Eventos Anais de eventos
COBEM 2021
26th International Congress of Mechanical Engineering
INTERACTION BETWEEN NATURAL CONVECTION AND SURFACE RADIATION IN A LOW ASPECT RATIO RECTANGULAR ENCLOSURE WITH DIFFERENT WALL EMISSIVITIES
Submission Author:
Eduardo Güths , SC
Co-Authors:
Leandro Manes, Eduardo Güths, Saulo Guths
Presenter: Eduardo Güths
doi://10.26678/ABCM.COBEM2021.COB2021-1827
Abstract
The prediction of heat transfer in differentially heated vertical cavities filled with air is important for a number of engineering applications, including heating and cooling in buildings, heat transfer through windows (e.g. double glazing systems), electronics cooling, thermal insulation, solar calorimeters, nuclear reactors and many others. The interaction of coupled surface radiation and laminar natural convection in a vertical cavity heated from the side and with horizontal adiabatic walls is analyzed numerically in this study. The numerical calculations were performed using the numerical software ANSYS CFX. Air flow in the cavity is governed by the Navier-Stokes equations under the Boussinesq assumption. Also, it is assumed that air is incompressible and radiatively non-participating, meaning that it doesn´t absorb the high temperature wall radiation. The study covers Rayleigh Numbers of 10^3 to 10^6 and aspect ratios of 1 to 10. The emissivity of the walls varies from 0 to 1 and the values differ or are the same for each wall depending on the case. The temperature of the cold wall was fixed at 288.5 K and that of the hot wall at 298.5. The results of this study show that the radiation heat transfer influences the overall heat transfer even for cavities with low emissivity. When the radiation was included, as expected, an increase in the emissivity of all walls resulted in greater radiative heat exchange and, consequently, higher values. It was interesting to note that an increase in leads to an increase out. The opposite was observed for in the case of pure convection. They also indicate that the total Nusselt number decreases with increasing aspect ratio. In the cases where the walls have different emissivity values it is clear that the influence of the horizontal wall emissivity was small compared to that of the vertical wall emissivity. The separate analysis of cold and hot wall surface emissivity shows that the increase in Nu_r with an increase in ε_h is slightly higher than that with the increase in ε_c. Although the effect of ε_h is greater, is it clear that Nu_r is strongly dependent on both ε_h and ε_c. Thus, in cases where the goal is to reduce the heat transfer, decreasing the emissivity of only one of the walls is effective.
Keywords
Heat transfer, convection heat transfer., Radiative heat transfer, Natural Convection
