Eventos Anais de eventos
COBEM 2021
26th International Congress of Mechanical Engineering
NUMERICAL ANALYSIS OF MIXED CONVECTION IN A RECTANGULAR CAVITY AT DIFFERENT ASPECT RATIOS
Submission Author:
Roberto Bigonha , DF , Brazil
Co-Authors:
Roberto Bigonha, Adriano Rosa
Presenter: Roberto Bigonha
doi://10.26678/ABCM.COBEM2021.COB2021-0403
Abstract
Natural or free convection can be defined as the form of heat transfer in which the flow occurs as a result of buoyancy forces in the presence of gravity and density variations between fluid layers. Forced convection depends on the presence of an additional mechanism which is responsible for driving the flow. Mixed convection occurs when the mechanisms of both natural and forced convection have significant influence on the behavior of the flow and the resulting heat transfer. The contribution of each motion depends on factors such as temperature gradient, type of flow, external source of movement and geometry. One largely known application for this kind of heat transfer is the cooling of electronic components using air. We analyze, in this work, the effects of mixed convection in a rectangular closed cavity filled with air. We investigate the influence of the aspect ratio on local and average Nusselt numbers for two situations, namely the assisted flow - when forced convection and natural convection go the same way - and the opposing flow - when forced and natural convection go in opposing ways. The left wall is kept at a constant high temperature and the right wall is kept at a fixed low temperature, while the lid and the bottom walls are both adiabatic. The lid is maintained at constant velocity, going right in the case of assisted flow and left in the case of opposing flow. Combinations of Reynolds and Grashof numbers are analyzed. A dimensionless parameter, the Richardson number (defined as the ratio between Grashof and Reynolds squared), is used to express which kind of convection is predominant. If the Richardson number is of order unity, both types are important; If it is much smaller than one, natural convection is unimportant; and if it is much greater than one, forced convection is unimportant. We take the fluid as incompressible and we restrain our investigation to the laminar flows. The governing equations are the Navier-Stokes equations with Boussinesq approximation and the energy equation. A first order projection method with a staggered grid was used to solve the governing equations. The results show that the local Nusselt number increases with the aspect ratio for both assisting and opposing flows. Plots of temperature and velocity profiles end the paper and help to elucidate the physics governing this category of fluid motion.
Keywords
numerical heat transfer, Mixed Convection, Rectangular cavity, Projection Method
