An analytical and experimental investigation of natural convection heat transfer in vertical channels with a single obstruction

This dissertation is concerned with the heat transfer due to natural convection flows of air in a vertical channel with a single obstruction. The work is separated into two sections: one dealing with experimental measurements and the other with the numerical solution of the two-dimensional conservation equations.

The numerical work consists of the solution of the governing differential equations. The equations which are derived using the Boussinesq approximation, are solved using a general purpose finite element computer code called NACHOS. The NACHOS program is based on the Galerkin form of the finite element method (FEM) and is designed for the solution of transient or steady state laminar flows described by the Navier-Stokes equations. Both thermal boundary conditions, uniform wall temperature (UWT) and uniform heat flux (UHF), are considered numerically.

In the experimental study, optical techniques were used to obtain measurements of both quantitative data (heat fluxes and temperatures) and qualitative data (flow visualization). The range of Rayleigh numbers for these tests was 2 x 103 < Ra ^ 105. Only uniform wall temperature (UWT) boundary conditions were investigated experimentally.

The main objective of this dissertation is to perform a combined numerical and experimental study of the effects of a single obstruction on natural convection flows in a vertical channel. It is found that the presence of the obstruction reduces the average Nusselt number by 5% at a Rayleigh number (Ra') of 104 to about 40% at a Rayleigh number (Ra') of 10, for a uniform wall temperature (UWT) boundary condition. It is also found that the location of the obstruction along the wall affects the rate of heat transfer. Moving the obstruction away from the entrance towards the exit reduces the the average heat transfer rate for the channel. For the case of uniform heat flux (UHF) boundary conditions; it is found that the maximum temperature occurs at the intersection of the top edge of the obstruction and the wall is only 4% higher than the maximum temperature for an unobstructed channel (which occurs at the exit of the channel).