal fields and on the heat transfer and its percentage improvement. The results obtained were summarized in the form of correlation equations of the average Nusselt number as a function of the Rayleigh number, the number of annular orifices and the volume fraction of the nanofluid. Through the results obtained in all the studies carried out in the frame of this doctoral thesis, a very important theorem has been reached on the improvement of heat transfer by natural convection in enclosures. The application of this theorem can revolutionize the field of performance improvement of various thermal engineering systems whose operating principle depends on heat transfer by natural convection.